1

INTRODUCTION

Saccharides and their derivatives are widely distributed in various forms of life as

essential moieties of glycoproteins, glycolipids, nucleic acids and polysaccharides. These

are found with an enormous range of complexity, from simple mono- to mega-dalton

polysaccharide structures. Because of conformational flexibility, saccharides play

significant roles in many biological processes such as signaling, cell-cell recognition,

molecular and cellular communication1-4

. To understand mechanisms of biological

processes, low molecular model compounds (e.g. alcohols, saccharides, peptides, nucleic

acid bases, nucleosides and nucleotides) have been studied due to the complexities of

biomolecules. Saccharides are important compounds due to their hydrophilic hydroxy

(–OH) rich periphery, coordinating ability, homochirality, stereospecificity, etc.

Saccharides with their well known stereochemistry are logical choices in stereo-selective

reactions for the synthesis of biologically active target molecules5-6

. The hydration

characteristics of saccharides in aqueous solutions are of direct relevance for

understanding the role of glycoproteins and glycolipids in molecular recognition7-11

.The

saccharide components of cell membranes are the receptors of biologically active

compounds (enzymes, drugs) etc. Saccharides consisting of an aliphatic moiety and polar

–OH groups are appropriate models for studying hydration properties of proteins and

nucleic acids. However, the understanding of the relationship between saccharide

structure and their biological function is still far behind that of proteins and nucleic

acids2,12

.

Saccharides act as stabilizing agents for enzymes, therapeutic proteins, hormones,

and antibodies, etc. because of their ability to enhance the structure of water, whereas

others have attributed this to the preferential hydration of proteins. Lee

&Timasheff13

studied the thermal transitions of α-chymotrypsin, chymotrypinogen and

ribonuclease in sucrose and argued that the sucrose stabilize proteins against thermal

damage. However, the mechanism by which stabilization is imparted still belongs to a

realm of speculation14-19

.

In mushroom, yeast, fungi and brine shrimp, intracellular ,-trehalose

contributes throughout its vitrification to their survival abilities against severe conditions

like freezing and desiccation. It is widely recognized that the known biological protective

2

function of trehalose has undoubtedly a close relationship with its physical features.

Although the disaccharides are structurally similar, their protective efficacies differ.

Several mechanisms for the antidesiccation properties of disaccharides have been

advanced, but their protective mechanisms are not well understood20-21

. The mechanistic

understanding of sweet taste chemoreception has been advanced by the micro- and

macro-scopic studies of sweetner-H2O interactions. The hydration characteristics of

saccharides in aqueous solutions are thus a key feature to understand their structural,

functional properties and the mechanism of taste chemoreception22-23

.

Since Angyal’s pioneering work24

, there has developed a great deal of interest in

the thermodynamic studies of saccharides in aqueous electrolyte solutions due to their

importance in several fields related to biology, medicine, catalysis and the environment.

Organisms exploit the water- structuring characteristics of the different saccharides in a

number of ways such as to modify the viscosity of cellular fluids and to protect against

freezing/dehydration25

. Trehalose forms viscous solutions and glasses at room

temperature which have proved useful in studies of protein dynamics. Trehalose coating

prevents thermal denaturation by damping large-scale fluctuations of protein-specific

motions20,21

. The presence of salts modifies important properties of aqueous saccharide

solutions related to their protective role such as viscosity, water sorption, and

crystallization rate and glass transition temperature21,26

. Due to the fact that saccharides

readily undergo isomerization and conformational changes their complexation with metal

ion result in the formation of mixture of products. In spite the weak metal-saccharide

complexes formed, their interactions are selective. Complexing abilities of different

saccharides are primarily dependent on conformation, stereochemistry and size of the

metal ion. Saccharides containing an ax-eq-ax sequence of three -OH groups in a

pyranose ring /cis-cis sequence in furanose ring (ribose isomers) have specific

interactions with some metal cations in solution. But the studies of metal ion-saccharide

interactions regarding binding preferences, structural and stereochemical features and

stabilization of the species formed are largely unexplored27-29

.

Thermodynamic and transport properties are very useful in the study of

hydration behaviour of saccharides. Many technological applications of saccharides

utilize the exotic rheological properties of their aqueous solutions, such as the control of

gelling processes, and the osmoregulation of tissues and organs in cryoprotective

3

provisions30-31

. Fundamental thermodynamic information is important to select the

reaction conditions (e.g. feasibility and optimization) of industrial processes.

Thermodynamic properties of aqueous saccharides at high temperature and pressure are

also applied in a variety of geochemical studies to assess the relative stabilities of

biomolecules at high temperature and pressure. The energetics of catabolism in

thermophiles and barophiles is one of the applications of such data32

. Saccharides have

substantial effects on tailoring phase tunable of ionic liquids (ILs) and water. These

systems are feasible to be used as extraction systems, especially in biological and

environmental engineering33

. In spite of the need, there is relatively little information

available on the enthalpy, heat capacity and volumetric properties for aqueous solutions

of saccharides as a function of temperature, pressure and concentration7,34-36

.

