Water, a Wonder Molecule

 R. Sanjeevi and B. Viswanathan

Water comprises about 70 per cent of the weight of living organisms and nearly 71 per cent earth's surface is water. Its uniqueness is reflected even at the molecular level. According to the Nobel Laureate Linus Pauling, the volume per water molecule in ice is about 32.2 A³ with a density of 0.92. On freezing the general tendency of matter is to contract. On the contrary water anomalously expands on freezing especially below 4 2^oC, and the ice thus formed floats in water. This phenomenon has important ecological implications in that if ice were to be denser than water, then the lakes, exposed to low temperatures would start freezing from the bottom up. This type of ice formation would then have prevented ice water fishes and other living organisms inclusive of vegetation from surviving due to extreme cold conditions. Further, the floating ice layer also has very good insulating effect and prevents the lake water from getting colder beyond a limit, thereby supporting life in the reservoirs. 

Further, as dielectric constant of water is very high, water is one of the most polar of all solvents. Consequently electrically charged molecules are easily separated in the presence of water. The heat capacity of water is also very high or in other words, a large amount of heat is needed to raise its temperature by a degree. This property gives a tremendous advantage to biological systems wherein the cells undergo moderate biological activity. Despite the fact that large amount of heat is generated by these metabolic activities the temperature of the cell-water system does not rise beyond reasonable limits.

The high heat capacity of water makes it possible for ocean currents to carry heat in a very efficient manner. The half-a-kilometer deep and 200-km-wide Gulf Stream has been estimated to carry energy equivalent to burning 160 billion kilograms of coal an hour. The heat carried by the ocean current thus prevents loss of aquatic life due to unfavorable temperature fluctuations. 

Water has a high heat of vaporization resulting in perspiration being an effective method of cooling the body. The high heat of vaporization also prevents water sources in the tropics from getting evaporated quickly. Surface tension is the most important physical parameter governing the clotting mechanism of blood. Clotting is efficient because of the high value of surface tension of water. The high conductivity of water makes nerve conduction an effective and sensitive mechanism of the body. It would appear that nature has designed the properties of water to exactly suit the needs of the living.

Water has higher melting point, boiling point, heat of vaporization, heat of fusion and surface tension than comparable hydrides such as hydrogen sulphide or ammonia or, for that matter, most liquids. All these properties indicate that in liquid water, the forces of attraction between the molecules is high or, in other words, internal cohesion is relatively high.

These properties are due to a unique kind of a bond known as the hydrogen bond. This bond is a weak electrostatic force of attraction between the proton of a hydrogen atom and the electron cloud of a neighboring electro-negative atom. In other words, hydrogen atom with its electron locked in a chemical bond with an electro negative atom has an exposed positively charged proton, which in turn electrostatically interacts with the electron cloud of the neighbor.

In 1920, American chemists proposed for the first time the concept of hydrogen bonds in their discussion of liquids like water and hydrogen fluoride, having dielectric constant values much higher than the anticipated values. Hydrogen bonding between water molecules occurs not only in liquid water but also in ice and in water vapor. It has been estimated from the heat of fusion of ice that only a small fraction, say about 10 per cent of hydrogen bonds in ice are broken when it melts at O^oC. Liquid water is still hydrogen bonded at 100^oC as indicated by its high heat of vaporization and dielectric constant.

That water is highly hydrogen bonded and still a fluid and not a solid is a paradox. It can be explained as follows: hydrogen bonds in liquid water are made and broken at a very fast rate. The half-life of the hydrogen bond in liquid water is about 10^-11 - 10^-12 seconds. The short range structure of liquid water is, therefore, statistical, since it is averaged over both space and time. Tremendous amount of research has gone into in the understanding of water and its structure.

Despite all this, it is surprising that the microscopic forces that define the structure of water is not fully known. Even now several publications aim at better understanding of the structure of water. For instance, in a report in Nature (December 1993), scientists have studied the details of the inter-atomic structure of water at super critical temperature using neutron diffraction. Recently they have shown that a minimum of six molecules of water are required to form a three-dimensional cage-like structure. Groups up to five water molecules and fewer form one-molecule- thick, planar structures (New Scientist, February 1997). 

Imagine a non-polar group in a cluster of water molecules. Since there is no interaction between water, a polar solvent and a non- polar group, water tends to surround this non-polar group resulting in higher ordering of water molecules. Consequently the entropy of the system lowers with increase in Free Energy. When yet another non-polar group is brought closer to the first non- polar group the energy of the surrounding water forces the two groups to be close to one another.

The water molecules surround the cluster of non-polar groups as is favored by thermodynamic conditions. This phenomenon was first observed by scientists in New Jersey in the early Fifties. They believed that `hydrocarbons actually prefer a non-polar environment to being surrounded by water'. He therefore called the phenomenon of non-polar groups getting clustered in water media as `hydrophobic bond'. But the term hydrophobic bond came under criticism under two grounds.

