The binding of alkane-α,ω-diols to α-cyclodextrin. A microcalorimetric study

Abstract

The binding of a series of alkanediols, HO(CH 2) N OH, n = 3 to 10, to α-cyclodextrin was studied by microcalorimetric titration at 288.15 K, 298.15 K, and 308.15 K. For n = 3 to 7, the results agreed well with a 1-1 binding model but for n = 8 to 10, a more complex stepwise-association model had to be assumed: the formation of both 1-1 and 1–2 (1 diol + 2 α-CD) complexes. Apparent values for the binding constants, and changes in the standard molar Gibbs energies, enthalpies, entropies, and heat capacities, were derived for the binding process. At all temperatures, enthalpy and entropy values show an initial zig-zag pattern as a function of the number of CH 2 groups whereas the CH 2 increment in the standard molar Gibbs energy change is regular up to C 8. The heat-capacity increments are regular up to C 6, after which they level off. The results are interpreted in terms of the expected “driving forces” leading to the binding and the observed pattern in the thermodynamic properties.