The Role of Structure in the Protein Dynamical Transition

The protein dynamical transition is investigated as a function of protein structure using terahertz time domain spectroscopy (THz-TDS). Measurements performed for native state and denatured hen egg white lysozyme (HEWL) show that protein structure is not necessary for the dynamical transition. We find the temperature dependence follows activated behavior and there is no evidence of a fragile to strong transition. Measurements of short chain poly alanine show a dynamical transition down to penta-alanine, however no transition is observed for di-alanine or tri-alanine. These measurements demonstrate that the temperature dependence arises strictly from the interaction of the side chains with the solvent. The lack of a transition for shorter chain polypeptides may indicate the temperature dependence arises from a net ordering of the adjacent water which scales with the length of the polypeptide chain.