Jennifer Marx investigates the active centers of the R2 proteins from Escherichia coli, Saccharopolyspora erythraea, and Geobacillus kaustophilus, discussing the spectroscopic differences among the Ia, Ic, and R2-like groups of ribonucleotide reductases. She demonstrates that their vibrational properties differ in nuclear inelastic scattering depending on the bound metals. Additionally, she identifies the regions of the partial vibronic density of states of iron that are sensitive to different metal occupancy. To better understand the vibrational properties in the low-frequency range, the author calculates a model of the entire protein in water.