Photoinduced Reductive Elimination of H2from the Nitrogenase Dihydride (Janus) State Involves a FeMo-cofactor-H2Intermediate.
N2reduction by nitrogenase involves the accumulation of four reducing equivalents at the active site FeMo-cofactor to form a state with two [Fe-H-Fe] bridging hydrides (denoted E4(4H), the Janus intermediate), and we recently demonstrated that the enzyme is activated to cleave the N≡N triple bond by the reductive elimination (re) of H2from this state. We are exploring a photochemical approach to obtaining atomic-level details of the re activation process. We have shown that, when E4(4H) at cryogenic temperatures is subjected to 450 nm irradiation in an EPR cavity, it cleanly undergoes photoinduced re of H2to give a reactive doubly reduced intermediate, denoted E4(2H)*, which corresponds to the intermediate that would form if thermal dissociative re loss of H2preceded N2binding. Experiments reported here establish that photoinduced re primarily occurs in two steps. Photolysis of E4(4H) generates an intermediate state that undergoes subsequent photoinduced conversion to [E4(2H)* + H2]. The experiments, supported by DFT calculations, indicate that the trapped intermediate is an H2complex on the ground adiabatic potential energy suface that connects E4(4H) with [E4(2H)* + H2]. We suggest that this complex, denoted E4(H2; 2H), is a thermally populated intermediate in the catalytically central re of H2by E4(4H) and that N2reacts with this complex to complete the activated conversion of [E4(4H) + N2] into [E4(2N2H) + H2].
Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States.
Dmitriy
D
Lukoyanov
Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States.
Nimesh
N
Khadka
Department of Biochemistry, Virginia Tech , 110 Fralin Hall, Blacksburg, Virginia 24061, United States.
Dennis R
DR
Dean
Pacific Northwestern National Laboratory , Richland, Washington 99352, United States.
Simone
S
Raugei
Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States.
Lance C
http://orcid.org/0000-0002-6457-9504
LC
Seefeldt
Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States.
Brian M
http://orcid.org/0000-0002-3100-0746
BM
Hoffman
D006859:Hydrogen / Q000737:chemistry / Q000378:metabolism*; D015394:Molecular Structure; D008983:Molybdoferredoxin / Q000737:chemistry / Q000378:metabolism*; D009591:Nitrogenase / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction; D055668:Photochemical Processes; D014466:Ultraviolet Rays
D008983:Molybdoferredoxin; D006859:Hydrogen; D009591:Nitrogenase