Observation And Analysis On The Stable Configurations Of Guanine And Cytosine

The assembly of the nuclei acid base molecules as the foundation for the evolution process is of great fundamental importance to study complex biological systems.In this thesis,the stable pairing configurations of guanine(G)and cytosine(C)have been investigated by the combination of theoretical calculations and experimental observations.The main findings are listed as below:1.All the planar homopairings and complementary pairings of guanine and cytosine are forecasted based the interactions between the binding sites.The density functional theory(DFT)calculations demonstrate that Watson-Crick(WC)GC pairing is the most stable configuration because of its lowest energy.2.The structures of C and G in the water environment have respectively observed on highly oriented pyrolytic graphite(HOPG)surface by in situ scanning tunneling microscopy(STM).Our STM observations indicate that C construct a well-ordered assembling structure composed of parallel wheat-like chains and G would form an orderly two-dimensional network structure connecting with hydrogen bonding.3.A series of nucleobases guanine(G)and cytosine(C)pairing configurations have been fabricated on HOPG surface by controlling the molar ratio of G and C in water solution.High resolution scanning tunneling microscopy images and density functional theory(DFT)calculations reveal that hydrogen bonding plays a dominant role in the formation of these pairing configurations.Watson-Crick(WC)base pairing governs the association of C and G nucleobases when the molar ratio of C/G is adjusted to 1:1.Nucleobases-rich are preferentially hydrogen-bonded to the sites exposed around WC motifs under the condition of cytosine-rich(guanine-rich).At a higher C/G molar ratio imbalance,the pairing configurations are controlled by the combination of interspace and sites of hydrogen binding between G and C bases.Therefore,it is predicted that the pairing structural polymorphism is determined by the competition between the priority and diversity of hydrogen-bonded configurations bonding G and C.