Mobility and structure of human serum albumin powders in water-acetonitrile mixtures by1H NMR relaxation and FTIR spectroscopy

The effect of acetonitrile on protein dynamics was investigated for solid human serum albumin samples at various hydration levels. Temperature dependences of¹H nonselective nuclear magnetic resonanceT ₁ andT ₂ relaxation times at 27 MHz have been measured and data were interpreted in terms of three kinds of internal motions in the protein. Microdynamic parameters of the motions were obtained within a “model-free” approach. It was found that acetonitrile hardly affects the fast motions but noticeably influences the slow motion of side chain groups, shortening the correlation time and increasing the amplitude of the motion. The acetonitrile effect on dynamics is likely based on the appearance of additional free volume as a result of the formation of rigid helical parts in the protein structure. Water, plasticizing the protein structure, promotes the action of organic solvent. A definite part of side chain groups, slowly moving in the same frequency window as the rest of protein side chain groups, performs less constrained “liquidlike” motion. The relative population of these highly movable protons is closely correlated with the increment of the helical structure induced by water and acetonitrile.