Escherichia coli ribonucleotide reductase (RNR) catalyzes the conversion of nucleoside diphosphates (NDPs) to deoxynucleoside diphosphates (dNDPs). This RNR is composed of two homodimeric subunits: R1 and R2. R1 binds the NDPs in the active site, and R2 harbors the essential di-iron tyrosyl radical (Y
![](/images/entities/bull.gif)
) cofactor. In this paper, we used PELDOR, a method that detects weak electron-electron dipolar coupling, to make the first direct measurement of the distance between the two Y
![](/images/entities/bull.gif)
's on each monomer of R2. In the crystal structure of R2, the Y
![](/images/entities/bull.gif)
's are reduced to tyrosines, and consequently R2 is inactive. In R2, where the Y
![](/images/entities/bull.gif)
's assume a well-defined geometry with respect to the protein backbone, the PELDOR method allows measurement of a distance of 33.1 ± 0.2 Å that compares favorably to the distance (32.4 Å) between the center of mass of the spin density distribution of each Y
![](/images/entities/bull.gif)
on each R2 monomer from the structure. The experiments provide the first direct experimental evidence for two Y
![](/images/entities/bull.gif)
's in a single R2 in solution.