Charge state dependent fragmentation of gaseous protein ions in a quadrupole ion trap: bovine ferri-, ferro-, and apo-cytochrome c

Abstract

The dissociation behavior of bovine ferri-, ferro-, and apo-cytochrome c ions formed by electrospray ionization has been studied as a function of precursor ion charge state under quadrupole ion trap collisional activation conditions. Tandem mass spectrometry data collected under solution conditions in which the oxidation state of the heme-iron is known show that cytochrome c ions dissociate by oxidation state dependent channels. Dissociation of precursor ions in the oxidized form [Fe(III)heme] yields predominantly b- and y-type ion backbone cleavages with preferential cleavages observed C-terminal to basic residues including lysine, arginine and histidine, C-terminal to aspartic and glutamic acids as well as N-terminal to proline. At the highest charge states studied, an abundant Leu–Met cleavage is apparent. The relative contribution of each type of fragmentation was found to be a function of precursor ion charge state. Dissociation of cytochrome c ions in the reduced form [Fe(II)heme] show, in addition to backbone cleavages, loss of protonated heme, which dominates the product ion spectrum for the [M+7H] 7+ and [M+8H] 8+ precursor ion charge states. Apo-cytochrome c ions dissociate via similar fragmentation channels to those observed in the product ion spectra for the oxidized form of cytochrome c. The results presented in this study are expected to reflect predominantly first generation cleavage products from cytochrome c precursor ions in the temperature range of 500–650 K.