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 ox LVINVLSQG, LVINVLSQGC ox , and LVINVC ox LSQG. 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 ox LVINVLSQC 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 ox LVINKLSQG, C ox LVINVLSQK, and C ox LVINVLSQR. Lastly, we rationalize the formation of b n-1 +H 2 O and a n-1 ions through a mechanism involving rearrangement of the C-terminal residue to form a mixed anhydride intermediate.

A study on the effect of cysteic acid position on the types of fragment ions formed by collisioninduced dissociation (CID) of [M -H] − ions is presented. Of particular note is the observation of d-type fragment ions for peptides that contain an N-terminal cysteic acid (fixed negative charge) and cleavable amino acid side chains possessing a β-γ carbon-carbon bond. For example, the CID mass spectrum of oxidized cys-kemptide (C ox LRRASLG) [M -H + O 3 ] − ions contains abundant series of d-type fragment ions, and similar results are observed for oxidized cysteinecontaining ribonuclease A proteolytic peptides. The d i fragment ions are assumed to arise by a charge-remote and/or charge-assisted fragmentation mechanism, which both occur at high collision energies and involve consecutive reactions (i.e., the formation of a i ions followed by the elimination of the side chain to form d i ions).

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