Electrochemical determination of the stability of complexes formed by proton-ionizable ligands of 3,5-disubstituted 1H-pyrazole with phenethylamine.
The application of two extreme models for diffusion in two-component systems to electrochemically determine equilibrium constants is discussed. The application of cyclic voltammetric, diferential pulse and rotating-disc electrode voltammetric data to elucidate the stoichiometry and formation constant of complex species by applying a generalization of the molar-ratio method is described. Molar-ratio experiments permit the distinction between the limiting diffusive regimes. The values of the equilibrium stability constants for complexation of phenethylamine and phenethylammonium ions by a 26-membered dioxotetraester crown of 3,5-disubstituted 1H-pyrazole as free ligand 1[L] and as dipyrazolate anion 1'[L(2-)]2Na(+), respectively, were determined.
Electrochemical and circular dichroism spectroscopic evidence of two types of interaction between [Ru(NH3)6]3+ and an elongated thrombin binding aptamer G-quadruplex
Abstract In the field of electrochemical biosensing based on DNA probes immobilised at electrode surfaces the [Ru(NH 3 ) 6 ] 3+ redox marker is commonly used for quantitative characterisation and/or detection purposes. Although it is generally assumed that the interactions between the hexaammineruthenium(III) cation and DNA are solely of electrostatic nature, and thus non-specific in essence, it is shown herein that this is not the case for a sequence containing a G-quadruplex part. The present work investigates the interactions of [Ru(NH 3 ) 6 ] 3+ with an oligonucleotide (21-mer) consisting of the thrombin binding aptamer sequence (a G-quadruplex) elongated by a 5′-overhang of six nucleotides. Combining circular dichroism and electrochemical analyses by differential pulse voltammetry, alternating current voltammetry and cyclic voltammetry, it is demonstrated that the overall interaction stoichiometry (number of [Ru(NH 3 ) 6 ] 3+ bound to one DNA strand) is lower than the 7 [Ru(NH 3 ) 6 ] 3+ per strand expected for a total charge compensation. Our results indicate that two types of interaction take place between [Ru(NH 3 ) 6 ] 3+ and the elongated TBA sequence: one is the conventional electrostatic binding of the positively charged redox marker with the negatively charged phosphate groups, while the other is connected with the G-quadruplex part of the sequence. This latter interaction influences the DNA conformation in solution and involves 2 [Ru(NH 3 ) 6 ] 3+ per strand, both in solution and with DNA immobilised at gold electrodes. In this configuration, the electron transfer rate constant is evaluated at 37 ± 5 s −1 on the basis of a Laviron plot.
Sterol metabolism. XLI. Cholesterol A-ring autoxidations
Abstract Chromatographic and spectral evidence is adduced for the presence of cholest-5-en-3-one, cholest-4-en-3-one, and cholest-4-ene-3,6-dione in samples of cholesterol aged naturally in air or subjected to irradiation in air by 60 Co gamma radiation. These findings establish an additional mode of air oxidation of cholesterol to A-ring 3-ketones. Moreover, the oxidation by air of cholest-5-en-3-one induced by 60 Co gamma radiation yielded cholest-4-en-3-one, cholest-4-ene-3,6-dione, and the epimeric 3-oxocholest-4-ene-6-hydroperoxides. Cholest-4-en-3-one was not altered by irradiation in air, nor was cholesterol isomerized to cholest-4-en-3β-ol upon irradiation. From these observations it is deduced that the radiation-induced A-ring dehydrogenation of cholesterol yields initially cholest-5-en-3-one which upon isomerization yields cholest-4-en-3-one not further oxidized and which by a second oxidation yields the epimeric 3-oxocholest-4-ene-6-hydroperoxides which decompose to cholest-4-ene-3,6-dione.