Negative ion fragmentation of cysteic acid containing peptides: Cysteic acid as a fixed negative charge

We present here a study of the collision induced dissociation (CID) of deprotonated cysteic acid containing peptides produced by MALDI. The effect of cysteic acid (C _ox) position is interrogated by considering the positional isomers, C _oxLVINVLSQG, LVINVLSQGC _ox, and LVINVC _oxLSQG. Although considerable variation between the CID spectra is observed, the mechanistic picture that emerges involves charge retention at the deprotonated cysteic acid side chain. Fragmentation occurs in the proximity of the cysteic acid group by charge directed mechanisms as well as remote from this group to form ions, which may be rationalized by charge remote mechanisms. Additionally, the formation of the SO ₃ ^-• ion is observed in all cases. Fragmentation of C _oxLVINVLSQC _ox provides both N- and C-terminal, y and b ions, respectively indicating that the negative charge may be retained at either of the cysteic acids; however, there is some evidence that charge retention at the C-terminal cysteic acid may be preferred. Fragmentation of tryptic type peptides containing a C-terminal arginine or lysine residue is considered through comparison of three peptides C _oxLVINKLSQG, C _oxLVINVLSQK, and C _oxLVINVLSQR. Lastly, we rationalize the formation of b _n-1 + H ₂O and a _n-1 ions through a mechanism involving rearrangement of the C-terminal residue to form a mixed anhydride intermediate.