Science in Society Archive

New age of water

Water has come of age. It is cool on everyone's lips. Decades of research on water is giving us remarkable insights into its dynamic collective structure, and changing our big picture of life and living process.

Organisms are seventy to eighty percent water. Is this water necessary to life? What vital functions does it serve?

Entire biochemistry and cell biology textbooks are still being written without ever mentioning the role of water. It is simply treated as the inert medium in which all the specific biochemical reactions are being played out.

Instead, recent findings are raising the possibility that it is water that's stage-managing the biochemical drama of life. Water is life, it is the key to every living activity. Some people will even say it is the seat of consciousness.

I-SIS brings you the latest revelations on water in this extended series that starts from the basics.


Is Water Special?

Water has a collective structure that's extremely flexible and dynamic, which may explain some of its 'anomalies'. Dr. Mae-Wan Ho explains

Water is simple, isn't it?

There is nothing simpler than water as a molecule. Its chemical formula, H2O, is almost the first thing in chemistry that one learns in school. However, its structure in the bulk is multifarious and changeable. There are 13 known crystalline structures of ice that appear under different temperatures and pressures. As a liquid, water forms dynamic 'flickering clusters' or networks of joined up molecules, with intermolecular bonds that flicker on and off at random. The basis for all this complexity lies in the ability of a water molecule to join up with its neighbours through a special kind of chemical bond, the hydrogen bond.

The hydrogen-bond

To understand how the hydrogen bond comes about, picture the water molecule consisting of an oxygen atom bonded to two hydrogen atoms. The water molecule has a shape approximating a tetrahedron, a three-dimensional triangle with four corners. The oxygen atom sits in the heart of the tetrahedron, the hydrogen atoms point towards two of the four corners and two 'electron clouds' belonging to the oxygen molecule point towards the remaining corners of the tetrahedron. The 'electron clouds' are negatively charged, and result from the atomic structures of oxygen and hydrogen and how they combine in the water molecule.

Oxygen has eight (negatively charged) electrons disposed around its positively charged nucleus, rather like the layer of the onion, two in an inner shell and six in the outer shell. The inner shell can only accommodate two electrons, so its capacity is filled. The outer shell, however, can hold as many as eight electrons. The hydrogen atom happens to have only one electron, so oxygen, by combining with two hydrogen atoms, completes its outer shell, while the hydrogen atoms each completes its first electron shell with two electrons, which it shares with the oxygen atom. That is how the usual 'covalent bond' of chemistry arises.

The oxygen nucleus has more positive charges than the hydrogen, so the shared electrons are slightly more attracted to the oxygen nucleus than to the hydrogen nucleus, which makes the water molecule polar, with two 'electron clouds' of negative charge at the opposite poles to the two hydrogen atoms, which are each left with a slight positive charge. (Though quantum mechanical calculations have shown that the two electron clouds are not really separate from each other.)

The positively charged hydrogen of one water molecule can thus attract the negatively charged oxygen of a neighbouring water molecule to form a hydrogen-bond (H-bond) between them. Each molecule of water can potentially form four H-bonds. Two in which it 'donates' its hydrogen atoms to the oxygen atoms of two other water molecules, and two in which its oxygen atom 'accepts' one hydrogen atom from each of two other water molecules. In other words, each molecule is capable of acting as hydrogen 'donors' and 'acceptors' for two other water molecules, so it has four bonded neighbours, or a '4-coordination'.

Ice structures

Water molecules in ordinary hexagonal ice crystals are close to the ideal tetrahedral structure described above. The hydrogen-bonded O-O distances are almost identical, varying between 2.759 Å and 2.761 Å (an angstrom is 10-10m), while the O-O-O angles also vary only slightly between 109.36o and 109.58o, which is close to the H-O-H angle of 104.52o of the individual water molecule.

However, there are many more forms of ice crystals (at least 12 others known) under different temperatures and pressures, where the bond lengths and angles vary much more widely. For ice II, which forms under moderate pressure of about 5 kbar (1kbar is equivalent to a pressure of ~ 1 000 atmospheres), the basic four-coordinated motif is maintained. But the bond length varies between 2.74 Å and 2.83 Å, while the bond angle varies between 80 o and 129 o.

In liquid water, there is much less constraint compared to a solid crystal lattice, and so the variations in bond length and bond angles take on a much wider continuous range. Instead of the regular hexagonal (6-member) ring structure of ordinary ice, a snapshot of the hydrogen-bonded network shows five, six and seven-member rings, and even smaller or larger rings. Instead of the 4-coordination motif, 2-, 3- and even 5-coordinations are possible, with the H of some water molecules in a 'bifurcated' schizophrenic state, seemingly bonded to two different neighbours.

Why is water special?

Why is water so special that life cannot exist without it? According to John L Finney of University College, London, the basic tetrahedral structure of the water molecule is central to the structural versatility of water in the condensed state (solid and liquid). It enables water to form extended, flexible networks of H-bonded molecules in liquid, allowing rapid coordinated molecular motions to take place. This same extended network also supports proton conduction, a flow of positive electricity that occurs much faster than the diffusion of ions.

Other substances might have some of those special characteristics, says Finney, but only water has them all, and that might be enough to make water especially 'fit' for life.

New insights into water structure

The picture of the structure of water just described has been obtained with powerful measurements techniques such as x-ray and neutron diffraction, which involve firing x-rays or neutron beams at water, and looking at the way the beams are deflected or scattered to make a diffraction pattern, which gives information about the structure of the atoms. These experimental techniques are combined with computer simulations (molecular dynamics) to give a consistent picture, which is supposed to form a firm molecular basis for all other investigations.

But in April 2004, an international team of scientists from universities and research institutes in the United States, The Netherlands, Sweden and Germany, have challenged this picture with the next generation of an even more powerful measurement technique.

They reported the behaviour of liquid water on a timescale of less than one femtosecond (one femtosecond is 10-15s) using a new x-ray absorption spectroscopy technique. This involves firing x-rays of different frequencies at water, and from the spectrum of frequencies absorbed - which is characteristic of each atom - making inferences concerning the structure of the water molecules.

They found that most molecules in bulk liquid water at room temperature are like those at the ice surface, with only two strong hydrogen bonds. The proportion of molecules with 4-coordination similar to bulk ice is very small. The contributions of the two different species - molecules with two H-bonds and those with 4 H-bonds - are 80% and 20% at room temperature, and increases to 85% and 15% at 90C with uncertainties of +15% and +20% in both cases.

As consistent with earlier results, the bond lengths and bond angles are found to vary widely from those in tetrahedral ice, attesting to the flexibility of the water structure in liquid.

They concluded: "Water is a dynamic liquid where H-bonds are continuously broken and reformed. The present result that water, probed subfemtosecond time scale, consists mainly of structure with two strong H-bonds, one donating and one accepting, nonetheless implies that most molecules are arranged in strongly H-bonded chains or rings embedded in a disordered cluster network connected mainly by weak H-bonds."

So, in a sense, it doesn't really alter the picture too much. But are these methods focussing too much on the individual molecules to reveal anything interesting? A growing number of water scientists are beginning to think so, and for good reasons.

Article first published 28/06/04



Sources

Finney JL. Water? What is so special about it? Royal Society Discussion Meeting, The Molecular Basis of Life: is Life Possible without Water? 3-4 December 2003, London.

Wernet Ph, Nordlund D, Bergmann U, et al. The structure of the first coordinated shell in liquid water. Science 2004, 304, 995-9.

Zubavicus Y and Grunze M. New insights into the structure of water with ultrafast probes. Science 2004, 304, 974-6.

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