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 fact or in the structures and reaction dynamics of biological molecules in the gas phase. In this paper, we further investigate salt bridge chemi stry in studies of the low-energy collision-induced dissociation (CID) of sodiated peptides. MALDI and electrospray ionization are used to g enerate singly and multiply charged sodiated peptides which are analyz ed by using an external ion source Fourier transform ion cyclotron res onance mass spectrometer. Of particular interest is the observation th at sodiated peptides exhibit highly selective cleavage at aspartic aci d residues. Sodiated peptides that lack acidic residues, however, unde rgo sequential cleavages from the C-terminus on low-energy CID. We pro pose 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 amid e nitrogen. This proposal is supported by ab initio calculations to qu antify the reaction energetics. In several instances the less selectiv e low-energy fragmentation processes of the protonated peptides have a lso been investigated for comparison.