1

INTRODUCTION

1.1 General Introduction

Saccharides and their derivatives are essential components of living systems. These are

important due to their hydroxyl (−OH) rich periphery, coordinating ability, homochirality and

stereospecificity. Saccharides interact with the aqueous environment through numerous

hydroxyl groups and build hydrogen bonds1-4

. Depending upon their molecular weight,

saccharides can be divided into three subgroups, such as mono-, oligo- and high molecular

weight poly-(cellulose, chitin, chitosan, starch, amylose, amylopectin, agarose, and xylan)

saccharides. As integral parts of some coenzymes, nucleic acids (DNA and RNA),

glycoproteins and glycolipids, they perform multiple roles from the storage and transport of

energy to participation in the immune system, fertilization, development, pathogenesis,

signaling, cell-cell recognition, and molecular and cellular communication. Saccharides

located at cell surface are receptors with regard to the bioactive structures of hormones,

enzymes, viruses, antibodies, etc5-12

.

To get a better understanding of biological phenomena, low molecular weight model

compounds such as alcohols, saccharides, peptides, nucleic acid bases, nucleosides and

nucleotides have been studied because of the complexities of biomolecules13

. The hydration

characteristics of saccharides in aqueous solutions are of direct relevance for understanding

the role of glycoproteins and glycolipids in molecular recognition. Therefore, an increasing

interest in biophysical and biochemical research is presently being directed toward the novel

subdiscipline “glycobiology”14-17

.

Saccharides and polyhydroxy compounds are well known protein / enzyme’s

stabilizing agents. In an aqueous environment, co-solutes such as disaccharides cause an

increase in the thermal denaturation temperature of proteins18

. Lee & Timasheff19

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 speculation20-23

.

There has been a growing interest for disaccharides especially concerning their

hydration, glass transition and biopreservation properties and much less work was devoted to

their crystallization in aqueous medium24-26

. Trehalose is well-known for its ability to

2

preserve life in cells, organisms, and biomolecules under conditions of extreme drought or

low temperatures20

. There are evidences that high concentrations of trehalose in the tissues of

certain insects, yeasts, lactobacillus, mushrooms, and desert plants allow them to survive for

decades or centuries, in a state of suspended animation under conditions of water deficiency,

resuming rapidly their active metabolism when rehydrated. Disaccharides such as maltose

and sucrose have shown similar properties but trehalose is found to be more effective.

Several mechanisms for the antidesiccant properties of saccharides have been proposed, but

their protective mechanisms are not well understood27-29

.

Saccharides are often used in pharmaceutical, food, and biomedical applications to

prepare a glassy matrix for long term storage of biological materials. Rheological properties

of saccharide solutions are applied for the control of gelling processes and osmoregulation of

tissues and organs in cryoprotection30-33

. Maltose consisting of two glucose units linked by a

glycosidic bond is widely used in the food industry. The micro- and macro-scopic studies of

sweetner-H2O interactions are useful to understand sweet taste chemoreception. Mechanism

of taste chemoreception is thought to depend upon the role of water, molar volume and

stereochemistry of the molecule, which affect their fit within the water structure34

. Various

solution properties (apparent specific volume and apparent molar isentropic compressibility)

of different sapid substances have been studied35-37

, but the mechanism of taste

chemoreception is still not fully understood.

The hydration characteristics of saccharides and their interactions with electrolytes

and non-electrolytes in aqueous media are of significant thermodynamic and biochemical

importance6,38-41

. Most biological systems contain ionic solutes (Na+, K

+, Ca

2+) which play a

vital role in various metabolic activities. It has been reported that synthesis, metabolism,

decomposition and trans-membrane transport of saccharides is related to the concentration of

Na+, H

+ and other metal ions in body fluid

42. The Mg-sugar complex has a stabilizing effect

on the DNA double-helix, sugar-metal complexes participate in the ATP cell energy cycle.

Due to the fact that saccharides readily undergo isomerization and conformational changes,

their complexation with metal ions result in the formation of a mixture of products, although

interactions involved are weak, but selective43-44

.

