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Hydrationforceandtheinterfacialstructureof
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ArticleinJournaloftheChemicalSocietyFaradayTransactions·September1991
DOI:10.1039/ft9918702733
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J. CHEM. SOC. FARADAY TRANS., 1991, 87(17), 2733-2739
2733
Hydration Force and the Interfacial Structure of the Polar Surface
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Gregor Cevc
Medizinische Biophysik-Forschungslaboratorien Urologische Klinik und Poliklinik, Klinikum r.d .I.,
lsmaningerstr. 22,Technische Universitat Miinchen, W-8000 Munchen 80, Germany
Solvation, in particular the hydration force, for simple surfaces is believed to decay approximately exponentially
on t h e length scale of a few tenths of a nanometre. For phospholipid membranes its range varies, however, with
t h e surface at least a s strongly as with t h e solvent characteristics. One possible reason for this is fine interfacial
structure, most notably, t h e finite interfacial thickness. From a generalized non-local electrostatic model of
hydration discussed in this work, the spatial decay of t h e interfacial hydration is concluded to be sensitive to t h e
volume distribution and to the transverse fluctuations of t h e surface polar residues. If the effective interfacial
width significantly exceeds t h e solvent-structure decay length, t h e interfacial width a n d t h e interfacial polarity
distribution may ultimately become more important for the solvation range t h a n t h e solvent characteristics
themselves. For a n y relatively thick, for example biological, interface s u c h effects are prone to dominate t h e
hydration force and act as a messenger of t h e interfacial structure. Approaching or interacting macromolecules
or membranes, consequently, can sense and pick up mutual surface patterns throughout t h e intermediate
solvent, provided that t h e surface polarity profile has a 'tail' longer t h a n t h e solvent-structure decay length. This
may be one of the reasons for t h e biological significance of the hydration force. On t h e one hand, a rationale is
t h u s proposed for the extraction of detailed structural information from t h e macroscopic hydration-force data. On
t h e other hand, one possible reason is identified for the non-linear dependence of t h e maximal repulsion of
hydration as a function of t h e overall surface hydrophilicity.
1. Introduction
Molecular or macromolecular solvation and especially the
solvation force are well established for aqueous'*2 as well as
non-aqueous, but associating,
Surface hydration
results in interfacial pressures of up to 1OOOO atm,6 affects
molecular conformation, and causes phase transition shifts
for the lipid membranes of up to 100°C.7*8Owing to its universality, the interfacial solvation is an ideal tool for the regulation of function and for the colloidal stabilization of
various macromolecules (such as deoxyribonucleic strands'
or proteins), molecular aggregates (such as membranes I ) and
whole organisms (see also special issue of Chernica Scripta' O).
Indeed, hydration, or its variation, has been shown or
hypothesized to govern the organization and interactions
between numerous oligo- and poly-peptides, polysaccharides,
and nucleic acids at small separations. Repulsion between
aggregates of the non-ionic surfactants, phospholipids, and
glycolipids is governed by the energetics of molecular dehydration; the latter has also been postulated to play a role in
regulating the function of certain integral membrane proteins,
such as ion channels; reduction of the hydration force is
required for the induction of cell fusion, viral infection etc.
2. Origin of Hydration Force
Hydration force is commonly visualized as a very strong but
relatively unspecific repulsion with a steep fall-off on the
length scale of ca. 0.19 and up to 0.4 nm or more.'' Such
force may have multiple roots. First, the layering of solvent
molecules near a molecularly smooth surface can cause an
oscillatory repulsion which decays rapidly with the separation from the interface and exhibits maxima and minima at
approximately the separation of one solvent diameter.12
Force of this type is observed for all solvents, being a result
of the steric, 'hard-core' repulsion between the layered,
bound solvent molecules. The oscillatory force, consequently,
is typical for all rigid, very regular surfaces, such as swollen
mica or clays.'2,'3 Secondly, solvation force could be a manifestation of the solvent polarization caused by the surface
dipolar and multipolar m ~ m e n t s . ' ~ ' 'Force of this type
would have to depend strongly, however, on the surface
dipolar moment as well as on the bulk relative permitt i ~ i t y . ' ~ ,For
' ~ lipids at least, the latter dependence is not
observed e ~ p e r i m e n t a l l y . ~Purely
. ~ . ~ ~ polarizational hydration force, consequently, is probably small, except when the
correlation-dependent interactions become important.' 8 * 1 9
The third, and in many instances the most important, reason
for the existence of hydration force lies with direct surfacewater and water-water interactions. These give rise to a sort
of 'chemical hydration' of a membrane surface.16 Such
hydration, by and large, involves partial charge transfer
between the surface polar residues and the water molecules,
in most cases along the surface-solvent hydrogen bonds.
