Resonant two-photon ionization study of jet-cooled amino acid: L-phenylalanine and its monohydrated complex

Abstract

L-phenylalanine (Phe), one of the aromatic amino acids, and its hydrated clusters were generated in supersonic expansion and investigated by resonant two-photon ionization. Excitation spectra of Phe and Phe–(H2O)1 were obtained near their S0–S1 origins. We found that, by comparing the experimental results with the density functional theory and ab initio calculations, the water in Phe–(H2O)1 tends to form a cyclic hydrogen bond at the carboxyl group while inducing little change in the corresponding monomer structure. No sign of water making bridged hydrogen bonds with both polar groups was found. In order to form the cyclic hydrogen bond, hydration takes place only with the conformers whose carboxyl hydrogen is free, i.e., not occupied in the intramolecular hydrogen bonding with the amino nitrogen in the monomer.