OVERALL AND INTERNAL PROTEIN DYNAMICS IN SOLUTION STUDIED BY THE NONSELECTIVE PROTON RELAXATION

A new algorithm for the analysis of nonselective proton relaxation dat a in protein solution is presented. T1 and T2 of protein protons in ly sozyme and RNase solutions were measured at three resonance frequencie s - 11, 27 and 90 MHz. In addition we measured water T1 dispersions in lysozyme solutions over the frequency range of 10 kHz - 10 MHz on a f ield-cycling installation. It was found that the correlation function of protein Brownian tumbling as a whole is nonexponential: in addition to a component with the usual correlation time tau(t) it contained al so a component with a correlation time exceeding tau(t) by approximate ly an order of magnitude and with a small relative amplitude. The expe riment shows that the parameters of the slow component of the tumbling correlation function depend both on the concentration and on the pH o f the protein solution. To explain the results obtained one must take into account the interprotein electrostatic interactions in solution. All protein molecules in solution experience electrostatic torques fro m their neighbors and this gives rise to an anisotropy in the protein Brownian tumbling. The lifetime of this anisotropy is controlled by th e translational diffusion of proteins.