Femtosecond photoionization of ion beam desorbed aliphatic and aromatic amino acids: Fragmentation via alpha-cleavage reactions

We report the photoionization of ion beam desorbed amino acids using femtos econd laser pulses at 195 and 260 nm. Ionization of ion-desorbed glycine, a lanine, valine, leucine, isoleucine, and phenylalanine using 195 nm laser p ulses is found to produce almost exclusively a decarboxylated ion fragment, while tyrosine and tryptophan produce a functional group cation. The adiab atic ionization potentials for the former amino acids correspond to the rem oval of an electron from the amine nitrogen atom, while for tyrosine and tr yptophan it corresponds to the removal of an electron from the aromatic fun ctional group. We find that fragmentation is initiated by the formation of a radical site which leads to an alpha-cleavage reaction and, depending upo n which electron is removed, results in the formation of a decarboxylated i on or a functional group ion. These results indicate that a significant fra ction of the amino acids are desorbed intact, but are fragmented when ioniz ed. Except for leucine, isoleucine, and phenylalanine, the mass spectra pro duced by 260 nm irradiation are similar to the mass spectra produced by 195 nm irradiation. When using these two wavelengths, the desorbed amino acids exhibit a 2- to 8-fold higher ion yield than is found using only secondary ions produced directly by the incident ion beam. Tyrosine exhibits up to a 40-fold increase in signal using 195 nm irradiation.