Capillary electrochromatographic evaluation of vitamin E-active oil constituents: tocopherols and tocotrienols

Kinetic study of the reaction of thiophene-tocopherols with peroxyl radicals enlightenings the role of O˙⋯S noncovalent interactions in H-atom transfer

Three new α-tocopherol thiophene derivatives were efficiently synthesized, characterized and used for the first time as chain-breaking antioxidants for the inhibition of the autoxidation of reference oxidizable substrates. The rate constant of the reaction with alkylperoxyl (ROO˙) radicals and the stoichiometry of radical trapping (n) for the thiophene-tocopherol compounds were determined by measuring the oxygen consumption during the autoxidation of styrene or isopropylbenzene, using a differential pressure transducer. The measurement of the reaction with ROO˙ radicals in an apolar solvent at 30 °C showed inhibition rate constants (kinh) in the order of 104 M-1 s-1. To rationalise the kinetic results, the effect of the thiophene ring on the H-atom donation by O-H groups of the functionalized tocopherols was investigated by theoretical calculations. The importance of noncovalent interactions (including an unusual O˙⋯S bond) for the stability of the conformers has been shown, and the O-H bond dissociation enthalpy (BDE(OH)) of these derivatives was determined. Finally, the photophysical properties of these new compounds were investigated to understand if the addition of thiophene groups changes the absorption or emission spectra of the tocopherol skeleton for their possible application as luminescent molecular probes.

A review of characterization of tocotrienols from plant oils and foods

Tocotrienols, members of the vitamin E family, are natural compounds found in a number of vegetable oils, wheat germ, barley and certain types of nuts and grains. Vegetable oils provide the best sources of these vitamin E forms, particularly palm oil and rice bran oil contain higher amounts of tocotrienols. Other sources of tocotrienols include grape fruit seed oil, oats, hazelnuts, maize, olive oil, buckthorn berry, rye, flax seed oil, poppy seed oil and sunflower oil. Tocotrienols are of four types, viz. alpha (α), beta (β), gamma (γ) and delta (δ). Unlike tocopherols, tocotrienols are unsaturated and possess an isoprenoid side chain. A number of researchers have developed methods for the extraction, analysis, identification and quantification of different types of vitamin E compounds. This article constitutes an in-depth review of the chemistry and extraction of the unsaturated vitamin E derivatives, tocotrienols, from various sources using different methods. This review article lists the different techniques that are used in the characterization and purification of tocotrienols such as soxhlet and solid-liquid extractions, saponification method, chromatography (thin layer, column chromatography, gas chromatography, supercritical fluid, high performance), capillary electrochromatography and mass spectrometry. Some of the methods described were able to identify one form or type while others could analyse all the analogues of tocotrienol molecules. Hence, this article will be helpful in understanding the various methods used in the characterization of this lesser known vitamin E variant.

CEC and EKC of natural compounds

In this review, an overview of CEC and EKC methods with their developments are summarized for different natural compounds. It is divided into three main parts. The first part elaborates the separation of lipophilic compounds without any charged groups. The second part constitutes CEC and EKC of lipophilic compounds containing ionizable functional groups whereas the third part contains hydrophilic compounds. Packed, monolithic, coated, or raw fused-silica (FS) capillaries are among the choice for stationary phases. Applications of these phases on the above-mentioned three classes of compounds, coupled with different detection methods, e.g. MS or LIF, are explored and their advantages and disadvantages are discussed.

Use of a polar-embedded stationary phase for the separation of tocopherols by CEC

A polar-embedded stationary phase (ULTIMA C18) has been investigated for the separation of alpha-, beta-, gamma- and delta-tocopherols by CEC in comparison with commercially available C(18) and C(30) n-alkyl RPs. The behavior of this stationary phase was tested for different mobile phases based on methanol, ACN, or mixtures thereof and different separation parameters such as retention factors and resolution were evaluated. The main feature of this stationary phase is the improved selectivity for the separation of beta- and gamma-tocopherols (positional isomers) when compared with the pure n-alkyl C(18) material, which was unable to resolve these compounds. Additionally, it is possible to observe a reversal in the elution order of the beta- and gamma-tocopherol isomers with respect to that obtained on the C(30) column. The resulting data indicate that the enhanced selectivity obtained with the polar-embedded stationary phase, with respect to the conventional C(18) material, is due to the participation of both hydrophobic and polar interactions: these latter are of the hydrogen bridge type with the amide group of the polar-embedded stationary phase, which increases the retention of the tocopherols and facilitates the discrimination between the beta- and gamma-isomers. Adequate separation of the four tocopherols was obtained by CEC using the polar-embedded stationary phase and 95:5 v/v methanol/water (5 mM Tris, final concentration) as the mobile phase.