Local dipolar moments or polarizability may also contribute
to this. At least for distances between 0.5 and 3 nm, the main
part of the integral hydration force is thus of 'chemical'
origin.
3. Range of Hydration Force
The range of hydration forces is commonly believed to be
determined by the solvent properties, but it is found to be
somewhat variable for obscure reasons. The maximum pressure of hydration is furthermore observed to depend, in an
equally unclear manner, on the interfacial properties. In this
contribution, an argument is extended which was briefly
introduced some time a g 0 , ~ 7 ~to' present one possible solution to these conundrums. It is shown that the effective decay
length of the hydration force (and in general of any middlerange solvation force) may depend more on the interfacial
characteristics and dynamics than on the solvent properties.?
It is noteworthy that quite recently Israelachvili and
t By doing so, a similar generalization is performed at the molecular scale as has been done previously at the level of the whole membrane. Allowing for the effects of surface fluctuations has been shown
to increase the range of intermembrane interactions.
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27 34
Wennerstrom2’ have reached a similar conclusion by using a
different model.
Moreover, one possible explanation is suggested for the
nonlinear relationship between the maximum pressure of
hydration and the interfacial hydrophilicity. Last, but not
least, the reason why different solvents for one surface give
rise to similar forces whilst various surfaces in the same
solvent exhibit relatively variable solvation behaviour is
rationalized. The theoretical basis for all quantitative arguments is a generalized non-local electrostatic model of hydration.
For a start, it is convenient to tackle the problem of solvation of lipid bilayers: this system being among the best investigated.’ Hydration of the lipid bilayers is predominantly
driven by the direct water binding. This chiefly ‘chemical’
nature of the lipid hydration is a consequence of the surface
polarity and, above all, of the interfacial and solvent capacity
for hydrogen-bond f ~ r m a t i o n . ~6 ., 2’2 This can be seen from
the fact that in non-structurable, typically aprotic, solvents,
lipids d o not self-aggregate into bilayers and interfacial solvation does not take place.4 For the simplest electrostatic
description of lipid hydration it is, consequently, enough to
consider the coupling between the atomic excess of charges
on all lipid polar residues and the corresponding charges on
the solvent molecules. (This is possible owing to the fortunate
circumstance that detailed electrostatic models accurately
mimic all essential features of the hydration even for polar
non-ionic surfaces.23
4. Electrostatic Hydration Model
In the generalized nonlocal electrostatic approach introduced
recently,21 the distribution of the local excess of charges at
thermodynamic equilibrium is evaluated by solving the
Poisson equation?
This states explicitly that the spatial variation of the
hydration-field, as well as the second derivative of the hydration potential +h(x), and the local density of the solventassociated local excess of charges [&@)], or of the surface
‘polarity charge’ [p,(x)], are mutually dependent.$ Parameters E , and &’ are the relative permittivity at high frequency and the permittivity of free space, respectively.§
~~~
t Abbreviations: l/,, , hydration potential; crp, surface local excess
charge density; decay length of the solvent structure; E and E , ,
static and high-frequency relative permittivity ; d , , interfacial thickness; pp and p h , volume density of the surface local excess
( = polarity) and of the solvent-associated local excess charge; P h ,
hydration pressure ; DPPC, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine; Chol, cholesterol.
