hypothesised to display similar chemistry as 4-fluoro-proline. ParcuCDPS can tolerate conservative derivatisations of proline however functional groups including hydroxyl and amines on the ring were not accepted. This may be due to positioning in the pocket causing steric clashes and forcing the amino acid into unfavourable conformations for cyclisation. Although ParcuCDPS also produces cHE (Supplementary Fig. ), it did not accept any glutamate analogues, demonstrating surprisingly narrow specificity.