N-H+ vibrational anharmonicities directly revealed from DFT-based molecular dynamics simulations on the Ala7H+ protonated peptide

The present investigation reports DFT-based Born-Oppenheimer molecular dynamics simulations of the gas phase Ala(7)H(+) protonated peptide, in order to unravel the structure and dynamics of the peptide and its Infrared vibrational signatures. At 350 K, the most statistically relevant conformations adopted by the globular folded Ala(7)H(+) peptide have the NH3+ N-terminus involved in two NH+ -> O=C charge-solvated hydrogen bonds. The dynamics performed here nicely provide a clear understanding of the IR-MPD features experimentally recorded, with an excellent matching of the dynamically simulated IR spectrum with the experiment in terms of band-positions and band-shapes. In particular, the diverse vibrational anharmonicities displayed by the N-H+ stretches depending on the number of simultaneous hydrogen bonds that the NH3+ can form at the N-terminus of the Ala(7)H(+) peptide chain, are remarkably reproduced by the present dynamics at finite temperature. This gives rise to a proper understanding of the different IR active bands, especially the supplementary band between 3100 and 3300 cm(-1) present for the Ala(7)H(+) peptide and absent for the smaller peptide chain lengths. Vibrational anharmonicities are naturally taken into account in the dynamical treatment of the movements, and this has once more been illustrated in the present work. Our results on the N-H+ stretching motions in relation with the number of hydrogen bonds formed by the NH3+ group, can be used as general guidelines in order to precisely interpret IR-MPD spectra of molecules containing NH3+ groups, taking into account vibrational anharmonicities. (C) 2011 Elsevier B.V. All rights reserved.