INTRA-IONIC INTERACTIONS IN ELECTROSPRAYED PEPTIDE IONS

Recent studies have demonstrated that interactions between cysteic aci d and arginine residues have a marked effect on the low energy decompo sitions of protonated peptides in the gas phase. This work extends the se findings to show that analogous interactions operate in the presenc e of other acidic amino acid residues, although the effects are less p ronounced. Thus, the presence of aspartic or glutamic acid residues (X ) in the sequence, RLXIFSXFR, attenuates the proton-sequestering prope rties of the arginine residues in the [M+2H](2+) ion, thereby promotin g charge-proximal fragmentation of the peptide backbone. Significant d ifferences in fragmentation behavior are observed for the doubly proto nated peptide incorporating two aspartic acid residues, in comparison with the glutamic acid-containing analogue. Low energy collisional act ivation of [M+2H](2+) ions of RLDIFSDFR, but not RLEIFSEFR, yields sig nificant d-type ions associated with cleavage at the acidic residues. The proposed mechanism invokes arginine/aspartic acid side-chain inter action and is blocked by the additional methylene group in the glutami c acid side-chain. b-Type fragmentation is promoted C-terminal to the aspartic acid residues; this is attributed to nucleophilic attack of t he side-chain carboxyl group on the carbonyl carbon of the adjacent pe ptide bond and is promoted by an interaction between the acidic side c hain and the guanidino group of the N-terminal arginine residue, This effect is not observed for the glutamic acid-containing analogue. Mole cular mechanics calculations indicate lowest energy conformations cons istent 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. (C ) 1997 Elsevier Science B.V.