The vertical ionization energy of (L)-phenylalanine was found to depend uniquely on the type of intramolecular hydrogen bonding in the neutral conformers. Between the two known subgroups of conformers, the ionization energy of those with a large backbone-residue interaction through a pi-hydrogen bond was considerably higher than the others because of the repulsion between the backbone and the residue in the cation. It was strongly noted from this study and others in the past that the neutral conformation uniquely determines such disparate properties as the cationic structure, the magnitude of the ionization energy, and the propensity toward hydration. (C) 2002 Elsevier Science B.V. All rights reserved.