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Interactions between macromolecules and ions: the
Hofmeister series
Yanjie Zhang and Paul S Cremer
The Hofmeister series, first noted in 1888, ranks the relative
influence of ions on the physical behavior of a wide variety of
aqueous processes ranging from colloidal assembly to protein
folding. Originally, it was thought that an ion’s influence on
macromolecular properties was caused at least in part by
‘making’ or ‘breaking’ bulk water structure. Recent timeresolved and thermodynamic studies of water molecules in salt
solutions, however, demonstrate that bulk water structure is
not central to the Hofmeister effect. Instead, models are being
developed that depend upon direct ion–macromolecule
interactions as well as interactions with water molecules in the
first hydration shell of the macromolecule.
Addresses
Department of Chemistry, Texas A&M University, College Station, TX
77843, USA
Corresponding author: Cremer, Paul S ([email protected])
Current Opinion in Chemical Biology 2006, 10:1–6
This review comes from a themed issue on
Biopolymers
Edited by Hagan P Bayley and Floyd Romesberg
[4–7], protein stability [8], protein–protein interactions
[9,10], protein crystallization [11], optical rotation of sugar
and amino acids [12], as well as bacterial growth [13].
Although Hofmeister effects for macromolecules in aqueous solution are ubiquitous, the molecular-level mechanisms by which ions operate are only beginning to be
unraveled [14].
In this short review we first examine several recent experiments which cast doubt on the notion that water structure
making and breaking by salts is central to the Hofmeister
series. This leads to the hypothesis that direct ion–macromolecule interactions are largely responsible for most
aspects of this phenomenon (Figure 2b). Second, we look
at the inclusion of dispersion forces in theoretical calculations of specific ion effects. Finally, we describe a recent
study on the lower critical solution temperature of poly(Nisopropylacrylamide). This final example employs a model
that only involves direct ion interactions with a macromolecule and its first hydration shell to explain hydrophobic
collapse.
Ions do not affect the bulk water properties
1367-5931/$ – see front matter
# 2006 Elsevier Ltd. All rights reserved.
DOI 10.1016/j.cbpa.2006.09.020
Introduction
Specific ion effects are ubiquitous in chemistry and
biology. Such effects exhibit a reoccurring trend called
the Hofmeister series [1,2], which is generally more
pronounced for anions than for cations. The typical
ordering of the anion series and some of its related
properties are shown in Figure 1. The species to the left
of Cl are referred to as kosmotropes, while those to its
right are called chaotropes. These terms originally
referred to an ion’s ability to alter the hydrogen bonding
network of water (Figure 2a) [3]. The kosmotropes, which
were believed to be ‘water structure makers’, are strongly
hydrated and have stabilizing and salting-out effects on
proteins and macromolecules. On the other hand, chaotropes (‘water structure breakers’) are known to destabilize folded proteins and give rise to salting-in behavior.
Recently, substantial attention has been paid to Hofmeister phenomena because of their relevance to a broad
range of fields. A few examples of physical behavior
obeying the Hofmeister series include enzyme activity
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To investigate the influence of anions on aqueous solution
structure, Bakker and co-workers [15,16–20] measured
the orientational correlation time for water molecules
in salt solutions by means of femtosecond two-color
pump-probe spectroscopy. The O–H stretch mode of
water molecules was excited with an intense ultrafast
mid-infrared pulse and the dynamics of the excitation were
followed using a second, weaker mid-infrared probe. The
dynamic behavior of water molecules in the liquid could be
observed on time scales shorter than the exchange time
between water in the ion solvation shell and in the bulk
liquid. The results showed that correlation times for first
hydration shell water molecules of Cl, Br, I, ClO4 or
SO42 were much slower than those for bulk water, as
might be expected. The experiments, however, clearly
demonstrated that these anions had no influence on the
dynamics of bulk water, even at very high concentrations
(up to 6 M) of both kosmotropic ions (SO42) and chaotropic ions (ClO4) [15]. In other words, the anions did
not alter the hydrogen bonding network outside the direct
vicinity of the anion. As a result, it was concluded that no
long-range structure-making or structure-breaking effects
for either kosmotropes or chaotropes take place. These
experiments provided a crucial challenge to the notion that
salt solutions affect bulk water structure.
