Contrasting Voltammetric Behavior of Different Forms of Vitamin A in Aprotic Organic Solvents

Conjugation Length Dependence of Free Radical Scavenging Efficiency of Retinal and Retinylisoflavonoid Homologues

Retinal (C20) and the C25 and C30 homologues were compared as radical scavengers together with their C22, C27, and C32 homologues linked with daidzein through a B'3 (isoflavonoid) to oxo-carbon (aldehyde) covalent bond. Oxidation potential in acetonitrile determined by cyclic voltammetry and ionization potential calculated by density functional theory for the aldehydes and dyads (conjugates), of which the two longer are new, decreased linearly with the wavenumber for absorption maximum. The logarithm of the second-order rate constant for scavenging of the ABTS^•+ increased linearly with decreasing oxidation potential suggesting that longer conjugation in the antioxidant increases the rate of electron transfer. A similar linear free energy relationship was found for the rate of scavenging DPPH^•, including daidzein, which may indicate involvement of hydrogen atom transfer from an isoflavonoid phenol. Prediction of radical scavenging efficiency from visible absorption spectra was demonstrated with the perspective of rational design of bifunctional amphiphilic antioxidants.

Electrochemical Study and Determination of All-trans-Retinol at Carbon Paste Electrode Modified by a Surfactant

The oxidation mechanism of all-trans-retinol (vitamin A₁) and its several esters in non-aqueous, aqueous organic mixture, and pure aqueous media was investigated by cyclic voltammetry. The oxidation occurred in several irreversible steps. The calculated highest density of electrons in retinoid molecules which are delocalized over carbon atoms of the five conjugated double bonds (C5-C14) was found in the part of the molecule involved in oxidation processes. The most sensitive oxidation peak (at +0.8 V vs. Ag/AgCl) was used for development of new direct voltammetric method based on differential pulse voltammetry for the determination of retinol at carbon paste electrode modified with surfactant sodium dodecyl sulfate (CPE/SDS). The results show that 30% (by mass) of modifier SDS exhibited optimal sensitivity and shape of voltammograms. Compared to commonly used glassy carbon electrode (GCE), the CPE/SDS showed significant progress in the retinol electroanalysis. The linear ranges for retinol determination were 1.5·10⁻⁶–1.8·10⁻⁴ M for CPE/SDS and 4.4·10⁻⁶–7.0·10⁻⁴ M for GCE with the detection limits of 1.3·10⁻⁶ and 4.6·10⁻⁷ M, respectively.

Square wave voltammetry at carbon paste electrode modified with surfactant for alpha tocopheryl acetate determination in cosmetics

The aim of this study was describe electrochemical properties of a carbon paste electrode (CPE) bulk modified with 30% (w/w) surfactant sodium dodecyl sulphate (CPE/SDS) and demonstrates its application in the determination of α‑tocopheryl acetate (α‑TAc), known as vitamin E acetate, in selected cosmetic products, especially body creams. In addition to anionic SDS, cationic hexadecylpyridinium chloride monohydrate (CPC) was also tested as possible modifier. It was found that selection of surfactant type and its content significantly affect an electrical conductivity and mechanical stability of these heterogeneous electroanalytical sensors in pure organic solvents. Under this study, it was found that CPC is a totally inappropriate mediator due to very high backgroundcurrent. Together with other lipophilic vitamins characterized by antioxidant activity (dominantly retionoids), this completely synthetic substance is widely used as significant cosmetic additive due its preservative properties. Monitoring of its content in cosmetic products is usually performed by high‑performance liquid chromatography (HPLC) with UV detection. This standard analytical protocol is always burdened with the complex and time‑consuming preparation of the sample before analysis. For that reason, robust and simple electroanalytical method based on anodic oxidation of the α‑TAc at CPE/SDS by square wave voltammetry (SWV) performed in pure organic electrolyte (99.8% acetonitrile containing 0.1 mol·L‑1 LiClO4) was developed. Moreover, simple dissolution of sample in supporting electrolyte using ultrasonic bath and subsequent filtering through a stacked filter included all the necessary procedures for sample preparation. The linear range from 0.1 to 1.2 mmol·L‑1and limit of detection 37 µmol·L‑1 were found at pulse amplitude 10 mV and frequency 10 Hz as optimum. In analysis of selected cosmetics, the developed electroanalytical method was not validated using comparison with standard HPLC. At least, the recovery was verified by analysis of model sample and value 95.8% was calculated.