Marked metal ion effects in electron transfer from reduced flavin to aromatic nitro compounds in ethanol

Abstract

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.

Catalysis by magnesium ion of the reduction of oxygen by the 1,5-dihydroflavin anion via electron transfer

“…Flavins are known to form complexes with Mg2+ in MeCN.9 The reduced flavin anion FIH-has also been reported to form a complex with Mg2+.10 Semiempirical MO calculations of FIH--Mg2+ (the ribityl acetate chain was replaced by Me) using the PM3 method," suggest that Mg2+ coordinates to C(4)=0 and N(5) as proposed previously.10 The adiabatic ionization potentials (Ei,, of FIH-and FIH--Mg2+ can be calculated as the difference between the enthalpy of formation (AHf) in the reduced form with the optimized structure and the half-oxidized (radical) form using the PM3 method with the RHF formalism. In contrast with FIH--Mg2+, the optimized structure of FIH'-Mg2+ indicates that Mg2+ coordinates to C(2)=0 and N (10) as the case of fully oxidized form, FI-Mg2+.9 The calculated Ei,, of FIH-(2.9 eV) is much smaller than that of FlH--Mg2+ (10.5 eV). Thus, the Mg2+ complex of FlH-may be a much weaker one-electron donor than free HH-.…”

Some Reactions and Properties of Nitro Radical-Anions Important in Biology and Medicine

“…The reduction of ArNO2 by flavins or thiols are catalyzed by trace quantities of Fe(II) (63,(73)(74)(75)(76). Oneelectron reduction by free thiols is thermodynamically much less favorable than by ascorbate, since E(RS/ RS-) must be much more positive than E(A/AH-) at pH-7, indeed higher than E(PZ+)/PZ) 0.8-0.9 V where PZ = common phenothiazines (77).…”