DISSOCIATION OF HEME-GLOBIN COMPLEXES BY BLACKBODY INFRARED RADIATIVEDISSOCIATION - MOLECULAR SPECIFICITY IN THE GAS-PHASE

The temperature dependence of the unimolecular kinetics for dissociati on of the heme group from holo-myoglobin (Mb) and holo-hemoglobin alph a-chain (Hb-alpha) was investigated with blackbody infrared radiative dissociation (BIRD). The rate constant for dissociation of the 9 + cha rge state of Mb formed by electrospray ionization from a ''pseudo-nati ve'' solution is 60% lower than that of Hb-alpha at each of the temper atures investigated. In solutions of pH 5.5-8.0, the thermal dissociat ion rate for Mb is also lower than that of HB-alpha (Hargrove, M. S. e t al. J. Biol. Chem. 1994, 269, 4207-4214). Thus, Mb is thermally more stable with respect to heme loss than Hb-alpha both in the gas phase and in solution. The Arrhenius activation parameters for both dissocia tion processes are indistinguishable within the current experimental e rror (activation energy 0.9 eV and pre-exponential factor of 10(8-10) s(-1)). The 9 + to 12 + charge states of Mb have similar Arrhenius par ameters when these ions are formed from pseudo- native solutions. In c ontrast, the activation energies and pre-exponential factors decrease from 0.8 to 0.3 eV and 10(7) to 10(2) s(-1), respectively, for the 9to 12+ charge states formed from acidified solutions in which at least 50% of the secondary structure is lost. These results demonstrate tha t gas-phase Mb ions retain clear memory of the composition of the solu tion from which they are formed and that these differences can be prob ed by BIRD. (C) 1997 American Society for Mass Spectrometry.