Dissociation constants of host-guest complexes of alkyl-bearing compounds with beta-cyclodextrin and ''hydroxypropyl-beta cyclodextrin''

Dissociation constants (K-d) of host-guest complexes formed from beta-cyclodextrin or ''hydroxypropyl-beta-cyclodextrin'' (beta-CD and Hp-beta-CD) and several types of aliphatic guests (alcohols, alkanesulfonate ions, alkylamines, and alpha-amino acids), with up to eight carbons in a chain, are reported. These constants were determined by inhibition kinetics and by a spectrofluorometric displacement method based on competition with 1-anilino-8-naphthalenesulfonate ion as a fluorescent probe. The value of K-d for a particular amine is close to that for the corresponding alcohol. For linear alkyl derivatives, there are strong correlations between pK(d) (= -log K-d) and the chain length of the guest, with slopes around 0.5, complementing trends that were noted earlier. Furthermore, the strengths of binding of various aliphatic derivatives to beta-CD and to Hp-beta-CD are close, with K-d values for the two CDs usually being within a factor of 2 of each other. Overall, for the binding of over 50 alkyl-bearing derivatives, there is a good correlation of pK(d) for Hp-beta-CD with that for beta-CD, with unit slope. These observations imply that the binding of simple aliphatic guests to Hp-beta-CD is not greatly influenced by the modification of the hydroxyl groups on the primary side of the beta-CD cavity but this may not be true for longer aliphatic derivatives (>C-8) or for aromatics that penetrate farther into the CD cavity.