Anharmonic vibrational spectroscopy of the glycine-water complex: Calculations for ab initio, empirical, and hybrid quantum mechanics/molecular mechanics potentials

Effects of intermolecular hydrogen bonding between glycine and one water mo lecule on the vibrational spectrum are investigated, using ab initio (at th e level of second order Moller-Plesset perturbation theory), empirical (OPL S-AA), and mixed ab initio/empirical quantum mechanics/molecular mechanics (QM/MM) potentials. Vibrational spectroscopy is calculated using the correl ation corrected vibrational self-consistent field method that accounts for anharmonicities and couplings between different vibrational normal modes. T he intermolecular hydrogen bonding interactions are found to be very strong and to affect vibrational frequencies and infrared intensities of both the glycine and the water molecule to a very large extent. The predicted ab in itio anharmonic spectra can be used to identify amino acids in complexes wi th water in experimental studies. The OPLS-AA potential is found to describ e hydrogen bonding between glycine and water incorrectly, and to predict er roneous vibrational spectra. Hybrid (QM/MM) techniques can, however, be use d to calculate more reliable vibrational spectra, in agreement with full ab initio treatment of the whole system, provided that the regions that conta in hydrogen bonds are described by ab initio potentials. (C) 2001 American Institute of Physics.