Remarkable enhancement of antioxidant activity of vitamin C in an artificial bilayer by making it lipo-soluble

Process optimization and kinetic evaluation for biosynthesis of D-isoascorbyl stearate

The synthesis of D-isoascorbyl stearate from D-isoascorbic acid and stearic acid with immobilized lipase (Novozym(®)435) as catalyst was studied. Response surface methodology and Box-Behnken design with six variables and three levels were employed to evaluate the effects of processing conditions on the conversion of D-isoascorbic acid. The results confirmed that the response surface method and statistical analysis were proved to be useful in developing optimal conditions for D-isoascorbyl stearate synthesis. The optimum conditions were predicted as follows: reaction temperature 48 °C, reaction time 17.7 h, immobilized lipase amount 50.0 % (w/w, of D-isoascorbic acid), substrate molar ratio 9:1 (stearic acid to D-isoascorbic acid), D-isoascorbic acid concentration 0.14 mol/L (based on solvent), 4A molecular sieve addition 200 g/L (based on solvent), and the optimal conversion was 90.6 %. Through the kinetics model fitting of the esterification, it was considered that the esterification conformed to a Ping-Pong bi-bi kinetic model with D-isoascorbic acid inhibition, and the obtained kinetic constants showed that the inhibition of D-isoascorbic acid and the enzyme affinity to substrate were abate with the increase of the reaction temperature.

Kinetic and thermodynamic investigation of enzymatic L-ascorbyl acetate synthesis

Kinetics and thermodynamics of lipase-catalyzed esterification of l-ascorbic acid in acetone were investigated by using vinyl acetate as acyl donor. The results showed that l-ascorbic acid could generate inhibition effect on lipase activity. A suitable model, Ping-Pong Bi-Bi mechanism having substrate inhibition, was thus introduced to describe the enzymatic kinetics. Furthermore, the kinetic and thermodynamic parameters were calculated from a series of experimental data according to the kinetic model. The inhibition constant of L-ascorbic acid was also obtained, which seemed to imply that enhancing reaction temperature could depress the substrate inhibition. Besides, the activation energy values of the first-step and the second-step reaction were estimated to be 37.31 and 4.94 kJ/mol, respectively, demonstrating that the first-step reaction was the rate-limiting reaction and could be easily improved by enhancing temperature.

Biocatalytic synthesis of ascorbyl esters and their biotechnological applications

Ascorbyl fatty acid esters act both as antioxidants and surfactants. These esters are obtained by acylation of vitamin C using different acyl donors in presence of chemical catalysts or lipases. Lipases have been used for this reaction as they show high regioselectivity and can be used under mild reaction conditions. Insolubility of hydrophilic ascorbic acid in non-polar solvents is the major obstacle during ascorbic acid esters synthesis. Different strategies have been invoked to address this problem viz. use of polar organic solvents, ionic liquids, and solid-phase condensation. Furthermore, to improve the yield of ascorbyl fatty acid esters, reactions were performed by (1) controlling water content in the reaction medium, (2) using vacuum to remove formed volatile side product, and (3) employing activated acyl donors (methyl, ethyl or vinyl esters of fatty acids). This mini-review offers a brief overview on lipase-catalyzed syntheses of vitamin C esters and their biotechnological applications. Also, wherever possible, technical viability, scope, and limitations of different methods are discussed.

Plasmatic antioxidant capacity due to ascorbate using TEMPO scavenging and electron spin resonance

BACKGROUND

Ascorbate is the most effective water-soluble antioxidant and its plasma concentration is usually measured by different methods including colorimetric assays, HPLC or capillary electrophoresis. Plasma antioxidant capacity is determined by indexes such as total reactive antioxidant potential, total antioxidant reactivity, oxygen radical absorbance capacity, etc. We developed an alternative method for the evaluation of the plasma antioxidant status due to ascorbate.

METHODS

TEMPO kinetics scavenging analyzed by ESR spectroscopy was performed on plasma samples in different antioxidant situations. Plasma ascorbate concentrations were determined by capillary electrophoresis. Ascorbyl radical levels were measured by ESR.

RESULTS

Plasma reactivity with TEMPO (PR-T) reflected plasma ascorbate levels. Average PR-T for normal plasmas resulted 85+/-27 micromol/l (n=43). PR-T during ascorbic acid intake (1 g/day) increased to an average value of 130+/-20 micromol/l (p<0.001, n=20). PR-T correlated with the plasmatic ascorbate levels determined by capillary electrophoresis (r=0.92), presenting as an advantage the avoiding of the deproteination step. Plasma ascorbyl radical levels increase from 16+/-2 to 24+/-3 nmol/l (p<0.005, n=14) after ascorbate intake.

CONCLUSIONS

PR-T could be considered as a measure of the plasmatic antioxidant capacity due to the plasma ascorbate levels and could be useful to investigate different antioxidant situations.

Effect of colloidal carriers on ascorbyl palmitate stability

Active compounds can be protected against degradation by incorporation into colloidal carrier systems. The stabilizing effect of carrier systems for ascorbyl palmitate (AP) was investigated using microemulsions (ME), liposomes and solid lipid nanoparticles (SLN). Analysis of chemical stability by HPLC showed that AP is most resistant against oxidation in non-hydrogenated soybean lecithin liposomes, followed by SLN, w/o and o/w ME, and hydrogenated soybean lecithin liposomes. The molecular environment of the AP-like nitroxide probe (C(16)-Tempo) in colloidal carriers was characterized using electron paramagnetic resonance (EPR) spectroscopy. We have found that the nitroxide groups are located in environments with different polarity and mobility. The hydrophilic part of AP is the reactive moiety, and high stability is obtained in systems in which this part is exposed to a less polar environment. Additionally, the determined accessibility of nitroxide groups to reduction correlated well with the chemical stability of AP. It is more deeply immersed in the interface when entrapped in a liquid-state carrier than when applied in gel-state particles. Encapsulation of AP in SLN core leads to greater stability. We conclude that the location of the sensitive group of the drug-molecule in a carrier system is crucial for its stability.

Making vitamin C lipophilic enhances its protective effect against free radical induced peroxidation of low density lipoprotein

The peroxidation of human low density lipoprotein (LDL) was initiated by water-soluble initiator 2,2'-azobis(2-amidinopropane hydrochloride) (AAPH) and copper and inhibited by vitamin C (VC) and its lipophillic derivatives ascorbyl-6-caprylate (VC-8), 6-laurate (VC-12) and 6-palmitate (VC-16), respectively. The peroxidation was monitored by oxygen uptake, by decay of alpha-tocopherol and by formation of lipid oxidation products. Kinetic analysis of the antioxidation process demonstrates that the VCs can work synergistically with endogenous antioxidants (vitamin E, ubiquinol-10, beta-carotene etc.) in LDL to suppress the peroxidation and that the synergistic effect of the lipophilic VCs is appreciably higher than that of their lipophobic parent molecule. It is also shown that VC and especially the lipophilic VCs, are much more effective in inhibition of copper-induced than AAPH-induced LDL peroxidation.