Time-Dependent Perspective for the Intramolecular Couplings of the N-H Stretches of Protonated Tryptophan.

: Quasiclassical direct dynamics simulations, performed with B3LYP-D3/cc-pVDZ electronic structure theory, are reported for vibrational relaxation of the three NH stretches of the -NH3+ group of protonated tryptophan (TrpH+), excited to the n = 1 local mode states. The intramolecular vibrational energy relaxation (IVR) rates determined for these states, from the simulations, are in good agreement with experiment. In accord with experiment, IVR for the free NH stretch is slowest, with faster IVR for the remaining two NH stretches which have inter-molecular couplings with an O-atom and a benzenoid ring. For the free NH and the NH coupled to the benzenoid ring, there are beats (i.e. recurrences) in their relaxations versus time. For the free NH stretch, 50% of the population remained in n =1 when the trajectories were terminated at 0.4 ps. IVR for the free NH stretch is substantially slower than for the CH stretch in benzene. The agreement found in this study between quasiclassical direct dynamics simulations and experiment, indicates the possible applicability of this simulation method to larger biological molecules. Since IVR can drive or inhibit reactions, calculations of IVR timescales are of interest for example in unimolecular reactions, mode specific chemistry and in many photochemical processes.