Photo-Induced Reactions of Biomolecular Ions in Gas Phase

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1.Electronic spectra of ammoniated ammonium radicals, NH_4(NH_3)_n, are investigated with a laser photodepletion spectroscopy. A drastic decrease of the excitation energy is observed, which is ascribed to the spontaneous ionization of NH_4 in ammonia clusters.2.In order to understand structure of biomolecular ions in the gas phase, mechanisms of microscopic hydrations, and role of hydrations for biological reactions, we develop a tandem mass spectrometer with electrospray ionization, and study the photo-induced reactions of biomolecular cluster ions.(1)Photo-induced reactions of multiply-charged cytochrome c ions are investigated. Photoionization is found to be the dominant process for the lower charged states and its yield decrease rapidly with increasing the charge. Model calculations of the Coulomb potential suggest that the structure of the polypeptide chain is completely elongated.(2)We develop the method to produce hydrated biomolecular cluster ions by improving electrospray ionization.(3)We examine the metastable decomposition of hydrated cluster ions containing amino acids and peptides to determine the incremental solvent binding energies and internal energies for the clusters using evaporative ensemble model. From these results, we find out that delocalization of the charge correlate with the binding energy.(4)Photo-induced reactions of [Fe(III)-protoporphyrin]^+ (hemin^+) ions solvated with water, pyridine, and methanol are investigated. The mass spectra of the fragment ions show the β-cleavage of carboxymethyl groups in addition to the evaporation of solvent molecules. Yield of the β-cleavage reaction is found to depend strongly on the excitation energy and the number of solvent molecules.(5)Photodissociations of solvated cluster ions containing tryptophan and glyciltryptophan are examined. Cleavages of -NH_3 and -COOH groups are observed, which are correlated with energy relaxation in the cluster ions.