water structure and properties.pdf

7/29/2019 water structure and properties.pdf

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Water: Structure andPropertiesKim A Sharp, E. R. Johnson Research Foundation, University of Pennsylvania,Philadelphia, Pennsylvania, USA

Water is a major component of all living things. It is anomalous in many of its physical

and chemical properties. Some are essential for life while others have profound effects on

the size and shape of living organisms, how they work, and the constraints within which

theymust operate. Many of watersbasic physicalproperties cannow be explained, at least

semiquantitatively, in molecular and structural terms, although in spite of intense study

it remains incompletely understood.

Introduction

Water is the material cause of all things (Thales, 624546 bc).

Water is a unique, ubiquitous substance that is a majorcomponent of all living things. Its nature and propertieshave intrigued philosophers, naturalistsand scientists sinceantiquity. Water continues to engage the attention ofscientists today as it remains incompletely understood inspite of intense study over many years. This is primarilybecause water is anomolous in many of its physical andchemical properties. Some of waters unique properties areliterally essential for life, while others have profoundeffects on the size and shape of living organisms, how theywork, and the physical limits or constraints within whichthey must operate. This was recognized by LawrenceHenderson in 1913 in his classic and still very readable

book, The Fitness of the Environment: An Inquiry into theBiological Significance of the Properties of Matter. Sincethen more has been learned about the structure andproperties of water at the molecular level, much of itthrough spectroscopic and thermodynamic experiments.The more recent discipline of computer simulation has alsoplayed a role, having achieved a level of sophistication inthe study of water in which it can be used to interpretexperiments and simulate properties not directly accessibleby experiment. Many of waters basic physical propertiescan now be explained, at least semiquantitatively, inmolecular and structural terms.

Basic physical properties

Selected physical properties of water are given in Table 1.To put these in context, comparison is made to the organicsolvents methanol and dimethyl ether, where one and twoof the hydrogen atoms are replaced by a methyl group,respectively. Water is a small solvent, occupying about0.03 nm3 per molecule in the liquid state at roomtemperature and pressure, yet it is highly cohesive becauseof the strong intermolecular interactions (hydrogen

bonds, or H-bonds) between the oxygen and hydrogeatoms. This is reflected in its high boiling point, the largamount of heat needed to vaporize it, and its high surfactension. Replacement of one or both of the hydrogendramatically weakens these intermolecular interactionreducing the magnitude of these quantities. The stroncohesive interactions in water also result in:

(1) a high viscosity, since for a liquid to flow interactionbetween neighbouring molecules must constantly bbroken; and

(2) a high specific heat capacity the ability to store a largamount of potential energy for a given increment ikinetic energy (temperature).

In part waters high specific heat and heat of vaporizatiorelative to other liquids results from its small size. Morintermolecular interactions are contained in a givevolume of water than comparable liquids. When this itaken into account by expressing the specific heat and heaof vaporization on a molar basis, methanol and water arcomparable. The surface tension of water, however, is stianomolously large after accounting for differences in sizeWater has one of the highest dielectric constants of annonmetallic liquid. It also has the remarkable properties oexpanding when it is cooled from 48C to its freezing poinand again when it freezes. Both the expansion of water anits high dielectric constant reflect subtle structural featureof liquid water at the molecular level.

Biological relevance of waters physicalproperties

Water, owing to its high boiling point, exists predominantlyin its liquidform in the range of environments wherlife flourishes, although the other two phases, ice anvapour, play an essential role in shaping the environmenThe high specific heat and heat of vaporization of wate