Comparison of different fluorimetric signals for the simultaneous multivariate determination of tocopherols in vegetable oils

This paper deals with the simultaneous determination of the quaternary mixture of tocopherols (alpha-, beta-, gamma-, and delta-T) performed using fluorimetric techniques and partial least squares (PLS-1) multivariate analysis. In this study, PLS-1 was applied to matrices made up of fluorescence excitation and emission spectra (EEM) and with fluorescence excitation, emission, and synchronous spectra (EESM) of tocopherols dissolved in hexane: diethyl ether (70:30 v/v). A calibration set of 55 samples based in a central composite plus a full factorial plus a fractionated factorial design was constructed. When synthetic samples were analyzed, recoveries around 100% were obtained and detection limits were calculated using EEM and EESM. For the analysis of the oils, the samples, diluted in hexane, were cleaned in silica cartridges and tocopherols were eluted with hexane: diethyl ether (90:10 v/v). The developed method was applied to different edible oils. The results are satisfactory for alpha-, beta-, and gamma-, but they are worse for delta-T.

Characterization of hydroxyaromatic compounds in vegetable oils by capillary electrophoresis with direct injection in an oil-miscible KOH/propanol/methanol medium

The separation of hydroxyaromatic compounds in vegetable oils, including synthetic antioxidants (3-tert-butyl-4-hydroxyanisol and 2,6-di-tert-butyl-4-hydroxytoluene), E-vitamers and other natural oil components, by nonaqueous capillary electrophoresis in an oil-miscible background electrolyte (BGE) was investigated. The BGE contained 40 mM KOH in a methanol/1-propanol (PrOH) mixture (15:85 v/v). The oil samples were 1:1 diluted with PrOH and directly injected in the capillary. Under negative polarity (cathode at the injection end), the anionic solutes moved faster than the electroosmotic flow, being well-resolved among them and from the triacylglycerols. Using virgin palm, extra virgin olive, wheat germ, virgin soybean and other oils, the capability of the procedure to quickly yield a characteristic profile of the biophenols present in the sample was demonstrated. The alpha-, (beta + gamma)- (as unresolved pair) and delta-tocopherols of a soybean oil sample were quantified.

Analysis of natural antioxidants by capillary electromigration methods

In this work, an exhaustive survey of capillary electromigration methods used to analyze natural antioxidants is presented together with some discussion of the use of these substances use as functional foods. This review provides an updated and exhaustive overview of the separation and identification by capillary electrophoresis of natural compounds with antioxidant activity found in natural matrices and/or foods. The compounds concerned are catechins, isoflavones, anthocyanins, phenolic acids, vitamins, as well as other less common natural substances that have shown antioxidant activity.

Rapid assay of vitamin E in vegetable oils by reversed-phase capillary electrochromatography

A rapid capillary electrochromatographic (CEC) method for the analysis of vitamin E in vegetable oils is reported. Vitamin E consists of a group of eight isomers, tocopherols (TOHs) and tocotrienols. The separation of four TOHs (alpha-, gamma-, delta-TOH), alpha-tocopherol acetate (alpha-TOH-Ac), and an antioxidant compound, butylated hydroxytoluene (BHT) used to prevent TOH autoxidation, was optimized. The CEC experiments were carried out in a 75 microm inner diameter (ID) fused-silica capillary, partially packed with 3 microm C(18 )stationary phase (33 cm total length, 8.4 cm and 7 cm effective and packed lengths, respectively). The optimum mobile phase was a polar organic phase composed of a mixture of methanol-acetonitrile in the ratio 50/50 v/v containing 0.01% ammonium acetate, applying a voltage and temperature set at -25 kV and 20 degrees C, respectively. The tocopherols and the BHT were successfully separated within 2.5 min using the short-end injection method. Under these experimental conditions, repeatability of retention time and peak area, analyte detection and quantitation limits, linearity, precision, and accuracy were studied. The CEC method was applied to determine the content of TOHs in different commercially available oils of virgin olive, hazelnut, sunflower, and soybean. The extraction of vitamin E isomers from oil samples was achieved using methanol and a methanol-isopropanol mixture.