The presence of salts modifies important properties of aqueous saccharide solutions

related to their protective roles such as viscosity, water sorption, crystallization rate and glass

3

transition temperature, Tg. The addition of disodium tetraborate (borax) to aqueous solutions

of saccharides and other polyols results in various complexes between the borate ion and the

saccharides45-47

. Aqueous solutions of sodium acetate are widely used in molecular biology

applications, purification and precipitation of nucleic acids, protein crystallization, staining of

gels in protein gel electrophoresis, and HPLC48-49

. Sodium gluconate occurs naturally in

plants and foodstuffs, is non-toxic, the gluconic acids salts are utilized in mineral

supplementation where the acid acts as a carrier of metal ions of therapeutic importance50

.

Physicochemical studies of saccharides in mixed aqueous solutions are of great

importance to understand the solvation behaviour of saccharides as a function of

concentration and temperature and how the solute-solvent interactions get modified in the

presence of co-solutes (additives)33,40

. Therefore, thermodynamic and transport properties of

solutions characterizing the solvation behavior of saccharides are needed which will be useful

for better understanding the various phenomena like protein stabilities, taste chemoreception,

antidesiccation properties, etc.

Saccharides have a very high affinity for water and their hydrogen bonding

interactions with aqueous solvents occurs through their hydroxyl groups. The hydrogen

bonding between saccharide and water is more extensive than that between water molecules

themselves and these interactions (hydrogen bonding) are stereo specific in nature.

Saccharides influence the surrounding water molecules and that in return, affects the structure

of the dissolved saccharide molecules51-52

. The nature of these interactions is responsible for

most biological functions of saccharides such as energy transport and storage, sweet taste

chemoreception, signal transmission, biomolecular recognition, protein stabilization, gel

phase formation, etc20,34-35,53

. So, for understating the role of water in characterizing the

hydration behaviour of saccharide in mixed aqueous solutions, a brief description about the

structure of water is needed, which is described in the next section.

1.2 Structure of Water

Water, due to its ubiquitous presence on the Earth and in living organisms, is the key

compound for our existence on this planet and it is involved in nearly all chemical, biological

and geological processes54-55

. In most cases, water contains dissolved salts and it is in such

aqueous solutions that the chemistry of life takes place. Despite the apparent simplicity of the

4

water molecule, the condensed phases of water, such as liquid water and ice, are the most

mysterious substances in our world56-58

.

Being the solvent of life, water has attracted the most scientific attention among

liquids, in the past several decades. In addition, water also plays an important role as a

mediator in interactions in proteins, which stabilize their structure59-60

. The most important

property of liquid water is its unique ability to form a network of self-associated molecules

through hydrogen bonding and that causes the anomalousness in physical properties of liquid

water61-62

. In the past years, a large number of studies have been carried out in order to

understand the structure of water and dynamics of the hydrogen bonding network in water

that give rise to different unique properties63-65

.

Bernal and Fowler66

and later Morgan and Warren67

, carried out the X-ray diffraction

measurements on liquid water and revealed that the water is tetrahedrally coordinated through

hydrogen bonds, similar to that in ice I (Fig. 1.1). Tammann68

suggested that there should be

as many different 'kinds' of water as there are different phases of ice. The complexity of

liquid water has given a great variety of proposed water models which are classified as; (a).

Continuum model, (b). Mixture models.

1.2.1 Continuum Model

It assumes almost completely hydrogen bonded water molecules in a continuous

network, where the distortion of hydrogen bonds results in a continuous distribution of

hydrogen bond distances, energies and angles. First structural theory work was done by Bernal

and Fowler66

, and later Pople69

established the modern continuum theory of water and this

theory accounts for the heat capacity and thermal energy of water. The thermodynamic

properties of liquid water will, of course, depend on the energetics of the bending and

stretching of the hydrogen bonds; it will also depend on the intra- and inter-molecular vibration

frequencies, which will be considerably altered by the deformations of the hydrogen bonds.

Recently, Random Network Model (RNM), which is a highly successful continuum

model, has been proposed by Rice and Sceats70

and further developed by Henn and

Kauzmann71

. It does not involve changes in the number of hydrogen bonds. RNM model is a

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

and also this model 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,

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

5

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.

Fig. 1.1 Schematic representation of crystal structure of ice I at low pressure

1.2.2 Mixture Models

These models72-78

visualize liquid water as a collection of different hydrogen bonded

species in which each water molecule can fluctuate through states where one or both of its H

atoms are engaged in hydrogen bonding. Depending on the type of species involved, the

mixture models have been classified as; (i) Interstitial Model proposed by Samoilov79

and

Forslind80

(ii) Water Hydrate Model81

proposed by Pauling (iii) Flickering Cluster Model

proposed by Frank and Wen72

, and Frank82

. The most popular and commonly used model,

especially in the field of solution chemistry is the ‘Flickering Cluster’ Model.

