Electron capture dissociation of gaseous multiply-charged proteins is favored at disulfide bonds and other sites of high hydrogen atom affinity

Disulfide bonds in gaseous multiply-protonated proteins are preferentially cleaved in the mass spectrometer by low-energy electrons, in sharp contrast to excitation of the ions by photons or low-energy collisions. For S-S cyc lized proteins, capture of one electron can break both an S-S bond and a ba ckbone bond in the same ring, or even both disulfide bonds holding two pept ide chains together (e.g., insulin), enhancing the sequence information obt ainable by tandem mass spectrometry on proteins ill trace amounts. Electron capture at uncharged S-S is unlikely; cleavage appears to be due to the hi gh S-S affinity for H* atoms, consistent with a similar favorability found for tryptophan residues. RRKM calculations indicate that H* capture dissoci ation of backbone bonds in multiply-charged proteins represents nonergodic behavior, as proposed for the original direct mechanism of electron capture dissociation.