Structure of Water and Its Dynamic Hydrogen Bond Network

1. Introduction

Water is essential to life as it is main constituent of cell in living organism. All biological macromolecules are almost inactive in its structural stability and functioning in absence of water.

1. Water-Role in Life

Water is key compound for our existence on this planet due to its ubiquitous presence on the earth and in living organisms. It is involved in all chemical, biological and geological processes. Due to its anomalous behavior it named as ‘matrix of life’, ‘solvent of life’. It plays a vital role from molecule and cell to tissues and organisms.1-4 In past several decades water has attracted the most scientific attention among the liquid due to its anomalous properties. It shows peculiar properties such as negative volume of melting, density maximum at 277 K, high melting and boiling point, high dielectric constant, minimum in the isobaric heat capacity and isothermal compressibility at 308 K and 319 K, respectively, high mobility transport for H⁺ and OH^– ions. The density of most liquids increases as it freeze but in case of water it expands about 11% due to which ice floats on water. It is the solvent of life and plays an important role in protein interactions and stabilization of protein structure. The work of Kauzmann gives the importance of water in protein folding and its interactions with water.5

1. Structure of Liquid Water

The anomalous behavior of water is due to its unique ability to form a network of self associated molecules through hydrogen bonding. To study the structure of water and its dynamic hydrogen bond network large number of studies has been carried out.1-12 Still many aspects of water are not fully understand at molecular level. Dyke and co-workers first reported existence of H-bonding in vapor phase experimentally and measured the H-bond length as 2.98 Å in water dimer using molecular beam resonance technique which is higher than water in solid (for ice H-bond strength 2.74 Å) and liquid (2.85 Å) indicates the H-bond strength is weaker in Gas phase.13

From X-ray diffraction study of Bernal and Fowler and Morgan and Warren it is revealed that water is tetrahedrally coordinated through hydrogen bonds similar to the structure of ice I (Figure 1.1). 14-15The number of theories for the water structure has been proposed based on different techniques such as X-ray, neutron diffraction, dielectric relaxation and Raman spectroscopy. 16-22. These theories are generally classified into two models as a) Continuum model and b) Mixture model.

Figure 1.1 Crystal structure of ice I at low pressure

1. Continuum model

In continuum model it is assumed that almost completely hydrogen bonded water molecules in a continuous network. Pople described the continuum model which is agreement with the observed variation of X-ray radial distribution function with temperature. 23According to Pople In continuous bonded network of the water bond bending and deformations occurs instead of bond water. Recently, Rice and Sceats 24proposed Random Network Model (RNM), which explains the continuum model and it is further developed by Henn and Kauzmann. 25This model is used for determining the heat capacity contribution due to water-water interactions.

b) Mixture model

In mixture model water consists of differently H-bonded species with zero, one or both hydrogens are engaged in hydrogen bonding. Franks and Wen 26 gives the “Flickering Cluster” model in which cooperative H-bonding is observed in water molecules. The co-operativity involves the hydrogen bond formation of one bonding site of water molecule contributes the delocalization energy to the molecule, which is involved in hydrogen bonding with another water molecule. According to Franks and Wen the clusters of the water molecules (bulk water) and free monomer molecules (dense water) are in equilibrium with each other. Samoilov 27 proposed the interstitial model in which water molecules are present in the cavities of ice lattice. Nemethy and Scheraga 28 used statistical thermodynamic model to calculate the Helmholtz free energy, internal energy and entropy as a function of temperature. Also the water hydrate model proposed by Pauling. 29 Now a days, theoretical techniques such as Monto Carlo, molecular dynamic simulation are used to study structural behavior of water. It provides most promising approach for the study of water at molecular level. Jorgensen 30 has developed transferable intermolecular potential functions (TIPS) suitable for use in liquid simulations for water. This potential has been used by Jorgensen and Madura 31 in MC simulation on liquid water to study the effect of temperature on vaporization, hydrogen bonding, density, isothermal compressibility and radial distribution functions. Number of water models such as SPC, TIP3P, TIP4P, TIP5P are developed for the molecular simulation of large biomolecular systems.

