Biological Water or Rather Water in Biology?

There has been a lot of discussion about biological water recently. Books 1-2 and reviews 3-5 were written in the past years and a special issue of the Journal of Chemical Physics was dedicated to the topic in 2014. 6 Interestingly, papers on biological water are mostly confined to chemistry and physics journals, being remarkably rare in biological or biochemical literature. So, what is actually the biological water, we physical chemists are so concerned about? Definitions vary from " soft " to " hard " ones. A soft definition describes biological water as any water around a biomolecule (i.e., protein, DNA or RNA, or a piece of a cellular membrane) that has properties distinct from those of the aqueous bulk. More hard definitions operate with mutual tailoring of thermodynamic and dynamic properties of the biomolecule and surrounding waters, 11-15 and even with covering the protein by a shell of functional water molecules which can " slave " its motions 5 and propagate to considerable distance. 16 Finally, the recent hardest interpretations invoke in their extreme form, which can hardly be considered as strictly scientific any more, the notion of cellular water as a distinct species which itself is able to carry the biological functionalities. 17-20 2 There is little doubt that a layer of non-bulk water exists around a biomolecule. 7 The relevant question is how thick such a layer is and to what extent its properties differ from those of the aqueous bulk. Let us first get a semi-quantitative estimate of the thickness, focusing on electrostatic interactions which dominate in water. The range of these interactions is governed by the Debye screening length, 21 which amounts to less than 1 nm for the physiological ionic strength of about 150 mM. The physiological solution thus has a remarkable ability to screen out electrostatic interactions, which could hardly propagate beyond some 1-3 water molecules from the surface of the biomolecule. Still, several solvent layers can represent a non-negligible fraction of available water in the crowded cellular environment. 4 Moreover, individual biomolecular functional groups can come close enough to each other such that topological characteristics of the protein or DNA surface can in principle combine with properties of interfacial water molecules in-between these groups. 22-23 For the above reasons, the relevant question is not only how many water molecules are influenced by the biomolecule, but also how much. Leaving aside the small …