1 In the simplest version of the nonlocal electrostatic model of
hydration the distribution of the solvent-associated local excess of
charges is assumed to be governed by a Boltzmann law. This yields:
5’ dZll/,(x)/dx2= t,hh(x), an expression which is formally similar to
well-known Poisson-Boltzmann equation of classical electrostatics:
2’ dzll/el(x)/dx2
=
However, conceptual differences between
both equations are profound. The former expression deals with the
interactions and the distribution of charges located on the atoms of
water molecules ; charges, which describe the hydration profile and
affect mainly the hydration potential of a membrane, ( J / h ) ; the latter
expression pertains to the net Coulombic charges associated with the
solution ions, the latter controlling the Coulombic electrostatic poten-
r,
tial
5 The value of the former parameter (i.e. E , ) depends on the
solvent model. For ,water, three major coupling modes have been
proposed :” the Debye mode involving correlated reorientations of
the water molecules (with a characteristic relative permittivity, E , =
78 and sub-nanometer correlation length < 0.3-0.5nm); the polar-
<
In the absence of ions, the variation of the local electric
field in an interfacial region is a manifestation of the distribution of the surface polar residues. Another important factor is
the ‘screening’ of these residues by the solvent molecules.
Any structural solvent perturbation near a polar surface, consequently, is accompanied by the redistribution of the local
excess of charges near a surface.1
The integral of the surface-associated local-excess charges,
by assumption,16 is taken to be compensated by the total
opposite charge contained in the solvation layers
c
A
This ensures electroneutrality and stability of the whole
system. It is noteworthy that in dry systems electroneutrality
is guaranteed by the intramolecular charge neutralization
between the positive and negative group on each zwitterionic
molecule; alternatively, a charged group and its associated
counterion can mutually compensate each other. For
hydrated lipids, however, and probably also for other types of
the amphiphilic or polar molecules, the same effect is
achieved largely via intermolecular charge neutralization. This
diminishes the importance of the macroscopic surface dipole
moments and involves many bound water molecules in the
charge neutralization at the surface.
To within a good approximation the interfacial hydrophilicity may be taken to be proportional to the total amount
(or density, ap) of the local excess charges on a hydrated
This density, in turn, determines the magnitude
of the interfacial hydration potential. But in any accurate
description of the interfacial solvation, it should be kept in
mind that molecular hydration depends on the magnitude as
well as on the distribution of the polar residues and charges.
This is seen directly from the detailed computer simulations
(see, for example, ref. 24) and should also be incorporated
into all advanced mean-field descriptions of hydration. In the
present hydration model, consequently, the distribution of
the solvent-associated charges as well as the distribution of
the surface local-excess of charges are examined and
described simultaneously. The concept of the interfacial
‘polarity’ profile proves convenient for this purpose.
The interfacial polarity profile corresponds, in the simplest
electrostatic approximation, to a distribution throughout the
interfacial region of the surface local-excess charges. The
latter arise from the polar surface groups or atoms. For a
planar hydrophilic surface with an infinitesimally thin interface the polarity distribution function thus differs from zero
only in the plane of the surface: dp = pp(x)6(x = 0), where
6(x) is a delta-function. The interfacial hydration strength for
such a surface from the nonlocal electrostatic model is found
to be a monotonic function of the surface local-excess charge
ization mode associated with orientational oscillations (librations)
and intramolecular vibrations which together are responsible for the
water absorption in the infrared ( E , = 5,
ca. 0.1 nm); and an
‘electronic polarization’ mode ( E , = 1.8, 5 z 0.05 nm). From the
present estimation for the decay length of hydration, < 0.1 nm, one
would conclude that in lipid hydration the second and especially the
third of these modes prevail. This is also consistent with the present
picture of ‘chemical hydration’. The relative permittivity value E, to
be used in nonlocal electrostatic description of hydration, consequently, is likely to be 1.8, rather than 5.
7 In many respects, the resulting regionally unbalanced distribution of the atomic excess charges can be envisaged as a special form
of the ‘generalized water polarization”6*26but, in contrast to the
situation with standard electrostatic polarization, the direction of the
generalized polarization is not important. This is due to the fact that
water binding is of chemical (‘quantum-mechanical electrostatic’)
rather than (Coulombic)electrostatic origin.