Article Contents

Introductory article

. Introduction

. Molecular Structure and Polarity

. Dielectric Constant

. Ionization

. Hydrogen Bonding

. Hydrophobicand HydrophilicInteractions: Water as

Solvent

ENCYCLOPEDIA OF LIFE SCIENCES 2001, John Wiley & Sons, Ltd. www.els.net


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have important consequences for organisms at the cellularand physiological level, in particular for the efficiencyof processes involving heat transfer, temperature regula-tion, cooling, etc. Viscosity is the major parameter ofwater that determines how fast molecules and ions can betransported and how rapidly they diffuse in aqueoussolution. It thus provides a physical upper limit to the

rates of many molecular level events, within whichorganisms must live and evolve. These include the ratesof ion channel conductance, association of substrateswith enzymes, binding rates, and rates of macromolecularassembly. It also sets an upper bound to the length scaleover which biological processes can occur purely bydiffusion. In many cases, for example in enzymesubstratereactions, evolution has pushed the components ofliving systems to the limits set by waters viscosity.

The high surface tension of water is relevant at twolevels. First, below a length scale of about 1 mm surfacetension forces dominate gravitational and viscous forces,and the airwater interface becomes an effectively

impenetrable barrier. This becomes a major factor in theenvironment and life style of small insects, bacteriaand other microorganisms. Second, at the molecular(0.1100 nm) scale the surface tension plays a key rolein waters solvent properties. The high dielectric constantof water also plays an important role in its action as asolvent. The biological significance of the expansion ofwater upon cooling below 48C and upon freezing, thoughcrucial, is largely indirect through geophysical aspects

such as ocean and lake freezing, the formation of thpolar ice cap, and in weathering by freezethaw cycles.

Molecular Structure and Polarity

The geometry of the water molecule is illustrated iFigure 1a. It consists of two OH bonds of length 0.096 nm

at anangleof 104.58. Other basic properties of water are isize, shape and polarity. Atoms that are not bonded wirepel each other strongly if brought close enough that theelectron orbitals overlap. At larger distances two atomattracteach other weaklydue to an induced dipole-inducedipole (London dispersion) force. The combination orepulsive and attractive interactions is termed the van deWaals interaction. The point at which the repulsive anattractive forces balance is commonly used to define thdiameter of an atom, which for oxygen and hydrogen ar0.32 nm and 0.16 nm, respectively. The water molecule thus approximately spherical. Water is electrically neutrabut because the electronegativity of oxygen is much greate

than that of hydrogen the electron distribution concentrated more around the former, i.e. water electrically polarized, having a permanent dipole momenof 6 102 30 C m in the gas phase. The dipole moment even larger (c. 8 102 30 C m) in liquid and ice becausneighbouring water dipoles mutually polarize each otherA useful way to represent the polarity of a molecule is tassign a partial charge to each atom, so as to reproduce thmolecules net charge, dipole moment, and possiblhigher-orderelectrical moments (Figure1b). The magnitud

Table 1 Selected physical properties of water

Values at 293 K unless indicated.aIn the gas phase.

Property Water Methanol Dimethyl ether

Formula H2O CH3OH (CH3)2O

Molecular weight (g mol1) 18 32 46

Density (kgL1) 0.998 0.7914 0.713

Boiling point (K) 373 338 248Molecular volume (nm3) 0.0299 0.0420 0.107

Volume of fusion (nm3) 0.0027 Negative Negative

Liquid density maximum (K) 277 None None

Specific heat (J K1 g1) 4.18 2.53 2.37

(J K1mol1) 75.2 81.0 109.0

Heat of vaporization (kJg1) 2.3 1.16 0.40

(kJ mol1) 41.4 37.1 18.4

Surface tension (mN m1) 72.8 22.6 16.4

Viscosity (Pa s) 1002 550 233

Dielectric constant 78.6 33.6 5.0

Dipole moment (Cm 1030)a

6.01 5.68 4.34

Water: Structure and Properties

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of an atoms partial charge is a measure of its polarity. Forwater there is about 1 0.5 on each hydrogen, and a chargeof opposite sign and twice this magnitude on the oxygen. Incontrast, the hydrogens of an apolar molecule such asmethane have a partial charge of % 0.1, and methanesdipole moment is zero. Thus water is a very polar moleculewith the ability to make strong electrostatic interactionswith itself, other molecules and ions.

