Antioxidant activity and calcium binding of isomeric hydroxybenzoates   全文下載

Antioxidant activity and calcium binding of isomeric hydroxybenzoates

The association constant for calcium binding to hydroxybenzoates in aqueous 0.16 M NaCl at 25 ℃ was found electrochemically to have the value Kass = 280 mol L -1 with ΔH° = -51 kJ mol -1, ΔS° = -122 J mol -1 K -1 for the 2-isomer (salicylate), Kass = 7 mol L -1 with ΔH° = 39 kJ mol -1, ΔS° = 116 J mol -1 K -1 for the 3-isomer, and Kass = 8 mol L -1 with ΔH° = -51 kJ mol -1, DS° = -155 J mol -1 K -1 for the 4-isomer. The 3- and 4-isomers were found more efficient as antioxidants than the 2-isomer in decreasing oxygen consumption rate in a peroxidating methyl linoleate emulsion and less sensitive to presence of calcium. All isomers were found prooxidative for iron-catalyzed initiation of oxidation due to enhanced radical formation as shown by electron spin resonance spectroscopy. Calcium salicylate was found to have low solubility with a solubility product Ksp = 4.49$10 -6 based on activity with ΔH° = 67 kJ mol -1, ΔS° = 123 J mol -1 K -1 for dissolution in water, when corrected for the strong complex formation. Calcium in food and beverages may thus lower antioxidant activity of plant phenols through complexation or by precipitation.