Oxidation of KGAILC(far)GIALK 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. Insets show structures of (b) 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 sulfur atom, (○) water losses, (*) ammonia losses, (lightning bolts) species subjected to CID, and (□) fragment ions that have lost the HOSR group.