Figure 1.2 Frank-Wen Flickering Cluster Model of Liquid Water

1.2 Hydrophobic Hydration and Hydrophobic Interactions

The weak non-covalent interactions like van der Waals forces, H-bonding, ion-dipole, hydrophobic interactions are responsible for change in the structure of water around the solute molecule. The hydrophobic interaction is the prominent factor in the solvation of apolar or non-polar molecule. When a non-polar solute is dissolve in water there is large negative change in entropy. The disruption in the normal H-bonded structure is occurred and new H-bonded cage-like structure is formed around the solute molecule. So the structure formed is more ordered than the ordinary water. The term hydrophobic hydration is used when non-polar solute solvated by the cage of the solvent molecule around it. The short lived aggregates are formed around the solute molecule. The formation of polymeric aggregates strengthens the hydrogen bonding which gives negative contribution to ΔH⁰. 10

Get Help With Your Essay

If you need assistance with writing your essay, our professional essay writing service is here to help!
Find out more about our Essay Writing Service

The hydrophobic interactions are important in a field of biochemistry for the purpose of conformational stability of biological macromolecules, protein folding, aggregation, ion transport, drug delivery as well as in industry. Usually hydrophobic hydration occurs in non-polar compounds such as alcohols, ethers, and amines. The tetraalkylammonium (TAA) salts with larger cation also shows the hydrophobic hydration effect. Kustov gives the effect of size of cation on the hydrophobic hydration. He studied the specific heat of solution for the higher size cation TAA salts and observed that as the size of cation in salt increases the specific heat of solution and hydration increases upto the tetrapentylammonium salts and then decreases. As the specific heat of solution increases the hydrophobic hydration increases. For the hexyl and heptyltetraalkylammonium salts the ΔC⁰_p decreases so the hydrophobic hydration weakens. Thus hydrophobic hydration depends on the size of cation of TAA. The hydrophobic interaction is best explained by Goring et al. by studying the interaction of non-electrolytes in aqueous solutions by dilatometrically. They compared the apparent specific volume (Ï•₂) relative to apparent specific volume at 0 ºC as function of temperature for non-electrolytes and showed that 1-butanol behaves like hydrophobic compound and acts as structure maker in aqueous solution while glycerol with polar groups disrupts the structure of water. The hydrophobic compound shows the slope dÏ•₂/dT is less than the corresponding thermal expansions of pure compound while it greater for the hydrophilic compounds. Madan and Sharp explained that non-polar solutes have large capacity heat of hydration ΔC_p while for polar solutes it is small negative. The large change in heat capacity at high temperature is due to unfavorable enthalpic interacions and not due to entropy change. The effect of salt on the hydrophobic hydration was carried out by Talukdar and Kundu and observed that hydrophobic cation induce more hydrophobic hydration in aqueous NaNO₃ solution than in pure water. Rossky et al. with the help of computer simulation studied the hydration properties of the interfaces between the water and the hydrophobic surfaces for the active peptide melittin in its monomeric and dimeric form and concluded that hydrophobic hydration is depends on the surface topography of biomolecule.

1.3 Spectroscopic study of water

Due to its various anomalous properties and great importance in the several field water is the most studied compound. To study the structure of water, number of spectroscopic techniques such as IR, Raman, neutron diffraction, X-ray scattering, NMR spectroscopy etc. have been used still today. The spectroscopy and scattering studies provides the structural information of water at molecular level. Bernal and Fowler analyzed the X-ray diffraction of water and investigated water as distorted quartz-like. The hydrogen bond network in water is found to be tetrahedral in nature and each water molecule can be bound with another four water molecules i.e. each water molecule is double proton donor and double proton acceptor. While recently, Wernet et al. studied the structure of water by soft X-ray absorption spectroscopy and X-ray Raman scattering and investigate that hydrogen bond network in the water consists of only two strong hydrogen bonds and one act as proton door and another as proton acceptor. This controversial result of structure of water from earlier study makes the scientist to study the water structure more interesting. In this context, number of scientists have been studied the water structure by X-ray absorption spectroscopy.