On the basis of energetics, there is an attraction between hydrocarbons and water rather than phobia; and the interactions have none of the characteristics that distinguish chemical bonds from Van der Waal's force. Joel H. Hildebrand, the worst critic of the concept of hydrophobic bond, suggested that the phenomenon can be explained by simpler terminologies like `interaction free energy' or `interaction energy' or `interaction entropy'. He went on to say, ``I do not find it necessary to invent fluorophobic bonds in order to handle the thermodynamics of the limited solubility of heptane in perfluoroheptane''. Other scientists argued that the tendency of apolar groups to cluster together in compact arrangement in proteins is primarily due to the `lipophobicity' of water and suggested the name `lipophobic bond'. The New Jersey scientists argued their case by saying that though Van der Waal's interaction favour mixing, the energy factors overshadowed by large, negative and unfavorable unitary entropy change upon mixing. In the water-hydrocarbon system the source of immiscibility is an entropy factor. Therefore it differs qualitatively from other systems of immiscibility. In view of this, the term `hydrophobic' was well taken and got entrenched into scientific literature. However, the objection to the word `bond' was sustained and consequently the terms `bonding' or `interaction' have come into vogue. Hydrophobic interaction has important implications on the stability of biological macromolecules.

In 1962, N. N. Fedyakin of Kostrama Technical Institute in the then Soviet Union, was studying the effect capillaries on the physical properties of water. In these experiments, Fedyakin suspended 1-10 micrometer capillaries over a supply of water to which a small amount of sulphuric acid had been added in order to reduce the relative vapor pressure to about 0.98 of the saturation pressure. The thermal expansion of the water condensed was different from that of normal water and Fedyakin concluded that these capillaries modified the structure of water. He prepared numerous samples of this water and called it `polywater'. This polywater, made by exposing very fine glass capillaries to a saturated atmosphere of water vapor, was very viscous, dense and had a refractive index similar to that of glass. 

The initial analysis of the infrared absorption and Raman scattering spectra of this viscous material suggested that it was a new form of water characterised by a symmetrical hydrogen bond and having a repetitive structure like a polymer. In 1969, Maylard researchers reported that several of the experimental results on polywater had very poor reproducibility. This triggered the enthusiasm of several research workers who found that the impurities present in polywater alone were responsible for its properties. After a complete evaluation of all available evidences it was unanimously concluded by the scientific community at large that it was extremely unlikely that a polymer of water had been discovered and the phenomenon had a natural death.

The importance of water is further enhanced as it is expected to be the source of energy in the future. Hydrogen, which is expected to be an energy carrier, can be obtained from water using any primary energy source like solar energy, electricity or thermal energy or a hybrid system consisting of more than one of these primary energy sources. Hydrogen, a secondary energy carrier, can be converted to produce water and this water appears to be an endless source of energy. 

The importance of water to life can be gauged from the fact that cellular life, evolved in water billions of years ago. The cells are filled with water and are bathed in watery tissue fluids. Water is the medium in which the cell's biochemical reactions take place. The cell surface, a lipid-protein-lipid is stabilized by hydrophobic interaction. Moreover, the proteins and membranes in cells are hydrogen bonded through water which protects them from denaturation and conformational transitions when there are thermal fluctuations. Transportation of ions from cell to cell is possible only because of the presence of water. Water is extremely important for structural stabilization of proteins, lipids, membranes and cells. Any attempt to remove water from these structures will lead to many changes in their physical properties and structural stability.

This then raises the question whether biological systems can survive without water or precisely, can there be any `life without water'. Antonie van Leeuwenhoek, pioneering microscopist, observed in 1702 that dry sediments of `animalcules', expected to be dead, were brought back to life when exposed to rain water. The phenomenon observed by him is now called `anhydrobiosis'. Later David Keilin of the University of Cambridge termed this phenomenon as `life without water'. There are several research groups engaged in this area. Human needs in relation to medicine, agriculture and other fields are the main focus of attention of these groups. Studies which center around the kinds of organisms that exploit anhydrobiosis include work on early developmental stages of a variety of microbes, plants, spores of bacteria and fungi, larvae of certain insects and the cysts of brine shrimps. A classic example of this phenomenon of anhydrobiosis is the germination of lotus seeds found buried in a peat bog in Manchuria, where they reputedly had survived for more than 2000 years.

Extensive studies showed that trehalose (a disaccharide having two molecules of glucose) and sucrose (having one molecule each of glucose and fructose) are involved in preserving the cells of anhydrobiotic organisms during dehydration. The mechanism by which trehalose and sucrose stabilize dry biomolecules is that the sugars form hydrogen bond with the hydrophilic regions on the dry biomolecules. In other words the disaccharides, trehalose and sucrose effectively replace water while preserving the hydrogen bond for the stabilisation of the structure. Further research studies have led to the possibility of a long lived dry blood substitute that can be rehydrated rapidly when the need arises. With the water content being reduced to the lowest possible limit, anhydrobiotic organisms do not exhibit any metabolic activity.

This raises an important question - are these organisms alive when they are dried. Researchers argue that a living thing has organised structure and as long as the structural integrity is not violated, the organism is alive. Trehalose and sucrose maintain the structural organisation in anhydrobiotic organisms on dehydration. Now the most important question before the scientific community is whether the findings on life without water can be extended to enhance the longevity of the human race. The precise answer to this question will have to wait for our complete understanding of the structure of water and how the sugar molecules replace water in biomolecules.