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Isothermal titration calorimetry (ITC) technique was used to study the interactions of trypsin with bicyclic sunflower-derived trypsin inhibitor (SFTI-1) as well as with its new monocyclic (with disulphide bridge only) analogues (CHO)-SFTI-1 and (CHO)-SFTI-1. ITC measurements were run in 50 mM buffer solution of HEPES or Tricine of pH 8, containing 20 mM CaCl at 298.15 K. Based on calorimetric data, the equilibrium constants for the inhibitor-enzyme-binding processes, K, the binding stoichiometry, N (inhibitor-to-enzyme molar ratio), as well as thermodynamic parameters (Δ G, Δ H, Δ S) for the reactions were determined. The study revealed that the stoichiometry of the resulting complexes equals 1:1. The negative binding enthalpy (Δ H) and favourable entropy factor ( TΔ S) suggest an important contribution of hydrogen bonding as well as hydrophobic interactions to the inhibitor-enzyme affinity. Furthermore, the relationship between the modification of the peptide structure, the experimental conditions and the thermodynamic parameters has been discussed. [ABSTRACT FROM AUTHOR]