The equilibrium geometry and protonation state of a model of the sulfinylimine type radical observed in the reaction between the modified substrate 2＇-azido-2＇-deoxyribonucleotide-5＇-diphosphate and a cysteine radical at the active site of ribonucleotide reductase (RNR) are investigated by means of hybrid and gradient corrected density functional theory. The results indicate that the radical is nonplanar and either neutral or anionic. The calculated nitrogen and P-proton hyperfine couplings are highly similar for both these systems and agree very closely with the experimentally observed main couplings of 25 G for N-14 and 8-9 or 6.5 G for H-1(beta) (depending on RNR source). The experimental data available at present are hence insufficient for a unique determination of the radical protonation state. It is proposed that C-13 labeling of the cysteine residue in question will provide a unique determination of the protonation state of the radical.