<
<
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J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87
2735
Hydration Potential
400 1
The spatial decay of hydration is found either from the
p h e n o m e n ~ l o g i c a lor
~ ~from the nonlocal
models to be exponential and on the scale of the solvation
decay length
300
<
$h(x)
= ( - Op c / & m &0)exP(
- x/8
(1)
>
E
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1-
Eqn. (1) and all subsequent results are given in the linear
approximation in order to be analytic.
It is easy to perceive that such approximation is remote
from reality; all surface polar residues are never confined to a
single plane. Rather they are distributed throughout a meshlike region. The polar surface is thus an interphase rather
than an interface. Instead of eqn. (l), a more general result,
therefore, must normally be used which allows for the interfacial effects, albeit in a simple approximation
;.
200
Q,
+
0
100
0
6
3
0
9
12
XI<
rx
+ Jo p,(x’)sinh[(x
\
-
x’)/<] dx’
Eqn. (2) can be derived by analogy to a similar treatment for
the double layer near a charged surface.28 Compared to eqn.
(1) it offers the advantage of requiring no a priori knowledge
about the position of the interfacial plane; integration starts
in the region where no water or surface-associated atomic
charges are located, in the membrane interior, say, and
extends into the bulk.
Eqn. (2) states explicitly that the spatial variation of the
interfacial hydration potential (and consequently of all other
hydration-dependent thermodynamic quantities) depends on
the solvent structure as well as on the interfacial structure.
The latter is here described in terms of a decay length parameter and a polarity distribution profile p,(x), as expected.
To clarify this point, let us consider a simple hypothetical
interface with an exponential polarity distribution profile.
The interfacial polarity function for such a surface is p,(x) =
(oJd,)exp( -x/dp). From this expression and from eqn. (2) the
spatial decay of the hydration potential profile is found to be:
Fig. 1 Hydration potential profile [$&)I near a polar surface in
contact with water and its dependence on the interfacial thickness
(d,) for an assumed exponential surface polarity profile: Values of
d d < are: (a), 0; (b) 1.05; (c) 2.5; and (4, 5. Curves were calculated
from eqn. 3 by using the following parameter values: cP = 0.425 C
m-’, E , = 2, 5 = 0,075 nm. The distribution of the surface polar residues perpendicular to the surface is seen to increase the range of the
hydration effects and to decrease the maximum as well as the
extrapolated ‘surface potential’ values. The hydration potential
decays exponentially, only for a hypothetical infinitely narrow interface (dp = 0) as hitherto assumed, on the scale of the water structure
decay length, <.In all other cases the potential profile depends on the
distribution of the solvent and surface local-excess of charges and,
consequently, is a complex function.
Insert: the measured values of the decay length of hydration force
between phospholipid membranes as a function of the interfacial
separation in excess water (&). Modified from Tables 3(a) and 4(a)
of ref. 1.
(e)
<
- exp( - x/c)
= ( - up5/&
+d, + 5
&,X</d,)eXP( - x/d,);
d, %
r
(4)
It thus, in general, consists of two components. The range of
the first component is controlled by the solvation decay
length. The second component has the form of the interfacial
polarity profile; in our special case it falls off exponentially
on the length scale of the interfacial thickness and prevails at
large separations (Fig. 1).
in which the direct (hard-core) steric-interactions are
neglected.
Hydration and hydration force profiles at large separations
are inevitably governed by the longer-range component. As
long as the interfacial width is sufficiently greater than the
solvation decay length, and has a long tail [cf: eqn. (411, the
characteristics of fine interfacial structure will be felt even at
appreciable distances, and will be enhanced by the surface
hydration.
This can easily be demonstrated for our illustrative
exponential-form interface. From eqn. ( 5 ) and (3) it follows
that the hydration repulsion between two relatively distant
polar surfaces with an exponentially decaying surface polarity
profile is given by
+
Hydration Pressure
The spatial variation of the hydration repulsive pressure can
be deduced from the hydration potential by a straightforward
integration
Ph(x)
(
<
/AxdGh/dx)
= (1/0,)(1
[’$h(x,
- E,/E)(d/dX)
0)
do [pp(x’) dx’
(5)
(3
- exp( -x/<)exp(- x/d,)
The singularity at = d, is due to the special choice of the
distribution function p,(x) and is not a general feature of the
solution.
