Overall and internal protein dynamics in solution studied by the nonselective proton relaxation

A new algorithm for the analysis of nonselective proton relaxation data in protein solution is presented. T1 and T2 of protein protons in lysozyme and RNase solutions were measured at three resonance frequencies--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 field-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 also a component with a correlation time exceeding tau t by approximately an order of magnitude and with a small relative amplitude. The experiment shows that the parameters of the slow component of the tumbling correlation function depend both on the concentration and on the pH of 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 from their neighbors and this gives rise to an anisotropy in the protein Brownian tumbling. The lifetime of this anisotropy is controlled by the translational diffusion of proteins.