To understand the behaviour of saccharides in aqueous and mixed aqueous

solutions, an understanding of the structure of water, ionic and polar group hydration,

hydrophobic hydration and hydrophobic interactions, etc. is a prerequisite.

Consequently in the following sections, these aspects are being briefly discussed.

1.1 Structure of Water

Water is the most important and widely studied solvent due to its ubiquitous

in biological and industrial processes37-39

. This has evoked a lot of interest to study

the current status of the structure of water and aqueous solutions in biological

systems, particularly the manner in which a solute modifies the short-range and long-

range order existing in water40

. Considerable effort has been devoted towards an

understanding of structure of water, both by experimental and theoretical probes41-49

.

Inspite of all these efforts, the structure of water is still a matter of conjecture37,48

.

Water owes its unique and unusual personality to certain anomalous

physicochemical properties, which distinguish it from other simple fluids50-51

. The most

striking examples of its anomalous behaviour are contraction upon melting (shown

only by Germanium and Bismuth among all the known substances44

), density

maximum, negative pressure coefficient of viscosity, high melting and boiling points,

high latent heat of fusion and evaporation, high dielectric constant, and high molar

heat capacity.

4

The pioneering work on solution X-ray diffraction measurements by Bernal

and Fowler52

and later by Morgan and Warren53

reveal that water is tetrahedrally

coordinated through hydrogen bonds, similar to that in ice I (Fig.(1.1)). The

various hypothetical models put forth to depict the water structure are classified into

(i) Continuum Model ; (ii) Mixture Models.

Fig. 1.1 Schematic representation of Crystal Structure of Ice I at Low Pressure

1.1.1 Continuum Model

The continuum or uniformist model assumes that liquid water is uninterrupted,

three dimensional lattice of tetrahedrally coordinated hydrogen bonded molecules52-

57. Thermal, electrostatic and steric perturbations are thought to produce hydrogen bond

stretching and bending deformations rather than bond breakage. The recently highly

successful continuum model, also known as Random Network Model (RNM) has

5

been proposed by Rice and Sceats56

and developed by Henn and Kauzmann57

. It does

not involve changes in the number of hydrogen bonds. RNM model is not only a good

model for determining the heat capacity contribution due to water-water interactions

but also gives a good insight into the reorganization of water around the non polar and

polar solutes. This model also provides a simple mechanism for energy absorption and

variations of dielectric properties and X-ray radial distribution function with

temperature as a variable. However, in view of the restrictions placed on molecular

motions, this model fails on logic to explain the trend in entropy data and the hydration

properties of non polar molecules.

1.1.2 Mixture Models

These models visualize liquid water as collection of differently hydrogen bonded

species in which each water molecule can fluctuate through states where none, one or

both of its hydrogen atoms are engaged in hydrogen bonding58-67

. Depending on the type

of species involved, the mixture models have been classified as follows: (i) Interstitial

Model proposed by Samoilov68

and Forslind69

; (ii) Water Hydrate Model proposed by

Pauling70,71

; (iii) ‘Flickering Cluster’ Model proposed by Frank and Wen

58, and Frank

72.

The most popular and commonly used model, especially in the field of solution

chemistry is the 'Flickering Cluster' Model.

Flickering Cluster Model

Frank and Wen58

, and Frank72

viewed water as a mixture of clusters of

hydrogen bonded water molecules with voids therein (bulk water) and free

unbonded monomers (dense water) in equilibrium with each other (Fig. 1.2). The

formation of hydrogen bonds and the disruption of hydrogen bonds in such a

polymeric complex is considered to be a cooperative phenomenon. The physical basis

of this cooperativity is that hydrogen bond formation involving one bonding site of a

water molecule contributes delocalization energy to the molecule, which favours

additional bonding at other potential hydrogen bonding sites. According to this

model, the presence of flickering clusters of water molecules (Fig. 1.2) of short half-

time (10-11

to 10-10

s) reflects local energy fluctuations. The occurrence of some type of

dynamic monomer-polymer equilibrium in water allows an acceptable

explanation of the free energy of activation of transport processes in water, whether

6

calculated from self-diffusion, viscous flow, dielectric relaxation, or ultrasonic

absorption data73

.