Structurally modified interfacial repulsion of solvation thus
contains three contributions. The first two depend separately
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2736
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on the decay lengths of the solvent and of the structural
surface excess-charge distributions, respectively. The third is
a mixed term which contains both characteristic decay
parameters. A pre-exponential factor of the 'Born type' is
proportional to the difference of the inverse relative permittivities.
If the interfacial thickness is much greater than the solvation decay length, the previous result simplifies to
(7)
which has a form similar to the standard non-local electrostatic expresssion for the evaluation of the hydrational repulsion between two infinitely narrow hydrophilic surfaces
The essential difference is that in the former case the characteristic interfacial thickness appears in place of the solvation
decay length. If the interfacial width is much greater than the
solvation decay length, the hydration force over significant
separations will decay on the length scale of the interfacial
thickness [cf: eqn. (7)]. For d, 9 moreover, the repulsive
force over large distances will exceed the force predicted by
the standard eqn. (8), owing to the increased range of repulsion.
It is remarkable that this ability and the interfacial hydration itself are suggested by eqn. (7) to disappear for nonstructurable solvents, this is, when = 0.t However, if the
inter-solvent correlations are too long-ranged, this is equally
disadvantageous. In such a case the solvent molecules are
sampling structures over relatively large areas and the
resolution by which the local structural details are picked up
is diminished. Water appears to be nearly optimal in both
respects.
r,
5. Interfacial Polarity Profile
The exponential interfacial polarity profile discussed hitherto
is just a special example. Obviously, other interfacial forms
are possible. Consider, for example, a single surface local
excess of charge fluctuating perpendicular to the hydrated
surface in an interfacial harmonic potential. This corresponds
to a linear force holding this charge near the surface
dU(x)/dx
=
-2ax
which is taken to be identical with the average surface plane.
For such a potential the distribution of several surface local
excess charges is given by Gaussian interfacial polarity profile
=
2u,(a/nkT)0*5exp(-ax2/kT)
For an ensemble of such surface polar residues located at
different positions the distribution function p,(x) is then
expected to be a function of the structural, intrinsic, as well as
dynamic, temperature dependent surface-characteristics. The
hydration force may mirror all such interfacial properties
simultaneously.
If the interaction between individual local excess charges,
arising, for example, from their solvation, is allowed for in the
previous equation, the coupling constant a depends on solvation. This then causes the distribution of surface polar resiThe fact that the hydration pressure given by eqn. (7) is predicted
to vanish if 5 + 0 is due to the fact that in this work the relatively
trivial direct steric force has been excluded from the model by
assumption.
dues to become wider with progressing solvation. From a
different point of view, this phenomenon is discussed in ref.
29.
From the present model it is easy enough to deduce that a
'Gaussian surface' should be the source of a hydration force
which decays non-exponentially ; the slope should be
approximately sigmoidal, reminiscent of the underlying
polarity profile. However, at quite large separations, this
profile is apt to be affected also by the surface fluctuations,
such effects tending to prolong the range of original repul~ i o n . ~ 'This
. ~ ~ makes it possible to achieve enforced good
agreement between the exponential and Gaussian approximations if an effective interfacial thickness for the former is
used that is somewhat smaller than the exponential decay
length. Quasi-exponential hydration force profiles published
to date, therefore, do not preclude the model advocated in
this work. On the contrary, some of the published hydration
curves show features similar to the ones just m e n t i ~ n e d . ~
An interface cannot extend to infinity, of course. The interfacial polarity distribution function, consequently, must terminate at some finite distance beyond which the hydration
profile must attain its purely solvational character. But such
a situation, while being possible, is not necessarily relevant. It
is proposed that the interfacial effects often extend to distances at which larger-scale surface fluctuations start to
dominate surface forces; i.e. to regions where the solvation
force tends to be a complicated function of the separati~n.~~,~'
6. Discussion
How relevant, then, are implications made so far for the real
systems? Available hydration and solvation data reveal that
the effective decay length of solvation force for the lipid
bilayers varies widely from case to case, by a factor of three
to four, at least. The insert to Fig. 1 demonstrates this. On
average, however, the decay length increases with the equilibrium separation between the interacting surfaces, d,, . It is
clear that the independent variable in Fig. 1 depends on the
repulsion as well as on the attraction between hydrated surfaces. However, because of the similarity of the systems investigated it is not possible to explain easily the observed
variability solely through variations of the interfacial van der
Waals attraction; due to the small interfacial separations it is
also improbable that surface fluctuations alone or in combination with changing van der Waals forces are fully
responsible for the observed scatter. I therefore infer that the
variability of the measured decay length of the hydration
force is a genuine effect and probably originates, at least
partly, from the interfacial structure.
