Tandem MS analyses were subsequently employed for identification of the amino acid residue(s) that were covalently modified. Samples of CAII modified by the resynthesised and purified SFs 2d and 2e were digested using trypsin and analysed by LC-MS/MS. This identified peptides 59ILNNGH*AFNVEFDDSQDKAVLK80 and 59ILNNGH*AFNVEFDDSQDK76 as the major site(s) of modification on His64 for both 2d (366 Da) (see Fig. ), and 2e (380 Da) (Fig. ), respectively. Further to this, minor modifications were observed on the Nterminal peptide 1MSHHWGYGK9 at His3 (2d and 2e), and Tyr7 (2d only).