Slow dynamics of solid proteins: Nuclear Magnetic Resonance relaxometry versus Dielectric Spectroscopy

1H 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 10kHz-40MHz for NMR relaxometry and 10-2Hz-20MHz 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 1H spin-lattice relaxation profiles have been decomposed into relaxation contributions associated with 1H-1H and 1H-14N dipole – dipole interactions. The 1H-1H relaxation contribution has been interpreted in terms of three dynamical processes of time scales of 10-6s, 10-7s and 10-8s. 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 1H relaxation. This finding suggest that for solid proteins there is a contribution to the 1H 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.