Nitrogenase Chemistry at 10 KelvinPhototautomerization and Recombination of CO-Inhibited α‑H195Q Enzyme

• Published in: Inorganic chemistry 2022-08, Vol.61 (30), p.11509-11513
• Main Authors: Gee, Leland B., Myers, William K., Nack-Lehman, Patrick A., Scott, Aubrey D., Yan, Lifen, George, Simon J., Dong, Weibing, Dapper, Christie H., Newton, William E., Cramer, Stephen P.
• Format: Article
• Language: English
• Subjects: Chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.2c00818

CO-bound forms of nitrogenase are N2-reduction inhibited and likely intermediates in Fischer–Tropsch chemistry. Visible-light photolysis at 7 K was used to interrogate all three known CO-related EPR-active forms as exhibited by the α-H195Q variant of Azotobacter vinelandii nitrogenase MoFe protein. The hi(5)-CO EPR signal converted to the hi-CO EPR signal, which reverted at 10 K. FT-IR monitoring revealed an exquisitely light-sensitive “Hi-2” species with bands at 1932 and 1866 cm–1 that yielded “Hi-1” with bands at 1969 and 1692 cm–1, which reverted to “Hi-2”. The similarities of photochemical behavior and recombination kinetics showed, for the first time, that hi-CO EPR and “Hi-1” IR signals arise from one chemical species. hi(5)-CO EPR and “Hi-2” IR signals are from a second species, and lo-CO EPR and “Lo-2” IR signals, formed after prolonged illumination, are from a third species. Comparing FT-IR data with CO-inhibited MoFe-protein crystal structures allowed assignment of CO-bonding geometries in these species.