The binding of myo-inositol hexasulfate to an N-terminal truncated 132-amino-acid human acidic fibroblast growth factor form was studied by isothermal titration calorimetry. The technique yields values for the enthalpy change and equilibrium constant, from which the Gibbs energy and entropy change can also be calculated. Experiments in different buffers and pH values show that the proton balance in the reaction is negligible. Experiments at pH 7.0 in the presence of 0.2-0.6 M NaCl showed that the enthalpy and Gibbs energy changes parallel behaviour with ionic strength change, with values in the -21 to -11 kJ x mol(-1) range in the first case and in the -31 to -22 kJ x mol(-1) range in the second. No dependence of entropy on ionic strength was found, with a constant value of approximately 35 J x K(-1) x mol(-1) at all ionic strengths studied. The results can be interpreted in molecular terms by a model in which competitive binding of 3-4 chloride ions to the myo-inositol-binding site is assumed. Isothermal titration calorimetry was also performed at different temperatures and yielded a value of -142+/-13 J x K(-1) x mol(-1) for the heat-capacity change at pH 7.0 and 0.4 M NaCl. Using different parametric equations in the literature, changes on ligand binding in the range -100 to -200 A2 in solvent-accessible surface areas, both polar and apolar, were calculated from thermodynamic data. These values suggest a negligible overall conformational change in the protein when the ligand binds and agree closely with calculations performed with NMR structural data, in which it is shown that the most important negative change in total solvent-accessible surface area occurs in the amino acids Ile56, Gln57, Leu58 and Leu149, in the high-affinity receptor-binding region of the protein.