Modelling Enzymatic Electrostatic Field Effects with Coordination Chemistry

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There is a fundamental gap in understanding the degree to which electrostatic fields within metalloenzymes are able to influence the reactivity of transition metal-containing cofactors. A growing body of evidence implicates electrostatic fields as the origin of a significant portion of enzymatic activity, but to date, only theoretical models have evaluated such electrostatic effects at the active sites of metalloenzymes. The continued existence of this knowledge gap represents a critical shortcoming in understanding the means by which metalloenzymes are able to perform strong-bond functionalization reactions, like N2 reduction to NH3 and selective C–H bond oxidation. The overall objective in this application is to synthesize model compounds that will allow the magnitude and direction of electrostatic fields to be correlated to critical steps in metalloenzyme-mediated transformations. Research will be performed to test the central hypothesis, that strong, local, electrostatic fields alter the valence electron distribution within metal-substrate interactions, directing reactivity to the substrate and catalyzing bond activation reactions. This hypothesis has been formulated on the basis of both literature precedent from the organometallic and enzymology communities as well as preliminary data produced in the applicant’s laboratory, which includes the synthesis of a new ligand framework capable of stabilizing a mononuclear Cu:O2 analog of the CuM site in peptidylglycine α-hydroxylating monooxygenase (PHM) in a manner consistent with the influence of strong, local electrostatic fields. Guided by this background information, the central hypothesis will be tested by first creating a library of model complexes in which non-Lewis acidic charged residues are appended in the secondary coordination sphere of a central metal center. The synthetic versatility of these ligands will allow for systematic changes to the location of the charged residues with respect to the active-site of the metal center. Next, vibrational Stark spectroscopy will be used to quantify the electrostatic field strength within the substrate-binding pocket of the model complexes. The union of these data with investigations into coordination chemistry and reactivity studies on O2-, NO-, and N2-bound model systems will be used to demonstrate the ability of charged residues to shift the electron distribution within the valence manifold of metal complexes. Overall, this work complements the PI’s broader research program focused on seeking fundamentally new methods for controlling the reactivity of transition metal complexes; this program includes a range of investigations into the use of electrostatic fields to influence coordination chemistry and catalysis as well as the synthesis of multinuclear cluster complexes that model the surface chemistry occurring on heterogenous catalysts. The approach described in this application is innovative, in the applicant’s opinion, because it provides a straight-forward means of both controlling and measuring the magnitude of strong, local, electrostatic fields, while using these data to understand the action of metalloenzymes. The proposed research is significant, because it is expected to provide data that will contribute to understanding of the origin of enzymatic catalysis.