Effect of ligands and other protein modifiers

In the presence of sodium dodecyl sulphate, the BSA (which has about 70 binding sites for SDS, 10-11 of which showing larger affinity than the others) undergoes a biphasic denaturation two endotherms appear in the DSC trace which are related to protein denaturation and binding equilibrium, respectively, (rather than to denaturation of different protein domains) [199]. 

Urea and guanidinium hydrochloride are conunonly used as protein denaturants, but the mechanism of their action is not yet fully understood. It is indeed matter of discussion whether the action of these agents is direct, i.e., related to the binding of denaturant molecules onto the protein surface, or indirect, i.e., mediated by a change in the properties of the solvent [200-204]. 

The effect of a ligand on thermal stability of protein can be explicitly accounted for by relating the binding equilibrium to the thermal denaturation process [205]. 

This analysis can be tentatively based on the denaturant binding model by Aune and Tanford [206], The model assumes that urea or GuHCI can bind to different independent sites of native and denatured conformations of the protein. Following this line, the DSC trace obtained in the presence of these denaturants can be simulated (see Appendix A4.2) according to the canonical partition function formalism [162,197], 

---