Unique characteristics of biological water

It is very difficult to directly observe water molecules in vivo at the surfaces of proteins or lipid bilayers or in the grooves of DNA. X-ray studies of protein crystals cannot provide full information as only a fi action of water molecules remain on the surface and even then in restricted positions. Also, the hydration layer is mobile in solution. One would like to know about the structural and dynamical characteristics of these water molecules in the active state, ideally within biological cells. Such detailed information is still not available in most cases. Much of our current understanding of water in biological systems has come from study of proteins and DNA in aqueous solution. 

Initial information about the protein hydration layer came from relaxation studies. Dielectric relaxation (DR) and NMR studies were the first to reveal the existence of water molecules in the restricted environments. Dielectric relaxation measurements show the existence of an additional dispersion in protein solutions with time constants in the 40-50 ps time range (to be contrasted with 8 ps for bulk water), while NMR estimates have varied from system to system, with claims ranging from slow (with lifetimes in excess of 300 ps) to fast (with lifetimes 2-5 ps). The general consensus now appears to be consistent with the DR data. 

The dielectric relaxation spectra of various DNA solutions are also quite complex, actually more complex than those observed for protein solutions. One of the reasons for such complex relaxation behavior of DNA solutions is the presence of free and bound counter ions in solution. Water molecules in the major and minor grooves of water behave differently from those in the buUc. The presence of the positively charged counter ions in turn influences the response of many other water molecules. Negatively charged phosphate ions also influence the surrounding water molecules. 

Below we list the common characteristics of water observed in/at constrained environments, such as on the surfaces of biomolecules or within cells and tissues. 

All of the above characteristics need not be uniformly observed in all kinds of 

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