IR spectroscopy of cationized aliphatic amino acids: stability of charge-solvated structure increases with metal cation size

Abstract Gas-phase structures of alkali metal cationized (Li+, Na+, K+, Rb+, Cs+) proline (Pro) and N-methyl alanine have been investigated using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser and computational modeling. Measured IRMPD spectra are compared to spectra calculated at the B3LYP/6-311++G(2d,2p) level of theory to identify individual conformers. Calculations indicate that the stability of the salt bridge (SB; zwitterionic) conformer relative to the most stable canonical structure with a single formal charge site (charge solvation; CS) of aliphatic amino acids (e.g., Pro, N-methyl alanine, N-methyl glycine, and glycine) does not increase with size and polarizability of the alkali metal cations, in contrast to the trend commonly found for functionalized amino acids. In fact, the relative stability of SB over CS conformers reaches a maximum at [amino acid + Na]+. A uniform SB structure and two characteristic CS conformers are identified by theory to be relevant for alkali metalized Pro, N-methyl alanine, and N-methyl glycine. For CS structures, the alkali metal cation is either coordinated to the nitrogen and the carbonyl oxygen of the acid functionality (Li+, Na+) or is solely interacting with the carboxylic acid oxygens (K+, Rb+, Cs+). The IRMPD spectra exhibit clearly distinguishable bands for the CO stretching modes of the carboxylic acid moiety in CS structures and for the carboxylate moiety in SB structures, allowing reliable structure assignments for all complexes investigated. The IRMPD spectra clearly exhibit the presence of mixed populations of SB and CS structures with the contribution of CS increasing toward the larger metal cations, in good agreement with the predictions from computational modeling. The special trend regarding formation and stability of individual gas-phase ion structures of aliphatic amino acids, lacking functionalized α-side chains, can be rationalized with the concept of hard and soft Lewis acids and bases. Furthermore, calculations show that the trends with metal cation size found for aliphatic amino acids with secondary amines are similar for ordinary aliphatic amino acids (Gly, Ala).