Redox-active proteins covalently modified with photosensitizers have served as important model systems for elucidation of kinetic principles controlling biological electron transfer (ET). To date, the fastest rates observed in these systems were ~10ns and involved donor acceptor (DA) distances of &gt;0.8nm. This leaves a significant gap in rates and distances between the model systems and the native photosynthetic proteins. To this end, we developed a series of ~30 cysteine mutants of PpcA, a 3-heme cytochrome from Geobacter sulfurreducens. Due to its relatively small size (71 amino acids) we expected to obtain a number of mutants with DA distances of 0.4-0.5nm and subnanosecond ET rates. These proteins were successfully expressed in E.coli, isolated, and covalently labeled with Ru(bpy)2(bpy-Br). With time-resolved nanosecond and ultrafast transient absorbance spectroscopy we have identified ~10 constructs with apparent photoinduced ET time constants of 20 ps or faster. The is a significant result as up to this point only natural photosynthetic systems demonstrated such a fast initial charge separation. To understand molecular principles responsible for such a dramatic acceleration of electron transfer rates, we used small- and wide-angle X-ray scattering and molecular dynamics simulations. We are currently attempting to obtain X-ray crystallographic and NMR structures of ultrafast constructs. Based on the obtained results we conclude that that photo-induced ultrafast charge transfer in proteins requires a tight binding of photosensitizers and effective dissipation of heat energy by protein frameworks. In contrast, DA distances and a small number of covalent bonds between the donors and acceptors do not reliably predict ultrafast ET rates.

Mimicking natural photosynthesis:  Ultrafast charge transfer in PpcA-Ru(bpy)3 complexes

ACS Spring 2022

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