A second challenge to bulk water structure ‘making’ and
‘breaking’ theories came from thermodynamic studies by
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2 Biopolymers
Figure 1
The Hofmeister series. Adapted from: http://tinyurl.com/ed5gj with permission.
Pielak et al. [21]. Bulk water can be conceptualized as a
mixture of two rapidly interconverting species: a less
dense, more structured form, and a more dense, less
structured arrangement [22]. This two-state mixture
approximation has proven useful in understanding the
volumetric properties of solutions. It has therefore been
assumed that the difference between bulk water and
hydration water arises from the varying ratio between these
two forms. Specifically, a ‘structure-making’ solute should
increase the fraction of the less dense architecture at the
expense of the more dense form in the solute’s hydration
water. On the other hand, a ‘structure-breaking’ solute
should have the opposite effect. The sign of ð@C p =@PÞT ,
where C p represents the partial molar constant pressure
heat capacity, indicates whether a solute makes
[ð@C p =@PÞT < 0] or breaks [ð@C p =@PÞT > 0] water structure
[23]. To probe this, Pielak and co-workers employed
pressure perturbation calorimetry, which measures the
heat transfer resulting from a pressure change above the
sample solution. The resultant sign of ð@C p =@PÞT and its
temperature dependence should therefore provide a direct
probe of structure making and breaking properties of
stabilizers and denaturants. Data were collected as a function of temperature to provide the coefficient of thermal
expansion a ¼ 1=V ð@V =@T Þ p , where V is the partial molar
specific volume. Measuring a simplified the determination
of ð@C p =@PÞT by using the Maxwell relation:
@C p
@ðV aÞ
¼ T
@T P
@P T
A total of 17 solutes (including guanidinium ions and
urea) were chosen for their well-known stabilizing or
destabilizing effects on proteins. The results showed
no obvious correlation between known stability effects
and the sign of ð@C p =@PÞT . Thus, the notion of stabilizers
and denaturants as bulk water structure makers and
breakers seems to be disproved on thermodynamic as
well as spectroscopic grounds.
In a third series of experiments, Cremer et al. [24]
directly followed the influence of Hofmeister anions on
the phase transition of a surfactant monolayer while
simultaneously observing the adjacent water structure.
Figure 2
Anions and water structure. (a) Organized water beyond an anion’ first hydration shell would be needed for structure-making and breaking
effects to occur. (b) The direct interaction of an anion with a macromolecule in aqueous solution. The relative sizes of water molecules,
macromolecules, and the anion are not generally to scale. However, the relative size of the anion with respect to the water molecules is
approximately correct for SO42.
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Interactions between macromolecules and ions: the Hofmeister series Zhang and Cremer 3
In this case, an octadecylamine monolayer was spread on
salt solutions at the air/water interface and monitored by
using vibrational sum frequency spectroscopy (VSFS).
VSFS can provide crucial information because it is capable of examining interfacial water structure as well as the
ordering of the surfactant molecules. Specifically, VSFS is
sensitive to alkyl chain conformation in the monolayer
[25] and can be exploited to ascertain the propensity of
any given anion to induce gauche defects into the film’s
alkyl chains under constant temperature and pressure
conditions. Vibrational spectra were collected from monolayers spread on D2O subphases containing various
sodium salts. The ratio of the oscillator strengths from
the methyl symmetric stretch and the methylene symmetric stretch was obtained for each anion. The higher
this ratio, the more ordered the monolayer is under a
given set of conditions [25,26]. The ratios for the anions
followed the Hofmeister series from most ordered to least
ordered monolayer as: SO42 > Cl > NO3 > Br >
I > ClO4 > SCN. The basis for this ordering is
related to an individual anion’s ability to penetrate the
headgroup region of the monolayer, thereby disrupting
the hydrocarbon packing (Figure 3) [27,28]. These results
showed excellent correlation with complementary surface
potential measurements made under the same conditions.
