Low Frequency Dynamics of Confined Proteins

ABSTRACTWe present the frequency dependence of the proton spin-lattice relaxation rate 1/T1 in variously hydrated proteins. We present also the case of proteins confined in heavily hydrated gels where the rotation has been immobilized. The relaxation efficiency increases according to a power law at low frequencies. The temperature dependence of the protein protons T1 demonstrates that relaxation results from a direct spin-phonon process instead of a Raman process at temperatures above 273K. We propose a theory that accounts for experiments and depends on the dynamical distribution of states, the localization of the disturbances along and transverse to the peptide chains, and the spatial distribution of hydrogen in the structure. In hydrated and confined proteins, the motions of the backbone that dominate the relaxation process are transverse rather than along the peptide chain. We show that the protein structure adjusts to hydration from the lyophilized state to the fully hydrated state in small increment steps.