Gas-phase infrared spectroscopy of the protonated dipeptides H(+)PheAla and H(+)AlaPhe compared to condensed-phase results

The influence of the physical environment on the structures of biomolecules is considered here for the dipeptide model H(+)AlaPhe cation, by making use of infrared multiple-photon dissociation (IRMPD) spectroscopy complemented by DFT calculations. The gas-phase structures of this peptide are also compared to the related peptide cations H(+)PheAla and H(+)AlaAla. The gas-phase IRMPD spectra of the Phe-containing cations are compared to previous studies, including the X-ray-crystallographic crystal structure for the H(+)AlaPheCl(-)center dot 2H(2)O salt,a recent IRMPD spectrum of H(+)AlaAla,and a recent determination of the IR absorption spectrum of the H(+)AlaPheCl(-) salt in a liquid-crystal host matrix, as well as recent cryogenic ion trap results for H(+)TyrAla and H(+)AlaTyr. Between the gas-phase H(+)AlaPhe ion and the H(+)AlaPheCl(-)center dot 2H(2)O crystal a conformational switch is observed, induced by hydrogen bonding with a water of crystallization, involving a 180 degrees rotation of the COOH group. The hoped-for comparison of the gas-phase IR spectra with the liquid-crystal matrix IR spectrum was frustrated, because the literature matrix spectrum seems most likely to be that of a protonated homodimer of the dipeptide rather than the protonated monomer. The IRMPD spectra of H(+)AlaPhe and H(+)PheAla are very similar, with only minor peak shifts suggesting small differences in local interactions within a similar overall architecture. The H(+)AlaAla spectrum was also similar, and no significant reorganization of the structure seems to result from the presence or position of the aromatic ring. The spectra give highly satisfactory matches to the predicted IR spectra computed for the most stable conformers of the protonated dipeptides. It is suggested that the NH3+ proton is shared through hydrogen bonding to the amide C=O, giving a distinctive broadening of the associated H-bending mode. (C) 2009 Elsevier B.V. All rights reserved.