Hydration profiles of aromatic amino acids: Conformations and vibrations of l-phenylalanine-(H₂O)_n clusters

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.