ION-ION PROTON-TRANSFER REACTIONS OF BIO-IONS INVOLVING NONCOVALENT INTERACTIONS - HOLOMYOGLOBIN

Multiply protonated horse skeletal muscle holomyoglobin and apomyoglob in have been subjected to ion-ion proton transfer reactions with anion s derived from perfluoro-1,3-dimethylcyclohexane in a quadrupole ion t rap operated with helium as a bath gas at 1 mtorr. Neither the apomyog lobin nor holomyoglobin ions show any sign of fragmentation associated with charge state reduction to the 1 + charge state. This is particul arly noteworthy for the holomyoglobin ions, which retain the noncovale ntly bound heme group. For example, no sign of heme loss is associated with charge state reduction from the 9+ charge state of holomyoglobin to the 1+ charge state despite the eight consecutive highly exothermi c proton transfer reactions required to bring about this charge change . This result is consistent with calculations that show the combinatio n of long ion lifetime and the high ion-helium collision rate relative to the ion-ion collision rate makes fragmentation unlikely for high m ass ions in the ion trap environment even for noncovalently bound comp lexes of moderate binding strength. The ion-ion proton transfer rates for holo- and apomyoglobin ions of the same charge state also were obs erved to be indistinguishable, which supports the expectation that ion -ion proton transfer rates are insensitive to ion structure and are de termined primarily by the attractive Coulomb field. (C) 1997 American Society for Mass Spectrometry.