Spectrophotometric study of complexation between betulin 3,28-diphthalate and β-cyclodextrin

Host–guest complexation of betulin 3,28-diphthalate (DPhB), that is poorly soluble in water and is an amphiphilic compound, with β-cyclodextrin (β-CD) was studied by spectrophotometry in 10 mM sodium tetraborate solution at pH 9.18 (ionic strength of 20 mM). This interaction was found to lead to a bathochromic shift of the DPhB absorption bands and an increase in absorption. Using the first derivative spectra of solutions with a fixed concentration of DPhB (0.01 mM) and a variable concentration of β-CD (0–10 mM), the logarithms of binding constants for 1:1 and 1:2 complexes of DPhB with β-CD were determined with 95% confidence intervals at 25 °C: 4.13 (4.03–4.20) and 6.30 (4.72–6.59). To eliminate systematic errors, the assumption that the equilibrium CD concentration in solution was equal to its total concentration was not used in this study. The obtained value for the 1:1 complex is consistent with the value determined earlier by affinity capillary electrophoresis (ACE), while the constant for the 1:2 complex is slightly lower than that from ACE, which can be explained by a higher concentration of DPhB; however, the use of this concentration is reasonable to reduce the measurement error. DPhB was found to form micelles at pH 9.18 and the critical micelle concentration (CMC) measured was 0.13 mM. This is a possible reason why the experimental points obtained at higher concentrations of DPhB than CMC are not described within the framework of theories that take into account the formation of 1:1 or both 1:1 and 1:2 complexes.