Evaluation of the ONIOM method for interpretation of infrared spectra of gas-phase molecules of biological interest.

The prediction accuracy of the ONIOM method for the interpretation of infrared spectra of gas-phase molecules of biological interest has been investigated. With the use of experimental results concerning amino acids, small peptides, and sugars taken from the literature, mode-specific local scaling factors have been determined for different high-layer/low-layer couples. A significant improvement is noticed when using local scaling factors with respect to global factors. The B3LYP/6-31G*:AM1 level turns out to offer the best trade-off between computational expense and accuracy. In the case of the RGD peptide, the B3LYP/6-31G*:AM1 and the B3LYP/3-21G levels require similar computational expense, but the former yields structures and predicted spectra comparable to those obtained from pure B3LYP/6-31G* calculations with a factor of 2 in time-saving gain. The experimental infrared spectrum of doubly charged gas-phase vancomycin ions has been recorded in the 1000-2000 cm(-1) range and compared to predicted spectra of three different conformers at the B3LYP/6-31G*:AM1 level. This demonstrates the possible interpretation of IR spectra of relatively large systems (178 atoms) with the use of rather modest computational means.