Neutral, singly- and doubly protonated tetraglycines have been studied by semiempirical (MNDO) and ab initio (3-21G) methods to model the mass spectral behaviour of singly- and doubly protonated oligopeptides. The favourable bond cleavages in the ions were investigated by using bond orders and energy partitioning values. The most favourable protonation site in singly charged tetraglycine is the N-terminal amine nitrogen. In the doubly protonated forms, protonation at the N- and C-terminal residues is the most favourable energetically, minimizing Coulombic repulsion. Protonation at the amide nitrogen results in a significant weakening of the corresponding amide bond, leading to b+ ion formation from the singly protonated forms, and y+ and b+ or alternatively b2+ ion (ion nomenclature according to P. Roepstorff and J. Fohlman, Biomed. Environ. Mass Spectrom., 11, 601 (1984)) formation from the doubly protonated forms. The critical energy of fragmentation is much lower in the doubly protonated species compared to the singly protonated ones showing the effect of charge repulsion. The relative positions of the two protonated sites also influences the bond cleavages. Most surprisingly, the extended beta-sheet structure, in which the distance between charges is maximized, is not stable, and collapses into a folded conformation, where internal solvation competes successfully with charge repulsion. This suggests that multiply protonated peptides may, generally, have folded and not extended ("linear') structures in the gas phase.
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Vékey et al. (1996) studied this question.