Exchange Coupling Approach to the Radical Disposition of an Intermediate in Escherichia coli Ribonucleotide Reductase

Abstract

Recent rapid freeze-quench EPR and Mossbauer studies have given evidence 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 ribonucleotide reductase (Ravi, N.; Bollinger, J. M.; Huynh, B. H.; Edmondson, D. E.; Stubbe, J. J. Am. Chem. SOC. 1994, 116, 8007). In the present study the radical disposition of this intermediate is investigated from the exchange-coupling perspective. A spin trimer with SI = S2 = 5/2 and S3 = l12 is analyzed in the framework of the Heisenberg-Dirac-Van Vleck formalism considering the most general case in which all the exchange-coupling constants are assumed to be unequal. The acceptable range, 0.66 < J13lJ23 < 1, for the ratio of the exchange-coupling constants between the radical and the two iron sites strongly suggests that the radical ligand asymmetrically bridges the two iron atoms.