Studies On Recognition Of Nonclassical Nucleic Acid Structures By Macrocyclic Compounds And Functional Applications

Nucleic acid is the main carrier of genetic information.Small molecules can regulate the biological process of cells by combining with nucleic acids.Therefore,small molecules that can specifically identify nucleic acid structure（especially non-classical nucleic acid structure）have potential application prospects in disease treatment and drug development.Macrocyclic compounds withπ-conjugation have attracted extensive attention due to their unique antiviral and antitumor activities and excellent binding ability to nucleic acids,and have potential applications in photodynamic therapy（PDT）of cancers.In this thesis,we selected the naphthalene dianthrone macrocyclic compound Hypericin（Hyp）as the recognition molecule and the non-classical structure of human telomere i-motif as the object.We found that the binding ability of Hyp was strongly dependent on the formation of i-motif structure.Based on this behavior,we developed a visible light-driven DNAzyme and the fluorescence recognition method of i-motif structural polymorphism.The main research contents are as follows:1.Visible light-driven i-motif-based DNAzymesDNA foldings provide variant possibilities to develop DNAzymes with remarkable catalytic performance.In spite of fruitful reports on G-quadruplex DNAzymes,four-stranded cytosine-rich i-motifs have not been explored as the potential skeletons of DNAzymes.In this work,we developed a visible light-driven DNAzyme based on human telomeric i-motifs using a natural photosensitizer of hypericin（Hyp）as the cofactor and dissolved oxygen as the oxidant source.The i-motif folding in acidic solution caused the distal thymine overhangs at the 3’and 5’ends to approach each other to provide a favorable binding site for Hyp via an interaction of fully complementary hydrogen bonding.The binding event converted Hyp from the fully dark state to the emissive state under visible light illumination.Subsequently,the excited Hyp had an opportunity to transfer energy to dissolved oxygen.Resultantly,singlet oxygen（¹O₂）was generated to initiate the substrate oxidation.Our developed i-motif-based DNAzyme can be driven by almost the whole range of visible lights,suggesting broad applications in the photocatalytic fields,for example,as an alternative strategy in developing biodevices.2.A C-C⁺fluorescent probe for exploring length-dependent human telomeric i-motif structure polymorphismThe i-motif structure（iM）has attracted much attention due to its in vivo bioactivity and wide in vitro applications.Herein,the length-dependent folding of cytosine-rich repeats of the human telomeric 5′-（CCCTAA）_n-1CCC-3′（iM-n,n=2-8）was fully explored.We found that iM-4,iM-5,and iM-8 mainly form the intramolecular monomer iM structures,while a tetramolecular structure populates only for iM-3,and iM-6 and iM-7 have the potential to fold as well into the bimolecular iM structures besides the monomer ones.The natural hypericin（Hyp）was used as a selective probe to recognize the iM structures.Interestingly,only iM-3,iM-6,and iM-7 can efficiently switch on the Hyp fluorescence.It is expected that Hyp can specifically bind with the outmost C-C⁺base pairs in these iM structures that are exposed directly to solution.However,those iM structures that are otherwise usually stacked by the loop sequences are unavailable for the Hyp binding.This work provides a versatile fluorescence strategy to explore the polymorphic iM structures dependent of the repeat length,which has important implications for studying the structure and function of i-motif.