The aggregation of cyclodextrins as studied by photon correlation spectroscopy

Photon correlation spectroscopy has been used to study the aggregation processes of natural and some modified cyclodextrins (CDs) in diluted aqueous solutions. alpha-, beta-, and gamma-CD form large, polydisperse aggregates in water, although the aggregation capability is different depending on the macrocycle considered. gamma-CD solutions filtered through 0.2 mum give a single-modal distribution of aggregates of 224 nm in size. The monomeric gamma-CD can be isolated by filtering through 0.1 mum. alpha-CD displays a bimodal distribution (monomer + aggregates) with both pore sizes. At the concentrations studied (0.012 M) the contribution in mass of the aggregates with both CDs is negligible. beta-CD is much more persistent in its aggregation, even after sieving its solutions through 0.02 mum filters, and time dependent. The aggregation displayed by CDs with partial substitution of the OH groups (Methyl-beta-CD and Hydroxypropyl-beta-CD) is much weaker, indicating the implication of the hydrophilic rims of the CDs in the process. High temperatures, addition of urea or electrolytes and ionisation of the OH groups by raising the pH, prevent the aggregation.