Electrochemistry of purine derivatives. 1: Direct determination for the antiviral drug 9-[(1,3-dihydroxy-2-propoxy)methyl]guanine by anodic differential pulse voltammetry

Electrochemical behavior of an antiviral drug acyclovir at fullerene-C(60)-modified glassy carbon electrode

Electrochemical oxidation of acyclovir at fullerene-C(60)-modified glassy carbon electrode has been investigated using cyclic and differential pulse voltammetry. In pH 7.4 phosphate buffer, acyclovir showed an irreversible oxidation peak at about 0.96V. The cyclic voltammetric results showed that fullerene-C(60)-modified glassy carbon electrode can remarkably enhance electrocatalytic activity towards the oxidation of acyclovir. The electrocatalytic behavior was further exploited as a sensitive detection scheme for the acyclovir determination by differential pulse voltammetry. Effects of anodic peak potential (E(p)/V), anodic peak current (I(p)/μA) and heterogeneous rate constant (k(0)) have been discussed. Under optimized conditions, the concentration range and detection limit were 9.0×10(-8) to 6.0×10(-6)M and 1.48×10(-8)M, respectively. The proposed method was applied to acyclovir determination in pharmaceutical samples and human biological fluids such as urine and blood plasma as a real sample. This method can also be employed in quality control and routine determination of drugs in pharmaceutical formulations.

Electrochemical studies of ganciclovir as the adsorbed catalyst on mercury electrode

Although ganciclovir (gan) as a purine analogue is a compound of biological interest (antiviral drug), it has been rarely electrochemically studied. In this paper surface catalytic electrode mechanism based on the hydrogen evolution reaction is analyzed under conditions of square-wave voltammetry and differential capacity curves of double layer measurements. The electrode mechanism is assumed to involve a preceding chemical reaction in which the adsorbed catalyst (ganads) is protonated at the electrode surface, i.e., ganads + H+aq → ganH+ads. The protonated form of the catalyst (ganH+ads) is irreversibly reduced at potential about –1.35 V vs Ag|AgCl, yielding the initial form of the catalyst and atomic hydrogen, i.e., ganH+ads + e → ganads + Haq. Changes of zero charge potential and surface tension point to the adsorption of ganciclovir molecule directed with guanine group to the mercury surface and suggests that ganciclovir molecules are not placed flat on the mercury surface. The effect of adsorption on mercury electrode was studied in detail in respect to analytical usefulness of the obtained results. A new catalytic method for voltammetric determination of ganciclovir was developed. The detection and quantification limits were 1.3 × 10–7 and 4.3 × 10–7 mol l–1 for square-wave voltammetry, and 1.4 × 10–7 and 4.7 × 10–7 mol l–1 for linear-sweep voltammetry.

Water soluble complexes of the antiviral drugs, 9-[(1,3-dihydroxy-2-propoxy)methyl]guanine and acyclovir: the role of hydrophobicity in complex formation

We investigated water-soluble complexes of various ligands with the antiviral drugs, 9-[(2-hydroxyethoxy)methyl]guanine (acyclovir) and 9-[(1,3-dihydroxy-2-propoxy)methyl]guanine (DHPG). For comparison, we also examined the "parent" compounds, guanine and guanosine, as substrates for complex formation. Using the phase-solubility technique, we measured formation constant (K1) values at 23 degrees C in pH 7 buffer. For a single substrate, formation constants with different ligands varied in the order: caffeine greater than pyridoxine approximately cytidine greater than nicotinamide greater than sucrose. With caffeine as the ligand, formation constants with different substrates varied in the order: guanine greater than guanosine approximately acyclovir greater than DHPG. The largest formation constant observed was 58 M-1 (for guanine-caffeine), and the smallest formation constant was 0.29 M-1 (for DHPG-sucrose). Examining the literature for formation constant data on compounds related to DHPG, and comparing literature data with our own, reveals a significant correlation between formation constants and ligand hydrophobicity. For 41 substrate-ligand pairs, least squares linear regression analysis of log K1 values versus various parameters reflecting donor-acceptor abilities (e.g., substrate and ligand HOMO and LUMO values, or substrate oxidation potentials) failed to significantly correlate. We conclude that ligand hydrophobicity is a general determinant of water soluble complex formation, but not necessarily the exclusive or dominant controlling factor for all complexes. Charge-transfer interactions are not important determinants of complex formation for the substrate-ligand combinations that we have considered.

Electrochemistry of purine derivatives. 2: Correlation of anodic differential pulse peak potentials with Hammett substituent constants

Differential pulse voltammetry was used to measure anodic peak potentials at the glassy carbon electrode (in pH 2 to 11 aqueous buffer versus Ag/AgCl) for 14 purine bases and nucleosides. The tested compounds included the antiviral drugs 9-[(1,3-dihydroxy-2-propoxy)-methyl]guanine (1) and acyclovir (2) plus six analogues of 1. At pH 7, representative peak potential values were as follows: 1, 0.97 V; 2, 1.01 V; 6-amino-6-deoxy-1 1.00 V; guanosine, 1.03 V; 2'-deoxyguanosine, 1.03 V. The observed potentials at pH 7 and related literature values correlated with Hammett pi(para) substituent constants. For 24 purines, stepwise multiple linear regression provided the relationship: P7 = (1.08 +/- 0.055) + (1.13 +/- 0.13) sigma 8 + (0.338 +/- 0.061)sigma(2 + 6) + (0.281 +/- 0.043)D (with n = 24, r2 = 0.930, CV = 11.1, and F = 88) where P7 is the pH 7 oxidation potential, the subscripts refer to purine ring substitution position, and D is an indicator variable for substitution at purine N9 by furanoside or glycoside analogue moieties. The observed relationship permits predictions of anodic peak potentials for other purines the substitution patterns of which are known.