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Chemical Reaction: An Update on Water
By: Steve Herman
Posted: October 26, 2006, from the October 2006 issue of GCI Magazine.
“Water is life’s matter and matrix, mother and medium. There is no life without water.”—Albert Szent-Gyorgyi
To see one of nature’s greatest miracles, turn on the tap. The properties of water pouring forth defy total understanding. Cosmetics are mostly water. The cells you try to beautify, protect and repair are mostly water. Simply saying that water is “H20” obscures its true nature, and attaching a simple label to it is likely to discourage the further pursuit of its mysteries. It is similar to wearing a t-shirt of Einstein sticking out his tongue and thinking that it implies a profound grasp of the theory of relativity by its wearer.
The structure of liquid water traditionally has been viewed as a looser form of ice, in an overall tetrahedral arrangement. Conventional wisdom is that water is unique among all molecules, not because it is polar and forms hydrogen bonds, but because each oxygen bonds to four hydrogen atoms, giving it unusual strength. Two bonds are covalent and two are polar.
However, researchers, using X-ray absorption and X-ray Raman scattering, imply that liquid water molecules take part in only two hydrogen bonds, not four, and that these are in a planar arrangement.1 This result suggests that water exists in hydrogen-bonded chains or large rings. Researchers found an asymmetry in the charge distribution of the water molecule—one hydrogen has significantly more electron density than another. The hydrogen with greater electron density would not be able to participate in a hydrogen bond, thus countering the traditional view that all hydrogen atoms are involved in hydrogen bonds in a tetrahedral arrangement. Instead, they believe that this asymmetry implies a linear structure, with a hydrogen atom on one side of the central oxygen participating in a hydrogen bond while an electron pair on the opposite side participates in a hydrogen bond with the next molecule on the chain.
Proteins in cells exist in an aqueous environment. Computer modeling allows a closer look at how water affects the structure, and consequently the function, of biological molecules.2 In 1986, the dark ages of data crunching, two weeks on a supercomputer was used to model bovine pancreatic trypsin inhibitor in water. The particular protein chosen is relatively small and had previously modeled in vacuo. The differences in the results with and without water showed the importance of water for protein structure and activity.