Nonadiabatic vibrational dynamics and spectroscopy of peptides: A quantum-classical description

A quantum-classical description of the amide I vibrational spectrum of peptides in aqueous solution is given, which is concerned with the effects of nonadiabatic couplings between vibrational eigenstates. It consists of a classical molecular dynamics simulation of the conformational distribution of the system, density functional theory calculations of the conformation-dependent and solvent-induced frequency fluctuations, and a semiclassical description of the vibrational line shapes. The study shows that the adiabatic approximation usually employed in semiclassical line shape theory is generally not valid, because it assumes a time scale separation between the dynamics of the amide I mode (with a period of ≈20 fs) and the motion of the solvent and the peptide (which also exhibits sub-100 fs dynamics). A practical and general computational scheme is presented, which allows for the calculation of spectroscopic response functions by directly solving the nonadiabatically coupled time-dependent Schrödinger equation. Adopting trialanine and heptaalanine as representative examples, it is shown that nonadiabatic interactions may considerably change the overall shape as well as local details of the amide I spectrum.