Photoredox Chemistry Using Protein Scaffolds for Energy Storage and Medicine

Environment and health are related in many ways. Research and development of green strategies for energy storage requires a clean source of input energy; one of the options is solar energy conversion by splitting water into dihydrogen and dioxygen by visible light in aqueous solutions. The development of therapies that selectively treat a disease without perturbing their surroundings to minimize systemic toxicity and side effects are also important societal goals.

Previously our lab reported the development of platforms that can convert light energy to produce H2 for energy storage, and also to drive release of Fenton-reactive iron to cancer cells in a targeted fashion. These two platforms were both constructed using the native and bioinorganic properties of a bacterial ferritin called bacterioferritin (Bfr), which functions naturally as iron storage protein. The photosensitizer for these platforms was Zn(II)-protoporphyrin IX (ZnPP) substituted into the native heme binding sites of Bfr.

Clark and Kurtz reported photosensitized H2 production using a Zinc Porphyrin-substituted protein, platinum nanoparticles, and ascorbate with no electron relay (Clark, E. R., et al. Inorg. Chem. 2017, 56, 4584−4593). The present work describes structural, photophysical, and photochemical properties of the ZnPP in the ZnPP-Bfr dimer. The results are consistent with an oxidative quenching pathway involving electron transfer from 3ZnPP* to Pt NPs. However, the low quantum yield for H2 production (∼1%) in this system could make reductive quenching difficult to detect, and can, therefore, not be completely ruled out.

Cioloboc et al. reported photosensitized reductive iron release from the ferric oxyhydroxide mineral core of ZnPP-Bfr (Cioloboc et al. Biomacromolecules 2018, 19, 178−187). The present work describes structural and photophysical properties of the ZnPP-Bfr, and the effects of the iron core on these properties. In this case the evidence supports electron transfer from the singlet excited state, 1ZnPP* to the ferric oxyhydroxide core, forming The ZnPP•+ and leading to release of the Fenton-reactive Fe2+.

This study also developed a simple but more efficient system for photosensitized production of H2 using human ferritin and the xanthene photosensitizer, eosin Y (EY). This system features a novel method for inserting Pt NPs into the protein shell using the native iron redox chemistry of ferritin. In this case the chemistry likely occurs through reductive quenching of the triplet state, 3EY*. The quantum yield for H2 production in this system was 18%.