Electrochemical Reduction of 6-Substituted Purines Correlation with Structural and Energetic Characteristics

Abstract

The electrochemical reduction of 6‐substituted purines has been examined over the available pH range by direct‐ and alternating‐current polarography, and the effects of substitution evaluated in respect to significant electrochemical characteristics. Where the 1,6 normaland 3,2N=C bonds are available in the pyrimidine moiety, a four‐electron polarographic wave due to hydrogenation of these bonds is observed; purine itself and 6‐methylpurine give two twoelectron waves. The energy‐controlling step involves reduction of the protonated 1,6normalthinsp;N=C bond. The current is essentially diffusion controlled at low pH where the wave is relatively constant in height and kinetically controlled at higher pH where it begins to disappear. Correlation of the ease of reduction, as measured by E½ , with total polar substituent constant false(σpfalse) and polar substituent constant (σ*) indicates either an absence of mesomeric and steric effects due to the substituents or their transmittal by an inductive mechanism and, further, that none of the substituents exerts any specific effect different from those exerted by the other substituents. Substitution in the 6‐position influences mostly the Nfalse(1false)‒Cfalse(6false) bond order and the electronic charge distributions on N(1) and N(3), which, in turn, largely determine the reducibility; E½ correlates linearly with the quantum mechanically calculated magnitudes of these quantities, but not with other bond orders and charges. E½ also correlates linearly with electron‐acceptor properties as represented by the calculated energy of the lowest empty molecular orbital ( ki coefficients) and with pKa for proton acquisition (the protonation, which occurs prior to electron transfer, seems to have no significant role in the potential‐determining step except in the region where the wave has nearly disappeared). Isoguanine generally deviates from the various correlations. Adsorption involving an uncharged site in the molecule is markedly greater with derivatives having bulky substituents.

Differential Pulse Polarographic Analysis of Chlorpyrifos Insecticide

“…As the pH approaches the moderately acidic and neutral range the reduction response decreases analogously to an acid dissociation curve (Fig. 2) because the protonation kinetics under these circumstances contribute progressively to the control of the overall rate of the reduction [17,18]. Therefore, it can be concluded that pH 2.2 is the optimum pH for the determination of the investigated compound using DPP at the DME.…”

“…Similar behavior has been reported for quinoline [14], pyrimidine [15], and purine [16] compounds. According to the general mechanism established for such N-heterocyclic compounds [17,18], the --C=N of the heterocyclic system undergoes in acid to neutral media a totally irreversible electrode reaction with a total uptake of 2e…”

The electrochemical reduction of the purines guanine and adenine at platinum electrodes in several room temperature ionic liquids

“…The electrochemical behaviour of guanine and other purine derivatives have received a lot of attention with the aim either of developing analytical methods to monitor nucleosides and their metabolic products, or to aid in interpreting the incompletely understood biological reductions and oxidations of these complex molecules [1][2][3][4][5][6][7][8][9].…”

“…The electrochemical reduction of purine (adenine, and other 6-substituted derivatives) on mercury electrodes is well documented in the literature [2,3,7]. In general they are irreversibly reduced in aqueous media either in two successive steps, where each step involves the addition of two electron and two protons to form the hydrogenated species or in a single 4-electron reduction involving the hydrogenation of the two N C double bonds.…”

“…In general they are irreversibly reduced in aqueous media either in two successive steps, where each step involves the addition of two electron and two protons to form the hydrogenated species or in a single 4-electron reduction involving the hydrogenation of the two N C double bonds. In the case of adenine (2-aminopurine), relatively slow elimination of ammonia from the reduced form has also been described [1][2][3][4][5][6][7][8][9]. The electrochemical reducibility in aqueous media depends on protonation prior to electron transfer [1][2][3][4][5][6][7][8][9].…”