Formation of yellow, orange, and red pigments in the reaction of alk-2-enals with 2-thiobarbituric acid

The reaction of four to eight carbon straight-chain alk-2-enals with 2-thiobarbituric acid (TBA) produced yellow 455-nm-, orange 495-nm-, and red 532-nm-absorbing pigments depending upon the reaction conditions. The 1:1 reaction of the aldehydes with TBA in 15% acetic acid at 100 degrees C produced the yellow pigment at 0.25 h and the red at 6 h. The reaction of the aldehydes with TBA in excess at 100 degrees C produced the yellow at 0.25 h, the orange at 2-6 h, and the red at 0.25-6 h. The formation of these pigments required molecular oxygen. These pigments could be separated from each other on HPLC. The red pigment formed from the aldehydes could not be distinguished from the red 1:2 malonaldehyde-TBA adduct by absorption spectrum and HPLC. The red color yield was the highest in the 1:1 reaction and retarded in the reaction with TBA in excess. The red color due to these aldehydes may contribute in part to the color formed in the general TBA test of lipid oxidation. The 1:1 reaction initially produced colorless 1:1 adducts X, which were subsequently converted into the yellow and red pigments under aerobic conditions. The reaction of the aldehydes with TBA in excess might initially produce X and then another colorless 1:2 adducts Y; the latter being converted into yellow, orange, and red pigments under aerobic conditions.