Flickering Cluster Model

According to Frank and Wen72

, and Frank82

water is 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 as 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-life 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

data.

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

Nemethy and Scheraga73

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 al.83

and Lentz et al.84

have modified this

model to remove some major inconsistencies prevalent in it as pointed out by Nemethy85

.

Walrafen74

and Hartman86

provide the strongest support for the mixture model from the

Raman Spectroscopic studies of 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.

Recently Robinson and Cho87

have also reported that on average, 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

7

successful in predicting selected liquid water and ice polymorph properties, there is still a need

to construct a model of general applicability88-89

.

Fig. 1.3 (a) Argon-lon-Laser-Raman spectrum from a 6.2 M solution of D2O in H2O at 25ºC.

(b) Quantitative mercury-excited Raman spectra from a 6.2 M solution of D2O

in H2O at a series of temperature.

1.2.3 Computer Simulations

Different experimental techniques such as neutron scattering, X-ray scattering, infrared

absorption, and NMR have been used to probe the microscopic structure of water90-94

. The

information gathered from these experiments and from theoretical calculations95-96

has led to

the development of around twenty "models" that attempt to explain the structure and behavior

of water. With the advent of the supercomputers, computer simulation methods based on

statistical thermodynamic and molecular dynamic theories of mixtures, to understand the

structure of liquid water have become accessible97-98

.

Most commonly used computer simulation methods are: (1) Monte Carlo (MC)99

, (2)

Stochastic Dynamics100

(3) Molecular Dynamics (MD)101

(4) Normal mode analysis102

(5)

Energy minimization103

. Among these, Monte Carlo and Molecular Dynamics provide the

most promising approaches for structural characterization of liquid water101

. The former

8

method offers the possibility of generating canonical ensembles of given temperature, but it is

entirely restricted to a study of static structural properties. MD, which is nominally

microcanonical, can probe both static and kinetic behavior, so in this 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 the liquid water91

.

Results of both MC and MD 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 pair wise 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 none of these pair potentials can account for both gas

and condensed phase properties of water103

. It is evident from quantum mechanical

calculations on water trimers, tetramers and pentamers that hydrogen bonding in these

clusters is stronger than in dimer, which suggests the cooperative nature of hydrogen bonding

interactions104

.

Alder and Wainwright105

were the first researchers to perform the MD simulation,

followed by Barker and Watts106

and later by Rahman and Stillincer107

and they published the

first computer simulations of liquid water. Rahman and Stillincer107

using MD technique

examined both static structural properties and the kinetic behavior at nominal temperature,

34.3 ºC. They concluded that the model employed in the research work can be adapted to

simulation of a simple solute.

Sokol et al.57

carried out the MD simulation of water in solid, liquid and gaseous

state. They performed simulations for two density cases, normal ( = 1g.cm

−3) and lower

density ( = 0.251 g.cm

−3), for four different temperatures. The calculated structural and

dynamical properties were also compared to earlier experimental and simulated results.

Krishnan et al.108

carried out the computer simulation based on sophisticated and

empirical model of interaction developed by Rick et al. 109

. Krishnan and coworkers108

have

obtained the minimum energy structures of water clusters up to a size of ten and reported that

the cluster structures agree well with experimental data. They were able to identify the

hydrogens that form hydrogen bonds based on their charges alone, a feature that was not

9

possible in simulations using fixed charge models. They also used fluctuating charge model

in order to study the structure of liquid water at ambient conditions.

Beveridge et al. 110

used metropolis MC method to study the hydrophobic interaction

of alkyl and phenyl groups in water and solvent effects on the conformational stability of the

alanine dipeptide and the dimethyl phosphate anion in water.

Jorgensen95

has developed transferable intermolecular potential functions (TIPS)

suitable for use in liquid simulations for water. This potential has been used by Jorgensen and

Madura111

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 Jorgensen112

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

the temperature of maximum density. The obtained results further support a two state mixture

models for the liquids.

Wang et al.113

have performed DFT (density functional theory)-based AIMD (ab

initio molecular dynamics) simulations of liquid water with different GGA (generalized

gradient approximation) and vdW (van der Waals density) functionals and different densities,

comparing the structural and diffusive properties. It was concluded that a better treatment of

the exchange interaction together with vdW correlations might provide a better agreement

with experimental results in terms of structure, density, and diffusivity of liquid water.

