Lisa Olshansky

Proton-Coupled Electron Transfer in Natural and Artificial Metalloproteins

Metalloproteins have evolved to mediate the transfer of electrons and protons with exquisite control. Understanding, and ultimately replicating this control in synthetic systems represents a key goal in the development of renewable energy technologies. To this end, structural and kinetic factors affecting proton-coupled electron transfer (PCET) were examined in natural and artificial metalloproteins (ArMs). A series of ArMs containing Co4O4 cubane active sites and featuring various secondary coordination sphere effects were prepared. Structure-function correlations were performed by X-ray diffraction and electrochemical techniques. These studies revealed that in the presence of an H-bond between the metallocofactor and a proximal Tyr residue, multi-e⎯/multi-H+ PCET occurs that is distinct from the 1e⎯/1H+ PCET observed in the corresponding Ser and Phe variants. In a separate set of studies, the kinetics of PCET in the metalloenzyme ribonucleotide reductase (RNR) were examined. RNR undergoes long-distance (~35 Å) PCET via a pathway of redox-active amino acids. To bypass rate limiting conformational changes, a photo-triggered version of RNR was prepared and PCET events examined by transient optical spectroscopy. Replacing Tyr residues with unnatural fluorotyrosines, and using halogenated and deuterated substrates, the pKas, E°s, and BDEs along the PCET pathway were modulated and correlated with changes in rate. These studies allowed determination of rate constants, equilibrium values, isotope effects, and Marcus parameters for individual PCET events in RNR. In all, these findings identify dynamic and structural factors critical to orchestrating efficient and high fidelity PCET reactions.