A Thermodynamic Model for Redox-Dependent Binding of Carbon Monoxide at Site-Differentiated, High Spin Iron Clusters
Binding of N_2 and CO by the FeMo-cofactor of nitrogenase depends on the redox level of the cluster, but the extent to which pure redox chemistry perturbs the affinity of high spin iron clusters for π-acids is not well understood. Here, we report a series of site-differentiated iron clusters which reversibly bind CO in redox states Fe^(II)_4 through Fe^(II)Fe^(III)_3. One electron redox events result in small changes in the affinity for (at most ~400-fold) and activation of CO (at most 28 cm^(-1) for ν_(CO)). The small influence of redox chemistry on the affinity of these high spin, valence-localized clusters for CO is in stark contrast to the large enhancements (10^5-10^(22) fold) in π-acid affinity reported for monometallic and low spin bimetallic iron complexes, where redox chemistry occurs exclusively at the ligand binding site. While electron-loading at metal centers remote from the substrate binding site has minimal influence on the CO binding energetics (~1 kcal·mol^(-1)), it provides a conduit for CO binding at an Fe^(III) center. Indeed, internal electron transfer from these remote sites accommodates binding of CO at an Fe^(III), with a small energetic penalty arising from redox reorganization (~ 2.6 kcal·mol^(-1)). The ease with which these clusters redistribute electrons in response to ligand binding highlights a potential pathway for coordination of N_2 and CO by FeMoco, which may occur on an oxidized edge of the cofactor.
© 2018 American Chemical Society. Received: February 15, 2018; Published: March 28, 2018. This research was supported by the NIH (R01-GM102687B). T.A. is grateful for a Dreyfus fellowship. C.H.A. gratefully acknowledges an NSF Graduate Research Fellowship. M.J.C. gratefully acknowledges a Caltech Environment Microbial Interactions (CEMI) Fellowship. We thank Michael Takase and Lawrence Henling for assistance with crystallography, Heui Boom Lee for assistance with SQUID magnetometry, and Paul Oyala assistance with EPR spectroscopy (NSF-1531940 for EPR instrumentation). We thank Graham de Ruiter, Joshua Buss, and Christopher Reed for insightful discussions. The authors declare no competing financial interest.
Supplemental Material - ja8b01825_si_001.cif
Supplemental Material - ja8b01825_si_002.pdf
Accepted Version - nihms-995850.pdf