Dissociation and denaturation behaviour of sesame alpha-globulin in sodium dodecyl sulphate solution

The effect of anionic detergent, sodium dodecyl sulphate, on the major protein, alpha-globulin of sesame seed (Sesamum indicum L.) has been investigated by gel filtration, sedimentation velocity, viscosity, optical rotation, difference spectra and fluorescence measurements. The detergent causes dissociation of the protein first and then denaturation. In the detergent concentration range of .175-4.0 X 10(-"3) M four components are observed in the ultracentrifuge. The specific rotation of the protein increases with the detergent concentration above 2.5 x 10 (-3) M detergent suggesting conformational change; above 8 X 10(-"3) M detergent the value of -[alpha] does not change. The reduced viscosity etared however, increases above .25 X 10(-3) M detergent and does not attain a plateau value. The difference spectrum of the protein indicates that both tryptophan and tyrosine groups have been affected by the detergent. The fluorescence intensity decreases and the maxima shifts towards red in the detergent solution resulting in an "isoemissive point" at 355 nm. The double difference spectra in sucrose-detergent protein system show that below 5-0 X 10(-3) M detergent, the difference absorption and fluorescence spectrum result from the binding of the detergent near the chromophoric groups and are not due to conformational change. Binding studies by equilibrium dialysis indicate the presence of 50 binding sites in the protein and binding constant of 3-0 X 10(3).