Recently, Kalinichev and Bass114

performed MC computer simulations under

thermodynamic conditions corresponding to available X-ray and neutron diffraction

measurements of the supercritical water structure and found a good agreement between their

results with other recent experimental and simulated data under similar thermodynamic

conditions.

From theoretical and simulation methods115-116

, various other workers also have

reported that linear molar volume of relevant inherent structures contributing to the liquid

phase increases with the decrease in temperature. Owicki and Scheraga117

observed a broad,

smooth distribution of hydrogen-bond energy for liquid water, rather than relatively discrete

sets of the bonded and unbonded interaction energies.

Grunwald118

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

10

derivatives, the dielectric constant, the probable number of hydrogen bonded nearest

neighbors, intermolecular vibration frequencies and dielectric relaxation dynamics.

Dore119

have emphasized that the simulation methods will need to be used in

conjunction with experimental data for more understanding of water and its anomalies.

Inspite of enormous experimental and theoretical studies to explore water structure, a

compact and useful description is yet to be achieved.

1.3 Structure of Aqueous Solutions

According to ‘Flickering Cluster’ model72

, water molecules can be considered to be in

dynamic equilibrium between the bulky tetrahedrally hydrogen bonded clusters 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 (10−11

s) of the clusters, 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’70

.

The concept of ‘structure-making’ and ‘structure-breaking’ by solutes has been

proved to be useful in understanding the effects of solutes on the water structure63,120

. 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.

1.3.1 Ionic and Polar Group Hydration

Ionic hydration is one of the fundamental physicochemical processes related with

many research fields in chemistry including stability of biomolecules and chemical reactions.

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 water, it interferes with the

11

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 Wen72

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 and this phenomenon is called as positive hydration. The water

in the second and third solvation layers is less 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

12

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

Wen72

, 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 Krishnan121

have proposed a model for aqueous ionic solutions

primarily based on the ideas of Frank and Evans122

. Gurney123

and Samoilov79

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

affected 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 Krishnan121

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

properties of solutions, while Frank and Wen model72

, and more recently Stillincer and Ben-

Naim model124

of ionic solutions being rather qualitative, are supported by the experimental

studies125

of kinetic properties, X-ray and neutron scattering. Friedman and Krishnan121

have

compiled the various thermodynamic properties and have given reasonable explanation of

these properties based on their cosphere model.

Polyfunctional solutes e.g. saccharides exhibit very complex behaviour in aqueous

solutions which is difficult3,126-127

to rationalize as both hydrophilic and hydrophobic types of

hydration are contributing.

1.3.2 Hydrophobic Hydration

Non polar solutes (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 water128-129

. 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

13

solution. 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’128

. This behaviour of non polar

solutes is apparently uniquely characteristic of aqueous solutions. The negative entropy 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 called a ‘structure increase’ or enhancement of structure of water130

(Fig.1.5). This region is also known as iceberg formation or hydration of second kind or

hydrophobic hydration although the structure is not necessarily identical with that of ice.

These models assume the formation of short-lived aggregates of water molecules around the

solutes, having more or less well defined structure, called as ‘icebergs’122

, ‘flickering

clusters’70,72

, ‘clathration shells’, ‘polyhedral cages’ similar to those found in the crystalline

hydrates formed by some small non polar molecules. These models are consistent with the

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

aggregates involves strengthened hydrogen bonding128

, which would give negative

contribution to tH. The solutes imposed structure melts away on heating the solutions

which involves breaking of some of the excess hydrogen bonds resulting in an increase in the

heat capacity131

.

Ben-Naim132

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. Frank82

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 Gill131

, 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 zero133

, which is around 20 °C

for a number of liquids, TS is the other universal temperature around 140 °C where entropy of

transfer of non polar compounds into water is zero, and the process of transfer is most

14

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 itself cannot be regarded as the cause of their hydrophobicity.

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)

Muller128

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 25

°C. This modified ‘hydration shell hydrogen bond model’ (HSHB) 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 ∆CºP,2.

15

According to Lumry et al.77,134

and Ben-Naim132

there is an exact compensation for

the entropy and enthalpy changes 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 Gill131

and Murphy et al.135

have pointed out

that large heat capacity change plays the key role in regulating the hydration free energy

change and the principal feature of hydration process.

