Differential scanning calorimetric studies have shown that small amounts of Sodium Dodecyl Sulfate (SDS) increase the thermal stability of Bovine Serum Albumin (BSA). Specific binding of up to 10 SDS molecules pr molecule of BSA induces a 20-25°C increase in the thermal stability of the protein [1;2]. Recently high sensitivity Isothermal Titration Calorimetry (ITC) has been used to elucidate the forces that drive the specific binding of SDS to BSA [3]. This method proved particularly effective in resolving the binding of SDS into separate classes of sites with different affinity.

In this study ITC has been used to obtain a fundamental understanding of the forces that are responsible for specific interactions between BSA and SDS. The measured binding curves could be rationalized as association to at least two classes (high affinity/low affinity) of sites comprising respectively 3 and 6 similar independent sites. The binding to both sets of sites is dependent of the temperature and pH-value, but shows very little dependence of ionic strength. High affinity binding involves both electrostatic and hydrophobic forces, whereas low affinity binding is dominated by hydrophobic interactions.

The results evidently demonstrate the usefulness and potential of ITC to characterize binding of small ligands to multiple classes of bindingsites on proteins.

[1] Giancola, C., De Sena, C., Fessas, D., Graziano, G., and Barone, G., Int. J. Biol. Macromol. 20, 193-204, 1997.

[2] Yamasaki, M., Yano, H., and Aoki, K., Int. J. Biol. Macromol. 14, 305-312, 1992.

[3] Nielsen, A. D., Borch, K., and Westh, P., Biochim. Biophys. Acta, In Press, 2000.