Photosensitized H2 generation from one-pot and two-pot assemblies of a zinc-porphyrin/platinum nanoparticle/protein scaffold

We report photosensitized H-2 generation using a protein scaffold that nucleates formation of platinum nanoparticles (Pt NPs) and contains built-in photosensitizers. The photosensitizers, zinc-protoporphyrin IX or zinc-mesoporphyrin IX (ZnP) were incorporated in place of the naturally occurring heme in the 24-subunit iron storage protein bacterioferritin (Bfr) when the ZnPs were added to the E. coli expression medium. We engineered a stable dimeric Bfr variant with two protein subunits sandwiching a ZnP. Ten glycines were also substituted in place of residues surrounding the vinyl side of the porphyrin in order increase access of solvent and/or redox agents. An optimized one-pot reaction of this glycine-substituted ZnMP-Bfr dimer with a Pt(IV) salt and borohydride resulted in a similar to 50 : 50 mixture of protein in the form of Pt-free glycine-substituted ZnP-Bfr dimers and re-assembled 24-mers surrounding Pt NPs formed in situ. H-2 production occurred upon visible light irradiation of this one-pot product when combined with triethanolamine as sacrificial electron donor and methyl viologen as electron relay. An analogous two-pot system containing mixtures of separately prepared Pt-free glycine-substituted ZnP-Bfr dimer and porphyrin-free Pt NP@Bfr 24-mer also showed robust photosensitized H-2 generation. The glycine-substituted-ZnP-Bfr dimer thus served as photosensitizer for catalytic reduction of methyl viologen by triethanolamine, and the reduced methyl viologen was able to transfer electrons across the Bfr 24-mer protein shell to generate H-2 at the enclosed Pt NP in a dark reaction. Our results demonstrate that Bfr is a readily manipulatable and versatile scaffold for photosensitized redox chemistry.