Infrared and Raman techniques are also the important sources of the information of hydrogen bonding in water. Above the absolute temperature all the atoms in the molecules are in continuously vibrating motion with respect to each other. Any molecule absorbs the radiation when frequency of a specific vibration is equal to frequency of the IR radiation directed on the molecule. Each atom has three degrees of freedom, corresponding to motions of the three Cartesian coordinate axes (x, y, z). Total no of coordinate values is 3N for a molecule containing N atoms. Thus, Water has 9 degrees of freedom with C_2v symmetry. It shows the two stretching vibrations (symmetric and asymmetric), one bending vibration, three hindered rotations (librations), and three hindered translations. Earlier, number of research papers has been published on the study of the structure of water in solid, liquid as well as in vapor phase by IR and Raman technique. The fundamental IR frequencies for the H₂O and heavy water is as shown in Table

Table: Fundamental vibrations of liquid ordinary water and heavy water

http://www1.lsbu.ac.uk/water/water_vibrational_spectrum.html

Walrafen investigated the structure of water by Raman spectroscopy in the intermolecular as well as intramolecular vibrational region. From Raman scattering it is observed that for liquid H₂O and D₂O a broad weak hydrogen bending band at 60 cm^-1 and it is observed to be decreases as temperature rise, the band near 170 cm^-1 is produced by the stretching motion of O-H band in water molecule. This is also decreases as increases in temperature which indicates the structural breakdown of water units. These vibrations are the intermolecular vibrations of water which are observed in the restricted translational region. The intramolecular vibrations of water occurs in the range of 2000-4000 cm^-1.Walrafen studied the Raman spectra of 50 mole % solution of H₂O and D₂O in the intramolecular region in which principle contribution of HDO vibrations are studied. The two maxima at 3415±5 cm^-1 and 2495±5 cm^-1 are referred due to OH and OD stretching vibrations of HDO, and of H₂O and D₂O. Also the weak band at 2860± 10 cm^-1 arises from the overtone of the fundamental intramolecular bending vibration of HDO near 1450 cm^-1. When H₂O, D₂O mixture studied at 32.2 to 93 ºC, the isosbestic point observed at 2570±5 cm-1 indicates the equilibrium exists between hydrogen bonded and nonhydrogen bonded OD stretching vibrations. Senior and Verrall observed same results when studied the HDO stretching at temperature 29 to 87 ºC by infrared spectroscopy. Bakker et al. studied the lifetime of the OH-stretching vibration in the water as a function of temperature by using femtosecond mid-infrared pump-probe spectroscopy and observed that it increases from 260±18 (at 298 K) to 320±18 (at 358 K)

Recently, molecular dynamic simulation is becomes the fast method for the structural detection at molecular level. Xantheas et al. used the ab initio method to obtain the vibrational frequencies as well as zero point energy for the water clusters and its isomers with the help of second-order Møller–Plesset perturbation level of theory (MP2) with the augmented correlation consistent basis set of double zeta quality (aug-cc-pVDZ).

1.3.1 Near-Infrared spectral study of water and aqueous solutions

Near-Infrared consists of the region 800-2500 nm (12500-4000 cm^‑1) in the electromagnetic spectrum. In this region molecule have energy sufficient to excite first (2ν), second (3ν), and higher overtones (nν) vibrations. The overtones observed in the molecule when the intermolecular vibrations of the molecules do not obey the Hook’s law. The band is more intense when the greater the anharmonicity. The combination bands are also observed in the same region.

