Intra-ionic interactions in electrosprayed peptide ions

Abstract

Recent studies have demonstrated that interactions between cysteic acid and arginine residues have a marked effect on the low energy decompositions of protonated peptides in the gas phase. This work extends these findings to show that analogous interactions operate in the presence of other acidic amino acid residues, although the effects are less pronounced. Thus, the presence of aspartic or glutamic acid residues (X) in the sequence, RLXIFSXFR, attenuates the proton-sequestering properties of the arginine residues in the [M+2H] 2+ ion, thereby promoting charge-proximal fragmentation of the peptide backbone. Significant differences in fragmentation behavior are observed for the doubly protonated peptide incorporating two aspartic acid residues, in comparison with the glutamic acid-containing analogue. Low energy collisional activation of [M+2H] 2+ ions of RLDIFSDFR, but not RLEIFSEFR, yields significant d-type ions associated with cleavage at the acidic residues. The proposed mechanism invokes arginine/aspartic acid side-chain interaction and is blocked by the additional methylene group in the glutamic acid side-chain. b-Type fragmentation is promoted C-terminal to the aspartic acid residues; this is attributed to nucleophilic attack of the side-chain carboxyl group on the carbonyl carbon of the adjacent peptide bond and is promoted by an interaction between the acidic side chain and the guanidino group of the N-terminal arginine residue. This effect is not observed for the glutamic acid-containing analogue. Molecular mechanics calculations indicate lowest energy conformations consistent with the mass spectrometric data; specifically, the additional methylene group in the glutamic acid side-chain markedly increases the distance between the carboxyl group and the adjacent peptide bond.