| dc.description.abstract | Some literature pointed out that guanine (G) can be used as a drug to treat tumors. It is thought that cage-like tetramer structure of G can inhibit the replication of cell telomerase. The G tetramer has been prepared in vacuum after annealing. This is followed by more research to show that co-adsorption of potassium ions can facilitate the formation of G tetramer, which show improved structural stability. But again these studies were performed in ultra-high vacuum systems, which differ markedly from physiological condition.
In this study, a potential-controlled scanning tunneling electron microscope (STM) was used to study the adsorption structure of G molecules on the gold electrode as a function of anion and cation, pH of the media, and electrochemical potentials. In acidic solutions, the co-adsorption of anions resulted in different adsorption structures. G molecules were physically adsorbed on the gold electrode at negative potentials, but transformed to chemisorption as the potential was made more positive. The orientation of G admolecule changed from horizontal to upright and the surface coverage increased with more positive potential. This was the first stage of G adsorption, leading to ordered G adlayers on Au(111) electrode, as revealed by molecular resolution STM imaging. The second stage of G adsorption yielded crooked linear structure, having morphology similar to the shape of a fingerprint. In neutral phosphoric solutions only one ordered structure was seen at negative potential and deposition of G to give multilayer. The solubility of G in alkaline solution is significantly higher, but protracted STM imaging showed that G adlayer was disordered at negative potential, and transformed into a fingerprint-like structure at positive potential. The effect of potassium ions on G admolecules on Au(111) was examined with samples prepared by immersing in sulfuric and perchloric acids containing equal concentration G and K+. In sir STM imaging showed that G and potassium ions were co-adsorbed in an ordered structure with a 1:1 ratio. The degree of ordering deteriorated with solutions with different G/K+ ratios.
The adsorption of cytosine (C) molecule on Au(111) electrode was also examined. In contrast to disordered C structure formed in high vacuum, several C structures were found, depending on the chemical identity of the electrolyte. Aided by the CV results obtained in perchloric acid, partial deprotonation occurred at some positive potential. STM imaging revealed a (3√3 × √13)–C structure in a wide potential range, but transformed into (3√3 × √19) at more positive potential.
Finally, the co-adsorption of C and G molecules on Au(111) was investigated. The obtained STM results are used to understand the hydrogen bond formation within the adlayer potential control and anion co-adsorption. The CV recorded in perchloric acid containing1:1 C/G had the same characteristics as that of pure G. STM imaging showed that ordered structures the same as that of G molecule was formed at negative potential, but a bi-layer film was formed with the upper layer forming molecular chains 5-20 nm long and 0.7 nm wide. This structure is ascribed to mixed C and G adlayer linked by intermolecular hydrogen bonds. C and G admolecules could be differentiated by their STM intensities; C was lower than G by 0.01 nm. The mixed adlayer was most populated at 0.1V. | en_US |