Putting all this together, one can picture a watermolecule as a slightly sticky sphere of radius 0.32 nm inwhich two positive charges of1 1/2 and a negative chargeof 2 1 are embedded at the hydrogen and oxygen atomiccentres respectively (Figure 1b). Many of liquid watersproperties, including its cohesiveness, its high heat ofvaporization,dielectric constant andsurface tension can beexplained with this simple molecular model. Other proper-ties such as the temperature dependence of the density needa more sophisticated model thatincludes waters flexibility,polarizability and quantum mechanical effects.

Dielectric Constant

The dielectric constant is a measure of howeasilya materialis polarized by an electric field relative to vacuum. It isdefined by the magnitude of the dielectric polarization(dipole moment per unit volume) induced by a unit field.Water has nearly 80 times the dielectric constant of

vacuum, and it is an order of magnitude more polarizablthan most organic solvents. The dielectric constant of polar liquid such as water depends on four major factorthe permanent dipole moment of the molecule, the densitof dipoles, how easily they can reorient in response to field, and how cooperative this reorientation is. Water haa high dipole moment, it is small so there are a larg

number of dipoles per unit volume, and in the liquid statthey are easily and rapidly (within 10 ps) reoriented. Iaddition, because water is extensively H bonded, thpolarization response is cooperative: water moleculecannot simply reorient independently of their neighbourThey effectively reorient in groups of about three. Finallythere is a small contribution to the dielectric constan(c. 23) from the polarizability and flexibility of water. Athese factors explain the very high dielectric constant owater. Decreasing the temperature increases the dielectriconstant since it reduces the randomizing thermal fluctuations that oppose dipole alignment by an electrostatic fieldInterestingly, the static dielectric constant of wate

continues to increase through the freezing point. The higdielectric constant of ice (Table 2) demonstrates thimportance of the cooperative effect of dipole reorientation, although the time scale of reorientation is six orders omagnitude longer.

Ionization

Because the OH bond of water is strongly polarized, thelectron density around the hydrogen atom is very low anthe OH bond is rather weak compared with most covalenbonds. Thermal fluctuations in the liquid often (every 20m

or so) result in sufficient further polarization of the OHbond that the hydrogen nucleus can dissociate as a protonor H1 ion. Water being an excellent solvent for ions, it casolvate the resulting OH2 and H1 ions, the latteprimarily as H3O

1 . As a consequence dissociated watehas a relatively long lifetime of about 100ms in pure watebefore recombination. The spontaneous ionization owater is characterized by a dissociation constant, deriveusing eqn [1].

HOH

H2O 1:82 1016 mol L1

With a water concentration of 55.6 mol L21, the concen

tration of H1 at 258C in pure water is 1.0 102 7 moL2 1and thus it has a pH of 2 log10([H1 ])5 7. Thhydrogen ion is highly mobile in liquid water, diffusinabout five times more rapidly than water itself (Table 2Remarkably, the mobility of a proton in ice is higher stilclearly demonstrating that proton transport occurs not smuch by movement of a single proton, but by a hoppinmechanism between H-bonded waters, whereby a watemolecule accepts a proton on one side, and releases proton on the other side. Since the lifetime of an individua

O

H H

H-bondlength

O

H H104.5

0.096nm

(a)

+0.5 +0.5

1

0.32nm

(b)

O

H

(c)

H

OOHH

H

O H

H

H

OIce I

109.50.2

75nm

0.45nm

H

H

O

O

H H

H

H

O

H

OH

HOH

O

H H

Tetrahedral waters

Tetrahedral waters

H

O

H

Hydrophobicsolute

(d)

Angle

H

O

H

O

H

H

OH

H

Polarsolute

HO

H

O

H H

(e)

Figure 1 Structure of water. (a) Definition of key lengths and angles. (b)

Modelof water. (c) Structure of Ice I. (d) Schematic of H bonding structurein liquid water, and in presence of an apolar solute. (e) Schematic of Hbonding structure around a positively charged ion of polar atom.



