Paramagnetically shifted resonances in 1H NMR spectra of ribonucleotide reductase from Escherichia coli.

The 400-MHz 1H NMR spectra of the subunit B2 of ribonucleotide reductase from Escherichia coli show paramagnetically shifted resonances at 24 ppm (exchangeable protons) and at 19 ppm (nonexchangeable protons). The protein contains an antiferromagnetically coupled dimeric iron center and a tyrosyl free radical. The paramagnetically shifted resonances must be due to the iron center, since they remain essentially unchanged in protein B2 with and without free radical. In analogy with recently published results for hemerythrin from Phascolopsis gouldii, which has a similar iron center, the 24-ppm resonance is suggested to arise from histidine ligands to the iron ions.