Exposing the inadequacy of redox formalisms by resolving redox inequivalence within isovalent clusters

Significance Identifying the redox states within polynuclear cofactors in biology is critical for elucidating substrate binding sites. Multiple-wavelength, anomalous X-ray diffraction (MAD) has been used to address the redox states within the polynuclear cofactors in nitrogenase. While MAD provides a site-specific analytical technique to resolve differences in relative redox states within molecular species, we note that rigorous application of this technique on well-defined synthetic iso- and mixed-valent clusters has not been performed. Our report illustrates the large variation observed for nominally isovalent sites, thus providing a much-needed calibration on what to expect for resolving redox load distribution within cluster materials.