Figure 4.

Oxidation of A-factor (sequence YIIKGVFWDPAC(far)VIA) via ion/ion reaction with IO₄⁻. Activation of (a) [M + 2H + IO₄]⁺, (b) [M + H + O]⁺, and (c) dehydroalanine product formed upon HOSfar loss from the [M + H + O]⁺ species. The control spectrum (d) is the activation of the [M + H]⁺ species not subjected to ion/ion reactions. Insets show structures of (a) oxidized farnesylated cysteine and (c) the dehydroalanine residue produced upon loss of HOSfar. Symbols are as follows: (red squares) loss of HOSR to form the dehydroalanine product, (blue squares) cleavage on the other side of the oxidized sulfur atom, (yellow squares) loss of the farnesyl group in the nonoxidized species, (○) water losses, (*) ammonia losses, (lightning bolts) species subjected to CID, and (□) fragment ions that have lost the HOSR group.