Vibrational Spectroscopy of Peptides and Peptide−Water Complexes:  Anharmonic Coupled-Mode Calculations

Quantum calculations are reported for vibrational states and energy levels of several peptide and peptide−water complexes, to provide insight into the spectroscopic properties of such systems and their dependency on presently available anharmonic force fields. A blocked di-l-serine−H2O complex and trialanine in an antiparallel β sheet configuration are the main systems treated. The calculations are for variants of the Amber force field and use the vibrational self-consistent field (VSCF) method, which includes effects of anharmonicity as well as interactions between modes. The main findings are as follows: (1) Distinct isomers of a single peptide−H2O complex corresponding to different hydrogen binding sites of the H2O are observed. (2). The H2O induces shifts of up to 50 cm-1 in the frequencies of fundamental transitions associated with the peptide modes. (3). Some of the “intermolecular” modes of the peptide−H2O cluster are of frequencies and geometry that suggest effective peptide-to-water energy transfer. (5). Changing the TIP3 water potential in the Amber force field into the Coker−Watts potential produces significant spectroscopic changes. (5). The peptide−H2O cluster is found to have several tunneling states, which are assigned in detail. At least some of these states should be spectroscopically observable by the magnitude of the splitting. (6) The trialanine calculations are compared with experimental data available for part of the transitions. For many of the “stiff” transitions good agreement is found, but the remaining differences suggest that the force field should be revised. It is argued on the basis of these results that high-resolution spectroscopy in jets should be an excellent tool for improving and developing biomolecular force fields.