Introduction
The redox reactions of the guanine nucleobase have received considerable attention because of the importance of these redox reactions in aging and metabolism [1], the potential utility of these oxidations in analytical techniques for detection of nucleic acids [2–5], and for use as donors in studies of electron-transfer reactions along the DNA double helix [6–8]. Electrochemistry is a powerful technique for studying the redox reactions of guanine because modern digital methods provide detailed information on kinetics and thermodynamics [9, 10], and because the small depth of the diffusion layer requires small quantities of material for analysis [11]. Thus, electrochemistry offers the possibility of performing detailed mechanistic studies on quantities of material much smaller than those required for optical methods, thereby allowing routine study of numerous sequences prepared using solid-phase synthesis or the techniques of molecular biology [12].
The observation of currents attributable to the faradaic electrochemistry of nucleic acids was pioneered by Palecek and coworkers who studied DNA adsorbed on mercury or carbon electrodes [13]. The signals detected by Palecek were attributable to oxidation of the purines, which produced signals indicative of irreversible processes involving adsorbed bases. These reactions were used as a basis for electrochemical analysis of DNA. Kuhr and coworkers later showed that similar strategies could be developed for analysis of nucleic acids via oxidation of sugars at copper electrodes [14–16].
This article will address research done in our group where complexes similar to $$ {\text{Ru}}{\left( {{\text{bpy}}} \right)}^{{2 + }}_{3} $$ (bpy=2,2′-bipyridine) are used to oxidize guanine and related nucleobases by a single electron upon generation of the Ru(III) state of the complex [17]. The reaction is readily monitored as a catalytic current in the oxidation of the complex to $$ {\text{Ru}}{\left( {{\text{bpy}}} \right)}^{{3 + }}_{3} $$ , and we have developed detailed methods for obtaining kinetic and thermodynamic information on the guanine–Ru(III) reaction from these signals [18, 19]. This information can be supplemented with parallel studies of the same reactions by following the optical absorption of the metal complex using stopped-flow absorption spectrophotometry or following the fate of the oxidized guanine by gel electrophoresis [12, 18]. This latter method provides sequence-specific information that reveals the effects of different DNA sequences and structures on the reactivity of the nucleobase. Although we have characterized the surface chemistry [20–24] and electrochemical mechanisms [25–27] of these reactions in detail, the discussion will be limited to those aspects that pertain to the general issues of long-range electron transfer, which is generally the subject of this volume. The electrochemical reactions we have studied occur mostly at short range, but many of the findings are relevant to questions surrounding the long-range systems. The final section will relate these findings to the issues in long-range electron transfer.