Association-dissociation and denaturation behaviour of an oligomeric seed protein alpha-globulin of Sesamum indicum L. in acid and alkaline solutions.

The association-dissociation and denaturation behaviour of the major protein fraction, alpha-globulin of sesame seed (Sesamum indicum L.), in acid and alkaline solutions in the ranges of pH 4.2-1.5 and pH 7-12 have been studied. The results of gel filtration, fluorescence and viscosity measurements indicate dissociation and denaturation of the protein up to pH approximately 3. The difference spectrum in this region arises from a combination of dissociation, denaturation and charge effect on the chromophore. In still stronger acid solution, reassociation of the dissociated fraction takes place by hydrophobic interaction. In alkaline solution dissociation takes place around pH 8, and above pH 10 dissociation and denaturation proceed simultaneously as has been evidenced by sedimentation, fluorescence, spectral change, optical rotation and viscosity measurements. The phenolic group (pKInt=10.6) in the protein is abnormal and denaturation in alkaline solution is irreversible. Above pH 11.5 further dissociation of the protein takes place. Characteristic pH values of transition from 10.6-10.8 indicate that the transition of the protein involves a single step in alkaline solution.