title: Hydration Differences Explain the Large Variations in the Complexation Thermodynamics of Modified γ-Cyclodextrins with Bile Salts.
abstract: The structure and thermodynamics of inclusion complexes of seven different γ-cyclodextrins (γCDs) and three biologically relevant bile salts (BS) were investigated in the present study. Natural γCD and six modified γCDs [two methyl-γCDs, one sulfobutyl ether-γCD (SBEγCD), and three 2-hydroxypropyl-γCDs (HPγCD)] and their complexes with BS were investigated by isothermal titration calorimetry, NMR, and molecular dynamics simulations. With the exception of the fully methylated γCD, which did not bind the BSs investigated, all of the γCDs formed 1:1 complexes with the BS, and the structures were similar to those with natural γCD; i.e., the modifications of the γCD had limited structural impact on the formation of complexes. Isothermal titration calorimetry was carried out over in the temperature interval 5-55 °C to enable the calculation of the stability constant (K) and the thermodynamic parameters enthalpy (ΔH°), entropy (ΔS°), and heat capacity (ΔCp°). The stability constants decreased with an increased degree of substitution (DS), with methyl substituents having a lower effect on the stability constant than the sulfobutyl ether and hydroxypropyl substituents on the stability constants. Enthalpy-entropy compensation was observed, since both enthalpy and entropy increased with the degree of substitution, which may reflect dehydration of the hydrophobic surface on both CD and BS. Calculations based on ΔCp° data suggested that each of the substituents dehydrated 10-20 (hydroxypropyl), 22-33 (sulfobutyl ether), and 10-15 Å(2) (methyl) of the BS surface area, in reasonable agreement with estimates from the molecular dynamics simulations. Combined with earlier investigations on modified βCDs, these results indicate general trends of the substituents on the thermodynamics of complex formation.
authors.affiliation: Pharmaceutical Science and CMC Biologics, H. Lundbeck A/S , Ottiliavej 9, DK-2500 Valby, Denmark.|NSM, Research Unit for Functional Biomaterials, Roskilde University , Universitetsvej 1, DK-4000 Roskilde, Denmark.
authors.forename: Jonatan
authors.initials: J
authors.lastname: Køhler
authors.affiliation: Pharmaceutical Science and CMC Biologics, H. Lundbeck A/S , Ottiliavej 9, DK-2500 Valby, Denmark.|NSM, Research Unit for Functional Biomaterials, Roskilde University , Universitetsvej 1, DK-4000 Roskilde, Denmark.
authors.forename: Christian
authors.initials: C
authors.lastname: Schönbeck
authors.affiliation: NSM, Research Unit for Functional Biomaterials, Roskilde University , Universitetsvej 1, DK-4000 Roskilde, Denmark.
authors.forename: Peter
authors.initials: P
authors.lastname: Westh
authors.affiliation: Pharmaceutical Science and CMC Biologics, H. Lundbeck A/S , Ottiliavej 9, DK-2500 Valby, Denmark.
authors.forename: René
authors.initials: R
authors.lastname: Holm
mesh_terms: D001647:Bile Acids and Salts / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D056004:Molecular Dynamics Simulation; D013816:Thermodynamics; D047408:gamma-Cyclodextrins / Q000737:chemistry*
chemical_list: D001647:Bile Acids and Salts; D047408:gamma-Cyclodextrins