90
Bulletin of Magnetic Resonance
THALLIUM NMR SPECTROSCOPY
J. F. Hinton
Department of Chemistry and Biochemistry
University of Arkansas, Fayetteville, AR 72701
Contents
I.
II.
Introduction
General Characteristics
A. Nuclear Properties of 205 Tl and 203 Tl
B. Chemical Shift
C. Spin-Spin Coupling
D. Line Broadening and Relaxation
E. Summary
III. Chemical Applications
A. Solution state studies
B. Solid state and melt studies
90
91
91
91
92
94
96
96
96
99
IV. Biochemical Applications
A. Biological properties of thallium
B. Biochemical studies using Tl NMR
103
103
104
V.
104
I.
References
Introduction
Dr. B. E. Mann has introduced the term "Cinderella Nuclei" to classify those spin 1=1/2 nuclei
that were very difficult to directly observe during
the early developmental years of nuclear magnetic
resonance spectroscopy (1). These nuclei, 187Os,
183
W, 109Ag, 107Ag, 103Rh, 89Y, 57Fe, became relatively easy to observe with the advent of high magnetic fields, the pulse Fourier transform technique
and inverse detection methods. For several of these
elements, NMR spectroscopy has become an important complementary probe for the exploration of
their rich and beautiful chemistry. Thallium (203Tl
1=1/2 and 205Tl 1=1/2), however, is very easy to
detect in the NMR experiment. By 1953, solid (2)
and liquid state (3) spectra had been obtained for
both nuclides. Consequently, thallium would not be
characterized as a "Cinderella Nucleus" according
to the above definition. There is a sense in which
thallium exhibits the Cinderella syndrome in that it
was almost twenty years after first being observed
in the NMR experiment before it found an appreciative constituency. This period of inactivity in
thallium NMR spectroscopy was associated with the
lack of readily available multinuclear NMR spectrometers to chemists, an unfocused attention to the
organic chemistry of thallium and, perhaps, with the
very toxic nature of thallium compounds (4). The
emergence of thallium NMR spectroscopy was most
probably the result of the juxtaposition of two factors. In the 1970's multinuclear NMR spectrometers became commercially available and thallium
compounds and intermediates became very useful
in organic synthesis (5), thus producing a wide variety of organothallium compounds for study. The
intent of this review is not to present the entire body
of the thallium NMR literature along with extensive
data tables but rather to present a selective segment
to function in a heuristic manner with the hope of
Vol. 13, No. 3/4
adding to that appreciative constituency. The emphasis will be placed on the general characteristics
of thallium NMR spectroscopy with selected applications to the solid and liquid state. An extensive
review of the thallium literature containing many
tables of data was published recently (6).
II.
A.
General Characteristics
Nuclear Properties of 205Tl and 203Tl
Because of their intrinsic nuclear properties,
both isotopes of thallium are well suited for the
NMR experiment. 205Tl (70.5% natural abundance)
and 2O3T1 (29.5% natural abundance) have spin of
1=1/2. The magnetic moments (/x//xN) for 205Tl
and 203 Tl are 2.7914 and 2.7646, respectively.
The resonance frequencies of 205 Tl at a magnet field
strength of 11.75T, 7.05T and 2.3488T are 288.474
MHz, 173.108 MHz and 57.708 MHz, respectively.
For the 203Tl nucleus the resonance frequencies at
the same magnet field strengths are 285.805 MHz,
171.506 MHz and 57.149 MHz. The relative receptivity of the more abundant isotope, 205Tl, is 0.1355
with respect to the proton which is assigned a value
of 1. This makes the 205Tl nucleus the fourth most
receptive spin 1=1/2 nuclide.
The very high receptivity of both 205Tl and 203Tl
nuclei allows them to be directly observed with very
little difficulty. Spectra of 205Tl in solution can be
obtained from samples with thallium concentrations
in the submillimolar range.
B.
Chemical Shift
Not only are the thallium isotopes relatively
easy to directly observe in the NMR experiment, the
physical properties measured (i.e., chemical shift,
spin-spin coupling and relaxation) are exceptionally
sensitive to the chemical (i.e., electronic) environment in which the thallium nucleus is placed. Thallium NMR spectroscopy, therefore, is a very useful technique for studying a wide variety of physical
and chemical interactions in the solid and solution
states. The T1(I) ion is also a legitimate surrogate
for the Na(I) and K(I) ions in many biological systems, thus providing a convenient spectroscopic alternative for the study of such phenomena as activation and regulation of enzymes and ion binding.
91
The outer electronic configuration of thallium,
6s 6p1, suggests that this element should exist in
the +1 and +3 oxidation states. Both oxidation
states are well known for thallium, however, oxidation potential data suggest that the +1 state
should be more stable in aqueous solution that the
+3 state where extensive hydrolysis and complexation occurs. In general, solution studies involving ion-solvent interactions, ion-pairing, for example, have been performed with T1(I) and organothallium compounds have been used to study Tl(III).
However, some Tl(III) compounds have been used
for solution studies.
2
Although the variety of thallium compounds is
not as extensive as that of other metals, such as Co,
the available oxidation states of (I) and (III) provide
an interesting array of electronic environments for
thallium and concomitant chemical shift groupings.
For example, the resonance frequency of Tl(III) is
normally higher than that of T1(I). The chemical
shift range of thallium is extremely large, covering
approximately 7000 ppm for Tl(III) species and over
3400 ppm for T1(I) species. The origin of this large
chemical shift range lies within the paramagnetic
term of the nuclear screening constant. The total
diamagnetic shielding of the thallium atom is quite
appreciable, being about 10000 ppm. However, in
chemical bonding the core electrons are relatively
unperturbed, the valence electrons being most affected. Therefore, the important quantity in determining the chemical shift is not the total shielding
but the shielding per valence electron. For thallium,
the diamagnetic contribution per valence electron is
of the order of 10-250 ppm. This is relatively small
compared to the observed chemical shift range of approximately 7000 ppm. This means that the paramagnetic term must be responsible for about 90%
of the variation observed in the thallium chemical
shift.
The direct measurement of the paramagnetic
shielding is a difficult problem. One method of determining the value of the paramagnetic term for
the thallium nucleus would be to obtain the shielding anisotropy from the nuclear magnetic resonance
spectrum of the nucleus in a linear molecule in the
solid state. Since the parallel component of the
paramagnetic term is zero in the linear molecule,
the measured anisotropy gives the value of the paramagnetic term. For the covalently bonded thallium
92
nucleus, the chemical shift anisotropy would probably be several thousand ppm. A value of 5550 ppm
for the paramagnetic term has been obtained for the
linear dimethylthallium cation (7).
