Electronic spectra, excited-state geometries and molecular electrostatic potentials of aromatic amino acids

Lowest singlet excitation and emission energies of the three aromatic amino acids (phenylalanine, tyrosine and tryptophan) that are known to absorb ultraviolet radiation strongly were obtained using gas phase ab initio calculations. The 3-21G basis set was used and the excited states were generated using configuration interaction involving singly excited configurations (CIS). The calculated excitation and emission energies reproduce the observed trends satisfactorily. The changes in geometry consequent to excitation take place in the molecules mainly in the corresponding six-membered rings. The rings of phenylalanine and tyrosine get somewhat expanded consequent to excitation. This is in qualitative agreement with experimental results on substituted benzenes obtained from high resolution spectroscopy. The changes in the geometry of tryptophan following excitation are somewhat different from those of phenylalanine and tyrosine. Ground and excited state molecular electrostatic potentials suggest that hydrogen bonding patterns of the three amino acids would not change appreciably following electronic excitation of the molecules which explains why structures and activities of proteins and enzymes are not seriously modified on ultraviolet irradiation. (C) 2000 Elsevier Science B.V. All rights reserved.