Separation of tocopherols by nano-liquid chromatography

Nanoliquid chromatography (nano-LC) was used for the separation of tocopherols (delta-, gamma-, alpha-TOH), alpha-tocopherol acetate (alpha-TOH-Ac) and an antioxidant compound, namely butylated hydroxytoluene (BHT) used to prevent TOHs autoxidation. The separation was carried out in a fused silica capillary of 100 microm I.D. and 375 microm O.D. packed in our laboratory with RP18 silica stationary phase of either 5- or 3-microm diameter (23-cm long). The mobile phase was composed by mixtures of methanol (MeOH), acetonitrile (MeCN) and water. Typical analyses time for the separation of all the five components of the mixture were 6-9 min depending on the composition of the mobile phase. Efficiency and resolution were strongly influenced by the particle diameter and the highest Rs and N/m values were observed using 3-microm RP18 particles. Experiments performed with capillaries packed with 3-microm RP18 particles provided good limit of detection (LOD) and limit of quantification (LOQ) (for delta-, gamma-TOH, alpha-TOH-Ac were 4 and 8 microg/ml, while for alpha-TOH were 6 and 10 microg/ml, respectively). The optimized method was applied to extracts of serum and pharmaceutical preparation containing alpha-TOH and alpha-TOH-Ac.

Separation of delta-, gamma- and alpha-tocopherols by CEC

In this study capillary electrochromatography (CEC) was used for the separation of three tocopherols (TOHs), namely delta-, gamma- and alpha-TOH and the antioxidant compound, butylated hydroxytoluene (BHT). The CEC experiments were carried out using an octadecylsilica (ODS) stationary phase packed, in our laboratory, in a fused-silica capillary (100 microm I.D., 365 microm O.D. x 33 cm of total length and 24.6 or 8.4 cm effective length). The mobile phase was composed by a mixture of methanol (MeOH) and acetonitrile (ACN), at different concentrations and 0.01% (w/v) of ammonium acetate. Retention time (t(R)), retention factor (k), resolution (R(s)) of the three TOHs were strongly influenced by the organic solvent composition of the run buffer and by the effective length of the capillary. Optimum experimental conditions were found even employing the short effective length of the capillary achieving the baseline separation of the studied analytes in a relatively short time (less than 5 min). The optimized method was applied to the qualitative analysis of vitamin E (alpha-TOH) present in a human serum extract.

Elution behavior of unsaponifiable lipids with various capillary electrochromatographic stationary phases

Capillary electrochromatographic (CEC) separations of unsaponifiable lipids, tocopherols (T), tocotrienols (T3), and plant sterols were studied under various conditions. Investigated stationary phases include pentafluorophenylsilica (PFPS), triacontylsilica (TCS), and octadecylsilica (ODS) phases. A baseline separation of four sterols (ergosterol, lanosterol, sitosterol and stigmasterol) on ODS was achieved and their elution order was found to be dictated by side-chain structures. CEC of the tocol-derived compounds on PFPS in aqueous methanol yielded the most satisfactory results with complete resolution of all components eluting in the order deltaT3>beta3>gammaT3>epsilonP>alphaT3>deltaT>zeta2T>betaT>gammaT>alphaT, while a reversal in elution of the epsilonT-alphaT3 pair was observed in aqueous acetonitrile. CEC with a TCS phase in non-aqueous methanol led to a different elution pattern deltaT3>gammaT3>betaT3>alphaT3epsilonT>deltaT>(zeta2+gamma)T>betaT>alphaT, despite favorable resolution of the (gamma-zeta2)T pair along with the observation of inseparable(beta-gamma)T and (beta-gamma)T3 pairs in non-aqueous dimethylformamide. Non-aqueous acetonitrile mobile phases provided unique selectivity for the (gamma-zeta2)T pair and isomer separations on TCS. Variations in separation and retention factors of relevant antioxidant species with CEC variables were evaluated. Examples of CEC quantification of unsaponifiable fractions of rice bran oils and soybean oils are presented.