Hydration and aggregation of lysozyme by extended frequency range depolarized light scattering

Extended frequency range depolarized light scattering is a spectroscopic technique operating in the GHz-THz range that, applied to aqueous solutions of biomolecules, is able to disentangle the dynamics of the solute from that of water, and relaxation processes of bulk from those of hydration water. Experiments performed on aqueous solution of a variety of biological systems of different nature, such as small hydrophobic and hydrophilic molecules, amino acids, dipeptides, and proteins, have shown that a significant increase in the dynamical retardation and in the extent of the perturbation of water surrounding solute molecules occurs at increasing chemical complexity of the solute. The behavior of aqueous solutions of lysozyme is here analyzed in detail, as a function of solute concentration. Our results provide evidence of a dynamical perturbation extending over more than three water layers in diluted solutions. We find a strong reduction in the average hydration number at increasing solute concentration that cannot be explained by the random superposition of hydration layers among lysozyme molecules in close proximity. This behavior is consistent with the formation of clusters in solution.