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.
ISIS brings you the latest revelations on water in this extended series
that starts from the basics. The articles will not be circulated consecutively,
so do watch out for them.
ISIS Report 28/06/04
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.
ISIS brings you the latest revelations on water in this extended series
that starts from the basics. The articles will not be circulated consecutively,
so do watch out for them.
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
Sources
for this report are available in the ISIS members site.
Full details here
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.
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