Although the 59 Co nucleus, with a chemical shift
range of 18000 ppm, appears to be more sensitive
to electronic effects than 205 Tl, the greater chemical
shift range of 205 Tl in compounds in which Co and
Tl are in similar environments suggests that 205 Tl
has the greatest NMR 'shiftability' of all the elements. Table 1 contains a selection of 2O5T1 chemical
shifts that illustrates the magnitude of the chemical shift range. The extent of this chemical shift
range can be appreciated from a consideration of
the solvent, phase change, substituent change and
temperature effects on the 2 0 5 Tl chemical shift. The
chemical shift of the 2O5T1(I) ion increases by about
2000 ppm as the solvent is changed from water to
liquid ammonia. The chemical shift of Me2TlNO3
increases by 240 ppm as the solvent is changed from
HMPA to n-butyl amine. The effect of environmental alteration, by phase change, on the chemical
shift can also be equally dramatic. For example,
the chemical shift of T1C1 increases by 925 ppm in
progressing from the melt to the solid phase. Substituent changes produce large chemical shifts: (1)
The difference in chemical shift between TlClsoijd
and Tllsoii<j is 920 ppm; (2) the difference in chemical
shift between Me 3 Tl and Et 3 Tl is 130 ppm and; (3)
The chemical shifts of C 6 H 5 T1(TFA) 2 , 4-Bu t C 6 H 4 Tl
(TFA) 2 , 4-Pr i C 6 H 4 Tl(TFA) 2 and 4-Pr n C 6 H 4 Tl(TFA) 2
are +2790 ppm, +2818 ppm, +2823 and +2816 ppm,
respectively (13). The chemical shift can also be
very temperature dependent, as illustrated by the
fact that, in liquid methylamine solutions, the resonance frequency of the T1(I) ion changes 5 ppm/K.
The temperature dependence of the 205 Tl resonance
frequency in the dimethylthallium cation in aqueous solution has been shown to be an accurate, convenient thermometer for determining the temperature of samples within the magnetic field of an
NMR spectrometer (8). These examples serve to
illustrate the fact that the 205 Tl chemical shift is
an exquisitely sensitive probe for subtle changes in
environment. This feature of the NMR properties
of 205 Tl makes it quite useful for investigating such
phenomena as ion-pairing, solvent-solute interaction,
phase changes, long-range substituent effects and
binding constants.
Bulletin of Magnetic Resonance
The chemical shift anisotropy has been measured
for the 205 Tl nucleus in a number of chemical environments. These are shown in Table 2. The values
extend from 55 ppm for 7.5% T1NO3 in KNO 3 to
5550 ppm for M 2 TlBr. Again, the magnitude of
this range of this chemical shift property epitomizes
the sensitivity of the 2 0 5 Tl nucleus to the chemical
environment in which it is placed.
C.
Spin-Spin Coupling
Just as the 2O5T1 nucleus exhibits a robust chemical shift range, the range in magnitude of the spinspin coupling constants involving this nucleus is also
very large. For example, one-bond coupling constants with the 13 C nucleus can be almost 11000
Hz and a six-bond 2O5T1-1H coupling constant of 66
Hz has been observed in certain organothallium(III)
compounds. Representative spin-spin coupling constants for Tl(III) are shown in Table 3. Because
of the highly ionic nature of T1(I) complexes, few
coupling constants have been observed for this oxidation state. An example of spin-spin coupling
with T1(I) is found in the Tl(I)-valinomycin complex where 2 J( 2O5 T1- 13 C) = 96 and 101 Hz (47).
The data contained in Table 3 illustrate a number of features of thallium coupling constants. As
mentioned above, one-bond coupling constants can
be thousands of Hz. For example, 1j(2O5Tl-3lP) =
3203 Hz, * J( 2O5 T1- 13 C) > 10000 Hz and l J(2O5T1-1H)
= 6144 Hz. Long range coupling constants ( 5or6 J)
can be about 100 Hz. Substituent effects are very
pronounced. For example, 2J(2O5T1-XH) changes by
70 Hz if the methyl group in Me 3 Tl is replaced by
ethyl groups and the 1 J( 2O5 T1- 13 C) coupling constant changes from 10718 Hz in C 6 H 5 Tl(OAc) 2 to
8841 Hz in 2,4,6-Me3PhTl(OAc)2- The effect of
change in isotope from 20S Tl to 203 Tl can also be observed. The 2 0 3 Tl coupling constants are less than
those of 2O5T1 (J(2O5T1-1H) = 387 Hz and 3 J( 2O3 T1X
H) = 384 Hz in the compound, T1N(CH2CO2")3),
as dictated by the difference in magnetogyric ratio.
Thallium coupling constants can also be solvent and
temperature dependent. The value of 2 J( 2O5 T1- 1 H)
for Me 3 Tl in acetone is -268.8 Hz and -250.8 Hz
when CH2C12 is the solvent. The value of 2 J( 2O5 T1X
H) for Me 3 Tl dissolved in CH2C12 at -30 °C, -50
°C and -70 °C is -232.8 Hz, -243 Hz and -250.8 Hz,
respectively.
The large coupling constants associated with thai-
93
Vol. 13, No. 3/4
Table 1:
COMPOUND
Tl 3 Fe(CN) 6
Me 3 Tl
Et 3 Tl
Ph 3 Tl
(Me 2 T10Et) 2
Me 2 TlNO 3
Me 2 TlNO 3
Me 2 TlNO 3
Me 2 TlNO 3
Me 2 TlNO 3
Me 2 TlNO 3
PhTlCl 2
PhTlCl 2
C 6 H 5 T1(TFA) 2
(NH4)3T1C16
T1C13
T1C13
TICI3
T1(NO 3 ) 3
T1(C1O4)3
T1(C1O4)3
Til
Til
TIBr
TIBr
T1C1
T1C1
T1F
T1F
T1NO3
T1NO3
T1NO3
T1NO3
TINO3
T1NO 3 /18C6
Tl/valinomycin
Cs 3 Tl 2 Br 9
[NBu 4 ]TlI 4
205
Tl Chemical Shifts"
SOLVENT
solid
n-pentane
n-pentane
Et 2 O
n-hexane
n-butylamine
H2O
Formamide
DMSO
DMF
HMPA
Pyridine
CH 3 OH
THF
solid
solid
melt
H2O
oo-dil. HNO 3
solid
melt
solid
melt
solid
melt
solid
melt
solid
melt
oo-dil.butylamine
oo-dil. (NH 3 ) liq
oo-dil. HMPA
oo-dil. H 2 O
Pyrrole
CHC13
solid
solid
solid
CHEMICAL SHIFT
+14000
+5093
+4963
+3814
+3753
+3586
+3514
+3477
+3450
+3386
+3346
+3087
+2989
+2872
+2220
+2485
+2515
+2307
+2592
+1965
+1885
+1845
+2170
+1100
+1620
+325
+1250
+1300
+1380
+1848
+1819
+502
0
-389
-160
-524
-1194
-1560
REFERENCE
9
10
10
10
10
11
11
11
11
12
11
10
10
13
14
15
15
16
16
15
15
15
15
15
15
15
15
15
15
17
18
17
18
17
19
20
21
21
"Positive values represent shifts to increasing frequency relative to the reference
of T l + at infinite dilution in H 2 O.
