The Chemical Evolution of a Nitrogenase Model, XIX Simulation of the Enzymatic Reduction of Cyclopropene

Abstract

Abstract The nonenzymatic reduction of cyclopropene in molybdothiol-and related model systems of nitrogenase yields cyclopropane and propylene in a pH-dependent fashion: In alkaline media, cyclopropane is formed exclusively. In acidic solutions, cyclopropane and propylene are produced. Cyclopropene is thus identified as a pH sensitive chemical probe of nitrogenase. Since substantial amounts of propylene are formed in the enzymatic reduction of cyclopropene, it follows that the active site in functioning nitrogenase is in a locally acidic environment. The protons required to sustain the acidic pH are presumably generated by ATP hydrolysis; addition of substrate amounts of ATP to nonenzymatic cyclopropene reducing systems also stimulates propylene-and cyclopropane production. The stereochemical course of cyclopropene reduction to cyclopropane is exclusively cis, both under enzymatic and nonenzymatic conditions. The formation of propylene from cyclopropene is not stereoselective and mechanistically consistent with acid catalyzed ring opening prior to reduction.