Construction Of Artificial Photoenzyme For Cycloaddition And Dehalogenation Reaction

Recent development of artificial enzymes and photocatalysis has brought novel strategies for sustainable organic synthetic chemistry.Excited states exhibit unique reactivity modes and can forge the formation of many challenging chemical bonds that cannot be addressed under classic thermal reaction conditions,therefore providing an indispensable synthetic approach.As a research frontier of synthetic organic chemistry,photobiocatalysis integrates the advantage of enzyme and photocatalysis and has shown great potential for the construction of important organic molecules.The limited number and structural complexity of naturally occurring photoreactive coenzyme are the bottleneck for developing artificial photobiocatalytic systems for organic synthesis.Constructing artificial photoenzymes through the introduction of novel non-natural photocatalytic active centers like small molecule photocatalyst into the enzymes by chemical methods or genetic engineering techniques is one of the significant ideas to expand the substrates types and reaction diversity of photobiocatalysis.This dissertation aims to develop novel high-efficiency photoenzymes for organic synthesis,using chemical modification and gene codon expansion to introduce non-natural photocatalytic active centers such as benzophenone and thioxanthone into a protein scaffold.We applied these artificial photoenzymes in significant organic reactions including photocatalytic olefin[2+2]cycloaddition reaction,aryl halide cross-coupling reaction,and direct dehalogenation reduction of aryl halide.The main contents of this dissertation are described as follows:(1)Thioxanthone(TXO)was covalently modified on the surface of bovine serum albumin(BSA)based on chemical methods to construct an artificial photoenzyme BSA-TXO,which addressed the aggregation-induced catalytic activity deterioration of thioxanthone in the organic phase.BSA-TXO displayed remarkable catalytic activity for the photobiocatalytic cross[2+2]cycloaddition reaction of olefin in the aqueous medium.This method effectively inhibited the E/Z isomerization of chalcone and gives rise to a series of functionalized cyclobutanes with excellent yield and enantioselectivity.(2)Green fluorescent protein(GFP)was modified into a photosensitizer metalloenzyme PSP-Ni^II(bpy)containing an organic photocatalytic active group and an anchored unnatural Ni cofactor was devised by genetic code expansion technology and site-directed mutagenesis.With PSP-Ni^II(bpy)we have realized the first photobiocatalytic C-O cross-coupling reaction of aryl halides.Mechanistic experiments such as fluorescence quenching and transient absorption verified that energy transfer from the excited photosensitizer could forge the reduction elimination of Ni-organometallic species.The spatial distance between these two catalytic entities could precisely controlled to elevate the synergism of dual catalysis.Moreover,the catalytic system can also be extended to the C-N bond coupling reaction of aryl halides.(3)We have further accomplished a direct reduction dehalogenation strategy based on artificial photoenzyme PSP(Photosensitizer protein)without metal cofactors constructed by gene codon expansion.This photoenzyme could be applied to reduce a series of aryl halides and drug molecules to the corresponding arenes by using sodium formate as a sacrificial reductant,and the substrate generality was much superior to that of natural dehalogenase.Electron paramagnetic resonance and transient absorption spectroscopic proved that PSP generates radical after light exposure and sodium formate serves as the hydrogen donor.Hydrogen radical may effectively promote the reduction of aryl halide catalyzed by PSP.This thesis investigated the strategy of devising artificial photoenzymes by introducing small molecule photosensitizers into protein scaffold and applied them in abiological reactions such as alkene[2+2]photocycloaddition,C-O coupling,and dehalogenative reduction of aryl halides.The success of these transformations validates our design of artificial photocatalytic center,and it will spur further development of more challenging photobiocatalytic organic transformations such as asymmetric catalysis.