Anharmonicity, mode-coupling and entropy in a fluctuating native protein.

We develop a general framework for the analysis of residue fluctuations that simultaneously incorporates anharmonicity and mode-coupling in a unified formalism. We show that both deviations from the Gaussian model are important for modeling the multidimensional energy landscape of the protein Crambin (1EJG) in the vicinity of its native state. The effect of anharmonicity and mode-coupling on the fluctuational entropy is in the order of a few percent.