Solubility and spectrophotometric investigation of α-cyclodextrin and poly(ethylene glycol) complexation

The formation of a crystalline complex between cyclodextrins and polymers involves three types of interactions-interactions between cyclodextrin and polymer which affect threading of individual cyclodextrin rings on polymer chains, interactions between adjacent threaded rings which may account for the cooperativity of the complexation and interactions between various strands of threaded cyclodextrins which are important for the solubility of the complex. The comparison of binding constants obtained from the solubility study and by competitive spectrophotometry confirms strong cooperativity of cc-cyclodextrin threading on poly(ethylene glycol) because the latter constant, which corresponds to threading of a single ring, is much lower. With a 2-hydroxypropyl derivative of cr-cyclodextrin, no precipitation can be induced by poly(ethylene glycol) at any concentration although competitive spectrophotometry not only confirms the threading of cyclodextrin rings in this case but shows that the threading of a single cyclodextrin ring on a poly(ethylene glycol) chain occurs to a comparable extent regardless of cyclodextrin modification. (C) 1998 Elsevier Science Ltd. All rights reserved.