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| Journal Article | FZJ-2015-03366 |
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2010
Elsevier
Amsterdam [u.a.]
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Please use a persistent id in citations: doi:10.1016/j.bbapap.2009.11.003
Abstract: Protein molecules and liquid water are very different systems both from a structural and a dynamic point of view. Proteins are long range ordered, but its local molecular motions are restricted by covalent bonds, which makes it solid-like. The dynamics of such a system is properly represented by barrier crossing within a well defined energy landscape. Water in contrast is short-range ordered, but long range diffusion allows it to flow, the essence of the liquid state. The energy landscape is thus fluctuating and motion is determined by collective reorganization of molecules forming transient holes, the landscape turns into a seascape. At the protein-water interface mutual interactions modify the properties of both systems. The protein landscape is modulated in time by density fluctuations in the liquid phase and interfacial water is transiently immobilized by rigid protein structure.The physical phenomenon of protein hydration is to some extent elusive, since there is no rigid shell of water around a protein molecule. Instead there is a fluctuating cloud which is thermodynamically and dynamically different from bulk water. A “hydrodynamic” definition of hydration water is to evaluate, how many water molecules near the protein surface have their displacement vector along the trajectory of the protein at any instant of time. This is determined by the strength of interaction with the protein and, hence, by the ratio of the time that is spent “attached” to the protein to the time spent in the bulk phase. The sum of the fractional contributions of all water molecules is the measured hydration. It is the mass of water, which migrates with the protein at any instant of time. When divided by the molecular weight of water, this yields the average number of molecules, which interact with the protein [1].An operational definition is to determine the amount of non-freezable water at low temperatures, which depends however on the cooling-rate if crystallization occurs. Below about 0.4 g water per g protein, crystallization is hindered. The hydration shell can be super-cooled until a glass transition interferes at 170 K. In such hydrated protein powders, the water molecules are permanently in contact with the protein surface, while protein translation and rotation is suppressed. Such surface-attached water molecules are still highly mobile, performing long-range diffusion, which is a property of the liquid state. The translational diffusion is arrested at the glass transition [2]. This dramatic slowing down of molecular motions of water is a useful approach to the coupling between water and protein dynamics
Keyword(s): Others (1st) ; Others (1st) ; Others (2nd)
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