Theoretical Design And Study Of Nucleic Acid Base Magnet With Magnetic Coupling Interactions

Nucleic acid bases play an important role in genetic variation of biological system,growth and protein synthesis of organisms.Nucleic acid is divided into DNA and RNA.Different nucleic acids play different roles in living organisms.DNA is the material basis for storing and expressing genetic information in organisms,and has the characteristics of self-assembly and self-replication.While,RNA is a genetic information carrier present in biological cells as well as some viruses and viroids.It can realize the expression of genetic information on proteins,and it is a bridge between genetic information and phenotypic transformation.In a word,their structural characteristics provide the structural basis and possibility for constructing the basic unit of nanoscale molecular devices.In recent decades,diradical organic molecules have received extensive attention because of their unusual electromagnetic properties.Two unpaired electrons are distributed at the center of the atom of the diradical.Its magnetic characteristics can be characterized by the intramolecular and intermolecular magnetic exchange constants.We have been inspired to introduce free radicals into the nucleic acid bases,which will inevitably provide a new way of thinking for designing novel nanoscale molecular ferromagnetic or antiferromagnetic molecular wires,based on DNA/RNA structures.This article has carried out a series of work around the topic of the modification of nucleic acid bases and the study of their diradical properties,and has achieved some significant research results.The main conclusions and innovations are briefly described as follows:1)Study on Magnetic Properties of A-T/G-C Diradical Base In our research group,a lot of more complete theoretical work has been done on the modification of novel magnetic DNA bases.In this work,we explore the magnetic properties of diradical base pairs by changing the access of cyclopentadienyl radicals to DNA bases.Using density functional theory(DFT)and CASSCF caculation,we study the structural characteristics,electronic properties and magnetic properties of expanded radicals modified by cyclopentadiene.The results show that:(a)the trans-r-bases are similar to the r-base structures.They all maintain stable structures and can pair with their complementary base pair to form monolayer diradical base pairs or bilayers diradicals or tetraradical base pairs,(b)We characterize the magnetic coupling interactions of di/tetraradical base pairs via magnetic exchange coupling constants(J).The results of the magnetic exchange coupling constant(J)show that most trans-r-base pairs have an open-shell(BS)biradical ground state.The magnetic coupling of single-layer intermolecular base pairs is weak,and the magnetic characteristics of double-layer bis/quaternary base pairs are more pronounced.This work theoretically verifies the feasibility of ring-expanded radical modificatiuon and provides theoretical guidance for the design and synthesis of magnetic DNA-based organic molecular materials.2)The Uracil(U)Radical Assembly and Magnetic Characteristics In the work of DNA bases modification,we have demonstrated that cyclopentadiene radicals have excellent biradical characteristics.Therefore,we apply this modification to the uracil bases that are unique to RNA.Similar to the guanine G-quadruplex structure,the modified uracil radical has three different hydrogen bonding sites.Based on this,we design a series of uracil diradical building blocks.The related structural features,electronic properties and magnetic properties are discussed in detail by quantum chemical calculation methods.The results of the study indicate that the ten kinds of uracil isomers can all exist stably.Both of them have open shell singlet or triplet ground state with diradical character and they have significant spin coupling anisotropy.Obviously,this provides a theoretical basis for the design of biofunctionalized multimeric magnetic organic molecular materials.3)Coupler-Controlled Diradical Base Pair Assembly and Magnetic Properties In the work of biradical base pairs,we found that because of the weak interaction between two radicals through hydrogen bonding,their magnetic coupling is not strong.Taking into account the magnetic properties of cyclopentadienyl radicals,we use azobenzene(AB)to combine the two radical bases into light-controlled magnetic structural units,and we retain their hydrogen bonding ability.Using azobenzene(AB)as a coupler,we insert free radicals at both ends of the methylene group to assemble different combinations of base pairs.The results of the relevant magnetic properties indicate that the magnetic coupling of the bis-radical base pair bridged by azobenzene(AB)is greatly enhanced,and the value of the magnetic coupling constant produces a large change.And the isomerization of azobenzene can not only adjust their magnetic size but also change their magnetic properties(FM(?)AFM).This work expands the idea of radicalized DNA bases,and presents promising applications for the design of magnetic photo-controlled novel photosensor devices and magnetic modulator devices.4)Study on Magnetic Properties of Diradicals Intra-Bases Considering that the magnetic properties of the biradical base pairs have been inhanced from the initial weak hydrogen bonding effect to the coupler effect,the value of the magnetic coupling constant value is not prominent.Therefore,we directly introduce nitroxide radicals into the DNA bases or oxidize amino group to nitroxide radical.Through the calculation of the quantum chemistry method,we prove that the magnetic characteristics of the nitroxodiradical bases are particularly prominent.Specifically,the magnetic coupling of intramolecular diradicals satisfies the law of alternating spins,which is more intuitive and easier to understand than the magnetic coupling interactions of intermolecular diradicals.This work provides us with a new perspective on the research of double-base nucleobases.In summary,in this work,firstly the cyclopentadienyl radicals are used to modify the nucleic acid bases,and the magnetic coupling characteristics of free radical base pairs under hydrogen bond weak interactions are studied,such as trans-(rA-rT/rG-rC)and rU-rU base pairs;magnetic properties of free radical base pairs under the action of azobenzene(AB)couplers;magnetic properties of nitroxide radical base pairs within bases.Through constant transformation to optimize the structure of bases,the magnetic characteristics of modified base pairs have changed significantly,and magnetic coupling has become more and more obvious.This work is expected to provide some help for the experimental synthesis of magnetic molecular materials.