Figure 5.

(a) A summary of the Cp1 data presented in this manuscript. At pH 8, the typical resting state X-ray diffraction structure and EPR signal are observed. At pH 5, a peptide flip and repositioning of the α-Arg357 side chain away from the Fe3,4,5,7 face of the FeMo-cofactor is observed as well as a S = 3/2 spin system with zero-field splitting parameters similar to those reported for one of the signals observed in the E₂ state. At intermediate pH, both structural conformations and EPR spin systems are observed. The EPR signals and X-ray structures are reversible and correlated. (b) The 1c peak has been attributed to the E₂ state and is hypothesized to result from protonation of the FeMo-cofactor. Our experimental conditions include only a proton source and not an electron source, so it is unlikely that these conditions achieve a reduced state, such as E₂. Consequently, we propose that our low pH conditions yield a protonated resting state, which we call “E₀H⁺”.