Fig. 1.2 Frank-Wen Flickering Cluster Model of Liquid Water

Nemethy and Scheraga60

used a statistical thermodynamic treatment to calculate

the Helmholtz free energy, internal energy and entropy as a function of temperature of

liquid water, assuming that each of the hydrogen bonded molecules could be

associated to a discrete energy level in the flickering clusters. Hagler et al73

. and

Lentz et al74

. have modified this model to remove some major inconsistencies

prevalent in it as pointed out by Nemethy75

. Walrafen61

and Hartman76

provide the

strongest support for the mixture model from the Raman Spectroscopic studies of

7

HOD. These studies reveal a pronounced shoulder in the high frequency side along

with the intense low frequency band. Further these studies show that as the water is

heated, the intensity of the high frequency component increases compared to that of

low frequency component. The high frequency component was associated to the

monomeric species and the main band of the low frequency to the hydrogen bonded

species. It was further argued that the increasing intensity of the high frequency

component with increase in temperature reflects the conversion of hydrogen-bonded

molecules to the non-hydrogen bonded molecules (Fig. 1.3). The mixture model thus

appears to be consistent with the vibrational spectroscopic data.

Fig. 1.3 (a) Argon-lon-Laser-Raman spectrum from a 6.2 M solution of D2O in

H2O at 250C and

(b) Quantitative mercury-excited Raman spectra from a 6.2 M solution of

D2O in H2O at a series of temperature.

8

Recently Robinson and co-workers77-78

have also reported from the studies on

properties of pure water as a function of temperature and pressure that on average,

the water is composed of dynamically transforming microdomains of two very

different bonding types i.e. regular tetrahedral water-water bonding similar to that of

ordinary ice i.e. Ih and other is a more dense non-regular tetrahedral bonding similar

to that in ice II. Although many water models are successful in predicting selected

liquid water and ice polymorph properties, there is still a need to construct a model

of general applicability37,39

.

1.1.3 Computer Simulations

Computational techniques are often used to model protein folding, ligand

recognition and binding, partition coefficients, solubility, reaction rates, pK a, and

tautomer ratios, all of which depend crucially on the accuracy with which solvation

effects can be modelled37

. Two types of approaches have been used for computer

simulation studies of the liquid water: (i) Monte Carlo (MC) method79-82

; (ii)

Molecular Dynamics (MD) method83-84,49

. In both these methods, the partition

function and hence the equilibrium properties are generated for an ensemble of water

molecules from the positive additive intermolecular potential energies. The

molecular arrangements in the liquid water are also studied by these two methods.

In MD method, the temporal evolutions of the translational and rotational

trajectories of the individual molecules are followed under the joint influence of the

forces and torques exerted on it by all other molecules. These trajectories then yield

the time autocorrelation function and hence various kinetic properties of liquid

water.

The MC method generates an ensemble of molecular configurations

representing the thermal equilibrium. The results of both MD and MC methods

indicate that there exists a marked tendency of liquid water toward hydrogen bond

order, but with much less predictable geometry. However, the study of ice by

crystallography alone would have suggested this. The potential used in the MC or

MD calculations are all pairwise additives, i.e., the energy of a given assembly is

expressed as a sum of the interaction energies between all pairs of molecules in the

assembly. It has been stressed that none85

of these pair potentials can account for

both gas and condensed phase properties of water. It is evident from quantum

9

mechanical calculations on water trimers, tetramers and pentamers that hydrogen

bonding in these clusters is stronger than in the dimer86

, which suggests the

cooperative nature of hydrogen-bonding interactions.

Jorgensen87

has developed transferable intermolecular potential functions

(TIPS) suitable for use in liquid simulations for water. This potential has been used

by Jorgensen and Madura80

in MC simulation on liquid water to study the effect of

temperature on vaporization, hydrogen bonding, density, isothermal compressibility

and radial distribution functions. Recently Mahoney and Jorgensen88

by applying

classical MC Statistical Mechanical Simulations optimized a five-site, fixed charge

model of liquid water with emphasis on improving the computed density as a

function of temperature and the position of temperature of maximum density. The

obtained results further support a two state mixture models for the liquids.

Various workers49,84,89-90

have used the theoretical and simulation methods to

study the behaviour of water. It was reported82

that linear molar volume of relevant

inherent structures contributing to the liquid phase increases with the decrease in

temperature. Owicki and Scheraga91

observed a broad, smooth distribution of

hydrogen-bond energy for liquid water, rather than relatively discrete sets of the

bonded and unbonded interaction energies. Chu and Sposito92

started with the

relaxing cage model for the dynamic structure of an atomic liquid and finally

generalized it in the case of liquid water. Grunwald93

developed a model for the

structure of the liquid water network using the concept of the two state models. The

model is consistent with free energy and its temperature derivatives, the dielectric

constant, the probable number of hydrogen bonded nearest neighbours,

intermolecular vibration frequencies and dielectric relaxation dynamics.

