Phenylalanine analogs were subjected to hydrogen/deuterium exchange (HDX) in both solution and the gas phase, and gas-phase infrared multiple photon dissociation spectra were obtained for each of the species. For sodium cation-attached N-acetylphenylalanine, gas-phase HDX took place at only one site. Comparison of spectra from both undeuterated and singly deuterated sodiated N-acetylphenylalanine showed band shifts for normal modes that involved mainly vibrations of the O–H group, indicating that gas-phase exchange occurs at the COOH hydrogen and not at the NH hydrogen. Conversely, HDX in solution did result in exchange of the NH hydrogen, even for the protected species O-methyl N-acetylphenylalanine and N-acetylphenylalanine O-methylglycine. Rate coefficients for gas-phase H/D exchange were measured for the single deuteration of sodiated N-acetylphenylalanine and all three deuterations of protonated N-acetylphenylalanine, and found to be in the range (1.5–3.6) × 10
−11 cm
3/s.
Density functional theory calculations predicted that the phenylalanine analogs, although of different size, have relatively similar structural features. These calculations showed that Na+ interacts with the phenyl ring and all available carbonyl oxygens, thus essentially locking the structures into one basic conformation. This behavior is quite distinct from other amino acids which are more flexible, and where gas-phase exchange also occurs at the amine (NH) group.
