Changes in water properties in serum albumin solutions induced by alterations in protein flexibility. NMR studies

Two types of changes in the spin-spin relaxation time of water protons (T2) corresponding to the decrease in the correlation time of water molecules have been found in human serum albumin solutions in the temperature range of 20-40 degrees using the NMR method. It follows from experiments with frozen HSA solutions that over 15% of H2O is pushed out from the HSA hydration shell in 10% D2O solution and about 30% of H2O in 20% D2O solution by DOD and DOH molecules. The T2 effects observed increase in the presence of D2O in the protein solution, however, and their maxima shift by 5--10 degrees toward a higher temperature range. An increase in the viscosity of HSA solutions by adding sucrose (20%) results in complete disappearance of the effects. The results were analysed and the conclusion was made that the anomalies observed in T2 result from thermally induced changes in the nature of the relative translational-rotational diffusion of three domains of HSA. Concomitant rapid exchange between the free solvent and water present in the protein cavities between domains can stimulate fluctuations in the solvent and destabilize its structure.