Liu and Ichiye94

have presented a new one-site effective pair potential model

for water intended for modeling the solvent effect in biological systems. The present

model, based on modifications of the hard-sphere, sticky dipole potential of Bratko,

Blum and Luzar (BBL)94-95

contains an Lennard-Jones type repulsive soft-sphere, plus a

tetrahedral ‘sticky’ potential and an embedded point dipole. This model gives an

intermolecular energy, hydrogen bond energy and heat capacity in good agreement with

results from experiments and other models. Therefore, it represents an improvement

over the BBL in both structure and intermolecular energy. While most of these

10

results79,96

describe liquid water as a continuum of bent or stretched hydrogen bonds

but at the same time the MD results of Stillinger and co-workers83

do indicate the

presence of non-bonded -OH groups whose life time is much longer than the water

molecular vibrational periods. Therefore, these studies lend indirect support to both

continuum and mixture models. Nevertheless, they have been able to explain the

various properties of liquid water like high heat capacity, a density maximum and

compressibility minimum. Recently Dore97

and Gorden and Hura98

emphasized that the

simulation methods will need to the used in conjunction with experimental data for more

understanding of water and its anomalies. In spite of enormous investment in terms of

experimental and theoretical studies to explore water structures, a compact and

useful description is yet to be achieved.

Supercritical water has attracted scientist's interest in recent years due to its

peculiar physicochemical properties and industrial uses37-38,48,99

. Ohtaki et al.48

from X-ray

diffraction and also from neutron diffraction studies proposed a classification of

supercritical water into three categories: (i) low density water; (ii) medium density

water; (iii) high density water, because density is a very important

thermodynamic quantity to describe properties of sub- and super-critical water. It

has been concluded that water at high temperature and high pressure consists of

mixture of clusters and dispersed phase contains low aggregates including

monomeric water molecules. They further stressed that the knowledge of hydration of

ions and polar substances in supercritical water is important to understand ion-

water and polar molecule-water interactions which relate solubility phenomena and

decomposition of the substances. Karamertzanis et al.37

have proposed

a novel,

anisotropic rigid-body intermolecular potential model to predict the properties of water

and the hydration free energies of neutral organic solutes. The proposed water model is

found suitable for modeling the structure, and a range of properties of ordered ice

polymorphs and liquid water not included in its parametrization.

1.2 Structure of Aqueous Solutions

In view of the lack of a complete satisfactory theory of water structure, there

have been continuous efforts to know the structure of aqueous solutions. The most

popular model that has emerged in the area of solution chemistry is the 'Flickering

Cluster' model56-58

. Though like other models this model is not entirely adequate, still it

11

has been able to explain the various properties of aqueous solutions more satisfactorily

than any other model.

According to 'Flickering Cluster' model58

, water molecules can be

considered to be in dynamic equilibrium between the bulky tetrahedrally hydrogen

bonded clusters (H2O)n and monomer molecules, as represented by (H2O)n

n(H2O), where (H2O)n and n(H2O) represent 'bulky' and 'dense' states, respectively. The

statistical degree of ice-likeness (or whatever its structure in water) is considered to

be proportional to the half-life of the clusters, which is of the order of 10-11

s in pure

water, which may shift the equilibrium in either direction. The solute which causes a

shift so as to increase the average half-life of the cluster is termed as a ‘structure

maker’ and a solute, which has an effect in the opposite direction, is called a ‘structure

breaker’56

.

Although the concept of structure-making and structure-breaking effects of

solutes is not very satisfactory, but it has proved useful in discussing the effects of

solutes on water structure100

. These effects can be reflected experimentally by observing

the changes brought about by the solutes in the kinetic properties of water such as

fluidity, reorientation time, viscosity, conductance, etc. For instance, structure

makers are shown to decrease the fluidity of water by causing an increase in

reorientation time and increase in viscosity and result in positive excess partial molar

heat capacities in water. The reverse is true for structure breakers. Hydration of

different kinds of solutes, depending upon their nature is classified as: (i) Ionic and

polar group hydration; (ii) Hydrophobic hydration ; (iii) Hydrophobic interactions.

1.2.1 Ionic and Polar Group Hydration

The ionic hydration is one of the most important fundamental

physicochemical processes, which bears close connection with many research fields

in chemistry including chemical reactions and stability of biomolecules. For example,

the crystal lattice of salts like NaCl is held together by very strong electrostatic

attractions between ions. However, water readily dissolves crystalline NaCl because of

very strong electrostatic attractions between Na+/or Cl

- ions and water dipoles. This

leads to very stable hydrated Na+

and Cl- ions, ion-solvent interactions greatly exceed

the tendency of Na+ and Cl

- ions to attract each other. When an ion is introduced into

12

water, it interferes with the local ordering of the water molecules in its neighbourhood

and tends to impose a new order on them, ions of simple electrolytes such as LiF and

MgCl2 having high charge density act as net structure makers and the ions of electrolytes

like CsI and KBr with low charge density behave as net structure breakers. In order

to explain these phenomena, Frank and Wen58

visualized a picture (Fig. 1.4) in

which an ion is surrounded by three concentric regions of water molecules as

described below:

(i) An innermost structure-formed region 'A' consisting of polarized, immobilized and

electrostricted water molecules.