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H3O1 ion is % 1 ps, about five orders of magnitude shorter

thanthe lifetime of dissociatedwater, many hopping eventsoccur before recombination. This lifetime is also shorterthan the molecular translation time, again indicating thatdirect diffusion of the H3O

1 cannot account for the highproton mobility. The ionization constant of water is ordersof magnitude higher than that of most organic solvents.

Waters unique ability to ionize easily and to solvate OH

2

and H1 ions allows it to partake in OH2 and H1

exchange with many polar solutes. Water can donate itsH1 to a base, or accept (solvate) H1 from an acid. Acidbase and proton exchange reactions are pervasive inbiology, occurring in protein folding, protein binding,enzyme catalysis, ion pumping, ion channel reactions,bioenergetic pathways, synthesis of ATP, and in thechemiosmotic mechanisms of energy transduction, toname a few. Communication of a biological signal ortransmission of energy via protons is also extremely rapiddue to the facile ionization of water and the high protonmobility.

Hydrogen Bonding

Water structure

Water exists in three phases: vapour, liquid and ice, the lastof which has at least nine known forms. For biologicalphenomena, the most important is the liquid phase. It isuseful, however, when describing its structure to use thesimplest form of ice, Ice I, as a reference. The structures of

both are dominated by the hydrogen-bonding interactionThe hydrogen bond (H-bond) is a strong bond formebetween a polar hydrogen and another heavy atom, usuallcarbon, nitrogen, oxygen or sulfur in biological moleculeIn the gas phase the strength of an H bond between twwaters is 22.7kJ mole2 1, although in liquids and solids itstrength is greatly dependent on geometry and th

surrounding molecules. It is sometimes characterized aintermediate between ionic and covalent bonds in character, although its energy as a function of the length anangle can be quite accurately described by a Coulombiinteraction between the partial atomic charges on thhydrogen, the heavy atom it is covalently attached to, anthe oxygen, nitrogen, carbon or sulfur atom with which itmaking the H-bond.

Ice I is a tetrahedral lattice where each water makeH-bonds to four other waters, which lie equidistant fromeach other at the vertices of a regular tetrahedron with edglengths of 0.45 nm (Figure 1c). The H-bonds are 0.275 nmlong measured from oxygen to oxygen, and linear (08 H

bond angle; Figure 1a). The H-bonding pattern of ice isymmetrical: each water makes two donor H bonds with ihydrogen atoms, and two acceptor H-bonds with thhydrogen atoms of neighbouring waters. The 2H-bonding symmetry is an important feature of wateCombined with an HOH bond angle very close to thideal tetrahedral angle of 109.58, and with the tendency fothe four neighbouring waters to repel each other electrostatically, it is sufficient to explain the tetrahedral H

Table 2 Selected physical proteins of liquid water and ice

Liquid (293 K) Ice I (269 K)

Coordination number 4.7 4

Dipole moment (Cm1030) 8.08.7 8.79.4

Polarizability (nm) 0.144 0.144

Static dielectric constant 78.6 93Ionization constant (molL1) 1.821016 3.81022

Dissociation rate (s1) 2.5105 3109

Dielectric relaxation time 9.5 ps 10 s

Molecular reorientation time 10 ps 10 s

Molecular translation time 20 ps 10 s

H3O+ lifetime 1 ps 0.1 ps

Hbond lifetime 1 ps

Diffusion constant (m2 s1)

H2O 2109 3.91015

H+ 9109 2108

Coordination water exchange timeAround water 1 ps

Around a typical ion 110 ns


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bonding pattern of Ice I and the persistence of thetetrahedral pattern in liquid water.

