Phonon-like excitation in secondary and tertiary structure of hydrated protein powders

Existence of sub-thermal collective excitations in proteins is of great interest due to its possible close coupling with the onset of their biological functions. We use high-energy resolution inelastic X-ray scattering to directly measure phonon dispersion relations and their damping in two hydrated proteins, α-chymotrypsinogen A and casein, differing in their secondary and tertiary structures. We observe that specific phonons in the Q range 28-30 nm ^-1 are markedly softened only above T _D = 220 K, the observed protein dynamic transition temperature. This might indicate that only phonon modes within the wavelengths in the length scale comparable to the secondary structure dimension could be linked to the onset of protein biological activity. We also infer that the presence of tertiary structure contributes little to the population of phonons, while the α-helix seems to be the major contributor to phonons propagation. © 2011 The Royal Society of Chemistry.