Alloxan-dialuric acid cycling: a complex redox mechanism.

Time-resolved kinetic studies involving the reactions of alloxan (A.H(2)O) with the reducing species superoxide and carbon dioxide radical anions and the reaction of dialuric acid (HA(-)) with sulphate radicals showed that the same radical (AH(.)) was formed either by the one-electron reduction of alloxan or by the one-electron oxidation of dialuric acid. A mechanism including several reversible reactions was proposed and validated. A detailed kinetic analysis yields the following bimolecular rate constants: k(A.H(2)O + [image omitted] ) < 10(5) m ( -1 ) s(-1), k(A.H(2)O + O(2) (-))=(3.4+/-0.5)x10(6) m ( -1 ) s(-1), k(HA(-)+[image omitted] )=(8+/-1)x10(7) m ( -1 ) s(-1) and k(AH(.)+AH(.))=(1.7+/-0.8)x10(8) m ( -1 ) s(-1). From these values, the redox potentials E degrees (A.H(2)O,H(+)/AH(.))=(-290+/-20) mV, E degrees (AH(.)/HA(-))=(277+/-20) mV and E degrees (A.H(2)O,H(+)/HA(-))=(-15+/-20) mV, were obtained.