A different way of describing the structure of Ice I is tocount up the number of neighbours each water has as afunction of distance. Starting at the central water depictedin Figure 1c and moving out, the first four neighbours arefound at a distance of 0.275 nm. The next set is encountered

at 0.45 nm: 12 waters that are H-bonded to the first fourneighbours. Continuing out, shells of water are encoun-tered at discrete distances. The resulting radial distributionfunction (rdf) is characteristic of a crystalline solid, andconsists of discrete peaks (Figure 2). The number of watersin the firstpeak definesthe coordination number,whichinIce I is four. Liquids do not have a single structure since themolecules are in constant motionbut the rdf is an extremelyuseful way to describe their average structure. Figure 2shows the rdf for liquid water obtained either from X-rayscattering experiments or computer simulations usingthe water model in Figure 1b. The rdf for liquids isnormalized to one so that the value at any point is the

average number of waters found at that distance relativeto the number expected if the distribution of watermolecules were completely random, i.e. if there were nostructure.The broad overlapping peaks decaying away to a constantvalue of one at large distances is characteristic of a liquid.The first peak or hydration shell indicates that thereis a high probability that two waters will be separatedby about 0.250.30 nm, the range of H-bonding distances.Beyond 0.3 nm there is a dip since waters at this distanceare likely to overlap with those in the first shell, then thereis a smaller peak at 0.45 nm, which is the remains of the

second layer seen in the Ice I structure. The area under thfirst peak gives the coordination number of water at 258Cas 4.7. This is somewhat higher than for Ice I, indicatinthat the lattice structure has partially collapsed, and eacwater on average makes an H bond to more than fouwaters. Organic solvents typically have coordinationumbers of 6 or higher, so by comparison water has a

open structure. Experiments and computer simulationshow that the open structure results from the high degreof angular ordering in liquid water. Figure 3 shows thprobability distribution of H-bond angles made by eacwater to its 4.7 neighbours. It is bimodal, and should bcontrasted with the H-bond angle distribution for IcI, which is a single peak at 08. Inliquid fourof the H bondare approximately linear (mean angle of about 128), anvery close to their length in Ice I. This indicates thamuch of the tetrahedral structure of Ice I persists in liquiwater, albeit in a distorted form (Figure 1d). The H-bond tthe additional neighbour(s) is more distorted, with aaverage angle of about 528, since these neighbours have t

sit in a face of the tetrahedron formed by the primarhydration waters.

The open tetrahedral structure is also responsible fothe anomalous temperature dependence of water densityWater contracts upon melting, and continues to contracuntil it reaches a temperature of 48C, above which expands like most liquids. In the contraction phase thcollapse of the open tetrahedral structure due to increasingly bent H-bonds outweighs the normal tendency fomaterials to expand because the molecules become furtheapart.

0.010 50

Relative

probability

H-bond angle ()

0.0

10 70 9

4 H bonds oflength 0.276 nm

30

Ice I

Purewater 0.7 H bonds of

length 0.31nm

Water nearapolar group

Water nearpolar group

Figure 3 Hydrogen bond angle probability distribution for Ice I and puwater (top), and for water around solutes (bottom).

1

0 0.4

Radialdistributionf

unction(

rdf)

R (nm)

1

0.2 0.6 0.8

Water withhydrophobic

solute

Water:X-ray scattering

simulation

Ice I

Figure 2 Radial distribution functions (rdf) of Ice I (bottom), pure water

(middle), and water around a hydrophobic solute (top). Lines, measured;circles, computer simulation.



