REACTIVITY OF FERRATE(VI) AND FERRATE(V) WITH AMINO-ACIDS

The oxidation of the essential amino acids by ferrate(VI) (Fe(VI)) and ferrate(V) (Fe(V)) has been studied by stopped-flow and pulse radiolysis techniques at pH 12.4 and 23-24-degrees-C. FeO43- was formed in these studies by reduction of Fe(VI) with radiation-generated reducing radicals. Both Fe(VI) and Fe(V) react preferentially with amino acids in which the alpha-amino group is protonated (RCH(NH3+)COO-). Rate constants (k5) for reactions of Fe(VI) with RCH(NH3+)COO- range from 10 to 10(3) M-1 s-1. The corresponding k6 values for Fe(V) are orders of magnitude higher: k6 = (0.01-5.0) x 10(7) M-1 s-1. Cysteine, a reducing species, reacts with Fe(VI) at a rate of k = 760 +/- 49 M-1 s-1 and with FeO43- at a nearly diffusion-controlled rate of k = (4.0 +/- 0.8) x 10(9) M-1 s-1; both rates were computed on the basis of [cysteine]tot. The ratio k6/k5 varies from 5 x 10(3) to 3 x 10(5); for cysteine this ratio is 5 x 10(6). The oxidation process of amino acids initiated by Fe(V) proceeds in presence of Fe(VI) by a chain reaction in which amino acid free radicals and Fe(V) are the chain carriers.