Next, the interfacial water structure was probed under
nearly identical conditions, but with H2O rather than
D2O in the subphase. The water structure data showed
a partial Hofmeister-like trend, but had significant deviations from the series. For example, a high degree of water
ordering was seen in the presence of ClO4, but not
SCN. The observed differences between the way anions
affected alkyl chain ordering and surface potential on the
one hand and water structure on the other further argues
against the hypothesis that the Hofmeister effect is
primarily due to the ions’ ability to influence bulk water
structure.
Additional corroboration for the idea that anion penetration into a Langmuir film is responsible for changes in
Figure 3
An octadecylamine monolayer at the air/salt solution interface. The
arrows compare the penetration of kosmotropic (green) and chaotropic
(red) anions into the headgroup region of the monolayer. The red x
indicates that the kosmotrope doesn’t effectively penetrate into the
headgroup region.
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alkyl chain ordering comes from Leontidis et al. [29].
These researchers demonstrated that chaotropic anions
could penetrate a lipid monolayer. They investigated a
DPPC (1,2-dipalmitoyl phosphatidylcholine) film at the
air/salt solution interface. The monolayer phase behavior,
morphology and structure of the lipid phase were interrogated by surface pressure-area isotherm measurements,
Brewster angle microscopy, grazing incidence X-ray
diffraction, and infrared reflection-absorption spectroscopy. The presence of chaotropic salts in the subphase
increased the surface pressure at a fixed area per molecule. The magnitude of this increase followed the order of
Hofmeister series: Cl < Br < NO3 < I < BF4 <
ClO4 < SCN. Therefore, at least for Langmuir monolayers, it is the direct interactions of the ions with the film
that are primarily responsible for the observed changes in
physical behavior, not indirect influences via changes in
water structure.
Dispersion forces need to be taken into
account
Derjaguin–Landau–Verwey–Overbeek (DLVO) theory
of interparticle interactions treats colloid stability in terms
of a balance of attractive van der Waals forces and
repulsive electrical double-layer forces [30]. One of the
major approximations in DLVO theory is the use of the
Poisson–Boltzmann equation to describe the electrostatic
interactions. This method treats ions in solution as point
charges, and consequently, ion specificity is lost. DLVO
theory works rather well at low salt concentrations (up to
0.01 M), where electrostatic forces dominate. The theory, however, loses its validity at salt concentrations of
biological interest or in any system where the ion concentration approaches 0.1 M or higher. To help remedy
this problem, Ninham et al. [31,32] have recently introduced a dispersion potential into DLVO theory and
treated it at the same level as the electrostatic forces.
By using this modified model, specific ion effects on the
interactions between charged particles could be examined. These authors have now employed their model to
help interpret many ion-specific phenomena, including
lysozyme behavior in salt solutions [33], ion binding to
micelles [34], counterion condensation on polyelectrolytes [35], pH measurements in buffers and protein
solutions [36–38], as well as the surface tension of electrolytes [39,40]. Moreover, Jungwirth and Tobias [41–
43,44] have performed simulations of air/electrolyte
interfaces in the presence of salt in which they included
the ions’ polarizabilities. Figure 4 shows sample ion
distributions from aqueous/air interfaces at 1.2 M sodium
halide solutions obtained by molecular dynamics simulations. This elegant work clearly shows that including
polarizability is critical to obtaining the correct ion distribution as a function of distance from the interface as
found by high pressure X-ray photoelectron spectroscopy
experiments [45]. On the other hand, Manciu and
Ruckenstein [46], have cast doubt on the ability of
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4 Biopolymers
Figure 4
Top and side views of aqueous/air interfaces for 1.2 M sodium halide solutions obtained by molecular dynamics simulations.
adding a polarizability term to correctly predict all interfacial properties at the aqueous/water interface in the
presence of salts. Also, Kunz et al. have found that that
including polarizability to describe ion effects at the
protein/water interface is less important than at the air/
water interface [7].
