EXCHANGE COUPLING APPROACH TO THE RADICAL DISPOSITION OF AN INTERMEDIATE IN ESCHERICHIA-COLI RIBONUCLEOTIDE REDUCTASE

Recent rapid freeze-quench EPR and Mossbauer studies have given eviden ce for the presence of an intermediate species, a spin-coupled trimer consisting of two high-spin ferric ions and a radical, in the cofactor assembly reaction of the R2 subunit in Escherichia coli ribonucleotid e reductase (Ravi,N.; Bollinger,J.M.; Huynh,B.H.; Edmondson,D.E.; Stub be,J.J. Am. Chem. Soc. 1994, 116, 8007). In the present study the radi cal disposition of this intermediate is investigated from the exchange -coupling perspective. A spin trimer with S-1 = S-2 = 5/2 and S-3 = 1/ 2 is analyzed in the framework of the Heisenberg-Dirac-Van Vleck forma lism considering the most general case in which all the exchange-coupl ing constants are assumed to be unequal. The acceptable range, 6.66 < J(13)/J(23) < 1, for the ratio of the exchange-coupling constants betw een the radical and the two iron sites strongly suggests that the radi cal ligand asymmetrically bridges the two iron atoms.