Ultrafast Limits of Photo-Induced Electron Transfer Rates in PPCA, a Multi-Heme C-Type Cytochrome

Abstract

Conversion of light energy to electrochemical equivalents is at the core of any photosynthetic system. In our attempts to build enzymes for artificial photosynthesis, we need to have complete control of all charge transfer rates, including the initial charge separation step. Here we demonstrate an unexpectedly large range (5x107 to 5x1011 s−1) of the observed photo-induced electron transfer rates between PpcA, a 3-heme member of cytochrome c7 family from Geobacter sulfurreducens, and Ru(bpy)2(4-MeBr-4′ - Mebpy), an artificial photosensitizer covalently-linked to genetically engineered cysteine residues in close proximity to PpcA hemes. HPLC-MS characterization confirmed purity of isolated proteins and their correct assembly, as well as stoichiometric binding of photosensitizers. Combined small- and wide-angle X-ray scattering studies performed at the Advanced Photon Source of Argonne National Laboratory demonstrated the absence of any significant structural changes from the initial compact globular shape in mutated and covalently-labeled protein forms. Ongoing NMR spectroscopy and all-atom molecular dynamics simulations provide further insight into protein structure and dynamics responsible for the large range of electron transfer rates. These results demonstrate the limits of tuning charge transfer rates and offer opportunities to use inexpensive photosensitizers based on abundant elements for the initial charge separation step in artificial photosynthesis.