EPR of the anaerobic ring contraction reaction. Concentrations of 150 μm CbiH60, 100 μm cobalt(II) factor III (CFIII), 1 mm SAM, and 10 mm DTT were used in all of the following reactions and incubated at 37 °C. A, spectrum i, CFIII; spectrum ii, CFIII + SAM; spectrum iii, CFIII + DTT. B, reaction of CbiH60 with CFIII recorded immediately after mixing, together with SAM (spectrum i), DTT (spectrum ii), or SAM + DTT (spectrum iii). C, reaction of CbiH60 with CFIII recorded after 24 h of incubation, together with SAM (spectrum i), DTT (spectrum ii), or SAM + DTT (spectrum iii). D, EPR spectra of purified intermediates. Spectrum i, CFIII; spectrum ii, anaerobically purified cobalt(II)-precorrin-4; spectrum iii, aerobically purified cobalt(II) factor IV. The asterisk indicates the presence of a small amount tetrapyrrole radical signal in spectrum iii) (estimated to be less than 1% of total tetrapyrrole).