The impact of guest compounds on cyclodextrin aggregation behavior: A series of structurally related parabens.

Several studies have demonstrated the presence of aggregates in aqueous cyclodextrin containing solutions. The presence of guest compounds has been shown to influence this cyclodextrin aggregation process. In an attempt to gain insight into the effect of the physicochemical properties of the guest compound on 2-hydroxypropyl-β-cyclodextrin aggregation formation, a series of structurally related parabens was selected as model compounds. Using nuclear magnetic resonance spectroscopy and phase solubility studies, these parabens, differing only in side chain length, were demonstrated to form inclusion complexes with 2-hydroxypropyl-β-cyclodextrin. Additional techniques were subsequently applied to evaluate the aggregation behavior of this cyclodextrin in presence of the selected parabens. Solutions containing a broad range of 2-hydroxypropyl-β-cyclodextrin concentrations were saturated with the guest compounds and were used as test media. Results obtained from dialysis experiments, dynamic light scattering and mass spectrometry revealed a positive effect of the side chain length of the parabens on aggregate formation: in presence of heptylparaben, more and larger aggregates were observed than in presence of parabens with shorter side chains such as methyl- and butylparaben. No clear connection could be demonstrated between the cyclodextrin concentration and the extent of aggregate formation in presence of the guest compound.