Confined molecular motions of globular proteins studied in powder samples and in solution

Internal molecular equilibrium fluctuations have been studied for alpha-amylase in two different types of samples. The comparison of incoherent neutron scattering from the enzyme in hydrated powders and in solutions gives insights into the protein - solvent interaction. Although the protein is already fully hydrated at h = 0.4 in a powder sample (the protein surface is completely covered with solvent), the proteins are characterised by a significantly larger structural flexibility in solution as compared proteins in powder samples. As obtained from model fits describing local diffusion, the results indicate that the higher mobility of solvent molecules in solution initiate additional fluctuations in the protein structure. Due to much more solvent surrounding the protein in solutions, the picosecond fluctuations of the enzyme seem to be damped indicated by smaller "amplitudes" and a weaker temperature dependence as compared to powder samples.