A synthetic cysteine oxidase eased on a ferrocene-cyclodextrin conjugate

We report a novel synthetic cysteine oxidase consisting of a ferrocene-beta -cyclodextrin conjugate in which the ferrocene moiety is bound to the secon dary hydroxyl side of the cyclodextrin cavity through an ethylenediamine li nker. Cysteine oxidation occurs after the ferrocene group is electrochemica lly oxidized to the ferricinium form, and this generates a voltammetric ele ctrocatalytic wave, the magnitude of which is related to the rate constant for cysteine oxidation. Comparison of cysteine oxidation rates for the prim ary and secondary beta-cyclodextrin derivatives (105 and 1470 M-1 s(-1), re spectively) shows that the secondary derivatives are more effective synthet ic enzymes. Substrate selectivity of the secondary derivative is demonstrat ed by comparison of oxidation rates for cysteine (1470 M-1 s(-1)) and gluta thione (260 M-1 s(-1)) at pH 7.0. The rate constant for cysteine oxidation was 8-fold higher at pH 8.0. With a constant synthetic enzyme concentration , electrocatalytic limiting currents increased linearly with increasing cys teine concentration to a maximum at 6 mM cysteine; above this concentration , the current decreased significantly. These and other results suggest that product inhibition of the catalytic cycle occurs as a result of cystine bi nding more strongly to the cyclodextrin than cysteine.