Carbohydrates and amino acids: Infrared multiple photon dissociation spectroscopy and density functional theory calculations

In addition to acting as repair agents, stabilizing protein folding or early defense systems in cellular systems, saccharides are especially important in energy storage and enzymatic reactions of proteins. All these areas of research require knowledge of the saccharide structure. Spectroscopic studies of monosaccharides and amino acids were undertaken to better understand structural conformations in the gas phase. Infrared spectra were obtained by using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) in conjunction with infrared multiple photon dissociation (IRMPD).;Sodiated phenylalanine analogs were subjected to H/D exchange before their IRMPD spectra were taken. The gas phase H/D exchange experiments of N-acetylphenylalanine indicate that of the two possible locations for exchange to occur, the O-H hydrogen is kinetically favored over the N-H hydrogen. For two larger species, O-methyl N-acetylphenylalanine and N-acetylphenylalanine O-methylglycine, exchange occurred at the N-H site since it was the only one available for exchange, but the D for H substitution only took place in solution and not in the gas phase. Theoretical calculations showed that the phenylalanine analogs, although of different size, have relatively similar structural features. The sodium cation is predicted to interact with the phenyl ring and also bind to the carbonyl oxygens.;In a second project, IRMPD spectra of N-acetylglycosamines showed that the frequency of the CO stretch was indicative of the particular glycosamine conformation. A band shift of about 10 cm-1 was seen between the anomers, alpha-D-methylglucosamine and beta-D-methylglucosamine, while an 11 cm-1 shift was seen for the galactosamine anomers. Calculations indicate that the O-methyl group's position (alpha and beta, or axial and equatorial, respectively) and its close proximity to the N-acetyl group cause the orientation of the carbonyl to change in order to minimize steric hindrance, and therefore a band shift is observed for the CO stretch.;A third project involved the setup and use of an optical parametric oscillator (OPO) laser to obtain IRMPD spectra of rubidium cation-bound glycosides. DFT calculations and experimental spectra showed that the anomers of D-glucoside and D-galactoside all have differing hydrogen bonding and locations of rubidium binding, consequently showing distinct spectra in the O-H stretch region.