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 23-24C. FeO{sub 4}{sup 3{minus}} 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(NH{sub 3}{sup +})COO{sup {minus}}). Rate constants (k{sub 5}) for reaction of Fe(VI) with RCH(NH{sub 3}{sup +})COO{sup {minus}} range from 10 to 10{sup 3} M{sup {minus}1} s{sup {minus}1}. The corresponding k{sub 6} values for Fe(V) are orders of magnitude higher: k{sub 6} = (0.01-5.0) {times} 10{sup 7} M{sup {minus}1} s{sup {minus}1}. Cysteine, a reducing species, reacts with Fe(VI) at a rate of d = 760 {plus minus} 49 M{sup {minus}1} s{sup {minus}1} and with FeO{sub 4}{sup 3{minus}} at a nearly diffusion-controlled rate of k = (4.0 {plus minus} 0.8) {times} 10{sup 9} M{sup {minus}1} s{sup {minus}1}; both rates were computed on the basis of (cysteine){sub tot}. The ratio k{sub 6}/k{sub 5} varies from 5 {times} 10{sup 3} to 3 {times} 10{sup 5}; for cysteine this ratio is 5 {times} 10{sup 6}. The oxidation process of amino acids initiated by Fe(V) proceeds inmore » presence of Fe(VI) by a chain reaction in which amino acid free radicals and Fe(V) are the chain carriers.« less