Structural changes of sinapic acid during alkali-induced air oxidation and the development of colored substances

Structural changes of sinapic acid were induced by air oxidation in aqueous solutions at pH 7-10 and followed by spectral and high-performance liquid chromatographic (HPLC) analysis. Color properties of the sinapic acid solutions were determined by taking the transmittance spectra, calculating the Commission Internationale de l'Eclairage (CIE) 1931 tri-stimulus values, and converting to Hunter L a b values. Reaction rate constants for sinapic acid were determined by a kinetic study based on the quantitative results from HPLC analysis. These reactions were first order with respect to sinapic acid and fit the appropriate equation with a coefficient of R-2 > 0.97. Sinapic acid was converted to thomasidioic acid with reaction rate constants (k) of 8.54 x 10(-6), 2.51 x 10(-5), and 4.87 x 10-5 (-1) in phosphate-boric acid buffers of pH 7, 8.5, and 10, respectively: Similar reactions in ammonium bicarbonate buffers were more than 10 times faster. With time, thomasidioic acid further converted to 2,6-dimethoxy-p-benzoquinone and 6-hydroxy-5,7-dimethoxy-2-naphthoic acid. Air oxidation of sinapic acid aqueous solutions caused darkening of the color for the system, with the 2,6-dimethoxy-p-benzoquinone as a major color contributor.