Inorganic Chemistry, Vol.34, No.5, 1040-1043, 1995
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 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 S-1 = S-2 = 5/2 and S-3 = 1/2 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, 6.66 < J(13)/J(23) < 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.
Keywords:IRON-SULFUR PROTEINS;GIGAS FERREDOXIN-II;METHANE MONOOXYGENASE;3-IRON CLUSTERS;ACTIVE-SITE;HYDROXYLASE;LIGANDS;EPR;OXO