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Effect of solutes

Solutes perturb the structure of water, primarily in thesolutes first hydration shell (the layer of water in contactwith the solvent), with a lesser effect on more distantwaters. Figure 2 shows the rdf for liquid water around thehydrophobic (water-avoiding) solute tetramethyl ammo-

nium. There is a slight sharpening of the distributioncaused by an increase in thefirst peak height and a decreasein the first dip. This is indicative of increased ordering ofwater in the first hydration shell, but the effect is not large.The rdf ofliquidwater isdominated bythe size and packingof the water, i.e. by the van der Waals interactions, andthese are relatively insensitive to the presence of the solute.In contrast, solutes have a large effect on the angularstructure of water (Figure 3). Apolar solutes and groupsshift the bimodal distribution of waterwater H-bondangles towards the more ice-like, linear form, effectivelyincreasing the ordering of water by decreasing the lessordered population of H-bonds. These solutes lack the

ability to make strong electrostatic interactions with water,and they interact primarily through the van der Waalspotential. Their effect is essentially geometric: they tend todisplace the more weakly H-bonded facial water in thecoordination shell (Figure 1d), thus reducing the populationof more bent H-bonds. Ions and polar solutes and groupshave the opposite effect. They shift the distribution ofwaterwater H-bond angles towards the more bent form.This is a consequence of the strong electrostatic interac-tions they can make with water. Waterdipoles tend to aligntowards or away from the atoms with large atomic partialcharges, consequently distorting the waterwater H-bond(Figure 1e).

Hydrophobic and HydrophilicInteractions: Water as a Solvent

Perhaps waters most important biological role is as asolvent. It can dissolve a remarkable variety of importantmolecules, ranging from simple salts through smallmolecules such as sugars and metabolites to very largemolecules such as proteins and nucleic acids. In fact wateris sometimes called the universal solvent. Practically all themolecular processes essential to life chemical reactions,association and binding of molecules, diffusion-driven

encounters, ion conduction will only take place atsignificant rates in solution, hence the importance ofwaters solvent properties. Equally important as watersabilities as a good solvent is its differential effect as asolvent the fact that it dissolves some molecules muchbetter than others. Figure 4 shows the relative solubilities inwater of a selection of solutes that are of biologicalimportance or are building blocks of biologically impor-tant macromolecules. The solubilities range over 50 ordersof magnitude! The high end includes ions and charged

amino acids such as arginine, and aspartic acid. Thessolutes are hydrophilic (water-loving). This category alsincludes some other neutral amino acids such as asparagine, the peptide backbone of proteins, the phosphatesugar backbone of nucleic acids, sugars and lipid headgroups. At the low solubility end are aliphatic amino acidsuch as leucine, the aromatic amino acids such a

phenylalanine, and the hydrocarbon tails of lipids. Thessolutes are hydrophobic. Other solutes such as nucleic acibases and the amino acid tryptophan have intermediatsolubility, and cannot be simply classified as hydrophobior hydrophilic.

Physical basis of solvation

The logarithm of the solubility of a solute is proportionato the thermodynamic work, or hydration free energ(DGhyd) necessary to transfer it into water from a referencsolvent (here cyclohexane). High water solubility corre

sponds to a negative (favourable)DGhyd

, low solubility topositive DGhyd (work must be performed to dissolve thsolute). DGhyd is directly related to the properties of thsolute, the water, and the strength of interactions betweewater and solvent. It is here that the high surface tensioand dielectric constant of water are crucial. The surfactension is the work necessary to create a unit area of watervacuum interface (units of force per unit length arequivalent to energy per unit area). Work is necessarsince interactions must be broken to bring water from thinterior to the surface. Hydrating a solute can be divideinto two steps:

. creation of a solute-shaped cavity in water, whicrequires work to be done against the surface tension owater;

. placing the solute in the cavity, which involves interactions of the solute with water molecules and restructuring of the water.

The first step always opposes dissolution of any solutes. Ithe interactions between the solute and water are weak, athey are for apolar solutes and groups, the cavity termdominates and the solubility will be low. The cavity termdrives aggregation of apolar molecules to reduce thsurface area in contact with solvent. This is known as th

hydrophobic effect. In contrast, when a polar or ionisolute is dissolved in water the electric field from thsolutes partial atomic charges induces a large polarizatio(reorientation) of the water dipoles resulting in aattractive electrostatic field (the reaction field) back athe solute. This results in a high solubility a consequencof waters high dielectric constant, and the reason it cadissolve a wide range of ionic and polar solutes.