94
Bulletin of Magnetic Resonance
Table 2:
205
SUBSTANCE
T1 / valinomycin
TICIO4
TICIO4
7.5% T1NO3 in KNO3
7.5% TINO3 in KNO3
TINO3
TIPO3 (glass)
Til
Til
(Tl 2 Se) x (As 2 Se 3 ) 1 _ x
T12O3
Me 2 TlBr
T1CN
Tl metal
Tl metal
Tl Chemical Shielding Anisotropy (ACT)
TEMPERATURE K
301
307
307
305
305
305
298
401
298
77
298
210
77
1.2
Hum NMR spectrocopy can, in some instances, present
experimental problems because of insufficiently large
spectral window, aliased peaks and computer-limited
resolution. In the observation of nuclei to which
thallium is coupled, it must be remembered that
there are two isotopes and that decoupling of one
might still leave a relatively complex spectrum.
D.
Line Broadening and Relaxation
Several line width and relaxation studies have
been conducted on 205 Tl and 203 Tl in the solid state.
Because 205 Tl and 203 Tl nuclides have large magnetogyric ratios, direct dipole-dipole interaction is expected to contribute significantly to the NMR line
widths of these nuclides. The solid state spectra of
thallium frequently exhibit line broadening as the
result of indirect spin-spin coupling. This spin-spin
interaction is independent of the magnetic field but
depends on the transmission of nuclear spin information directly through intervening electrons. Although the indirect coupling may be negligible for
light nuclides, the large hyperfine interaction is an
important source of line broadening in many cases
for thallium. The 205 Tl and 203 Tl line widths of
thallium metal and T12O as a function of the percentage of abundance of 2O5T1 have been investigated (27). It was found that the 205 Tl line was
(ppm)
150
117
>95
55
90
>95
560
1030
<730
~500
1870
5550
~1000
2500
1440
ACT
REFERENCE
20
22
22
23
23
19
24
25
19
26
27
7
28
27
28
relatively narrow in highly enriched 205 Tl samples,
whereas the 203 Tl line was broad. The opposite effect was observed with samples enriched with 203 Tl.
It was concluded that spin exchange between unlike nuclei ( 205 Tl and 203 Tl) contributes to the second moment or the line width, while spin exchange
between like nuclei ( 205 Tl and 205 Tl or 203 Tl and
203
Tl) does not affect the second moment. One,
therefore, has a very good method for detecting the
contribution to the second moment or line broadening from exchange interaction in samples containing
thallium at the natural abundance level. Because
203
Tl, whose natural abundance is 29.5%, is surrounded primarily by the unlike 20S Tl nucleus in the
solid, exchange interactions, if present, broaden this
line more than the line of 2O5T1 which surrounded
primarily by other 2 0 5 Tl nuclei. The favorable natural abundance and magnetogyric ratio of 205 Tl makes
this a simple and effective method for detecting the
presence of exchange broadening.
Line broadening resulting primarily from exchange
interactions frequently prevents one from observing
the chemical shift anisotropy (CSA) in the powder
spectrum of thallium compounds. One method of
eliminating this line broadening mechanism is by
dilution of the spin of interest in a magnetically
inert matrix. An example of this technique can
be illustrated with solid TINO3. Since the main
Vol. 13, No. 3/4
95
Table 3: Representative 205 Tl Spin-Spin Coupling Constants
COMPOUND
Me3Tl
COUPLING CONSTANTS (Hz)
2j(205 T 1 _i H ) = _268.8
lJ(205T1_l3c = 1 9 3 0
Et 3 Tl
2jJ2O5T1.lH) = _ i 9 g 2
3j(205 T1 _l R J = 3 9 g 1
2j(205 T1 _i H j = _ 3 2 4
2 2O5
J( T1- 1 H) = - 4 0 8
1 2O5
J( T1- 1 3 C) = 2513
3
J(' 2O5 T1- 1 H) = 912
4 2O5
J( T1- 1 H) = 350
Me2TlN(SiMe3)2
(Me)2TlOAc
C6H5Tl(OAc)2
5j(205 T1 .i H )
CH2=CHT1(C1O4)2
2
2O5
=
n
REFERENCE
29
29, 30
31
32, 33
34
o
1
J(
T l - H g e m ) = 2004
3j(205T1.lHtraas) = 3 7 5 0
3
T1HJ
T1N(CH 2 CO^) 3
J( 2 O 5 Tl- 1 H c i s ) = 1806
ij(205 T 1 _i H ) = 6144
35
3
2O5
1
J( T1- H) = 387
J( 2 O 5 T1- 1 H) = 384
2 2O5
J( T1-' 2O3 T1) = 2560
2 2O5
J( T1- 2O3 T1) = 1200
2j(205T1_203T1J = 5 3 6 2
36
36
3
(TlOEt) 4
(Me 2 T10Et) 2
(Me 2 TlNMe 2 ) 2
C 6 H 5 Tl(OAc) 2
2,4,6-Me 3 PhTl(OAc) 2
X
J( 2U5 T1- 13 C) = 10718
2j(205 T1 _13 C ) = 5 2 7
3j(205 T 1 .13 C ) = 1 Q 4 7
4 2O5
J( T1- 1 3 C) = -202
1 2O5
J( T1- 1 3 C) = 8841
2 J ( 205 T 1 _13 C ) = 512
37
38
38, 39
40
41
3
J( 2 O 5 T1- 1 3 C) = 968
4j(205 T1 _13 C) = ^ 1 6 g
5
J( 2 O 5 T1- 1 3 C) = 107
3j(205 T1 _13 CMe ) = 459
(C 6 F 5 ) 2 TlBr
3
J( 2 O 5 T1- 1 9 F) = 799
J( 2 O 5 T1- 1 9 F) = 343
5 2O5
J( T1- 1 9 F) = 99
J( 2 O 5 T1- U F) = 173
= 84
= 258
= <30
= 77
J(205 T 1 _U9 S n ) = 795
J( 2 0 5 Tl- 1 1 7 Sn) = 760
1 2O5
J( T1- 3 1 P) = 3203
l j ( 2 0 5 T 1 . 3 l p ) = 3144
42
4
(B 1 0 H 1 2 TlMe 2 )-
Sn 8 Tr 5
P2(TlMe2)2(Ph)4
Tl(I)/2,2,2 cryptand
T1(I) /valinomycin
J ( 2 U 5 T 1 - 1 5 N ) / 7 N 7 T I = 850
J(
2U5
13
T1- C) = 96, 101
43, 44
45
46
58
52
96
source of line broadening in pure TINO3 arises from
spin exchange between T1(I) ions, the dilution of
TINO3 in a matrix of KNO3 should produce significant line narrowing of the thallium resonance line
due to the presence of the small magnetic moments
of the potassium nuclides. As shown in Fig. 1, line
narrowing does occur with increased dilution to the
point that the CSA can be observed in the 205Tl
NMR line. The effect of a different environment on
the 205Tl chemical shift is also obvious.
