Structures and reactions of biomolecular ions produced with electrospray ionization

Photo-induced reaction of[Fe(III)-protoporphyrin]⁺(hemin⁺) ions solvated withdimethylsulfoxide (DMSO) is investigated by using a tandem massspectrometer with electrospray ionization. We measure thephotodissociation yields of mass-selectedhemin⁺(DMSO)_nclusters for n = 0-3. Themass spectra of the fragment ions show the -cleavage ofcarboxymethyl groups in addition to the evaporation of solventmolecules. Yield of the -cleavage reaction is found to dependstrongly on the excitation energy and the number of solventmolecules. We also examine photo-induced reactions ofmultiply-charged cytochrome c ions, (M + nH)^{n +} (n = 9-17). Photoionization isfound to be the dominant process for the lower charged states (n = 9-12) and its yielddecreases rapidly with increasing the charge. Thephotoionization is ascribed to the emission of electron bymultiphoton excitation of heme under the influence of Coulombattractive potential arising from the charges in the polypeptidechain. Model calculations of the Coulomb potential suggest thatthe structure of the polypeptide chain is completely elongated.