Borisover et al.136

suggested a new method of estimating the hydrophobic effect

contribution to the thermodynamic functions 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 Panov137

but contrary to Abraham’s opinion138

according to

which enthalpy is positive. Borisover et al.136

from these studies also indicated that the rare

gases exhibit hydrophobic hydration.

Costas et al.139

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 ∆CºP,2. 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.

Madan and Sharp129

have reported that non polar solutes have a large positive heat

capacity of hydration, CP, while polar groups have a small negative CP of hydration. The

increase in heat capacity of non polar solutes can be as large as 12 J K−1

mol−1

per first shell

water at 25 °C, a surprisingly large change when compared to the total heat capacity of water,

75 J K−1

mol−1

. As a consequence of the large heat capacity increase, the low solubility of

apolar groups in water at higher temperatures (>90 °C) is caused by unfavourable enthalpic

interactions and not because of unfavourable entropy changes. The hydration of polar groups

is accompanied by a small 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 Kundu140

have studied the salt effect on the hydrophobic hydration and

concluded that hydrophobic cations induce more hydrophobic hydration in aqueous NaNO3

solution than in pure water.

1.3.3 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

16

bonding’73,141

or ‘hydrophobic interaction’142-143

. 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 structures141,144

. 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 binding145

.

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 a semantic level146

.

Ide et al.147

have reported that non polar substitutents bring about structuring of water

surrounding these groups. NMR studies carried out by Bagno et al.148

also showed that water-

water bonding in the surrounding of hydrophobic side substituents are stronger than these

within bulk water. Hechte et al.149

has also reported the increase in water clusters around

alkyl side chain with increase in alkyl side chain length. 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 cannot be prevented. These aggregates are formed with a high cooperativity and

London dispersion energy contributes significantly to their stabilization. Pairwise

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 al.150

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. ΔH°

> 0, ΔS°

> 0,

ΔCp°

< 0, ΔG°

< 0 have opposite signs as compared to those of hydrophobic hydration56,73

.

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 water56,143

. The hydrophobic interactions

are found to be unusual as compared to normal processes because these are temperature

17

dependent. At 25 °C, the transfer of non polar solutes to water is not principally opposed by

the enthalpy, rather it is due to excess entropy131,133,144

. It arises from the water of solvation

around the solute rather than from solute-solute interactions. Water surrounding the non polar

solute prefers to hydrogen bond with other water molecules rather than to ‘waste’ hydrogen

bonds by pointing them towards the non polar species151-152

. 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 positive56,131,133

, which implies that the enthalpic and entropic contributions are highly

temperature 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 minimum. The free energy of transfer of

non polar solute into water is extrapolated to be most positive in the temperature range 130-

160 °C

133-135,153. Hydrophobic interactions are found to be at their maximum value at 140

°C

and are determined entirely by enthalpic contributions. Hence water ordering is not the

principal feature of the aversion of non polar residues of water.

1.4 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). Solvent molecules adjacent

to non polar solutes interact with their external atomic groups thereby becoming structurally,

dynamically and thermodynamically distinct from bulk water. 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.

The hydration characteristics of a saccharide molecule depend on its stereochemistry.

It has been proposed that the anomeric effect and the ratio of axial versus equatorial hydroxyl

groups of a saccharide govern its hydration154

. “Specific hydration model” emphasized that in

addition to the number of potential hydration sites, the relative position of the −OH groups at

C-2 and C-4 appears to be a key factor. This is fully consistent with quantitative studies of

kinetic medium effects3,16

and relevant thermodynamic measurements155

. Since the hydration

interaction competes with the intermolecular hydrogen bonding in water, solute molecules

18

which are able to interact with the solvent without major perturbations in the solvent structure

are likely to interact preferentially. Galema et al.156

have reported that molecular dynamics

(MD) studies in aqueous solutions provide important insights into the hydration

characteristics of saccharides.

Warner157

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

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

oxygen spacing (4.75 °A) in a hypothetical ice I lattice at 298 K. 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 could be cooperatively hydrogen bonded into water

structure without undue modification of the existing hydrogen bonded system (Fig. 1.6).

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.

Figs. 1.1 & 1.2 -Ref. 73, Fig. 1.3 –Ref. 74, Fig. 1.4 - 72, Fig. 1.5 - Ref. 56, Fig. 1.6 - Ref. 74.

Model - ice

Biotin

1,4-Quinone

-D-Glucose

Triglyceride

19

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