In solution, the spin-lattice relaxation time of
the T1(I) ion has been found to be very sensitive to
environmental effects. In an aqueous solution the
spin-lattice relaxation time for the T1(I) ion changes
from 1.8 s in the absence of dissolved oxygen to 0.024
s in the presence of 1 atm of oxygen (48). This
dependence of relaxation time upon oxygen can be
advantageous because it permits faster pulsing and,
therefore, more rapid spectral accumulation. The
spin-lattice relaxation time of the T1(I) ion is also
solvent dependent (49), as can be seen from the
data contained in Table 4. In aqueous solution, the
T1(I) spin-lattice relaxation time is dominated by
the spin-rotation mechanism (48,50). The 2O5T1(I)
and 2O3T1(I) relaxation times are equal and are independent of solvent isotopic substitution (H2O, D2O),
concentration (0.03 - 2.0 M), anion and resonance
frequency (48,50). However, in non-aqueous solution the spin-lattice relaxation time of T1(I) can be
very dependent upon the chemical shift anisotropy
mechanism (51). In the case the Tl(I)-antibiotic
complexes, the spin-lattice relaxation time is determined by contributions from the spin-rotation, dipolar and CSA mechanisms (52-54).
An excellent study of the relaxation of the
Tl + cation as a function of temperature and magnetic field strength has been published (55). Although the relaxation is dominated by the CSA mechanism, a contribution from the spin-rotation mechanism was also found to be present. Activation energies for the reorientational correlation time and
the angular momentum correlation time were determined to be 19.7 kJ/mol and 17.7 kJ/mol, respectively. At 298 K, the reorientational correlation
time was found to be 39.1 ps and a value of 5.05
x 10~15 s was obtained for the angular momentum
correlation time. The 205Tl spin-lattice relaxation
time of the (CH^Tl"1" cation in an aqueous glycerol solution has been studied as a function of tem-
Bulletin of Magnetic Resonance
perature and magnetic field strength. In this solution medium, the CSA relaxation mechanism was
the only effective relaxation mechanism (56). Theoretical investigations of the importance of the CSA
and spin-rotation mechanisms (57) and other mechanisms (58) for thallium relaxation have been made.
The spin-lattice relaxation time of the (CH3)2T1+
cation is sensitive to the presence of oxygen in solution, however, this dependence is not as great as
that found for the T1(I) cation (48).
E.
Summary
Not only are the isotopes of thallium (205Tl and
Tl) relatively easy to observe in the NMR experiment, the spectral parameters of chemical shift,
spin-spin coupling constant and relaxation are exceptionally sensitive to the chemical (i.e., electronic)
environment in which the thallium nucleus is placed.
Consequently, thallium NMR spectroscopy is a very
useful technique for the investigation of a wide variety of chemical and physical phenomena in both the
solid and liquid states. The T1(I) ion is also a legitimate surrogate for K(I) and Na(I) ions in biological
systems, thus providing a convenient spectroscopic
alternative for the study of cation binding, transport
and activation and regulation of enzymes.
203
III.
A.
Chemical Applications
Solution state studies
Perhaps the most fundamental type of solution system to investigate involves the interaction
between a thallium ion, T1(I) or Tl(III), and a single type of solvent molecule. In practice, this type
of investigation can be very difficult due to the proclivity of the thallium ion to form ion-pairs or higher
order aggregates. To eliminate this problem, one
must obtain the infinite dilution chemical shift of
the ion from an extrapolation to infinite dilution
of the curve representing the relationship between
the chemical shift and ion concentration. The extreme sensitivity of the thallium chemical shift to
the chemical and physical environment means that
one must exercise the utmost care in temperature
control of the NMR probe, the solvent purity and
the determination of the ionic concentration. For a
given solvent, it is advisable to use several thallium
salts whose anions are different to approach the in-
Vol. 13, No. 3/4
97
32 *c
TlNO3
m KNO5
100'
50%
24 '/.
20 V.
14 V.
7.5 %
3.6 '
0
i.
5I.9;5
- 4 0 0 ppm
-200
1
51.905
r~
51.895 M H ;
Figure 1: Solid state 205Tl NMR spectra of pure T1NO3 and TINO3 diluted in a matrix of KN0 3 .
finite dilution point from more than one position on
the chemical shift-concentration curve. A computer
analysis of the curves can then be used to determine
the infinite dilution chemical shift for a solvent.
The chemical shift - salt concentration relationship for a simple ion-pair equilibrium process such
cation, respectively. The Xf a n d Xip terms are the
corresponding mole fractions and Cf and Ct terms
are the free and total cation concentrations, respectively. The ion-pair formation constant for the association process described above may be written
as
as
T1+ + X- ^ Tl+X"
can be described by the equation
= (Cf/Ct)(£f - Sip) + 6ip (1)
where S{ and <5ip are the infinite dilution chemical
shifts of the free (solvated) ion and the ion-paired
ip
r r ri+'v—\l(n, A^rmi-Mry—1
[J-i - ^ J/vT±J V
JI/*- J
^o"\
V^j
where -y± is the mean activity coefficient. Equation
1 may be used to obtain the free cation concentration. From initial estimates of 6{ and <5jp and the
experimentally determined values of 6oks, Cf can be
calculated for each Ct value and then used to calculate, using the extended Debye-Huckel relationship,
7± for each value of C t . Equation 2 then yields K;p
Bulletin of Magnetic Resonance
98
Table 4: 2O5T1(I) Spin-Lattice Relaxation Time"
SOLVENT
Water
Pyrole
Methanol
Formamide
Dimethylformamide
Hexamethylphosphoramide
N-ethylformamide
Dimethylsulfoxide
Pyridine
N-butylamine
a
SPIN-LATTICE RELAXATION TIME (sec.)
