Far/mid-infrared signatures of solvent-solute interactions in a microhydrated model peptide chain

Far/mid-IR signatures of the first hydration step of a flexible biomolecule, the model peptide chain Ac-Phe-NH₂, have been investigated in the gas phase using the selective IR/UV double-resonance laser technique. The broad spectral region investigated with the free-electron laser FELIX (150-800 cm^-1/70-12 μm) provided a direct access to three intermolecular vibrational modes of monohydrates, in which the water molecule bridges neighboring NH and CO sites of the peptide backbone. The spectral features, analyzed with the help of quantum chemistry, are assigned to the IR activity of the libration and wagging motions of the water molecule together with a strongly mode- and conformer-dependent vibrational coupling between solute and solvent molecules. These resolved spectra obtained in a so far poorly documented spectral region provide benchmark data, which should enable theoreticians of molecular interactions to assess their methods, in terms of both intermolecular potentials and treatment of the vibrational anharmonicity.