Complexation of roccellin by β- and γ-cyclodextrin

Abstract

Measurements of the u.v.–visible, fluorescence and circular dichroic spectra of roccellin (RO) in the presence of α-, β- and γ-cyclodextrin (αCD, βCD and γCD) have been carried out. From equilibrium u.v.–visible spectral measurements, in the case of βCD, a single 1:1 complex was observed (298.2 K): RO +βCD⇌RO·βCD (K₁) where K₁=(7.20 ± 0.88)× 10² dm³ mol^–1.

In the case of γCD a single 2:1 complex was observed (298.2 K): 2RO +γCD⇌(RO)₂·γCD (K₁₂) where K₁₂=(9.0 ± 1.8)× 10¹⁰ dm⁶ mol^–2. No complexation was observed between roccellin and αCD. Measurements of induced circular dichroism and of fluorescence were consistent with the reaction schemes proposed above for both βCD and γCD, but no interaction was observed with αCD.

Measurements of the u.v.–visible spectra of aqueous solutions of roccellin allowed characterisation of the dimerisation equilibrium: 2RO⇌(RO)₂(K_d) where K_d is (1.64 ± 0.12)× 10⁴ dm³ mol^–1. Measurements of the u.v.–visible and fluorescence spectra of roccellin, in increasing concentrations of lithium sulphate up to 0.5 mol dm^–3, showed marked changes, consistent with the presence of increasing concentrations of aggregates of dye molecules with increasing electrolyte concentration.