1.80
1.05
0.91
0.68
0.66
0.61
0.40
0.18
0.09
0.08
Tl(I) was 0.2M T1NO3 and all solutions were degassed.
for each point; the average of these ion-pair formation constants may be used to calculate a theoretical
chemical shift at each point. Computer analysis can
then be used to determine Kip by varying 6f and <*>ip
to minimize variations between the calculated and
observed chemical shifts. This type of analysis gives
not only the infinite dilution chemical shift and the
K;p but also the ion-pair chemical shift. These parameters have been determined for a number of solvents (60). The value of Kjp was found to increase
with decreasing dielectric constant of the solvent.
A correlation between the infinite dilution chemical
shift of the T1(I) ion and the Gutman donor number (61), a measure of the basicity of the solvent, has
been made for a number of solvents (62). This relationship suggests that the more basic the solvent,
the higher the resonance frequency of the T1(I) ion.
The increase in resonance frequency with increasing
solvent basicity may be interpreted as a measure
of the strength of the interaction between the ion
and the solvent molecules, with the solvent acting
as a Lewis base and interacting both covalently and
electrostatically with ions. Transient orbital mixing
created by ion-solvent collision-induced polarization
of the ion electron cloud may also contribute to the
observed change in resonance frequency. The thermodynamic parameters for the ion-pairing process,
AH and AS, can also be determined by obtaining
Kjp as a function of temperature (63). One must exercise caution when comparing data on ion pairing
obtained by NMR techniques with that from conductance measurements on the same salt system.
Conductance measurements recognize as ion-pairs
all species in which the component ions are not free
to conduct, including contact, solvent-shared and
solvent separated ion-pairs. NMR more easily observes contact and solvent-separated ion-pair equilibria but differentiates less distinctly between free
ions and solvent separated ion-pairs. It is possible that in solvents of low dielectric constant, differences between conductance and NMR data reflect the fact that the NMR technique observes the
equilibrium between contact and solvent separated
ion-pairs while the conductance experiment monitors the equilibrium between free ions and ion-pairs.
Hence, the techniques are complimentary in nature.
The anion and concentration dependence of the
T1(I) chemical shift in aqueous solution and in nonaqueous solvents (3,9,10,14,64-74) has been extensively investigated and chemical shift changes of 10
to 100 ppm observed. The exceptional dependence
of the T1(I) chemical shift on solvent makes thallium NMR spectroscopy a very good probe for the
study of preferential solvation in mixed solvent systems. The chemical shift of 5 mM TINO3, in nine
binary solvent systems has been analyzed using a
nonstatistical distribution of solvate species to describe preferential solvation (75-79).
Thallium chemical shift, spin-spin coupling and
spin-lattice relaxation time studies of T1(I) com-
Vol. 13, No. 3/4
99
plexes with crown ethers (52-54,65,80-82,84) and cryptands (59,83,84) have been used to obtain information concerning ion-ligand binding and solution
structure of these complexes.
Although the Tl(III) ion tends to hydrolyze in
aqueous solution, a number of 2O5T1(III) NMR studies, many involving ligand exchange kinetics, have
been made (3,10,16,21,85,86). A combination of solution and solid state 205 Tl NMR experiments have
been used to study the formation and geometry of
(TlXn)3~n complexes, where X=C1, Br (21). Stability constants and the chemical shifts of the individual species were also determined. This study emphasizes the importance of using solid state NMR
data to better understand solution results. The
chemical shift range for pH, concentration and anion effects is about 2000 ppm. Although no specific
spin-lattice relaxation time measurements have been
made with the Tl(III) ion, a number of linewidths
have been reported for non-degassed solution and
they range from 10 to 5000 Hz (16,21). Recently,
205
Tl (III) NMR linewidth measurements were used
to study the kinetics of ligand exchange in the TICI3HC1O4(3 M) system (86). Rate constants for the
. reactions of
\3-n
\3-n
have been obtained and the activation energy parameters for the dissociatively activated and associatively activated interchange processes determined.
A proton NMR investigation of the Tl(III) complex with the nitrilotriacetate ion revealed a difference in 205Tl and 203Tl spin-spin coupling constants
with the methylene protons, 2O5T1-1H = 387 Hz and
203 T 1 _l R
3 8 4 H z
(87)
The concentration, anion, temperature and solvent dependencies of the NMR parameters of the
monomethyl and dimethylthallium(III) cations have
been investigated. In general, the temperature dependence of the chemical shift is greater than resulting from changes in anion or concentration for the
compounds, (CH3)2T1X (X=NO3, BF4, O2CCH3).
For a number of the (CH3)2T1(III) derivatives, a
linear correlation was observed between the 205Tl
chemical shift and the Drago solvent base parameters (88). Solvent isotope effects on the chemical shift the (CH3)2T1(III) cation in H2O and D2O
have been found to be concentration dependent (48).
Thallium chemical shifts have been obtained for phenylthallium(III) dichloride and its complexes with
PPI13 and dipydridine in methanol and pyridine (10),
diphenylthallium(III) chloride in liquid ammonia
(89), diphenylthallium(III) bromide in DMSO (10),
a series of substituted arylthallium(III) bis(trifluoroacetates) in a number of solvents (13) and triphenylthallium(III) in diethyl ether (10). In general, the
diaryls resonate at approximately 200 ppm to low
field of the monoaryls and approximately 600 ppm
to high field of the triaryls.
B.
Solid state and melt studies
Thallium is one element whose NMR history began with both solid state and solution state investigations. In 1953, seven years after the landmark
papers of Bloch et al. (90) and Purcell et al. (91),
the first solid state thallium NMR paper was published (2). The types of compounds studied by solid
state thallium NMR spectroscopy range from simple inorganic salts to thallium antibiotic complexes
to thallium species contained within zeolites to high
temperature superconductors.
The dominant factors that determine the chemical shift of the thallium nucleus are oxidation state
and electronic interactions with the environment.