(ii) An intermediate structure-broken region 'B' in which water is less ice-like i.e.

more randomly ordered than the normal water;

(iii) An outer region 'C' in which water molecules have the normal liquid structure,

which is polarized in the usual way as the ionic field at this range will be

relatively weak..

Fig. 1.4 Frank-Wen multizone model of ion-hydration in aqueous solutions

In the electrostrictive region, ionic charge on the ion completely orients the

surrounding water molecules, which become immobilized and firmly packed around

the ion in comparison to bulk water. Samoilov68

has called this phenomenon as

positive hydration. The water in the second and third solvation layers is less

13

immobilized and orientation of water molecules is diminished but it is enough to interfere

with the formation of normal bulk water. The decreased structure in region 'B' is

presumably due to approximate balance of the two competing forces, namely, the normal

structure-orienting influence of the neighbouring water molecules and the radially-

orienting influence of the electric field of the ion which acts simultaneously on any water

molecule in this region. The latter ionic influence predominates in region ‘A’ and the

former in region ‘C’ and between ‘A’ and ‘C’, according to Frank and Wen58

, there is

a region of finite width in which more orientational disorder should exist than in

either ‘A’ or ‘C’. Thus, water in this region has less structure ( or there are fewer

hydrogen bonds) and ion can be classified as structure-breaking or negative

hydration ion.

The relative size of these three regions depends on the charge, size and

composition of the ion. In general, the ions with low charge density have relatively

weak electrostatic fields which makes the region ‘A’ very small thereby causing the net

decrease in the structure. On the other hand, ions of high charge density increase the

region ‘A’ and might induce additional structure (entropy loss) of some sort beyond

the first water layer, thereby causing a net structure-making effect.

Friedman and Krishnan101

have proposed a model for aqueous ionic

solutions primarily based on the ideas of Frank and Evans102

. Gurney103

and

Samoilov68

with the assumption that around each solute particle xz (z=charge on the

ion) there is a region termed as cosphere, having the thickness of one solvent

molecule in which solvent properties are effected by the presence of the solute.

These effects are characterized by the thermodynamics of the process:

n[Solvent(pure bulk liquid)] → n[Solvent (in cosphere state next to xz)] where, n is the

number of solvent molecules in the cospheres. Friedman and Krishnan101

model

emphasizes on the contribution of the various cosphere states to the thermodynamic

properties of solutions, while Frank and Wen model58

, and more recently Stillinger and

Ben-Naim model104

of ionic solutions being rather qualitative, are supported by the

experimental studies of kinetic properties, X-ray and neutron scattering105

. Friedman and

Krishnan101

have compiled the various thermodynamic properties and have given

reasonable explanation of these properties based on their cosphere model.

14

Recent computer simulation studies106

have confirmed the presence of two types of

hydration shells. Inner hydration shell has a well defined hydration number (exceptions

are with the low charge densities) and in the outer hydration shell, the water-water

correlation depicts a decrease in structure.

Aqueous solutions of simple polar non electrolytes which have proton donor or

acceptor sites are very interesting since many of them are components of

biological fluids or very similar to monomeric units of biological macromolecules107

.

Urea, amides, alcohols, polyols etc. are the few examples where hydration of these

compounds involve hydrogen bonding, dipole-dipole and dipole-induced dipole

interactions between these and water molecules108-109

. Polyfunctional solutes e.g.

carbohydrates exhibit very complex behaviour in aqueous solutions which is

difficult22-24,110

to rationalize as both hydrophilic and hydrophobic types of hydration are

contributing. As all taste effects are mediated by water, thus the solution properties of

small saccharide molecules provide necessary clues to the possible mechanisms of

taste chemoreception109

. The apparent molar volumes of saccharide molecules

depend upon the axial and equatorial arrangement of their -OH groups111

and

saccharide molecules have been thought of as polarised with respect to water

structure just as they are oriented with respect to the taste receptor. In other words,

water molecules probably cluster preferentially around the hydrophilic end of the

saccharide molecule.

