Electron Capture Dissociation of Multiply-Charged Oxygenated Cations. A Nonergodic Process

Several mechanistic aspects have been proposed as important in causing the unusual ion chemistry induced in multiply-charged protein cations by electron capture. The 5–7 eV energy released by neutralization appears to induce cleavage before energy randomization (nonergodic), and the electron forms radical species whose activation energies for dissociation should be much lower. In contrast, electron capture by [HO(C₂H₄O)₂₄H + 2H]²⁺ ions from polyethylene glycol yields no radical ions, losing H^• consistent with the lower H^• affinity of the hydroxyl and ether groups vs the amide and S–S functionalities of proteins. However, the dominant product ions, [HO(C₂H₄O)_24–nH + H]⁺ (n = 2 to 8), do appear to be formed by nonergodic dissociation of the hypervalent (M + 2H)^1+• intermediate. The expected complementary alkoxy radical ion product is not found, possibly due to an energetic Franck–Condon relaxation. Precursors ionized with (NH₄)₂²⁺ and Na₂²⁺ yield ECD products that are analogous but of different size (n values). Those for Na₂²⁺ can be rationalized with structures proposed by Bowers and coworkers. ECD spectra of polyethers should be useful for sequencing.