In solids, the presence of greater covalency might
be expected to result in large shifts to low field of
solvated, ionic thallium species. This has been observed in a number of cases, however, this is in no
way an inviolable rule since the thallium resonance
in a crystalline salt may be shifted upfield from that
in solution by several hundred ppm. The relative
shifts are determined by the strength of the electronic interactions in the solid compared to those
in solution, which my be substantial. Within a series of solids, the highly ionic systems exhibit upfield
shifts while more covalent systems have resonances
at lower fields.
The thallium NMR spectra of solid samples usually contain very broad resonance lines. These broad
lines are most often described in terms of moments,
where the second moment is the mean square linewidth which has values that depend significantly
upon on the amount of Lorentzian or Gaussian character of the lineshape. Several factors determine
thallium NMR linewidths and second moments in
polycrystalline solids. The most important factors
are the chemical shift anisotropy, direct-through
100
space-dipolar interactions, indirect spin-spin interactions, coupling to quadrupolar nuclei and motional
averaging. In some cases, the chemical shift anisotropy will be significantly larger than the total linebroadening from all other sources. In such cases,
NMR spectra of powders or glasses are characteristic of powder patterns from which the value of the
chemical shift anisotropy and the values of the principle elements of the shielding tensor can be determined. If the line-broadening from chemical shift
anisotropy is smaller than that resulting from other
factors, then broad, symmetrical NMR line are observed. The magnitude of the chemical shift anisotropy can still be determined in this case from the
slope of a plot of the second moment as a function
of the square of the applied magnetic field strength.
Direct dipole-dipole interaction might be expected
to contribute significantly to thallium NMR line widths since the magnetogyric ratios of 205Tl and 203Tl
are large. If the structure of the sample is known,
the contribution of the direct dipole-dipole interaction to the second moment can be determined using
the method of Van Vleck (92). For those cases in
which structural details are unknown, the absence of
an orientation dependence of the linewidth of a single crystal suggests the absence of dipolar broadening. Dipolar broadening is independent of the magnetic field strength.
Line-broadening, as the result of indirect spinspin coupling, has been found to make a significant
contribution to the total line width of thallium NMR
lines. This type of interaction is independent of
the magnetic field strength but depends upon the
transmission of nuclear spin information indirectly
through intervening electrons. The electrons couple
with the nuclear spins via dipolar or hyperfine interactions and correlation between the electrons then
results in the indirect nuclear interactions. Two
types of indirect interactions are possible. When
only s electrons transmit nuclear spin information,
the interaction is of the scalar exchange type. When
s and p electrons are involved, the interaction becomes a tensor quantity and is of the pseudodipolar
type. If both electrons are in p orbitals, both exchange and pseudodipolar interactions exist. In the
single crystal rotation experiment, it is not possible
to separate directly the orientation dependence of
the second moment into dipolar and pseudodipolar
contributions. However, since the total orientation
life
Bulletin of Magnetic Resonance
dependence represents the sum of these, calculation
of the dipolar second moment using the Van Vleck
(92) approach allows the pseudodipolar contribution
to be estimated. Scalar exchange broadening is, of
course, independent of orientation. An example of
spin exchange broadening has been discussed previously for the TI2O3 system.
The effect of quadrupole coupling on the line
width of spin 1/2 nuclei in the solid state has been
investigated (93). It has been shown that quadrupole broadening may add a factor of up to 1.84
times the dipolar contribution to the second moment. Quadrupolar interaction will have the same
effect on both 205Tl and 203 Tl.
Motional narrowing can occur in both the melt
and solid state. Chemical shift anisotropy is averaged by fast rotational reorientation, as are dipolar and pseudodipolar interactions. Exchangebroadened lines may also be narrowed at the onset
of molecular motion. In this case, the narrowing results from the modulation of the distance between
interacting nuclei rather than from angular motion.
The abrupt narrowing of thallium NMR lines is usually taken as evidence for a phase transition in a
solid.
The NMR properties of metals are determined
predominantly by the interactions of nuclei with conduction electrons. The NMR shift of thallium in a
metallic sample arises from the coupling with the
magnetic moments which derive from the spins of
the conduction electrons themselves. This type of
NMR shift found in metals is known as the Knight
shift (94). The Knight shift for thallium in a variety of metallic systems has been measured. A
summary of these shifts can be found in reference
6. The Knight shift can exhibit an anisotropy just
as the chemical shift (95-97). Spin-lattice relaxation in metals and alloys is usually the result of
mutual spin-flips between nuclear magnetic dipole
moments and the magnetic moments of conduction
electrons at the Fermi level. When these electrons
are highly localized, the relaxation time is related
to the Knight shift by the Korringa equation (98).
For the case where conduction electrons localize on
a particular thallium site, the relaxation rate will
increase proportionally above that predicted by the
Korringa equation. Comparison of the experimentally determined relaxation time with that predicted
by the Korringa equation provides a useful method
Vol. 13, No. 3/4
for obtaining information about internal electronic
structure in the metal or alloy.
A number of NMR investigations of the Knight
shift and relaxation of thallium metal in the solid
state and melt have been performed (2,27,96,100106). NMR studies of thallium in a number of alloys
and inter metallic compounds such as, LiCdi_xTlx,
CaCd!_xTlx(0 < x <l), La3TlC, La3Tlo.5Ino.5,
La3Tlo.8Pbo.2, Tl!_xTex, TISe, Tl2Se3, TlAsSe2,
PSe2.5Tl, PSe4Tl, As2Se3-Tl2Se3, (Tl2Se)x-(As2
Se3)i_x, Ino.5Tlo.5, Tl0.3WO3, TlGaSe2, TlNb2O5F,
Tl x NbO 2+x F!_ x , Tl3.5Ta02.5Fo.5, TlPbI3, NaTl and
Na2Tl, have been reported (107-141). A number of
thallium NMR studies of the thallium barium calcium copper oxide superconductor have been made
(142-152) as well as that of the superconductor type,
La3Tl (108). The investigations with the superconductors involved the determination of the shift, spinlattice relaxation time, line shape and vortex lattice
formation.
