Determination of Microscopic Equilibrium Constants for the Complexation of Ditopic Guests by Cyclodextrins from NMR Experiments

Molecular structure study of a heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin complex of cholesterol

Heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (2) and cholesterol form a water-soluble complex 3. We performed several NMR studies, particularly ¹H, ¹³C, 2D NOESY and DOSY, at various temperatures on 500 and 950 MHz instruments. We discovered that the complex 3 is unstable above 57 °C in heavy water, while it is kinetically stable enough to be studied by NMR in detail at 1 °C. We demonstrated the formation of a face-to-face 2:1 complex with a binding constant of approximately 2.2 × 10⁶ M^-2.

Inclusion Complexes of PBN-Type Nitrone Spin Traps and Their Superoxide Spin Adducts with Cyclodextrin Derivatives: Parallel Determination of the Association Constants by NMR Titrations and 2D-EPR Simulations

(1)H NMR and electron paramagnetic resonance (EPR) titrations were used to determine the association constants of the complexes of alpha-phenyl-N-tert-butylnitrone (PBN) analogues and their superoxide spin adducts, respectively, with methylated beta-cyclodextrins. A 1:1 stoichiometry for the nitrones with randomly methylated beta-cyclodextrin and 2,6-di-O-methyl-beta-cyclodextrin and 1:1 and 1:2 stoichiometries for the corresponding cyclodextrin-nitroxide complexes were observed. After the superoxide radical spin trapping reaction, EPR titrations afforded the association constants of the corresponding cyclodextrin-nitroxide complexes. Two-dimensional EPR simulations indicated a bimodal inclusion of the nitroxide free radical spin adducts into the cyclodextrins. For all the nitrone-cyclodextrin and nitroxide-cyclodextrin complexes, the association constants were always higher for the nitroxide complexes than for the nitrone complexes. A cooperative system concerning the complexation of the nitroxide spin adduct with a cyclodextrin was evidenced by EPR titrations. The efficiency of the cyclodextrin inclusion technique to trap superoxide and to resist bioreduction by sodium l-ascorbate was also investigated.