Low-temperature molecular dynamics simulations of horse heart cytochrome c and comparison with inelastic neutron scattering data

Molecular dynamics (MD) simulation combined with inelastic neutron scattering can provide information about the thermal dynamics of proteins, especially the low-frequency vibrational modes responsible for large movement of some parts of protein molecules. We performed several 30-ns MD simulations of cytochrome c (Cyt c) in a water box for temperatures ranging from 110 to 300 K and compared the results with those from experimental inelastic neutron scattering. The low-frequency vibrational modes were obtained via dynamic structure factors, S(Q, omega), obtained both from inelastic neutron scattering experiments and calculated from MD simulations for Cyt c in the same range of temperatures. The well known thermal transition in structural movements of Cyt c is clearly seen in MD simulations; it is, however, confined to unstructured fragments of loops Omega(1) and Omega(2); movement of structured loop Omega(3) and both helical ends of the protein is resistant to thermal disturbance. Calculated and experimental S(Q, omega) plots are in qualitative agreement for low temperatures whereas above 200 K a boson peak vanishes from the calculated plots. This may be a result of loss of crystal structure by the protein-water system compared with the protein crystal.