1.2.2 Hydrophobic Hydration

Non polar solutes such as hydrocarbons and ions having non polar groups e.g.

substituted hydrocarbons and alkyl-ammonium salts, behave quite differently in water

as compared to the ionic solutes, as these are readily soluble in most non polar solvents,

but only sparingly soluble in water. The relatively low solubility of hydrocarbons in

water is easy to observe and determine, but unexpectedly difficult to explain112-114

. In

thermodynamic terms, the low solubility of non polar solutes means that the transfer of

the solute molecules from the pure liquid alkane to dilute aqueous solution and this

process is accompanied by large decrease in entropy which makes the free energy of

their solution positive in spite of their exothermic dissolution in water. It seems natural

to regard the insolubility as being ‘entropy controlled’113

. This behaviour of non polar

solutes is apparently uniquely characteristic of aqueous solutions. The negative entropy

15

and enthalpy change and positive change in heat capacity upon the addition of non polar

solute to water indicates that the packing of water molecules around the non polar

solutes is somewhat tighter than in pure water. This situation is called a ‘structure

increase’ or enhancement of structure of water112,115

(Fig.1.5). This region is also known

as iceberg formation or hydration of second kind or the hydrophobic hydration although

the structure is not necessarily identical with that of ice. These models assume the

formation of short-

Fig. 1.5 Diagrammatic representation of

(a) Hydrophobic hydration and (b)-(e) hydrophobic interactions;

(b) Kauzmann-Nemethy-Scheraga contact interaction;

(c) Globular protein folding;

(d) Proposed solvent-separated interaction; and

(e) Possible stabilization of helix by interaction as in (d)

16

lived aggregates of water molecules around the solutes, having more or less well

defined structure, called as ‘icebergs’116

, ‘flickering clusters’56,58

, clathration shells’,

polyhedral cages’ similar to those found in the crystalline hydrates formed by some

small non polar molecules117

. The most impressive evidence for the formation of some

kind of definite structure i.e. iceberg or cages in aqueous solution is probably the

existence of solid gas hydrates and clathrates. These models are consistent with the

view that considers, inert non polar solutes as ‘structure makers’. The formation of

polymeric aggregates involves increased or strengthened hydrogen bonding113

, which

would give negative contribution to tH. The solutes imposed structure melts away on

heating the solutions. This ‘melting’ involves breaking of some of the excess hydrogen

bonds resulting in an increase in the heat capacity118

.

Ben-Naim119

has also discussed the stabilization of water structure caused by the

addition of non electrolytes that arise from the difference in the change of the chemical

potentials of the clusters and of the monomeric water molecules. He also pointed out

that the penetration of solute molecules into the cavities of the relatively open structure

of the clusters enhances the stabilization effect, although filling up of the cavities is not

an essential requirement for the existence of a stabilizing effect. A number of hydrates

of the noble gas atoms, hydrocarbons, halogens and of many other simple molecules are

known to exist. Frank59

has pointed out that the induced ice-likeness, cannot consist of

clathrate formation in the liquid phase, although ice-like structure could be transitory

fragments of clathrate-like aggregates.

According to Privalov and Gill118

, there are two temperatures TH and TS of

universal nature that describe the thermodynamic properties for the dissolution of liquid

hydrocarbon into water. TH is the temperature at which heat of solution is zero120

, which

is around 200C for a number of liquids, TS is the other universal temperature around

1400C where entropy of transfer of non polar compounds into water is zero, and the

process of transfer is most unfavourable. Further at any temperature other than TS, the

net effect of hydration of non polar solutes favours the transfer of non polar molecules

from the compact state into water. Thus, according to their views, the hydration effect

of non polar solutes stabilizes the dissolved state and this in itself cannot be regarded as

the cause of their hydrophobicity.

17

Muller113

has given another view that at and above TS water retains though to a

lesser extent, the same capacity for structural reorganization which makes it a unique

solvent at 250C.This modified ‘hydration shell hydrogen bond model’ (HSHB)

112

assumes that the progressive breaking of hydrogen bonds on heating accounts for about

half of the observed heat capacity of bulk water, and the bond breaking process is

perturbed by the introduction of a solute that give rise to ∆C0

p,2 . Simulations confirm

Muller’s hypothesis that a larger fraction of the hydrogen bonds within the first

solvation shell are broken in comparison to the bulk solution.

According to Lumry et al.65,121

and Ben-Naim119

there is a exact compensation

for the entropy and enthalpy change that arises from change in the state of water in the

presence of non polar solutes i.e. ordering of water, and thus this effect does not

contribute to the hydration Gibbs energy. However, Privalov and Gill118

and Murphy et

al.122

have pointed out that large heat capacity change which plays the key role in

regulating the hydration free energy change and the principal feature of hydration

process.

Borisover et al.123

suggested a new method of estimating the hydrophobic effect

contribution to the thermodynamic functions (Gibbs energies and enthalpies) of

hydration of non electrolytes. Their results indicate that enthalpies of the hydrophobic

effect for alkanes are negative which are similar to that reported by Belousov and

Panov124

. However these results are contrary to Abraham’s opinion125

that this enthalpy

is positive. Borisover et al.123

from these studies also indicated that the rare gases

exhibit hydrophobic hydration.

