Guanine, adsorption

Ex situ measurements in the presence of dissolved oxygen have proved that the mixed monolayer was stable in the solution free of 6TG and guanine. Madueno etal. [Ill] have also studied adsorption and phase formation of 6TG on mercury electrode. At high potentials, the molecules were chemisorbed and were able to form a self-assembled monolayer. When the potential was scanned to more negative values, reductive desorption of the monolayer was observed. Cathodic voltam-metric peaks, which are typical of a 2D condensed phase transition, divided the potential window into two regions one, in which self-assembled monolayer was stable, and the second, in which a physisorbed state existed. 

Farias etal. [197] have presented cathodic adsorptive stripping voltammetry of guanine in the presence of copper at static mercury electrode. Cyclic voltammetry was also employed to characterize the interfacial and redox mechanisms. 

The electrochemical behaviour and the adsorption of nucleic acid molecules and DNA constituents have been extensively studied over recent decades [1-6]. Electrochemical studies demonstrated that all DNA bases can be electrochemically oxidized on carbon electrodes [7-13], following a pH-dependent mechanism. The purines, guanine (G) and adenine (A), are oxidized at much lower positive potentials than the pyrimidines, cytosine (C) and thymine (T), the oxidation of which occurs only at very high positive potentials near the potential corresponding to oxygen evolution, and consequently are more difficult to detect. Also, for the same concentrations, the oxidation currents observed for pyrimidine bases are much smaller than those observed for the purine bases. Consequently, the electrochemical detection of oxidative changes occurring in DNA has been based on the detection of purine base oxidation peaks or of the major... 

G. Dryhurst, Adsorption of guanine and guanosine at the pyrolytic graphite electrode, Anal. Chim. Acta, 57 (1971) 137-149. 

DNA can be easily immobilized on GEC by simple wet-adsorption onto GEC surface (Fig. 21.1(A2)). A small drop of DNA probe in acetate saline solution pH 4.8 [58] is put onto the surface of a GEC electrode in upright position. The immobilization of the probe was allowed to proceed for 15 min without applying any potential under static conditions. After the inosine-modified DNA probe immobilization, the DNA target was detected by the intrinsic DNA oxidation signal coming from the guanine moieties. Briefly, the procedure consists of the following... 

All measurements of this work were carried out using the ratio between guanine signal of the target and non-complementary sequences to control the non-specific adsorption of oligonucleotide sequences onto the graphite surface. 

Fig. 3.6. Curves obtained for I vs. adsorption time corresponding to the guanine ( ), and adenine ( ) peaks during surface modification (I) first conditioning (2) last conditioning.

The adsorptive and voltammetric characteristics of Cu(II) complexes with guanine, guanosine and adenosine were exploited [120] in order to detect these bases after separation by capillary zone electrophoresis, and the enzyme-mimic catalytic activity of a DNA-Cu2+ complex [121] was used to develop an amperometric quinacrine sensor using an oxygen electrode covered by the complex entrapped in polyacrylamide gel. 

Under these immobilization conditions, a significant guanine signal could be measured only after a sample s thermal denaturation the nonspecific adsorption of the nondenaturated sample was negligible. 

Electrochemical biosensing of DNA sequences using direct electrochemical detection of DNA hybridization, adsorptive striping analysis, metal complex hybridization indicators, organic compound electroactive hybridization indicators and renewable DNA probes have been considered [65,67,72,73]. With metal complexes and organic compound electroactive hybridization indicators, non-specific adsorption can influence the results [68,94]. Chrono-potentiometric detection was used to monitor the hybridization onto screen-printed carbon electrodes by following the oxidation of the guanine peak, which decreases in the presence of the complementary strand [64,68,73]. 

Guanine, having — NH2 and =0 as substituents, shows more complex adsorption behavior than adenine, with only the —NH2 group as a substituent. Deoxyribose and phosphate moieties of guanine and adenine derivatives seem to participate in their adsorption processes, mainly in the positive potential region. 

Figure 19 shows the A/l/i o — curves of cytosine, thymine, and their derivatives. In the case of cytosine, A // o begins to increase at about —0.5 V. The curve shows a quasi-bell shape having a maximum at the pzc at lower concentrations but aquires a somewhat more complex shape at higher concentration, with some increase in A // ol at potentials more positive than 0.1 V (curve b in Fig. 19A). This seems to indicate that the adsorption process is accompanied by reorientation or formation of the cytosine dimer on the positively charged surface. Since such complicated shapes are observed only in the curves of cytosine and guanine, having the same substituent groups, the interaction of —NH2 and =0 may be partly responsible for their complicated behavior on the electrode surface.

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