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The A denine and Thym ine M olecules.
Some E xcited Singlet and T riplet Levels
o f the Norm al and Tautom eric Forms
Julio M arañ on and Oscar M. S orarrain
Departam entos de Fisica y Quimica, F aeultad de Ciencias
Exactas, Universidad Nacional de La Plata
values given by S u tto n 6 for both molecules. We
have taken 30 configurations with a 0.2 Ä mash
for the movement of the H atom.
In Fig. 1, we show the relative position of the H
atom in both conform ations: normal and tauto­
meric.
(Z. Naturforsch. 32 c, 647 — 649 [1977] ; received
Decem ber 17, 1976/M arch 11, 1977)
Forms
:h
normal
tautomeric
Adenine, Thymine, Barriers, Singlets, T riplets
:
The shape of the barrier for the H transfer between the
norm al and tautom eric forms have been determ ined for the
adenine and thym ine molecules. The correlations for several
singlet and triplet states has been calculated using the
CNDO/2-CI method. The relative stability between both
conform ations and the agreem ent with some experim ental
values is also discussed.
h
:
h
Adenine
Introduction
The theoretical analysis for the molecules of
adenine and thymine has been undertaken by former
authors using semi-empirical and ab-initio meth­
ods 1-4. In these papers several results with their
biological applications has been reported for the
normal and tautom eric conform ers in both mole­
cules.
The relative stability for both conformers in each
molecule is one of the interesting problems con­
sidered by the mentioned authors. In this paper, we
have calculated the shape of the b arrier between
both conformers for a particular movement as an­
other point of view for the theoretical analysis of
the stability corresponding to the different molec­
ular conformers.
For this purpose, several excited singlet and trip ­
let levels have been obtained using the semiempirical CNDO/2-CI method.
M ethod and Results
In a previous p a p e r5 we have discussed the
relative stability of the norm al and tautomeric con­
formations in the ground electronic configuration.
Now, we have calculated the shape of the barrier
corresponding to the proton transfer between both
conformers in a group of singly excited singlet and
triplet states. We have used a standard CNDO/2-CI
program . The molecular param eters (bond angles
and interatomic distances) has been taken from the
Requests for reprints should be sent to Dr. 0 . M.
Sorarrain, Faeultad de Ciencias Exactas, Departam ento
de Fisica, Calle 115 y 49 C.C. No. 67, La Plata, Argen­
tina.
v/m ine
Thymine
Fig. 1. Normal and tautom eric conformers adenine and
thvmine.
The excited singlet levels calculated for the mole­
cule of adenine correspond to the first and third
spectral region following the classification given by
T inoco1. The energy levels for the first spectral
range are: 3.92, 4.31, 4.72, and 4.95 eV. For this
range the only available experimental value is
4.9 eV. In the third region (5.9 —6.7 eV) we have
determined the following values: 5.55, 5.92, 6.12,
6.44, and 6.87 eV.
The correlation between the excited energy levels
corresponding to the different H positions have
been determined following the criterion given by
A n h 7 (no crossing for levels with the same sym­
metry) . In general, there are difficulties in the case,
when is in the middle of the distance between both
conformers because there is also a contribution
from configurations of both symmetries (even or
odd) for some excited levels. Hence these dubious
cases we have left out from our figures. The results
are shown in Figs 2 —5.
In Fig. 2, we give the singlet o — n* and n — n*
transitions for the molecule of adenine.
The values for the n — n* and o —ti* triplet
transitions are shown in Fig. 3. The excited triplet
levels for the normal conformer are: 2.75, 3.26,
and 3.52 eV. These values obtained in our calcula­
tions should be compared with the only one avail­
able experimental value: 3.9 eV 9.
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648
F or the thymine molecule we have calculated
some lines for the normal conform er: 4.64, 5.82,
6.42, and 6.60 eV. The agreement with the experi­
mental values 4.67 and 5.94 eV is really satisfac­
tory 8. The correlation between both conformers for
the o — n* and n — n* excited singlet and triplet
states are given in Figs 4 and 5.
former authors 3> 4. Anyway, some results are inter­
esting enough to be pointed out, i. e.: the 1st o —n*
singlet transition predicts a more stable conform a­
tion of the normal molecule and the o — n* triplet
one of the tautomeric form.
Summing up: the values given in Figs 2 —5 show
the following: a) The tautom eric conformer of the
molecule of adenine is m ore stable for some singlet
and triplet states (see Figs 2 —3 ), i.e .: the normal
conform ation has low stability for some singlet
n — n* levels. On the contrary, the o — n* singlets
corresponding to the norm al conformer are more
stable than the tautom eric ones (there is one excep­
tion, the 1st excited level, see Fig. 2 ).
We have determined the intersection of the energy
surface (two coordinates are necessary for the move­
ment of the H in the molecular plane) with a plane
normal to the movement of the H in the molecular
plane. The results often show that the tautomeric
forms are more stable than the normal ones both
the adenine and thym ine molecules. If these con­
clusions are right it should be possible to determine
experimentally the tautom eric equilibrium constants
not only of the electronic ground state 10, but also
for some excited levels of the free nucleotide.
b) In the case of the two conformers of the
thymine molecule we have sim ilar troubles than the
Final Comments
A d e n in e
Thymine
singlets
or-~ 7 i* singlets
7.0
7.0
6.0
‘6.0
5.0
5-0
tautomeric
4.0 _ n o m n a l^ ^ s ^ ^ c > ^ < > -o ^
-
Movement of the H [X]
Fig. 2. Shape of the barrier between both conformers. Ex­
cited singlet levels, molecule of adenine.
0.8
0.0
1.0
4.0
normal
j
2.0
3.0 -0.8
0.0
Movement of the H [Ä]
/
^
tautomeric^
1.0
2.0
3.0
Fig. 4. Shape of the barrier between both conformers. E x­
cited singlet levels, molecule of thymine.
Th ym in e
A d e n in e
-0.3
0.0
1.0
2.0
3.0-0.8
0.0
Movement of the H [X]
1.0
2.0
3.0
Fig. 3. Shape of the barrier between both conformers. E x­
cited triplet levels, molecule of adenine.
Movement of the H [X]
Fig. 5. Shape of the barrier between both conformers. E x­
cited triplet levels, molecule of thymine.
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If the bases are within the DNA molecule a
radiative excitation process should lead to the tauto­
meric form s: A*, T*. These conform ers have an­
other pairing pattern than norm al one: A* —C and
G —T*. These errors can propagate at cell duplica­
tion following the schema given by L öw dinn . A
single excitation could cause thus changes in the
genetic information, but the whole process is very
complicated because the rate of decay from the
excited level to the ground state must also be con­
sidered.
1 A. Pullm an and B. Pullm an, Advances in Quantum
Chemistry, Vol. 4, 267, Academ ic Press, New York-London 1968.
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5 O. M. Sorarrain and E. A. Castro, Chem. Phys. Letters
19, 422 [1973].
6 L. E. Sutton, Tables of Interatom ic Distances and Con­
figuration in Molecules and Ions, P. M. 181, The Chemi­
cal Society, London 1965.
7 H. Trong Anh, Las reglas de W oodward-Hoffmann, E di­
torial Reverte, Barcelona 1974.
8 J. Ladik and K. Appel, Theoret. Chim. A cta (Berl.) 4,
132 [1965].
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Theoret. Chim. Acta (Berl.) 10, 231 [1968].
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Solid, Preprint B7, Q uantum Chemistry Group, U ppsala
University 1963.
We should like to express our gratitude to the Consejo
Nacional de Investigaciones de A rgentina and the O. E. A.
for their help during the development of this paper.
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