Ion effects on the solubility of poly(Nisopropylacrylamide)
Hydrophobic collapse of polymers and proteins is of
central interest in Hofmeister phenomena because it
represents a key step in protein folding. To explore
hydrophobic collapse, our laboratory [47] has monitored
the lower critical solution temperature (LCST) of poly(Nisopropylacrylamide) (PNIPAM), a thermoresponsive
polymer that becomes insoluble upon heating in aqueous
solution [48,49]. The LCST behavior of PNIPAM is
essentially analogous to the cold denaturation/renaturation point of proteins. The sodium salts of 11 anions
were studied and the ability of a particular anion to lower
the LCST followed the Hofmeister series. Significantly,
the effect of the anions could be completely explained on
the basis of three direct interactions of the anions with the
macromolecule and its immediately adjacent hydration
shell. First, kosmotropic anions were found to polarize
water molecules that were directly hydrogen bonded with
the amide moieties of PNIPAM (Figure 5a). Second, both
chaotropes and kosmotropes could increase the cost of
hydrophobic hydration (i.e. raise the surface tension of
the polymer/water interface; Figure 5b). Third, chaotropic anions could bind directly to the side-chain amide
moieties (Figure 5c). The first and second of these
interactions led to the salting-out of the polymer- thereby
lowering the LCST, whereas the third effect causes the
polymer to salt-in. Moreover, the first two phenomena
depend linearly on salt concentration [50] whereas the
third displays simple saturation behavior. These facts
allowed us to model the perturbation of PNIPAM’s
LCST by added salts using a simple equation that consists of a constant, a linear term, and a Langmuir isotherm:
T ¼ T 0 þ c½M þ
Bmax K A ½M
1 þ K A ½M
T0 is the LCST of PNIPAM in the absence of salt and [M]
is the molar concentration of salt. The constant, c, has
units of temperature/molarity. KA is the binding constant
of the anion to the polymer and Bmax is the increase in the
Figure 5
Interactions amongst anions, PNIPAM, and hydration waters. (a) Hydrogen bonding of the amide and its destabilization through polarization
by the anion, X. (b) The hydrophobic hydration of the macromolecule, which can be modulated by salt. (c) Direct binding of the anion to the
amide moiety of PNIPAM.
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Interactions between macromolecules and ions: the Hofmeister series Zhang and Cremer 5
phase transition temperature due to direct ion binding at
saturation. Using this equation, it could be shown that an
anion’s effect on the LCST correlated extremely well to
its hydration entropy (DShydr), surface tension increment
(s), and binding constant, KA. Moreover, only kosmotropes showed a correlation between the slope, c, of the
LCST and DShydr (note: polarization of water is related to
water ordering). On the other hand, the chaotropes
showed excellent correlation between c and the value
of s. Hence, it could be shown that the mechanisms of
action of the chaotropes and kosmotropes were fundamentally different in modulating the LCST. These
results indicate that, in certain instances, chaotropes
and kosmotropes work by separate mechanisms rather
than on a continuously graded scale.
It should be pointed out that a preliminary understanding
of the influence of anions on the physical properties of
macromolecules is really only a first step toward understanding the effects of solution conditions on biological
systems. A complete picture will inevitably involve an
integrated understanding of the role of cations (including
guanidinium ions) and osmolytes (such as urea and trimethylamine N-oxide) as well. There has been some
progress in these fields, although such subjects are generally beyond the scope of this short review.
Conclusion
5.
Pinna MC, Salis A, Monduzzi M, Ninham BW: Hofmeister series:
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Bauduin P, Nohmie F, Touraud D, Neueder R, Kunz W,
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7.
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This review describes recent research on the Hofmeister series.
Recent advances in understanding the mechanism of the
Hofmeister series have provided valuable insights for
several ion-specific phenomena. Experimental evidence
clearly shows that changes in bulk water structure by added
salts cannot explain specific ion effects. Instead, Hofmeister phenomena need to be understood in terms of direct
interactions between the ions and macromolecules.
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Acknowledgements
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This work was supported by the National Science Foundation (CHE0094332) and the Robert A Welch Foundation (Grant A-1421).
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6 Biopolymers
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