Co-assemble Of Short Peptide-porphyrin And Application In Photochemical Transformation

Developing clean and renewable sources of energy such as solar energy instead of traditional fossil fuel is a necessary step to resolve the shortage of energy and environmental issues.Efficient utilization of solar energy attracts great attention due to its sustainable and environment friendly essence.The conversion of light energy to heat energy,electric energy and chemical energy has become the focus of research.Photosynthesis,which occurs in green plants and certain bacteria,shows us a perfect model about the conversion of solar energy to chemical energy,and inspires people to explore the internal composition,structure and interaction of subunits in the natural photosynthesis system.On this basis,artificial photosynthesis devices were further biologically constructed.In order to simulate the structure of photosynthetic system in nature,here we developed a new type of photosensitive complex by integrating the light-harvesting property of porphyrin,self-assembly ability of short peptide,and electron separating and transferring functionality of Pt nanocrystals inspired by the natural photosynthetic units.We encapsulated it in semi-permeable silica capsules,a micrometer reactor similar to photosynthetic protocells was obtained,which can transform NAD⁺to NADH under weak visible light.This work shows the importance of porphyrin aggregation structure in light capture and photoelectronic conversion,and provides guidance for the biomimetic design of advanced artificial photosynthetic system.These results will be beneficial to the aggregation control of porphyrin and the biomimetic construction of novel photosynthetic devices.Firstly,using the cationic short peptide Ac-IIIIKK-NH₂（I₄K₂）preassembled nanofibers as templates,the anionic tetrakis（4-sulfonatophenyl）porphine（TPPS）was arranged in order on the surface of the peptide templates,and then the polypeptide-porphyrin different S-type aggregates were obtained.We systematically investigated the effect of peptide concentration,porphyrin concentration and solution pH on their aggregating behavior,and attempted to expose the relationship between the aggregation structure and the photoelectric property of the different S-type aggregates.The results indicate that the J-aggregates of TPPS can be obtained at a relatively mild condition in the presence of the cationic short peptide assemblies,and can be easily regulated by the solution pH or molar ratio of TPPS and I₄K₂.The photoelectric conversion property of the S-type aggregates are mainly contributed by the J-aggregates structure rather than the content of the chromophore（TPPS）.Then,Pt nanoparticles were in situ photo-reduced from its Pt^II salts and assembled on the surface of the S-type aggregates resulting in novel I₄K₂/TPPS/Pt complex formation.The I₄K₂/TPPS/Pt complex exhibited hierarchical structure and special visible light sensitivity in catalysing the reaction of NAD⁺to NADH.Furthermore,artificial protocell model with light-harvesting function were constructed by encapsulating the I₄K₂/TPPS/Pt complex into water dispersible silica colloidosomes that was capable to perform the generation of NADH from NAD⁺under normal day light.The protocells could transform light energy into chemical energy even in weak light conditions and found that the high light sensitivity of the protocells was associated with their hierarchical structure especially with the structure of the S-type aggregates.Finally,the formation process of this hierarchical structure and its photoelectric properties had been systematically studied.The molecular I₄K₂ and TPPS molecules were co-assembled and compared with those TPPS aggregate on the surface of the peptide assemblies.The regulatory behavior between them was observed by changing the concentration of polypeptides.It was found that the addition of porphyrin did not affect the second structure of the peptide,but the addition of peptide could regulate the porphyrin aggregation.With the change of I₄K₂ concentration,porphyrins first combine with peptide molecules to form small peptide-porphyrin units,then induce more TPPS aggregation on this site to form larger J-aggregates.With peptide concentration increasing and keep the porphyrin concentration constant,the aggregates changed from lamellar structure to rod shapes,and finally to tangled and bundled fiber-like structure.However,too much TPPS binds to the polypeptide results in less TPPS molecules to form J-aggregates when the concentration of the peptide is greater than 4mM.As we had expected that such C-type aggregates via co-assemble of I₄K₂ and TPPS possess higher photo stability compared with those formed via TPPS aggregating on the surface of the I₄K₂ assemblies.The NADH content increasing with the irradiation time and finally reached a equilibrium unlike that occurred in the C-type aggregates formed on the surface of the preassembled peptide assemblies NADH increased firstly and then decreased rapidly with further extension of irradiation time.