The inclusion complexes of four ligands binding to cyclodextrins (CDs) were studied by electrospray ionization mass spectrometry (ESI-MS) and the dissociation constants of the complexes were obtained. The 1 : 1 stoichiometric inclusion complex was found in the system of CD and fenbufen or aspirin. The obtained K_D values of the inclusion complexes of fenbufen binding to α-CD and to β-CD are 4.38 × 10^–4 mol L^–1 and 2.12 × 10^–4 mol L^–1, respectively. The K_D values of the inclusion complexes of α-CD-aspirin and β-CD-aspirin are 3.33 × 10^–4 mol L^–1 and 1.83 × 10^–4 mol L^–1, respectively. A non-linear least squares regression method was applied to validate the results which were consistent with each other. For the system of tetracycline hydrochloride and CD, the 1 : 1 and 1 : 2 stoichiometric inclusion complexes were found in the mass spectra. The K_D,1 and K_D,2 values of the 1 : 1 and 1 : 2 stoichiometric inclusion complexes of α-CD and tetracycline hydrochloride are 4.47 × 10^–4 mol L^–1 and 6.51× 10^–4 mol L^–1, respectively, and those of β-CD and tetracycline hydrochloride are 2.26 × 10^–4 mol L^–1 and 8.57 × 10^–4 mol L^–1, respectively. For the system of norfloxacin and CD, besides the 1 : 1 and 1 : 2 inclusion complexes, the 1 : 3 stoichiometric inclusion complex was also found. The K_D,1, K_D,2 and K_D,3 of α-CD and norfloxacin inclusion complexes are 4.61 × 10^–4 mol L^–1, 6.05 × 10^–4 mol L^–1 and 1.45 × 10^–3 mol L^–1, respectively. The three K_D values of β-CD and norfloxacin are 1.96 × 10^–4 mol L^–1, 4.93 × 10^–4 mol L^–1 and 1.15 × 10^–3 mol L^–1, respectively.

Keywords: dissociation constant; cyclodextrin; inclusion complex; electrospray ionization; mass spectrometry