Internal dynamics and hydration of solid proteins by1H NMR relaxation and FTIR spectroscopy

Temperature dependences of¹H nonselective nuclear magnetic resonanceT ₁ andT ₂ relaxation times measured at 27 MHz have been studied on solid human serum albumin (HSA) samples at various hydrations. The data were interpreted in terms of three kinds of internal motions in a protein and microdynamic parameters of the motions were obtained by a “model-free” approach. Two fast motions with correlation times lying in the range of tens to hundreds picoseconds were shown to be essentially insensitive to hydration. Unlike lysozyme and bovine albumin, HSA reveals relaxation transition due to slow motion in the room temperature range thus allowing one to obtain microdynamic parameters more precisely. Hydration leads to a shortening of the correlation time from hundreds to tens nanoseconds and to a less restricted movement. The comparison of the hydration dependence of relaxation parameters with infrared spectra of HSA side chain groups clearly shows that methyl protons are evidently involved in a slow motion, following the saturation of the protein globule surface by water. The same dependence correlating with solvent accessible surface areas was shown to exist for some other proteins. In addition to the main set of protons performing a solidlike movement, a small amount of much more mobile protons is also present with its proportion rising steeply with hydration and temperature. The origin of these protons is discussed.