In summary, the solubility is determined by two majocontributions:


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. The cavity contribution, which is unfavourable andapproximately proportional to area of the solute orsolute group(s) exposed to water.

. The electrostatic contribution, which depends on thestrength of the reaction field induced in water. Thisin turn depends on the magnitude of the partial atomic

charge, the dielectric constant of water, and how nearthe atomic charge is to the water (i.e. the atoms radius,and whether it is buried or exposed to solvent).

Role of solvation

Many biological macromolecules, such as proteins,nucleic acids and lipids, contain both hydrophilic andhydrophobic groups. Waters differential ability to solvatethe different groups produces a driving force for them toadopt structures or self-assemble in ways where thehydrophilic groups are exposed to water and the hydro-phobic groups are sequestered from water. This is a majorfactor in thefolding, assembly and maintainence of precise,complex three-dimensional structures of proteins, mem-branes, nucleic acids and proteinnucleic acid assemblies.For example, the hydrophobic effect promotes:

. formation of a buried apolar core of amino acids inprotein folding;

. helix formation in nucleic acids through base stacking;

. formation of lipid membranes with an apolar lipid tailregion;

. formation of macromolecular complexes such as multmeric proteins, proteinnucleic acid assemblies anmembrane proteinlipid assemblies;

. specific binding and recognition of molecules witcomplementary apolar surface groups.

Solvation of polar groups acts in a reverse fashion to thhydrophobic effect in the above processes: there is a strondriving force to keep the ionic and polar portions oproteins, lipids and nucleic acids on the surface in contacwith water. This is also the reason that the low dielectrilipid tail region of membranes is impervious to ions, a keproperty of biological membranes.

The delicate balance between polar and apolar solvatioforces contributes to a remarkable fidelity and accuracy oself-assembly. For example, related proteins of differensequence but with conserved patterns of amino acihydrophobicity/hydrophilicity can adopt structures thaare similar to 0.1 nm or better tolerance.

Further Reading

Eisenberg D and Kauzmann W (1969) The Structure and Properties o

Water. Oxford: Oxford University Press.

Gerstein M and Levitt M (1998) Simulating water and the molecules

life. Scientific American 279: 100105.

Henderson LJ (1913) The Fitness of theEnvironment: An Inquiry in to th

Biological Significance of the Properties of Matter. New Yor

Macmillan.

Rahman A and Stillinger F (1971) Molecular dynamics study of liqu

water. Journal of Chemical Physics 55: 33363359.

10 5040

Log10 (relative solubility)

200 30

PhosphateNa+

Cl

N-Propyl-guanidine (arg side-chain)Acetic acid (asp side-chain)

GlucosePropionic acid (glu side-chain)

Acetamide (asn side-chain/peptide)Butyl-amine (lys side-chain)

Propionamide (gln side-chain)Guanine (nucleic acid base)

4-Methyl imidazole (his side-chain)Cytosine (nucleic acid base)

Methanol (ser side-chain)Ethanol (thr side-chain)

Uracil (nucleic acid base)Adenine (nucleic acid base)Thymine (nucleic acid base)

P-Cresol (tyr side-chain)Hydrogen (gly side-chain)

Methanethiol (cys side-chain)Methane (ala side-chain)

3-Methyl-indole (trp side-chain)Ethyl, methyl sulfide (met side-chain)

Toluene (phe side-chain)Propane (val side-chain)

Isobutane (leu side-chain)Butane (ile side-chain)Hexadecane (lipid tail)

10

Figure 4 Solubilities of selected solutes in water, expressed as log10 of the solubility ratio between water and the apolar solvent cyclohexane.


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water structure and properties.pdf