'i i
The favorable NMR properties of thallium have
permitted a number of investigations of thallium
ions absorbed on surfaces and contained in zeolites
(153). A study of the chemical shift and relaxation
time of thallium in the zeolite, Tl3HGe7Oi6-nH2O
showed that thallium diffusion is rapid in the completely hydrated zeolite with the rate being essentially the same as that of water at room temperature
(154). The thallium chemical shift was also found
to be very dependent upon the amount of water
present (154,155). 205Tl NMR spectra of thalliumexchanged zeolites, X(Si|Al 1.2), Y(Si|Al>1.5) and
A(Si|Al 1.0) have been obtained as a function of
temperature and water content (156,157). The spectra of zeolite A was interpreted as being a superposition of three resonance lines and the observed
changes in the lineshape were related to the thermal
motion of thallium cations located near the center of
the eight membered rings. The frequency of jumps
between the four cation sites in the plane of the ring
was found to increase with increasing temperature
and water content. Out of the plane jumps lead to
translational diffusion of thallium cations through
the crystal. 205Tl NMR studies of zeolite A have
also been conducted in which the resonance line was
associated with at least two specific types of cationic
site (158). Additional evidence for the existence
of several thallium cation sites has been found and
the determination of the correlation times allowed
101
a detailed description of the motion of the thallium cation as a function of water content and temperature (159). In general, the effect of hydration
is to increase the mobility of the thallium cations.
The chemical shift and spin-lattice relaxation time
of thallium ions adsorbed on hydrated silica have
been studied (159). The chemical shift of adsorbed
T1(I) ions was found to be about 80 ppm upfield of
aqueous T1(I) acetate. The shift to high field was
attributed to dissociation of ion pairs with acetate
upon binding to the surface. The spin-lattice relaxation measurements suggested the dominance of the
dipolar relaxation mechanism for the adsorbed T1(I)
ions.
The structures of thallium silicate and germanate
glasses of varying thallium content have been investigated using thallium chemical shifts and linewidths
(160-162). Large Tl-Tl scalar exchange interactions
suggest the presence of thallium clusters in these
glasses. Thallium borate glasses have been investigated and at least two basic thallium-containing
subunits apparently exist in these systems, one ionic,
spherically-symmetric and the other more covalent
and less symmetric (163,164). The proportions of
these subunits were found to be dependent upon
the thallium content. A chemical shift anisotropy
of 500-1000 ppm was found for the low-symmetry
subunit. The effect of temperature on the thallium
chemical shift has been measured and explained in
terms of thermally induced overlap of cation and anion wavefunctions (165,166). A number of thallium
phosphate glasses have been studied (24,167). The
thallium chemical shift anisotropy in T1PO3 glass
has been determined to be 560 ppm although the
isotiropic chemical shift is -50 ppm upfield. Thallium
sites in T1PO3 glasses are moderately ionic but not
as ionic as that found in the polycrystalline form.
Thallium NMR spectroscopy has been used to
study a number of paramagnetic compounds such as
TlCoF3, TlMnF3, TlMnCl3, TlNiClg, Tl3Fe(CN)6,
TlFe(SO4)2 and Tl3Co(CN)6 (9,168-170). Unpaired
electron spin density on thallium in these compounds
produce very large shifts to low field, as much as
+14000 ppm for Tl3Fe(CN)6. As might be expected,
thallium linewidths are very large in these compounds,
ranging from 2.5 kHz to 74 kHz.
Solid state and melt thallium NMR studies of
many of the common T1(I) salts have been performed. A number of Tl(III) compounds and a few
102
organothallium compounds have been investigated.
In general, the effect of changes in temperature on
the thallium chemical shift in solids and melts is often very pronounced. For example, the 205Tl chemical shift in solid T1(I) acetate varies by 5 ppm per
degree. It must be mentioned that for the large
number of compounds studied, none exhibits a shift
that has a negative temperature dependence. The
positive dependence of the chemical shift upon temperature arises from the enhanced vibrational overlap of the thallium-anion wavefunctions, resulting
in additional paramagnetic shielding with increasing temperature. Because of the extreme sensitivity
of the thallium chemical shift to changes in chemical
environment, a change in the chemical shift would
be expected to occur at the temperature of a phase
transition. In fact, a change in chemical shift of several hundred ppm can occur at the phase transition.
If sufficient data are available, the shift discontinuity
and temperature dependence of the chemical shift
can be used to calculate the distance of closest approach on ions in melts (15,171). The magnitude
of the chemical shift discontinuity may be used as a
measure of the overall difference in the two phases,
where large discontinuities are associated with the
greatest dissimilarities. For this reason it appears
that the thallium environments in melts of Tl(III)
salts may be more like those in the solids. It seems
likely that strong cation-anion interactions in these
salts persist upon fusion and that the basic units
in the melt may be relatively covalent. Thallium(I)
carboxylates exhibit little discontinuity at the melting point (15,172). The absence of linewidth discontinuity at the melting point also suggests great
similarities between the structures of the solid and
the melt in these compounds (i.e., the car boxy late
anions bind tightly to the thallium cations in both
the solid and melt).
There are cases where thallium linewidths are
very sensitive to phase changes. An excellent example of this is observed with T1NO3. The 2O5T1 linewidth in this compound is essentially independent
of temperature for a given phase, a, j3 or 7. However, a sudden narrowing of the resonance line occurs at the transition from orthorhombic 7 to hexagonal p and again at the transition from (3 to cubic a
(167,173,174). The (3 to a transition is accompanied
by a 100-fold increase in electrical conductivity and
marks the onset of T1(I) diffusion in the solid (173).
Bulletin of Magnetic Resonance
Further support of this was obtained from thallium
relaxation time studies (175,176). Ions, by their
very nature, exhibit only weak electronic interactions with their surroundings. Therefore, the degree
of ionic character limits the magnitude of the chemical shift anisotropy which can be observed for an ion.
Comparatively small chemical shift anisotropies are
anticipated for highly ionic systems, while the potential exists for much greater anisotropies if stronger
ionic interactions are present. There is little doubt
as to the highly ionic nature of 205 Tl in pure T1NO3
since its resonance line lies some 135 ppm upfield
from that of aqueous 2O5T1+. The covalency of this
salt has been estimated to be only 0.6% (15). The
chemical shift of -300 ppm indicates even greater
ionic character for T1(I) isolated in the KNO3. Yet,
at high temperatures even this very ionic T1(I) ion
exhibits a rather remarkable chemical shift anisotropy of 91 ppm. The isotropic chemical shift of 205 Tl
in TICIO4 is quite far upfield suggesting that this is a
very ionic salt. Nevertheless, polycrystalline TICIO4
was found to present a classic powder pattern characteristic of axial symmetry with a chemical shift
anisotropy of 117 ppm (22). Dilution of TICIO4 in
a KCIO4 matrix causes the anisotropy to increase to
149 ppm. That a salt with such high ionic character
can still exhibit a chemical shift anisotropy of that
magnitude testifies to the extreme sensitivity of the
thallium chemical shift to small electronic effects.
