Complexation of tropaeolin 000 No. 2 by β- and γ-cyclodextrin

Abstract

Measurements of the temperature-jump and equilibrium u.v.–visible and induced circular dichroic spectra of tropaeolin 000 No. 2 (TR) in the presence of α-, β- and γ-cyclodextrin (αCD, βCD and γCD) have been carried out. Three complexation steps are detected in the presence of γCD through the temperature-jump data (298.2 K): TR +γCD ⇌ TR·γCD fast (K₁), [graphic omitted], γCD +(TR)₂·γCD ⇌(TR)₂·(γCD)₂ fast (K₃), where K₁, K₂ and K₃ are (4.18 ± 1.47)× 10², (1.68 ± 0.54)× 10⁶ and (1.77 ± 1.54)× 10² dm³ mol^–1, respectively; k₂=(2.27 ± 0.61)× 10⁹ dm³ mol^–1 s^–1 and k_–2=(1.35 ± 0.23)× 10³ s^–1. In the presence of βCD the third complexation was not detected and K₁=(7.1 ± 0.7)× 10² and K₂=(4 ± 7)× 10⁶ dm³ mol^–1; k₂=(5 ± 6)× 10⁹ dm³ mol^–1 s^–1 and k_–2=(1.3 ± 1.5)× 10³ s^–1. No complexation reactions were detected in the presence of αCD. The equilibrium u.v.–visible spectra and the circular and linear dichroic spectra are consistent with these reaction schemes, but suggest different orientation and penetration for the (TR)₂ dimer included in (TR)₂·βCD compared with (TR)₂·γCD.

For the equilibrium [graphic omitted] the equilibrium constant K_d=(9.10 ± 4.28)× 10² dm³ mol^–1 has been determined by u.v.–visible spectroscopy, and k_–d=(2.24 ± 0.40)× 10³ s^–1 has been estimated from temperature-jump experiments. Hence k_d=(2.0 ± 1.0)× 10⁶ dm³ mol^–1 s^–1. Thus the increase in stability of (TR)₂ included in (TR)₂·γCD and (TR)₂·βCD over that observed in the absence of cyclodextrin is a consequence of k₂≈ 10³k_d.