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

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 Kd=(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 kd=(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≈ 103kd.