Cytosine, adsorption

In contrast, the adsorption of hydrogen increases the reflectivity of Pt and Pd electrodes [4, 33, 81, 82]. The increase in the reflectivity by hydrogen adsorption is attributed to the increase in free-electron density of the surface [4, 86). In the case of cytosine adsorption on Au shown above, the charge transfer is from the electrode to the adsorbed molecule as is seen from the CV, and thus the reflectivity of the Au electrode decreases. The basehne is also sensitive to the applied potential and is minimal at the pzc of the electrode [15, 43]. 

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... 

Berg et al. 711 proposed that the adenine and cytosine residues in native DNA are reduced by a so-called electron hopping mechanism, the only condition for this being adsorption of protonated DNA at the electrode surface at the reduction potential of these bases. It was also assumed that the DNA is adsorbed in its A-form, exhibiting semi-conducting properties. There is consequently no surface denaturation of the DNA. 

In accordance with the foregoing, i.e. that substitution at Nt does not affect the reduction pathway, the nucleoside and nucleotide react at the mercury electrode essentially like the bases36 37 but adsorb more strongly than cytosine at a potential more positive than —1.6 V 37,48). The EI/2 for the reduction wave in the cytosine series becomes more positive in the order base > nucleotide > nucleoside, and is linearly pH-dependent 37,53). The mechanism for electrochemical reduction of cytosine, cytidine, CMP and CpC have been considered in terms of their structure, association in solution and adsorption 37). It was concluded that the deamination step for CpC occurs very slowly or not at all 37). 

In situ STM has provided detailed structural models for the adsorption of cytosine, uracil and thymine at a number of electrode surfaces.These studies have included thymine adsorbed on Au(lll), Au(lOO) and Au(210), uracil adsorption on Au(lll), Au(lOO) and Ag(lll). These pyrimidine bases show a particularly interesting phase behaviour, which can be seen from cyclic voltammograms (CV). A CV for the... 

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.

Figure 9 shows the SERS spectra of the dinucleoside monophosphates adenylyl-(3 -5 )-uridine (ApU) and adenylyl-(3 -5 )-cytidine (ApC) at different adsorption potentials in the characteristic spectral range of the ring-breathing modes. The adsorbed base modes alone on the electrode surface are at 736 cm " in adenine, 798 cm in cytosine and 795 cm in uracil. The corresponding frequencies in the dinucleoside monophosphates ApC and ApU are at 734 cm for adenine, 792 cm for cytosine and 796cm for uracil. 

The SERS spectrum of methylated DNA shows new Raman bands at 656 cm , 700 cm and 1360 cm" which correspond to characteristic vibrations of 7-MeGua residues in adsorbed methylated DNA (cf. Table 4). Furthermore, the decrease of the band at 1200 cm and the 13(K)cm" shoulder of the band centered at 1332 cm" in the SERS spectrum of native DNA upon methylation can be related to a conformational change of DNA at the location of modified nucleic base pairs 7-MeGua-cytosine. Thus, at a rather positively charged surface, the SERS spectra reveal substantial changes in the adsorption behaviour of methylated DNA. 

The DNA bases, guanine, thymine, cytosine, and adenine, were studied by STM on several substrates. Some authors report hydrogen bonding or stacking of the molecules on Au(lll). The Cu surfaces are often used as substrates for adsorption of the DNA bases. Hydrogen-bond patterns were analyzed on this surface. 

Furthermore, the results helped to characterize how nucleic acid molecules are adsorbed on clay minerals including kaolinite [28, 29], For example, double stranded DNA molecules that differ in their guanine-cytosine content were adsorbed in equal amounts by clays including kaolinite [49]. Linear DNA was revealed to be adsorbed on illite and kaoUnite to a greater extent than on montmorillonite [60]. This emphasizes the influence that positive charges of the lattice edges and the microorganization of clay particles have on the mechanism of DNA adsorption. 

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