Thermodynamic study of the dimerization of 8-anilino-1-naphthalenesulfonic acid by isothermal titration calorimetry
Montserrat Andujar-Sanchez, Ana Cámara-Artigas, Vicente Jara-Perez *
Dpto. Química Física, Bioquímica y Química Inorgánica, Universidad de Almería, La Cañada de San Urbano, 04120 Almería, Spain
a r t i c l e i n f o
Article history:
Received 8 May 2009
Received in revised form 20 July 2009
Accepted 2 September 2009
Available online 7 September 2009
a b s t r a c t
Dimerization of the fluorescence probe, 8-anilino-1-naphthalenesulfonic acid (ANS) in aqueous media have been studied by isothermal titration calorimetry (ITC). ITC experiments carried out at different pHs show that dimerization constants are highly pH dependent, decreasing their values with increasing pH. No dimerization is detected over pH 7. Analyzing the dependence of Kdim on pH, using a model that only considers dimers between zwitterionic molecules of ANS, a value of 5.6 for the pK of anilinium moi-ety is obtained. It is in agreement with the pK determined spectrophotometrically. The dimerization pro- cess is enthalpically disfavored and entropically driven at all pH and temperatures studied, indicating that hydrophobic effect has an important role on the formation of dimers. Although dimerization con- stants are low, dimerization equilibria must be taken into account when the energetics of the interaction
of ANS to a protein is studied at pH below 7.
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