Salt bridge chemistry applied to gas-phase peptide sequencing: Selective fragmentation of sodiated gas-phase peptide ions adjacent to aspartic acid residues

Salt bridge chemistry has recently been realized as a determining factor in the structures and reaction dynamics of biological molecules in the gas phase. In this paper, we further investigate salt bridge chemistry in studies of the low-energy collision-induced dissociation (CID) of sodiated peptides. MALDI and electrospray ionization are used to generate singly and multiply charged sodiated peptides which are analyzed by using an external ion source Fourier transform ion cyclotron resonance mass spectrometer. Of particular interest is the observation that sodiated peptides exhibit highly selective cleavage at aspartic acid residues. Sodiated peptides that lack acidic residues, however, undergo sequential cleavages from the C-terminus on low-energy CID. We propose a mechanism for cleavage at aspartic acid residues that involves a salt bridge intermediate in which the sodium ion stabilizes the ion pair formed by proton transfer from aspartic acid to the adjacent amide nitrogen. This proposal is supported by ab initio calculations to quantify the reaction energetics. In several instances the less selective low-energy fragmentation processes of the protonated peptides have also been investigated for comparison.