Dissociation reactions of gaseous ferro-, ferri-, and apo-cytochrome c ions

Electrochemical reduction of the iron bound in the heme group of cytochrome c is shown to occur in the nano-electrospray capillary if the protein is s prayed from neutral water using a steel wire as the electrical contact. Qua drupole ion trap collisional activation is used to study the dissociation r eactions of cytochrome c as a function of the oxidation state of the iron. Oxidized (Fe(III)) cytochrome c dissociates via sequence-specific amide bon d cleavage, while the reduced (Fe(II)) form of the protein dissociates almo st exclusively by loss of protonated heme. Apo-cytochrome c, from which the heme has been removed either via gas-phase dissociation of the reduced hol e-protein or via solution chemistry, dissociates via amide bond cleavage in similar fashion to the oxidized hole-protein. (C) 2001 American Society fo r Mass Spectrometry.