Hydration profiles of aromatic amino acids: conformations and vibrations of l-phenylalanine–(H2O)n clusters

Abstract

IR-UV double resonance spectroscopy and ab initio calculations were employed to investigate the structures and vibrations of the aromatic amino acid, L-phenylalanine–(H₂O)_n clusters formed in a supersonic free jet. Our results indicate that up to three water molecules are preferentially bound to both the carbonyl oxygen and the carboxyl hydrogen of L-phenylalanine (L-Phe) in a bridged hydrogen-bonded conformation. As the number of water molecules is increased, the bridge becomes longer. Two isomers are found for L-Phe–(H₂O)₁, and both of them form a cyclic hydrogen-bond between the carboxyl group and the water molecule. In L-Phe–(H₂O)₂, only one isomer was identified, in which two water molecules form extended cyclic hydrogen bonds with the carboxyl group. In the calculated structure of L-Phe–(H₂O)₃ the bridge of water molecules becomes larger and exhibits an extended hydrogen-bond to the π-system. Finally, in isolated L-Phe, the D conformer was found to be the most stable conformer by the experiment and by the ab initio calculation.