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.