Binding Delocalization in Polymer Inclusion Complexes of Ring Molecules: Pseudopolyrotaxanes of α-Cyclodextrin and Poly(ethylene glycol)

With polymer substrates, the continuous variation method based on monomer unit concentration underestimates the number of monomer units covered by a low-molecular-weight ligand. Accordingly, gel permeation chromatography confirms that an α-cyclodextrin (α-CD) molecule occupies more than previously claimed two ethylene glycol units in solid α-CD/poly(ethylene glycol) inclusion complexes. Consequently, the poly(ethylene glycol) chain cannot be modeled as an array of distinct binding sites corresponding to two monomer units and no preferred positions, i.e., no distinct binding sites probably exist on the chain for α-CD threading. The effect of such binding delocalization can be assessed using the theory of binding large ligands to a finite one-dimensional lattice [I.R. Epstein: Biophys. Chem. 8, 327 (1978)]. Binding delocalization slightly decreases the average occupancy with highly occupied chains but strongly promotes the occupancy in the case of weak binding. This may be an additional reason for observed high yields of precipitated CD/polymer complexes.