Ribonucleotide reductase (RNR) catalyzes conversion of nucleoside diphosphates (NDPs) to 2'-deoxynucleotides, a critical step in DNA replication and repair in all organisms. Class-Ia RNRs, found in aerobic bacteria and all eukaryotes, are a complex of two subunits: α2 and β2. The β2 subunit contains an essential diferric-tyrosyl radical (Y122O(•)) cofactor that is needed to initiate reduction of NDPs in the α2 subunit. In this work, we investigated the Y122O(•) reduction mechanism in Escherichia coli β2 by hydroxyurea (HU), a radical scavenger and cancer therapeutic agent. We tested the hypothesis that Y122OH redox reactions cause structural changes in the diferric cluster. Reduction of Y122O(•) was studied using reaction-induced FT-IR spectroscopy and [(13)C]aspartate-labeled β2. These Y122O(•) minus Y122OH difference spectra provide evidence that the Y122OH redox reaction is associated with a frequency change to the asymmetric vibration of D84, a unidentate ligand to the diferric cluster. The results are consistent with a redox-induced shift in H-bonding between Y122OH and D84 that may regulate proton-transfer reactions on the HU-mediated inactivation pathway in isolated β2.