The reduction of p-chloronitrobenzene by a reduced flavin has been investigated kinetically in the presence of divalent metal ions in EtOH under anaerobic conditions. A large rate acceleration due to the metal ionswasobserved; l(none):32(Zn2+) :420(Ni2+) :I 000(Co2+):12(MnZ+).Theroleof themetal ions is proposed. The reducing ability of metal-chelated flavin radicals has been briefly examined by employing 2,4-dinitrochlorobenzene as an electron acceptor. Some flavoproteins are known which contain a metal ion as a prosthetic group that participates in electron transfer reactions.' Thus, investigations of the interactions between flavins and metal ions are of primary importance in understanding the functions of metalloflavoproteins. For instance, McCreery er af. have shown on the basis of electrochemical studies that the oxidation-reduction potential of riboflavin is significantly affected by complexation with a variety of metal ions.* The stability constants for complexes between transition metal ions and riboflavin and FMN have been determined by potentiometric t i t r a t i ~n . ~ These studies suggest that metal ions interact with the isoalloxazine ring at the G O ( 4 ) and N(5) positions. In fact, X-ray analysis of the bis-( 10-methylisoalloxazine)-copper(ii) complex has confirmed that Cu" complexes with the GO(4) and N(5) positions of the isoalloxazine nu~leus.~ Flavin radicals (flavosemiquinones) are known to be stabilized by metal co-ordination giving metal-chelated flavin radicals, which have been extensively investigated by means of u.v.-visible and e.s.r. ~pectroscopy.~To the best of our knowledge, however, there have been no investigations of the interaction of metal ions with a reduced ffuuin. Furthermore, kinetic studies of the effects of metal ions on flavin-mediated oxidation-reduction seem to be very 1 imi ted.?In this paper, we describe kinetic investigations on the reactions of (i) p-chloronitrobenzene with a reduced flavin in the presence of metal ions (Zn2+, Ni2+, C o 2 + , Mn2+, and Mg2+), and (ii) 2.4-dinitrochlorobenzene with metal-chelated flavin radicals in absolute ethanol under anaerobic conditions (Scheme 1).Aromatic nitro compounds were chosen as substrates t Note udded it1 proof: S. Shinkai et d. have reported kinetic studies by employing a metal co-ordinative flavin; Bull. CIiem.

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