2-thiobarbituric acid test for lipid oxidation in food:: Synthesis and spectroscopic study of 2-thiobarbituric acid-malonaldehyde adduct

Synthesis, purification, elemental analysis, and spectroscopic studies were undertaken to characterize the structure of the red adduct 2:1 thiobarbituric acid (TBA)-malonaldehyde involved in the evaluation of oxidative rancidity in fats and oils. Thin-layer chromatography, infrared and ultraviolet-visible absorption,H-1 (H-1 NMR) and C-13 nuclear magnetic resonance (NMR) spectra were used. A yield of 93% was obtained in the synthesis. The results of elemental analysis agree with the formula for the chloro-monohydrated form, C11H11N4O5S2Cl. Three characteristic absorption maxima at 532, 310, and 245 nm,respectively, were shown in acid aqueous medium (pH 2.9). The characteristic vibrations assigned to the -NH, -OH, (CH)-H-alpha (exocyclic) and -C=S groups were confirmed in the infrared spectra. There was no evidence of thioenolization. H-1 NMR data at delta 5.10 (-CONH- group, H2O and HCl molecules); delta 11.54 (-OH group of keto-enol tautomer, -NH group); and delta 176.4 (-CONH- group) also were observed. The experimental results obtained were consistent with the existence of two spectral equivalent tautomeric structures. The colored adduct was compared with other TBA-aldehyde compounds.