D-Amino acid recognition of tripeptides studied by ultraviolet photodissociation spectroscopy of hydrogen-bonded clusters.

To understand the roles of D-amino acids, evaluating their chemical properties in living organisms is essential. Herein, D-amino acid recognition of peptides was investigated using a tandem mass spectrometer equipped with an electrospray ionization source and a cold ion trap. Ultraviolet (UV) photodissociation spectroscopy and water adsorption of hydrogen-bonded protonated clusters of tryptophan (Trp) enantiomers and tripeptides (SAA, ASA, and AAS, where S and A denote L-serine and L-alanine, respectively) were carried out at 8 K in the gas phase. In the UV photodissociation spectrum of H+(D-Trp)ASA, the bandwidth of the S1-S0 transition, which corresponds to the ππ* state of the Trp indole ring, was narrower than those of the other five clusters, H+(D-Trp)SAA, H+(D-Trp)AAS, H+(L-Trp)SAA, H+(L-Trp)ASA, and H+(L-Trp)AAS. In the UV photoexcitation of H+(D-Trp)ASA(H2O)n, which were formed via water adsorption on gas-phase H+(D-Trp)ASA, the evaporation of water molecules was the main photodissociation pathway. An NH2CHCOOH-eliminated ion and H+ASA were observed in the product ion spectrum. By contrast, water molecules adsorbed on the other five clusters remained on the product ions for NH2CHCOOH elimination and Trp detachment after the UV photoexcitation. The results indicated that the indole ring of Trp was located on the surface of H+(D-Trp)ASA, and the amino and carboxyl groups of Trp formed hydrogen bonds in H+(D-Trp)ASA. For the other five clusters, the indole rings of Trp were hydrogen bonded in the clusters, and the amino and carboxyl groups of Trp were present on the cluster surfaces.