Costas et al.126

have found that if the combinatorial contribution to the Gibbs

energy and entropy of transfer is subtracted from the experimental values, all non polar

solutes in water behave in a universal manner, i.e. all of their thermodynamic transfer

functions can be studied with their molecular surface area as the only parameter. They

have considered that the water molecules around solute undergo a relaxation process,

which lowers the Gibbs energy, enthalpy and entropy of the system and is responsible

for large heat capacity of transfer. The origin of hydrophobicity lies in high cohesive

energy of water being counteracted by relaxation of water molecules in the

neighborhood of the hydrophobic solute.

18

Madan and Sharp114

have reported that non polar solutes have a large positive

heat capacity of hydration, Cp, while polar groups have a smaller negative Cp of

hydration. The increase in heat capacity of non polar solutes can be as large as 12 JK-1

mol-1

per first shell water at 25C, a surprisingly large change when compared to the

total heat capacity of water, 75 JK-1

mol-1

. As a consequence of the large heat capacity

increase, the low solubility of apolar groups in water at higher temperatures (>90C) is

caused by unfavourable enthalpic interactions and not because of unfavourable entropy

changes. The hydration of polar groups is accompanied by a smaller but significant

decrease in Cp. Thus the solubility of polar groups in water is driven by favourable

enthalpic interactions at both higher and lower temperatures. Talukdar and Kundu127

have studied the salt effect on the hydrophobic hydration in aqueous salt solutions. They

concluded that hydrophobic cations induce more hydrophobic hydration in aqueous

NaNO3 solution than in pure water.

1.2.3. Hydrophobic Interactions

The tendency of non polar groups to aggregate in aqueous solutions in order to

minimize energetically unfavourable interactions with water is termed as the

‘hydrophobic bonding’128-129

, ‘hydrophobic interaction130-134

. These are solvent

induced interactions between a polar moieties and are an outstanding feature of aqueous

solutions. They are driven by the urge of water in overlapping hydrophobic hydration

shells to regain the properties of water in the bulk. The entities can either be parts of

independent molecules or ions, e.g., glycerides, phospholipids, surfactants, and

dyestuffs, or side chains, covalently bonded to a common backbone, e.g., polypeptides

or proteins. They play fundamental role in the stabilization of micelles, biomembranes,

and native globular protein structures41,118,129,135-136

. These interactions are generally

involved in many molecular recognition processes such as the specific binding of

substrates in the active site of enzymes, quaternary protein structure formation, protein-

DNA interactions and host-guest binding135,137

. Hydrophobic interaction is also one of

the important factor in hydrophobic column chromatography, a novel methodology for

the purification of a variety of biological and industrial materials.These subjects are

however problematic even on semantic level138-140

.

From spectroscopic measurements, Ide et al.141

have reported that non polar

substitutents bring about structuring of water surrounding these groups. NMR studies

19

carried out by Bagno et al.142

also showed that water-water bonding in the surrounding

of hydrophobic side substituents are stronger than these within bulk water. Hechte et al.

143 has also reported the increase in water clusters around alkyl side chain with increase

in alkyl side chain length.

Commonly a distinction is made between bulk and pairwise hydrophobic

interactions in water. Bulk hydrophobic interactions, which lead to surfactant

aggregates, such as micelles and lipid bilayers, are the resultant of the fact that

insufficient water is available for the formation of independent hydration shells, so that

association can not be prevented. These aggregates are formed with a high cooperativity

and London dispersion energy contributes significantly to their stabilization. Pair wise

hydrophobic interactions on the other hand do take place via destructive overlap of

independent hydration spheres. London dispersion interactions play a much less

important role, since for these direct interactions, the hydration of solute is required.

Ludemann et al144

have reported very recently that till date there is no evidence for a

qualitative difference between pair and bulk hydrophobic interactions.The association

of non polar moieties is assumed to be partial reversal of the solution process. The

thermodynamic parameters for hydrophobic interactions i.e. ΔH0

> 0, ΔS0

> 0, ΔCp0

<

0, ΔG0

< 0 have opposite signs as compared to those of hydrophobic hydration42,60

.

The hydrophobic interaction is accompanied by disordering of water structure i.e.

partial melting of the so called ‘iceberg’ or clathrate like structure that are produced

when the separate non polar species are first introduced into water42,133

. The

hydrophobic interactions are found to be unusual as compared to normal processes

because these are temperature dependent. At 250C, the transfer of non polar solutes to

water is not principally opposed by the enthalpy, rather it is due to excess

entropy41,118,120

. It arise from the water of solvation around the solute rather than from

solute-solute interactions145-147

. Computer simulation supports a molecular

interpretation that at 250C non polar solutes are surrounded by ordered water molecules.

