The Reseach On DNA-Based Hybrid Catalytic System With Benzimidazole Ligand

DNA-based hybrid catalysts emerge in the field of artificial metalloenzymes due to their efficient and highly selective catalytic performance.Although it has been developed since 2005,its catalytic mechanism is still unknown.In order to reveal its catalytic mechanism,various strategies have been devised to explain it.Based on the previous work,we designed two very similar ligand N,N-(benzimidazolylidene)-amine and N,N-(benzimidazolylidene)-acetacetamide(IDB and IDB(O-acetyl))to achieve significant differences in catalytic reactions,and by exploring the reasons for this difference to try to explain the factors that affect the catalytic reaction.The catalytic performance of two benzimidazoles based ligands was evaluated by Diels-Alder reaction and Friedel-Crafts reaction.For Diels-Alder reaction,IDB(O-acetyl)can achieve stereoselectivity,where the substrate has the highest ee value of 27.8%for the p-nitrobenzene-substituted aza chalcone,and the lowest is phenyl-substituted aza chalcone with 15.6%ee.However,IDB has no stereoselectivity,all substrate getting the racemate.For Friedel-Crafts reaction,IDB(O-acetyl)can also achieve stereoselective catalysis with highest ee 24.4%,and IDB not only without stereoselectivity and also completely loss the catalytic capacity,so no product can be detected.In order to explore the reasons for this difference,a series of spectral characterization and dynamic simulation were carried out in this paper.In the IR spectrum data,the carbonyl has a significant red shift,indicating that there are some interactions between the carbonyl and copper ions.The spatial configuration of two kinds of copper ion complexes is obtained by kinetic simulation of energy minimization.IDB(0-acetyl)-Cu2+ is similar to "S"shape and IDB-Cu2+ is similar to "W" shape.In circular dichroism,IDB-Cu2+affects the absorption at the vicinity of 245 nm representing the helix DNA,while IDB(O-acetyl)-Cu2+ mainly affects the absorption at the vicinity of 275 nm representing the base stacking.In the UV-visible spectrum,for IDB(O-acetyl)-Cu2+,a little bathochromic shift was observed after the addition of st-DNA,while,only hypochromism without detectable bathochromic shift occur in the case of IDB-Cu2+.These results implied that IDB(O-acetyl)-Cu2+could bind to DNA base pairs more in intercalative mode while IDB-Cu2+combined with phosphate backbone by electrostatic interaction.The differences between the two ligand copper ion complexes and DNA binding patterns were determined,which explain the reasons for the different catalytic performance.In this paper,IDB and IDB(O-acetyl)were employed as ligands for DNA-based asymmetric catalysis for the first time.Different catalytic results were obtained by modifying the ligands and the reason of the difference was discussed.The current finding may provide a new strategy to explore the reaction mechanism of DNA-based asymmetric catalysis.