ascorbic-acid and 1-4-dioxane

The oxidation of ascorbate with the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical in water and water-dioxane mixed solvent has been demonstrated to be a proton-coupled electron transfer (PCET) process, involving hydrogen tunnelling at room temperature. The magnitude of the kinetic isotope effect (KIE) k(H)/k(D) in the reaction increases with decrease of the solvent polarity. The evidence comprise: (a) the spectroscopic and kinetic evidence for the interaction of ascorbate and TEMPO; (b) the observation of KIEs k(H)/k(D) of 24.2(0.6) in water and 31.1(1.1) in 1:1 v/v water-diox. (diox = dioxane), at 298 K; (c) the observation of isotope effect on the Arrhenius prefactor, A(H)/A(D) of 0.6(0.2) in the reaction in water and 1.2(0.2) in 1:1 v/v water-diox solvent; (d) the observation of isotope differences in the enthalpies of activation in water and D(2)O, Delta(r)H(double dagger) (in H(2)O) = 31.0(0.4) kJ/mol, Delta(r)H(double dagger) (in D(2)O) = 40.0 (0.5) kJ/mol; in 1:1 v/v water-diox and 1:1 v/v D(2)O-diox, Delta(r)H(double dagger) (in H(2)O/diox) = 23.9(0.2) kJ/mol, Delta(r)H(double dagger) (in D(2)O/diox) = 32.1(0.3) kJ/mol; (e) the temperature dependence of the KIEs in water and 1:1 v/v water-dioxane; these KIEs range from 27.3 at 285.4 K to 19.1 at 317.4 K in water and from 34.3 to 24.6 at the corresponding temperatures in 1:1 v/v water-diox, respectively; (f) the observation of an increase of the KIE in 10-40% v/v dioxane-water solvents relative to the KIE in water alone. There is a weak solvent dependence of the rate constant on going from water to 1:1 v/v water-diox. solvent, from 2.20(0.03) mol(-1) dm(3) s(-1) to 5.50(0.14) mol(-1) dm(3) s(-1), respectively, which originates from the mutual compensation of the enthalpy and entropy of activation.

Topics: Ascorbic Acid; Cyclic N-Oxides; Deuterium; Dioxanes; Isotopes; Kinetics; Solvents; Temperature; Thermodynamics; Water

6-O-dodecanoyl-2-O-α-D-glucopyranosyl-L-ascorbic acid (6-sDode-AA-2G) was synthesized from 2-O-α-D-glucopyranosyl-L-ascorbic acid and vinyl laurate with a protease from Bacillus subtilis in 30% dimethylformamide (DMF)/dioxane with a low water content. The addition of 3% (v/v) water to DMF/dioxane dramatically enhanced the 6-sDode-AA-2G synthesis. The optimum reaction conditions enabled 6-sDode-AA-2G to be synthesized in a yield of 38.1%.

Topics: Acylation; Ascorbic Acid; Bacillus subtilis; Catalysis; Dimethylformamide; Dioxanes; Endopeptidases; Industrial Microbiology; Solvents; Water

In the NO2-exposure of tyrosine in 70% dioxane/phosphate buffer (pH 7.4), beta-carotene enhanced the degradation of tyrosine and/or 3-nitrotyrosine produced, whereas alpha-tocopherol and ascorbyl palmitate inhibited the transformation of tyrosine into 3-nitrotyrosine. Generation of certain active species in the interaction of beta-carotene with NO2 was suggested. Ascorbyl palmitate effectively and alpha-tocopherol slightly inhibited the transformation of tyrosine in the NO2-exposure in the presence of beta-carotene. In the reaction of tyrosine with ONOO-/ONOOH, beta-carotene enhanced the degradation of 3-nitrotyrosine produced suggesting generation of certain active species, whereas alpha-tocopherol and ascorbyl palmitate completely suppressed the transformation of tyrosine into 3-nitrotyrosine.

Topics: Ascorbic Acid; beta Carotene; Dioxanes; Dose-Response Relationship, Drug; Free Radical Scavengers; Nitrogen Dioxide; Nitrous Acid; Peroxynitrous Acid; Time Factors; Tyrosine; Vitamin E