Water surrounding the non polar solute prefer to hydrogen bond with other water

molecules rather than to ‘waste’ hydrogen bonds by pointing them towards the non

polar species145-146

. However at higher temperatures the aversion of non polar solutes

for water becomes ordinary and less entropy driven. The heat capacity change for

transfer of non polar solutes from the pure liquid to water is large and positive41,101,118

,

which implies that the enthalpic and entropic contributions are highly temperature

20

dependent. The free energy is a nonlinear function of temperature, increasing at low

temperature and decreasing at higher temperature. Hence there will be a temperature at

which solubility of non polar species in water is a minimum41,118,120,148

. The free energy

of transfer of non polar solute into water is extrapolated to be most positive in the

temperature range 130-1600C

118-119,122,149. Hydrophobic interactions are found to be at

their maximum value at 1400C and are determined entirely by enthalpic contributions.

Hence water ordering is not the principal feature of the aversion of non polar residues of

water.

1.3 Specific Molecular Hydration

The hydration effects in aqueous solutions of non polar solutes have been shown

to be largely non-specific and are characterized by microheterogeneities (reflected in

marked concentration dependence of physical properties of these solutes). The opposite

is the case for highly polar solutes where specific interactions between proton donor or

acceptor sites (e.g. -OH, -N-, -O, >C=O) on neutral molecules and water molecules

occur. However, interactions which predominate in aqueous solutions of polyfunctional

solutes are of rather different nature from those occurring in both hydrophobic and

polar solutes.

Because of large number of structures present in saccharides, these can be very

good model compounds to explore the hydration behaviour of polyfunctional

hydrophilic solutes. The hydration of a saccharide molecule depends on its

stereochemistry, but how the hydration of a saccharide depends on its hydroxy

topology is a matter of debate and controversy8,150

. Generally there are three points of

view. It has been proposed that the anomeric effect of a saccharide governs its

hydration151

. Studies of Suggett and co-workers50,152

have shown the absence of any

marked solute-solute interactions in aqueous solutions of saccharides even at

relatively high concentrations. Tait and Frank153

have proposed the 'specific

hydration model' which emphasises that, in addition to the number of potential

hydration sites, hydration interactions depend on relative distance between, and mutual

orientations of these hydration sites. Since the hydration interaction competes with

the intermolecular hydrogen bonding in water, solute molecules which are able to

interact with the solvent without major perturbations in the solvent structure are likely to

interact preferentially. In this context, Warner154

from his model building studies,

21

pointed out that in many biological molecules, the spacing of ether, ester, hydroxyl and

carbonyl oxygen atoms is about 4.8 0A, which is very close to the next-nearest

oxygen spacing in a hypothetical ice I lattice at 298 K (4.750A). This suggests that,

provided there exists in liquid water some resemblance to an ice-like structure,

molecules like biotin, 1,4-quinone, β-D-glucose and triglycerides (Fig. 1.6) could be

cooperatively hydrogen bonded into water structure without undue modification

of the existing hydrogen bonded system.

Fig. 1.6 Correlation between oxygen spacings in organic molecules and in the ice

lattice. Numbers in the formulae indicate interaction points with the

correspondingly numbered oxygens in the ice lattice.

Model - ice

Biotin

1,4-Quinone

-D-Glucose

Triglyceride

22

In addition, Franks150,155

postulated the stereospecific hydration model, which

assumes that an equatorial hydroxy group is better hydrated than an axial hydroxy group;

hence, the hydration of a saccharide was proposed to depend on the ratio of equatorial

versus axial hydroxy groups. Although this theory has found considerable support156-160

,

doubts have arisen concerning its general applicability161

.

Recently, a modified stereospecific hydration model has been proposed23,162-163

,

emphasizing that the hydration of a saccharide depends predominantly on the relative

distance of the next-nearest neighbour oxygens of a saccharide compared to the oxygen-

oxygen distances of water. The relative position of the -OH groups at C-2 and C-4

appears to be a key factor. This theory is fully consistent with quantitative studies of

kinetic medium effects23,162

and relevant thermodynamic measurements163

.

Galema et al.8 have reported that molecular dynamics (MD) study of saccharides

in aqueous solutions provides important insights into their hydration characteristics.

There has been, and still is, a lively discussion as to the way in which molecular

dynamics simulations should be performed for saccharides in aqueous solution, because

most saccharides have a hemiacetal functionality that is subject to mutarotation.

Figs. 1.1 & 1.2 – Ref. 60, Fig. 1.3 – Ref 61, Fig. 1.4 – 58, Fig. 1.5 – Ref. 42,

Fig. 1.6 – Ref. 61.

23

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