But is it likely that the interfacial thickness exceeds the
natural value of the solvation decay length? To answer this,
we must inspect the structural as well as the dynamic properties of the polar parts of various molecules. Phosphate
groups, for example, are the main hydration sites of phospholipid membranes as well as of deoxyribonucleic acid. These
groups alone can occupy a region of appreciable thickness; in
lipid crystals they cover a width of ca. 0.15 nm, and the whole
polar region in such systems is even wider. In the case of
phospholipid bilayers it includes the non-phosphate polar
residues of the glycerol at least ; acyl-ester moieties sometimes
also belong to it. For deoxyribonucleic acids, both grooves
are parts of the polar region.24 In monoglycerides, the glycerol residues and the carbonyl groups, together more than 0.4
nm thick, contribute to the interface. The width of the hydrophilic zone of lipid membranes and deoxyribonucleic acid
strands therefore is likely to be of the order of 0.5 +_ 0.2 nm
or greater.
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J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87
Quoted values for the interfacial thickness tend to increase
with the surface hydration. The first reason for this is the
penetration of water into the apolar region; the second arises
from the thermally excited, hydration-enhanced, out-of-plane
fluctuations of the hydrated molecules. For a decanoldecanoate bilayer such an effect has been inferred from
molecular dynamics simulations to produce an interface as
thick as 1 nm.32This is probably an overestimate. For a fatty
acid monolayer at the air-water interface the corresponding
value has been directly measured as greater than 0.3 nm;33
for phospholipid monolayers the width of the phosphate-rich
region has been reported to be 1.1 and 0.8 nm in the fluid and
gel phases, r e ~ p e c t i v e l y ;the
~ ~ amplitude of the out-of-plane
fluctuations of phosphatidylcholines in fluid multi-bilayers,
finally, has been measured to be 0.2-0.6 nm.35 This means
that lipid head groups, at least transiently, may fill ca.
60-70% of the water-fillled region (dp d 0.7dW/2). The
‘interfacial width’ of a typical polar system, therefore, is at
least comparable to, and normally greater than, the reported
hydration decay lengths. Consequently, it may not be
neglected in any realistic hydration model.
This is even more obvious upon considering the lower limit
for the intrinsic value of the solvation decay length. Take, for
example, systems with truly narrow interfaces, such as simple
alkali ions. From the non-local electrostatic analysis of the
corresponding ion-hydration data one obtains x 0.07 nm.27
For more complex systems, such as lipid bilayers, the situation is less clear. Linear regression of the data shown in the
insert to Fig. 1 suggests that the decay length for relatively
apolar, weakly hydrophilic (and thus narrow) bilayer surfaces
should be close to 0.07 nm. Because of the experimental
uncertainty associated with the latter data set, and in light of
the low value of the correlation coefficient (0.67), such
extrapolation can be taken merely to show trends. To be on
the safe side, one can only say that the intrinsic length of the
solvation decay is between the model estimates and the smallest reliably measured values: 0.07 d </nm d 0.13. Even this
conservative guess is appreciably lower than the estimated
interfacial width.
