Antioxidant activity and calcium binding of isomeric hydroxybenzoates

The association constant for calcium binding to hydroxybenzoates in aqueous 0.16 M NaCl at 25 °C was found electrochemically to have the value K(ass) = 280 mol L(−1) with ΔH(o) = −51 kJ mol(−1), ΔS(o) = −122 J mol(−1) K(−1) for the 2-isomer (salicylate), K(ass) = 7 mol L(−1) with ΔH(o) = −39 kJ mol(−1), ΔS(o) = −116 J mol(−1) K(−1) for the 3-isomer, and K(ass) = 8 mol L(−1) with ΔH(o) = −51 kJ mol(−1), ΔS(o) = −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 K(sp) = 4.49·10(−6) based on activity with ΔH(o) = 67 kJ mol(−1), ΔS(o) = 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.