The Aggregation of Cyclodextrins as Studied by Photon Correlation Spectroscopy

Abstract

Photon correlation spectroscopy has been used to study theaggregation processes of natural and some modified cyclodextrins (CDs) in diluted aqueoussolutions. α-, β-, and γ-CD form large, polydisperse aggregatesin water, although the aggregation capability is different depending on the macrocycle considered. γ-CD solutions filtered through 0.2~μm give a single-modaldistribution of aggregates of 224 nm in size. The monomeric γ-CD can be isolated by filtering through 0.1 μm. α-CD displays a bimodaldistribution (monomer + aggregates) with both pore sizes. At the concentrations studied (0.012 M) the contribution in mass of the aggregates with both CDs isnegligible. β-CD is much more persistent in its aggregation, even after sievingits solutions through 0.02 μm filters, and time dependent. The aggregation displayed byCDs with partial substitution of the OH groups (Methyl-β-CD and Hydroxypropyl-β-CD) is much weaker, indicating the implication of the hydrophilic rims ofthe CDs in the process. High temperatures, addition of urea or electrolytes andionisation of the OH groups by raising the pH, prevent the aggregation.