Collective motions and anharmonicity in protein terahertz response

Andrea Markelz (University of Buffalo, SUNY)

The presence of structural collective motions in picosecond dynamics is investigated through terahertz (THz) time domain spectroscopy and molecular dynamics simulations of cytochrome c as a function of oxidation and hydration. For both oxidation states the measured THz response rapidly increases with hydration up to ~.25 h, and then saturates above this level. A large increase in the picosecond response occurs with oxidation, with the largest flexibility increase for lowest hydrations and highest frequencies. Quasi-harmonic vibrational modes and dipole-dipole correlation functions are calculated from the molecular dynamics trajectories. The measured hydration dependence is reproduced by the vibrational collective mode density of states demonstrating the existence of these collective motions. The large oxidation dependence is reproduced only by the dipole-dipole correlation function, indicating the contrast arises diffusive motions. This change in flexibility is consistent with structural contrast occurring in the vicinity of buried internal water molecule.

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