Neutron Spin Echo Studies of Dynamics in Hemoglobin and Myoglobin

Abstract

Neutron spin-echo (NSE) spectroscopy was used to study structural fluctuations that occur in hemoglobin (Hb) and myoglobin (Mb) in solution. Using NSE data to very high momentum transfer, q ( ∼ 0.62 A-1), the internal dynamics of these proteins were characterized at the level of the dynamical pair correlation function and self-correlation function in the time range of several picoseconds to a few nanoseconds. Comparison of data from the two homologous proteins collected at different temperatures and protein concentrations was used to assess the contributions to the data made by translational and rotational diffusion and internal modes of motion. The temperature dependence of the decay times can be attributed to changes in viscosity and temperature of the solvent as predicted by the Stokes-Einstein relationship. This is true for contributions from both diffusion and internal modes of motion indicating an intimate relationship between the internal dynamics of the proteins and the viscosity of the solvent. Viscosity change associated with protein concentration can account for changes in diffusion observed at different concentrations, but is apparently not the only factor involved in the changes in internal dynamics observed with change in protein concentration. Comparison of data from Hb and Mb at low q indicate an unexpectedly rapid motion of the hemoglobin αβ-dimers relative to one another. These observations are consistent with the notion that movements of structural elements along paths of intrinsically low free energy - as may form during evolution to expedite conformational changes between different functional states - are a major factor in determining the dynamic behavior of proteins in solution.