Ribonucleotide reductase (RNR) from Escherichia coli catalyzes the conversion of nucleotides to deoxynucleotides and is composed of two homodimeric subunits : R1 and R2. 2’-Azido-2’-deoxyuridine S-diphosphate (N3UDP) has previously been shown to be a stoichiometric mechanism based inhibitor of this enzyme. Inactivation of RNR is accompanied by loss of the tyrosyl radical on the R2 subunit concomitant with formation of a new nitrogen centered radical. The X-band EPR spectrum of this radical species exhibits a triplet hyperfine interaction of similar to 25 G arising from one of the three nitrogens of the azide moiety of N3UDP and a doublet hyperfine interaction of 6.3 G which has been proposed to arise from a proton. High frequency (139.5 GHz) EPR spectroscopic studies of this nitrogen centered radical have resolved the peaks corresponding to all three principal g-values : g(11) = 2.01557, g(22) = 2.00625, and g(33) = 2.00209. In addition, the nitrogen hyperfine splitting along g(33) is resolved (A(33)(N) = 31.0 G) and upper limits (similar to 5 G) can be placed on both A(11)(N) and A(22)(N). : R1 and R2. 2’-Azido-2’-deoxyuridine S-diphosphate (N3UDP) has previously been shown to be a stoichiometric mechanism based inhibitor of this enzyme. Inactivation of RNR is accompanied by loss of the tyrosyl radical on the R2 subunit concomitant with formation of a new nitrogen centered radical. The X-band EPR spectrum of this radical species exhibits a triplet hyperfine interaction of similar to 25 G arising from one of the three nitrogens of the azide moiety of N3UDP and a doublet hyperfine interaction of 6.3 G which has been proposed to arise from a proton. High frequency (139.5 GHz) EPR spectroscopic studies of this nitrogen centered radical have resolved the peaks corresponding to all three principal g-values : g(11) = 2.01557, g(22) = 2.00625, and g(33) = 2.00209. In addition, the nitrogen hyperfine splitting along g(33) is resolved (A(33)(N) = 31.0 G) and upper limits (similar to 5 G) can be placed on both A(11)(N) and A(22)(N).