It is therefore concluded that because of intermolecular
coupling the interfacial hydration for all polar surfaces can
act as a molecular amplifier and promoter of the interfacial
effects.? Specific solvent structure thus permits an ‘in
advance’ recognition of the characteristic interfacial features
during molecular approach.
It is thus inferred that published hydration profiles bear an
imprint of the interfacial structure and molecular dynamics.
This is especially true for the very polar systems which
strongly imbibe water in the interfacial region38 since such
systems have a thick interfacial region. Direct evidence for
this comes from the measured ‘water structure correlation
length’ near polyoxyethylene bilayers, the solvation decay
length for such aggregates having been measured to be
approximately proportional to the number of oxyethylene
residues in each molecular headgroup and >0.42 nm in the
case of dodecyltetraethylenoxide (C, 2E04).1
The relatively small value of the ‘natural’ hydration decay
length, CQ. 0.1 nm, makes water an ideal molecular amplifier
and promoter of the interfacial effects ; hydration phenomena
increase the range of interfacial recognition without notably
masking the fine structural details. Salt effects do not interfere
<
~
This is tantamount to saying that the spillover of the atomic
local-excess of charges from the surface into the solution (which is
possible because of the capability of the polar residues and of the
solvent molecules to mediate charge-transfer) can effectively mask the
intrinsic solvent structure. Phenomenologically such spill-over resembles the electrochemical concept of j e 1 l i ~ m . j ~
2737
with this significantly. For the present model it is concluded
that salts affect the solvation decay length only slightly in the
submolar concentration range owing to the small intrinsic
values of the bulk water structure decay length <. This may
explain the hitherto mysterious absence of salt effects on
decay length of solvation.
The short intrinsic range of the solvation phenomena,
moreover, can explain why different surfaces in water show a
high variability of the hydration decay lengths. This variability is at least as great (60%’)or greater than the diversity
of the decay lengths measured in different associating solvents for similar surfaces (45%’). If the experimental values of
the solvation decay length were truly a function of the solvent
properties only, one would have expected the solventdependent variability to be much smaller than the lipiddependent variability, however.
Within the framework of the present model, one can argue
that the maximum values of the hydration pressure need not
increase monotonically with the surface hydrophilicity.
Imagine, for example, a slightly hydrophilic surface containing only a few polar residues with, normally, a narrow polarapolar interface. The range of the hydration force in such a
system is then short, and the magnitude of the force is proportional to the number of the polar residues. To increase
interfacial hydrophilicity, more surface polar residues are
brought to the surface; alternatively, previously hidden residues can be exposed at the interface. In either case, the entire
volume occupied by the polar residues, as well as the interfacial thickness, both increase. Consequently, the range of the
surface hydration is extended and the integral surface hydration is enhanced; the maximum water-binding strength may
decrease simultaneously, however (see Fig. 1). Indeed, the
maximum pressure of hydration for very polar surfaces is
often relatively small but of long range [cf: eqn. (3)]. In order
to increase the interfacial hydration and avoid its saturation,
the interface escapes, so to speak, in the third dimension.
The experimental and theoretical results of Fig. 2 document this example. The pressure of hydration for pure phosphatidylcholine bilayers at short separations is higher than
the corresponding value for mixed phosphatidylcholinecholesterol systems.’ This happens despite the fact that for
phosphatidylcholine the maximum amount of membranebound water is less.37 However, the range of hydration for
the mixed system is greater than for the pure phosphatidylcholine bilayers. Similar apparent inconsistencies have
been reported for other types of membranes in the gel and
fluid phase. Specific examples are given in a recent review.’
The ideas introduced in my previous work and advocated
in this contribution may have far reaching practical consequences. When the intrinsic value of the hydration decay
length is precisely known or can be estimated reliably, equations similar to the ones presented in this paper can be used
to decipher the interfacial polarity profile from the experimental hydration data. If the experimental accuracy is suffciently high, information about the lateral compressibility
and changing headgroup conformation as well as knowledge
about the amplitude of the molecular fluctuations can be
obtained from a comparison of the experiment-derived interfacial polarity profiles and the corresponding profiles calculated from the crystal-structure data. Eqn. (2), preferably in
combination with some additional plausible simplifications, is
a good starting point for this type of analysis.
