Ribonucleotide reductase hyperfine coupling

A specific example is the combined multifrequency EPR and 35 GHz ENDOR study of the dLiron center in the R2 subunit of Escherichia coli ribonucleotide reductase (RNR) Samples in natural isotopic abundance (0.038% 0) and using H2 0 (34.9% 0) or 02 (85.5% 0) were prepared. ENDOR provided 0 (/ = 5/2) hyperfine coupling constants and these values were used to quantify the line broadening seen in 0-enriched versus natural abundance samples, which showed that the intermediate X contained two oxygen atoms, initially derived from dioxygen, as opposed to an alternate model with only one such oxygen atom. 

Fig. 11. Numbering scheme and ENDOR-derived spin-density distributions (shown in parentheses) in the tyrosyl radical of ribonucleotide reductase. The ring carbon values are symmetry-related. Hyperfine couplings from the ring 3,5 and the /3-carbon protons are responsible for the structure of the radical EPR signal. (Adapted from Babcock and coworkers.

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