Valence photoionization and photofragmentation of aromatic amino acids

The valence photoelectron spectra of the aromatic amino acids phenylalanine, tyrosine, tryptophan and 3-methylindole in the gas phase have been investigated by soft X-ray radiation. The photoemission spectra of the outer valence regions are similar to previously reported He I spectra, although relative peak intensities are different due to the different photon energy. The spectral range has been extended to the inner valence region and new information has been obtained. The photofragmentation mass spectra of the aromatic amino acids were measured after ionization by noble gas resonance radiation at energies from 8.4 to 21.2 eV. The main peak in the mass spectra of phenylalanine corresponds to loss of the aromatic functional group, while for tyrosine and tryptophan it is due to the loss of the carboxylic and amino groups. Lower photon energies lead to 'softer' ionization, with reduced fragmentation and increased parent ion signal.