Determination of Dissociation Constants of Cyclodextrin—Ligand Inclusion Complexes by Electrospray Ionization Mass Spectrometry

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.