Thermal decomposition of a gaseous multiprotein complex studied by blackbody infrared radiative dissociation. Investigating the origin of the asymmetric dissociation behavior

The blackbody infrared radiative dissociation technique was used to study t he thermal decomposition of the gaseous B-5 pentamer of the Shiga-like toxi n I and its complexes with the P-k trisaccharide and a decavalent P-k-based oligosaccharide ligand (STARFISH, S). Dissociation of the protonated penta mer, (B-5 + nH)(n+) = B-5(n+) where n = 11-14, proceeds almost exclusively by the loss of a single subunit (B) with a disproportionately large fractio n (30-50%) of the parent ion charge. The degree of charge enrichment of the leaving subunit increases with increasing parent ion charge state. For n = 12-14, a distribution of product ion charge states is observed. The yields , of the complementary pairs of product ions are sensitive to the reaction temperature, with higher temperatures favoring greater charge enrichment of the leaving subunit for +13 and +14, and the opposite effect for +12. Thes e results indicate that some of the protons are, rapidly exchanged between subunits in the gas phase. Dissociation of B-5(14+). S proceeds exclusively by the loss of one subunit although the ligand increases the stability of the complex and lso reduces the degree of charge enrichment in the ejected monomer. For B-5(12+)(P-k)(1-3), the loss of neutral P-k competes with loss of a subunit at low temperatures. Linear Arrhenius plots were obtained fro m the temperature-dependent dissociation rate constants measured for the lo ss of B from B-5(n+) and B-5(14+).S. The magnitude of the Arrhenius paramet ers is highly dependent on the charge state of the pentamer: E-a = 35 kcal/ mol and A = 10(19) s(-1) (+14), 46 kcal/mol and 10(23) s(-1) (+13), 50 kcal /mol and 10(26) s(-1) (+12), and 80 kcal/mol and 10(39) (+11). The E-a and A for B-5(14+).S are 59 kcal/mol and 10(30) s(-1), respectively. The reacti on pathways leading to greater charge enrichment of the subunit lost from t he B-5(14+) and B-5(13+) ions correspond to higher energy processes, howeve r, these pathways are kinetically preferred at higher temperatures due to t heir large A factors. A simple electrostatic model, whereby charge enrichme nt leads to Coulombic repulsion-induced denaturation of the subunits and di sruption of the intersubunit interactions, provides an explanation for the magnitude of the Arrhenius parameters and the origin of the asymmetric diss ociation behavior of the complexes.