Modeling hydrogen evolution from the Fe4S4 and Fe8S9X (X = N, C) clusters. Can a Fe--S high-spin cluster serve as a surrogate for the FeMo cofactor?
A high-spin model of nitrogenase with a Fe(8)S(9)X(+) cluster (X = nitrogen or carbon) is used to test a mechanism for molecular hydrogen production, which is known to accompany ammonia production. The reaction proceeds with a series of protonation-reduction (PR) steps which are considered to be spontaneous if the calculated hydrogen-cluster bond energy exceeds 35-40 kcal/mol. The novel features of this mechanism include the opening of the cluster when one of the bridging sulfides undergoes two PR steps and the direct participation of the central atom when it undergoes a PR step. After the sixth PR step, a cluster is formed which has a low barrier for loss of molecular hydrogen in an exothermic reaction step. The central atom (nitrogen or carbon) has only a minor effect on the reaction steps.
McKee
Michael L
ML
Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849, USA. mckee@chem.auburn.edu
D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D008958:Models, Molecular; D008983:Molybdoferredoxin / Q000737:chemistry*; D013816:Thermodynamics
D007506:Iron-Sulfur Proteins; D008983:Molybdoferredoxin; D006859:Hydrogen