Near infrared spectroscopy is the basic tool to study the hydrogen bonding in molecule. Earlier, the scientist Luck studied water and alcohol in the NIR region and observed that the strength of cooperativity of H-bond in water is about 250 % stronger than H-bond in a monomeric water. Different species of water present in the cooperative H-bond such as H-non bonded, H-bond strong and H-bond weaker. Ozaki et al. studied the structure of water by using two analytical techniques such as two dimensional correlation spectroscopy and principal component analysis in which they showed the two-state water model by measurements of the water at different temperatures from 6 to 80 ºC. Two bands are observed at 1412 and 1491 nm due to two different species of water i.e. weak H-bond and strong H-bond respectively. The species observed at 1438 nm which has no much effect of temperature which suggested may be due to distorted two-state model of water. The water at high temperature and pressure remarkably exhibits different properties than at ambient temperature. It becomes good solvent for hydrophobic substance such as benzene and hydrocarbons which are non-polar gets completely miscible at certain temperature and pressure. The effect of high temperature as well as pressure has been given by Ikawa et al. in the range of 5500 to 7800 cm^-1. They observed the band at 7000 cm-1 gradually shifts to higher wavenumber is due to free OH vibrations and at 673 K and 400 bar pressure the absorption band retain the rotational features i.e. water molecule quite rotate freely though there is collision with other molecules.

Recently, Near-Infrared spectroscopy has been used extensively for chemical analysis and characterization. The applications of NIR spectroscopy in various fields have attracted the scientific community. It is also used in the determination of moisture content in food samples. It can be used to probe the hydration effects in aqueous solutions of salt. Wu et al. have studied the effect of ethanol on the structural organization of aqueous solutions of [Bmim][BF₄] and [Amim][Cl] using one-dimensional and 2D correlation NIR spectroscopy. They showed that hydrogen bonding between water and ILs gets reduced in presence of high concentration of ethanol³² and can be used to remove water as an impurity in hygroscopic ILs. They also used this technique to study aggregation behavior of ILs in water. NIR spectroscopy has been used previously for the study of hydration by McCabe and Fisher in which they have studied the hydration of perchlorate and alkali halides in aqueous solutions by using excluded volume. Koga et al. have given the excess molar absorptivity in the range of 4600-5500 cm^-1 i.e. (ν₂+ν₃) combination band of water for the Na halides and concluded that the Br^‑ and I^– form the hydrogen bond directly with the water network which is different than the Cl^– ion. Bonner and Woolsey have obtained the hydration number for some alkali halides by using the 958 nm (2ν₁+ν₃) combination band of water. By applying their method, Hollenberg et al. calculated the hydration number for amino acids and carbohydrates

The new concept introduced by Noda in 1993 i.e. two dimensional correlation spectroscopy has attracted many scientist to study effect of solutes on the structure of water by IR as well as NIR spectroscopy. This technique becomes powerful tool for the elucidation of spectral changes induced by temperature, time and concentration. Noda et al. studied the structural and crystallization dynamics of poly(L-lactide) during isothermal cold crystallization by two dimensional correlation spectroscopy.

• An interpretation of the evolution with temperature of the ν2+ν3 combination band in water V. Fornés and J. Chaussidon, J. Chem. Phys. 68, 4667-4671 (1978)
• Near-infrared spectroscopic study of water at high temperatures and pressures Yusuke Jin and Shun-ichi Ikawa J. Chem. Phys., 119(23), 12432-12438, 2003.
• The importance of cooperativity for the properties of liquid water W.A.P. Luck Journal of Molecular Structure, 448 (1998) 131 – 142.

• Studies on the Structure of Water Using Two-Dimensional Near-Infrared Correlation Spectroscopy and Principal Component Analysis V. H. Segtnan, S. Sasic, T. Isaksson, Y. Ozaki Anal. Chem. 2001, 73, 3153-3161