The insert to Fig. 2 illustrates one simple application of
this sort. It represents the distributions of surface polar residues in the hydrated phosphatidylcholine and phosphatidylcholine-cholesterol mixed multi-bilayers at room
temperature. The curves were obtained by first assuming that
the interfacial polarity profile resembles one half of a Gauss-
<
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J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87
of the present conclusions, however. Solvation phenomena,
therefore, may be postulated to act, in general, as heralds for
the interfacial structure. Owing to the fact that the interfacial
thickness is probably usually greater than the structural
solvent correlation length, the structural interfacial information is propagated through the solvent with the aid of all
bound or associated solvent molecules. This puts current
understanding of the hydration phenomena in a new light. It
provides one possible explanation for the fact that atomic
force microscopes reliably monitor the macromolecular
surface despite the fact that the probe tip is not in direct
contact with the organic material; it points to one plausible
reason for the involvement of interfacial hydration in molecular recognition and mutual structural reorganizations; but it
also offers a means for determining the fine interfacial structure from the macroscopic hydration.
0
1
2
xlnm
3
4
Fig. 2 Spatial profile of the repulsion ('hydration pressure')
between two polar, hydrophilic surfaces as a function of the interfacial thickness and interfacial separation (x). The density of the
surface polar residues is assumed to fall off exponentially on the
length scale of d,. Parameter values as in Fig. 1. Experimental data
for : + , dipalmitoylphosphatidylcholine (DPPC); and *,
dipalmitoylphosphatidylcholine-cholesterol (1 : 1) mixed bilayers
(DPPC-Chol) at 25 "C from ref. 39.
In general, the hydration profile depends on parameters 5 as well
as d , . For d,/< ca. 1 the slope of the hydration profile is nearly that
predicted by the standard non-local electrostatic hydration model,
except for the fact that the curve is offset towards the bulk solution.
The spatial decay of the hydration pressure becomes mainly a function of the interfacial thickness, however, if the latter is appreciably
greater than the value of 5. The hydration profile in such event
mirrors the distribution of the polar surface residues.
Insert: Effective interfacial polarity profile for A, phosphatidylcholine; and B, phosphatidylcholineholesterol bilayers. The
assumed Gaussian polarity distribution functions @), were optimized
by deconvoluting the experimental hydration data for the main figure
body. (Changes in the interfacial structure occurring upon separation
variation were not considered.) Horizontal bar gives the corresponding interfacial width deduced from neutron diffraction experiments
for DPPC.38
ian distribution. (This is similar to the distribution of polar
residues in a harmonic-well combined at the centre with the
hard-wall potential.) The optimal width of the Gaussian was
then fixed by comparing the calculated polarity profile with
the measured hydration pressure data displayed in the main
body of the figure. The width of the calculated profile is in
excellent agreement with the corresponding result of neutron
diffraction experiments, shown in the insert as a horizontal
bar.38
While more detailed analyses of the phospholipid and
other systems are in progress, at least one more implication
of the present model deserves immediate mention. From what
has been said, the ion- or pressure-induced decrease of the
decay length of the deoxyribonucleic acid hydration9 can be
tentatively interpreted as a sign of a conformation change in
the interfacial region of DNA bio-polymers. Halving the
decay length of the hydration force between DNA strands
suggests that this change is perhaps accompanied by a
decrease in the effective thickness of the polar/apolar interface, maybe because of formation of trans-groove water connection~.~~
Qualitatively similar conclusions hold for water and
organic structurable (protic) solvents' and for all polar surfaces investigated so far. Because of the short range of the
hydration force, the gross geometrical features, such as a
deviation from surface planarity, do not restrict the validity
Supported by DFG through grants SFB266/C8 and Ce19/2I. Discussing the final version of this typescript with M. A.
Vorotyntsev and A. A. Kornyshev was very useful.
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Published on 01 January 1991. Downloaded by Bayerische Staatsbibliothek on 18/04/2014 16:06:33.
Paper 1/00222H; Received 15th January, 1991