Intermolecular Interactions between Aromatic Amino Acids Investigated by Ultraviolet Photodissociation Spectroscopy of Hydrogen-Bonded Clusters of Histidine and Tryptophan Enantiomers

Abstract

Intermolecular interactions between aromatic amino acids were investigated by ultraviolet photodissociation spectroscopy of hydrogen-bonded protonated clusters of histidine (His) and tryptophan (Trp) enantiomers in the gas phase. Product ion spectra and photodissociation spectra in the wavelength range of the S₁–S₀ transition of Trp at several temperatures (8–100 K) were obtained using a tandem mass spectrometer equipped with an electrospray ionization source and a cold ion trap. l-Trp detachment forming protonated His was the main pathway. Two bands observed at 288 and 285 nm in the photodissociation spectra of heterochiral H⁺(d-His)(l-Trp) indicated the coexistence of two types of conformers. The bands at 288 and 285 nm were attributed to the conformers formed from stronger and weaker intermolecular interactions, respectively. In the spectra of homochiral H⁺(l-His)(l-Trp), only the band due to the stronger interactions was observed at 288 nm. The intermolecular interactions of l-His with l-Trp were stronger than those of d-His with l-Trp.