Structural, Photophysical, and Photochemical Characterization of Zinc Protoporphyrin IX in a Dimeric Variant of an Iron Storage Protein: Insights into the Mechanism of Photosensitized H2 Generation

Some of us have previously reported the preparation of a dimeric form of the iron storage protein, bacterioferritin (Bfr), in which the native heme b is substituted with the photosensitizer, Zn(II)-protoporphyrin IX (ZnPP-Bfr dimer). We further showed that the ZnPP-Bfr dimer can serve as a photosensitizer for platinum-catalyzed H-2 generation in aqueous solution without the usually added electron relay between photosensitizer and platinum (Clark, E. R., et al. Inorg. Chem. 2017, 56, 4584-4593). We proposed reductive or oxidative quenching pathways involving the ZnPP anion radical (ZnPP center dot-) or the ZnPP cation radical, (ZnPP center dot+), respectively. The present report describes structural, photophysical, and photochemical properties of the ZnPP in the ZnPP-Bfr dimer. X-ray absorption spectroscopic studies at 10 K showed a mixture of five- and six-coordinated Zn centers with axial coordination by one long Zn-S gamma Met distance of similar to 2.8 angstrom and,similar to 40% having an additional shorter Zn-S distance of similar to 2.4 angstrom, in addition to the expected 4 nitrogen atom coordination from the porphyrin. The ZnPP in ZnPP-Bfr dimer was prone to photosensitized oxidation to ZnPP center dot+. The ZnPP center dot+ was rapidly reduced by ascorbic acid, which we previously determined was essential for photosensitized H-2 production in this system. These results are consistent with an oxidative quenching pathway involving electron transfer from (ZnPP)-Zn-3* to platinum, which may be assisted by a flexible ZnPP axial coordination sphere. However, the low quantum yield for H-2 production (similar to 1%) in this system could make reductive quenching difficult to detect, and can, therefore, not be completely ruled out. The ZnPP-Bfr dimer provides a simple but versatile framework for mechanistic assessment and optimization of porphyrin-photosensitized H-2 generation without an electron relay between porphyrin and the platinum catalyst.