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

The associationdissociation and denaturation behaviour of the major protein
fraction, a-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- 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 (pKlnt = 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 10.6-1 0.8
indicate that the transition of the protein involves a single step in alkaline
solution.