Thermal dissociation of the protein homodimer ecotin in the gas phase.

The influence of charge on the thermal dissociation of gaseous, protonated, homodimeric, protein ecotin ions produced by nanoflow electrospray ionization (nanoES) was investigated using the blackbody infrared radiative dissociation technique. Dissociation of the protonated dimer, (E2 + nH)(n+) congruent to E2(n+) where n = 14-17, into pairs of monomer ions is the dominant reaction at temperatures from 126 to 175 degrees C. The monomer pair corresponding to the most symmetric charge distribution is preferred, although 50-60% of the monomer product ions correspond to an asymmetric partitioning of charge. The relative abundance of the different monomer ion pairs produced from E2(14+), E2(15+), and E2(16+) depends on reaction time, with the more symmetric charge distribution pair dominating at longer times. The relative yield of monomer ions observed late in the reaction is independent of temperature indicating that proton transfer between the monomers does not occur during dissociation and that the different monomer ion pairs are formed from dimer ions which differ in the distribution of charge between the monomers. For E2(17+), the yield of monomer ions is independent of reaction time but does exhibit slight temperature dependence, with higher temperatures favoring the monomers corresponding to most symmetric charge distribution. The charge distribution in the E2(15+) and E2(16+) dimer ions influences the dissociation kinetics, with the more asymmetric distribution resulting in greater reactivity. In contrast, the charge distribution has no measurable effect on the dissociation kinetics and energetics of the E2(17+) dimer.