Chemical properties of inner and surficial regions of hydrogen-bonded clusters of biological molecules: ultraviolet photodissociation and water adsorption analyses in the gas phase.

Abstract

The chemical and physical properties of cold, gas-phase hydrogen-bonded clusters of L-alanine (L-Ala), L-trialanine (L-Ala3), L-tetraalanine (L-Ala4), and tryptophan (Trp) enantiomers were investigated using tandem mass spectrometry with an electrospray ionization source and cold ion trap. From the ultraviolet (UV) photodissociation spectra at 265-290nm, the electronic structures of homochiral H+(L-Trp)(L-Ala) at 8K were found to be different from those of heterochiral H+(D-Trp)(L-Ala) and protonated Trp. The number of water molecules adsorbed on the surface of gas-phase H+(D-Trp)(L-Ala) was larger than that of H+(L-Trp)(L-Ala), indicating stronger intermolecular interactions of L-Ala with H+(L-Trp) than those with H+(D-Trp). The product ion spectrum obtained by 265nm photoexcitation of H+(L-Trp)(L-Ala3)(H2O)n formed via gas-phase water adsorption on H+(L-Trp)(L-Ala3) showed that the evaporation of water molecules was the main photodissociation process. In the case of H+(L-Trp)(L-Ala4)(H2O)n, signals of H+(L-Ala4) (H2O)n formed via L-Trp evaporation were observed in the product ion spectra, and the cross-section for UV photoinduced L-Trp evaporation became larger as the number of adsorbed water molecules increased. This observation indicates that water molecules were selectively adsorbed on the H+(L-Ala4) side of H+(L-Trp)(L-Ala4) and weakened the intermolecular interactions between L-Trp and H+(L-Ala4) in the hydrogen-bonded cluster.