Nuclear magnetic relaxation times of water sorbed by proteins. Lysozyme and serum albumin

Abstract

Spin-spin and spin-lattice relaxation times have been measured for water sorbed by proteins as a function of temperature and of the amount of water on the surface. The relation between the two relaxation times is consistent with a motional relaxation process which is anisotropic. For lysozyme, the initial linear segment of the plot of T 2 versus coverage extrapolates to an amount of water corresponding approximately to one molecule per polar side chain group. For both lysozyme and bovine serum albumin, Arrhenius plots based on T 2 show two segments at higher surface coverages, the upper temperature range corresponding in slope to a very small or even negative apparent activation energy. This behavior may result from the transfer of protons between a tightly bound surface phase and a more liquid-like layer, or from a change in the structure of the protein.