Microscopic Protein Diffusion at High Concentration MRS Bulletin (2006) in press, QENS Proceedings

The self-diffusion of myoglobin in concentrated solutions was investigated up to volume fractions of 0.4 by neutron back-scattering spectroscopy. The quasi-elastic spectrum can be decomposed into two Lorentz curves: (1) a narrow line, where the width increases with Q, which is assigned to translational diffusion, and (2) a broad Q-independent line, reflecting proteininternal motions. The apparent diffusion coefficient decreases with increasing concentration and wave-vector, suggesting that protein diffusion deviates at high Q (1.75 Å) from its long-time Brownian limit. Jump diffusion, sample heterogeneity and time-dependent diffusion are discussed as possible explanations.