Slow dynamics of solid proteins - Nuclear magnetic resonance relaxometry versus dielectric spectroscopy.

H Nuclear Magnetic Resonance (NMR) relaxometry and Dielectric Spectroscopy (DS) have been exploited to investigate the dynamics of solid proteins. The experiments have been carried out in the frequency range of about 10 kHz-40 MHz for NMR relaxometry and 10Hz-20 MHz for DS. The data sets have been analyzed in terms of theoretical models allowing for a comparison of the correlation times revealed by NMR relaxometry and DS. The H spin-lattice relaxation profiles have been decomposed into relaxation contributions associated with H-H and H-N dipole - dipole interactions. The H-H relaxation contribution has been interpreted in terms of three dynamical processes of time scales of 10s, 10s and 10s. It has turned out that the correlation times do not differ much among proteins and they are only weakly dependent on temperature. The analysis of DS relaxation spectra has also revealed three motional processes characterized by correlation times that considerably depend on temperature in contrast to those obtained from the H relaxation. This finding suggest that for solid proteins there is a contribution to the H spin-lattice relaxation associated with a kind of motion that is not probed in DS as it does not lead to a reorientation of the electric dipole moment.