Nonequilibrium molecular-dynamics study of the vibrational energy relaxation of peptides in water

A nonequilibrium description of the vibrational-energy relaxation of solvated flexible molecules such as small peptides in aqueous solution is outlined. Having in mind to employ standard biomolecular molecular-dynamics program packages, several methodological developments are introduced. To calculate the vibrational normal-mode energies for a system undergoing large-amplitude motion, an instantaneous normal-mode analysis is employed. To mimic the laser excitation of a given vibrational mode in its excited states, a computational scheme is proposed which allows us to calculate the nonequilibrium phase-space initial conditions for the solute and the solvent atoms. It is demonstrated that the vibrational relaxation dynamics sensitively depends on the accurate representation of the initially excited normal mode. In particular, effects of the quantum-mechanical zero-point energy contained by the initial state are investigated, thus elucidating the importance of quantum fluctuations. To study the validity and the performance of the method, the laser-induced amide I nu=1-->0 energy relaxation of N-methylacetamid in D2O is considered. The vibrational energy relaxation rate obtained from the nonequilibrium simulations is in qualitative agreement with experiment, whereas a Landau-Teller-type calculation underestimates the rate considerably. The virtues and problems of the nonequilibrium description are discussed in some detail. (C) 2003 American Institute of Physics.