A number of thallium NMR studies have been
made of the T1(I) halides. In addition to the determination of the chemical shift of 205Tl in T1F,
it has been shown that the 2O5T1 linewidth is due
mainly to direct dipolar interactions between 205Tl
and 19F (14,15,177-179). Contributions from indirect spin-spin interactions are approximately four
times smaller than those from direct dipolar interactions in this compound (179). Thallium NMR
studies of TlCl suggest that the thallium linewidth
results mainly from scalar interactions between thallium nuclei, however, the relative importance of contributions from other relaxation mechanisms remains
unclear (177,180). Spin-lattice relaxation time studies of TlCl as a function of temperature indicate
that the rapid relaxation observed can be attributed
to hyperfine contact interactions with transient unpaired electrons originating from thermal dissociation (181). For TIBr the thallium NMR linewidth
is significantly larger than that observed for TlCl.
Vol. 13, No. 3/4
Indirect spin exchange has been shown to be a very
important source of line broadening for TIBr (177,
178, 182). The thallium chemical shift and linewidth for TIBr have been found to be independent
of pressure up to 8000 kB cm"1, probably because
of the incompressibility of the salt (177). Relaxation in molten TIBr is apparently dominated by
transient interactions with unpaired electrons and
paramagnetic Tl(II) generated from thermal dissociation of T1(I) (183). T1(I)I exhibits a phase transition from orthorhombic to cubic form at about
160 °C. Thallium NMR spectra of the two forms
are significantly different. The 205Tl resonance lines
in both modifications are shifted far downfield of
aqueous T1(I), with the cubic form to lower field
(25). The thallium linewidths of orthorhombic and
cubic Til are quite different. Upon transition to
the cubic phase, the 205Tl resonance line narrows to
about 30% of its width in the orthorhombic form
and further narrowing occurs with increasing temperature (25). It is of interest to note that in the
orthorhombic phase the 203Tl linewidth is the same
as that of 205Tl but in the cubic form the 203Tl resonance line is about 50% broader than that of 205Tl.
This suggests an important contribution from TlTl exchange interactions in cubic Til but little in
the orthorhombic form (25,184). Indirect Tl-I spin
exchange has been suggested as a major source of
line-broadening in both forms of solid Til, in addition to quadrupole broadening and chemical shift
anisotropy (25). 205Tl chemical shifts have been obtained for molten T12C14 (74,185) and Tl2Br2 (74).
In both cases, two signals were observed which were
attributed to equal amounts of T1X and TIX3. For
the compound, TI2CI3, two signals of relative intensity 3:1 were observed which were attributed to the
presence of T13(T1C16) (14). 205Tl NMR studies of
mixed-salt halides in the solid and melt state have
been performed to explore the effect of covalency on
the thallium chemical shift (74,186,187).
The effect of phase change on the NMR parameters of thallium in the carbonate (14,188), formate
(14,189,190), acetate (188,190) cyanide (28) and thiocyanate (191) salts has been extensively studied.
A number of Tl(III) salts have been studied in
the melt and solid state using 205Tl NMR spectroscopy. The chemical shift of T1(C1O4)3 has been
determined as a function of temperature in the melt
and solid state (15). This salt exhibits the only
103
known negative change in chemical shift upon fusion, implying a decrease in covalency in the melt.
Chemical shift and linewidth measurements have
been made on the T1(C1O4)3-6H2O (15), T1C13 (15,
37) and T1C13-4H2O (21) salts have been studied.
Solids containing the T1X^~ and TIX^2 moiety, such
as Zn(TlCl4)2, KT1C1, KTlBr4-2H2O, Na2TlCl5-4H2O,
Cs2TlCl5-H2O and [NBu4]TlI4 (21). The [NBu4]TlI4
salt exhibits a remarkable chemical shift of -1560
ppm which is about 1000 ppm upfield from the nearest T1(I) salt. Compounds containing the TIX^T3
and Tl2Xg anions have also been studied, where X
is a halogen atom (21).
IV.
A.
Biochemical Applications
Biological properties of thallium
Alkali cations participate in a variety of biological processes. These include transmission of
nerve impulses, initiation and maintenance of muscular activity, synthesis of proteins, regulation of
metabolism, activation and regulation of enzymes.
Most knowledge concerning the biological chemistry
of sodium and potassium has come from standard
biochemical assays, electrochemical experiments and
the use of radioactive tracers. With the very high
magnetic field NMR spectrometers currently available, NMR spectroscopy of the alkali cations in biological systems is quite feasible. However, there
are cases for which the direct observation of some of
these cations is not sufficient to solve the biochemical problem of interest because of the presence of
very low concentrations or because the process of
interest does not produce an adequate response in
the observed NMR property. A number of probe
ions with more favorable spectroscopic properties
can be successfully substituted for investigative purposes (192). A number of investigations indicate
that thallium is a good surrogate for potassium and,
to a lesser extent, sodium in biochemical systems
(192). The relative receptivities to the NMR experiment for 39K(I) and 205Tl are 0.000473 and 0.1355,
respectively. The sensitivity of the chemical shift to
environmental changes of thallium is much greater
than that of potassium or sodium. These facts suggest that 205 Tl is a good spectroscopic replacement
probe for potassium and sodium in biochemical systems.
Bulletin of Magnetic Resonance
104
B.
Biochemical studies using Tl NMR
The structure of the binding site for the monovalent cation activator of S-adenosylmethionine synthetase from Escherichia coli has been characterized by 205 Tl NMR spectroscopy (193). 205 Tl NMR
spectroscopy has been used to determine that the
monovalent cation binding site in (Na/K)-ATPase is
very near the divalent cation binding site (194,195).
The effect of the presence of pyruvate kinase and its
substrates on the chemical shift and relaxation of
2O5
T1(I) has been investigated (196,197) as well as
the interaction of T1(I) with dipalmitoyl-phosphatidylcholine (198). The interaction between the T1(I)
ion and antibiotic molecules, such as gramicidin,
which acts as ion-transporting agents in membranes,
has been studied with 205Tl NMR spectroscopy (199204). The equilibrium binding constants of the Group
I and II cations with gramicidin A incorporated into
model membranes have been determined with a combination of 2O5T1 NMR spectroscopy and competition binding (205-207). 2O5T1(III) NMR spectroscopy
has also been used to study the two binding sites for
Fe(III) in transferrin (208).
Acknowledgement: The financial support of
the NSF through grant DMB-9003671 is gratefully
acknowledged.
V.
l
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