The R2 subunit of ribonucleotide reductase (RNR) contains a stable tyrosine radical coupled to an adjacent diferric center. The spin-lattice relaxation rate of the tyrosine radical is greatly enhanced above 20 K due to the paramagnetic excited states of the diferric center. By using saturation-recovery electron paramagnetic resonance (EPR) spectroscopy, we have examined the spin-lattice relaxation dynamics of the tyrosine radical in R2 proteins from mouse, herpes simplex virus type 1, Escherichia coli, and Salmonella thyphimurium within the temperature range of 4-70 K. These measurements yield the diferric exchange coupling as well as the radical-metal coupling, which contains both exchange and dipolar components. In all four species, the ground state of the diferric center is diamagnetic, indicating that the two Fe(III)s are antiferromagnetically exchange-coupled. The diferric exchange interaction (H = -2JS(1)S(2)) is found to vary from J = -66 cm(-1) (herpes simplex virus type 1) to J = -92 cm(-1) (E. coli). Measurements on samples in deuterated buffer suggest that the variation of the diferric exchange coupling among species may result from differences in hydrogen bonding to the mu-oxo bridge between the ferric ions. An interpretation of the observed spin-lattice relaxation channels of the tyrosine radicals on the basis of the spectroscopic data, as well as the published three-dimensional structure of the R2 protein from E. coli, is offered.