13C and 15N NMR study of the hydration response of T4 lysozyme and alphaB-crystallin internal dynamics

The response to hydration of the internal protein dynamics was studied by the means of solid state NMR relaxation and magic angle spinning exchange techniques. Two proteins, lysozyme from bacteriophage T4 and human alphaB-crystallin were used as exemplars. The relaxation rates R1 and R1rho of 13C and 15N nuclei were measured as a function of a hydration level of the proteins in the range 0-0.6 g of water/g of protein. Both proteins were totally 15N-enriched with natural 13C abundance. The relaxation rates were measured for different spectral bands (peaks) that enabled the characterization of the dynamics separately for the backbone, side chains, and CH3 and NH3+ groups. The data obtained allowed a comparative analysis of the hydration response of the protein dynamics in different frequency ranges and different sites in the protein for two different proteins and two magnetic nuclei. The most important result is a demonstration of a qualitatively different response to hydration of the internal dynamics in different frequency ranges. The amplitude of the fast (nanosecond time scale) motion gradually increases with increasing hydration, whereas that of the slow (microsecond time scale) motion increases only until the hydration level 0.2-0.3 g of water/g of protein and then shows almost no hydration dependence. The reason for such a difference is discussed in terms of the different physical natures of these two dynamic processes. Backbone and side chain nuclei show the same features of the response of dynamics with hydration despite the fact that the backbone motional amplitudes are much smaller than those of side chains. Although T4 lysozyme and alphaB-crystallin possess rather different structural and biochemical properties, both proteins show qualitatively very similar hydration responses. In addition to the internal motions, exchange NMR data enabled the identification of one more type of motion in the millisecond to second time scale that appears only at high hydration levels. This motion was attributed to the restricted librations of the protein as a whole.