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.