the chemistry
of renewable energy
The coupling of a proton to an electron, proton-coupled electron transfer (PCET), is the basic mechanism of chemical and bio-energy conversion. Small-molecule activation, redox-driven proton pumps, and radical initiation and transport all involve the coupling of electrons to protons. However, the mechanistic details of how the electron and proton couple remain largely unresolved. We have developed methods that allow, for the first time, the electron and proton to be measured and in turn allow us to define how PCET is manifested in radical-based biology and in the catalysis of small molecules of consequence to biology and energy.
Electron-proton donor-acceptor systems are designed in which the electron and proton transfer may be initiated by an ultrafast laser pulse. This allows us to time the electron and proton with respect to each other. In making these kinetics measurements, the Nocera group provides a foundation for developing a theoretical underpinning to PCET as well as a foundation on which to build catalysts and biological systems that rely on PCET for their function.
The mechanistic insights acquired from PCET studies in model and natural systems provide a roadmap to develop new platforms for catalytically activating small molecule substrates by PCET. Reactions of interest include the conversion of water to oxygen and hydrogen (solar energy conversion), and the reverse reaction, the reduction of oxygen by protons and electrons to water (fuel cell reaction), along with transformations promoted by oxygenases and reductases. In biology, nature has designed active sites for PCET reactions that utilize the acid/base functionality of protein amino acid side chains in the secondary coordination to couple protons to the redox chemistry of metallo-cofactors. We have captured this biological function with the design of novel "Hangman" platforms (porphyrins, salens and corroles). The "Hangman" architecture poises an acid/base functionality over the redox cofactor to serve several purposes: (1) as a hydrogen bond scaffold for assembly of substrate, (2) as an intramolecular proton donor/acceptor to provide kinetic enhancement when proton transfer to substrate is rate-limiting, and (3) to provide hydrogen bonds to the substrate that can lower the activation energy for bond-making, bond-breaking reactions. These attributes result in enhanced catalytic activity by PCET.
The PCET mechanisms of biological systems are a study focus in the Nocera group, with particular emphasis on enzymes utilizing amino acid radicals as redox active cofactors and charge transfer intermediates. As a case study, a collaborative effort between our group and the Stubbe group seeks to define the PCET reactivity of ribonucleotide reductase (RNR), which converts ribonucleosides to deoxyribonucleosides necessary for DNA synthesis through a radical-dependent mechanism. RNR is composed of two subunits: R1 contains the enzyme active site and R2 contains the assembled diiron-tyrosyl radical cofactor. For each turnover, the radical in R2 must transiently oxidize an active site cysteine residue in R1 located more than 35 Å away. The Nocera group has developed methods to permit tyrosine radicals to be triggered with a pulse of laser light; these radical triggers may be incorporated at chosen sites within RNR. Using these "photo-RNRs", the radical transport pathway in RNR is being disentangled. The mechanistic insight provided by these studies allows us to measure and monitor the effects of antiviral and anticancer drugs on RNR.