João P. S. Farah

Assessing the separation of neutral plant secondary metabolites by micellar electrokinetic chromatography

In this work, partition coefficients (Pwm) and solute-micelle association constants per monomer (Km/N) were measured using micellar electrokinetic chromatography in tetraborate-sodium dodecylsulfate electrolytes for 18 important plant secondary metabolites (coumarin, verbenone, camphor, eucalyptol, carvone, alpha-terpineol, linalool, jasmone, bergapten, rose oxide, geraniol, t-anethole, citronellal, citronellol, p-cymene, limonene, caryophyllene and nerol) of wide occurrence in herbal extracts and essential oils. Caryophyllene presented a retention time longer than anthracene (micelle marker) and its set of constants could not be determined accurately. Pwm and Km/N were generated by the non-linear data fitting of both partition and solute-micelle association models for the 17 solutes under consideration (caryophyllene excluded). Pwm varied from 147 (coumarin) to 13175 (limonene) while Km/N varied from 37 (coumarin) to 3721 (limonene). Under optimal conditions, the separation of the selected compounds was attempted successfully in commercialized samples of rose, anise and geranium essential oils.

Development and validation of a liquid chromatography method for anthocyanins in strawberry (Fragaria spp.) and complementary studies on stability, kinetics and antioxidant power

A RPLC-DAD method for the analysis of eight anthocyanins was developed, validated and applied to strawberry extracts. The chromatographic method was conducted under gradient elution in acidulated water-methanol mobile phase and octadecyl-silica columns. An ultrasound extraction procedure was optimized by a 3(2) factorial design (%HCl in methanol, temperature, and time) and response surface methodology. Method validation was performed according to the following parameters: linearity (R(2)>0.99, p-value<10(-4), F>725), LOD (3-7 μmol L(-1)) and LOQ (9-22 μmol L(-1)), selectivity/specificity (baseline separation of all analytes and peak purity), instrumental precision (<6.4%CV), repeatability (<6.3%CV) and intermediate precision (<9.9%CV), recovery (83-99%), robustness (mobile phase pH, column temperature and flow rate) and stability (high temperatures and storage; 1st order kinetics). The antioxidant power of anthocyanins was measured on-line (ABTS(+) reaction; Trolox as reference). Ten strawberry extracts were quantified (average values: 24.2 μg/g for cyanidin-3-glucoside and 49.1 μg/g for pelargonidin-3-glucoside).

Using quantum chemistry to predict solubilization in detergent micelles

Experimental Ks values for the solubilization of 66 non-ionic solutes in micelles of the anionic detergent sodium dodecyl sulfate can be reproduced using quantum chemical surrogate parameters, the calculation of which requires only a knowledge of the solute molecular structure. The parameters evaluated include the solute molar volume (Vx), the solute HOMO and LUMO energies, the magnitudes of the charges of the most positive hydrogen and most negative atom of the molecule (q+ and ¦q−¦, respectively), the molecular dipole moment and the solute “dipolarity” (the molecular polarizability divided by Vx). The resultant linear solvation free energy relationship (LSER) based on these surrogate parameters (log Ks = −0.487 + 1.27q+ −3.76¦q−¦ + 2.97Vx) is chemically reasonable and consistent with the LSER for micellar solubilization based on experimentally-derived solute parameters. The use of quantum chemical parameters to develop LSERs should thus prove to be a convenient approach for predicting solubilities in organized media from solute molecular structure.

Transition State Structure for the Reversible Hydration of 1, 3-Dichloroacetone in the Presence of Aerosol-OT Reversed Micelles in Hexane

Rate and equilibrium constants, and the kinetic orders with respect to water for the reversible hydration of 1, 3-dichloroacetone (DCA) in the presence of bis-2-ethylhexyl sodium sulphosuccinate (Aerosol-OT or AOT) reversed micelles in hexane differ from those observed in aqueous dioxane. The transition state (TS) structure for the micellar reaction was probed by subjecting the reaction to a proton inventory study. The results show that the TS structure depends on the molar ratio water/surfactant (R). At a low R value (1.3) a TS including the ketone and one water molecule probably H-bonded to the surfactant, is implied. Proton inventories at higher R values (4.2, 8.3, and 11.1) indicate that a second water molecule is participating as a “general base”. In all cases, the results are compatible with an acyclic TS with one proton “in flight”, and the degree of proton transfer to the second water molecule increases as a function of increasing R. The activation parameters at different water concentrations agree with the proposed TS structures, and show that the micellar rate enhancement is mainly due to entropy increase.