Isothermal microcalorimetric studies of toluidine blue O/sulfated polysaccharides interactions

Isothermal microtitration calorimetry was performed to analyze the interaction of fucoidan and dextran sulfate with toluidine blue O (TB) to achieve relevant and characteristic thermodynamic information. Thermodynamic data obtained were referred to the anionic binding sites of the sulfated polysaccharides (SPS) and were not related to the polysaccharide concentration in general. Enthalpic values achieved by the two SPS titrations in TB solutions could be approximated using a single fit curve. The same also applies for the TB titrations in SPS solutions. Evaluation of the data obtained for forward and reverse titrations could not be performed since it results in inconsistent enthalpic and entropic contributions. An appropriate modeling could only be performed assuming two non-symmetric binding sites at the SPS although the thermodynamic data revealed a 1:1 stoichiometry. A 1:2 model accounts for the deviation generated by the stacking of the TB molecules. Data obtained for the binding of a single TB to a sulfate ester indicate highly negative values (−9.7 to −9.4 kcal mol−1) for the change in Gibbs energy and thus a spontaneous enthalpy-driven reaction. Formally, further thermodynamic values contain information about the second binding site at SPS. In this case, the information provided can be equated with the TB-TB interaction or dimerization that is entropically favored (T∆S = 13.7-16.5 kcal mol−1). Equilibrium constant for TB binding at the sulfate esters values to 0.7-1.4 10−7 mol−1, and TB-TB interaction has a binding constant of 2.1-3.0 10−4 mol−1, indicating an equilibrium which is mainly positioned at the dimers.