Chronocoulometry diffusion coefficients as a measure of cyclodextrin–estradiol complex association

The formation of inclusion complexes between estradiol and β-cyclodextrin (CD), sulfobutyleter-β-CD (SBCD) and hydroxypropil-β-CD (HPCD) was studied by chronocoulometric and differential pulse voltammetry. In presence of CD there is a decrease of the cathodic peak current with the increase of the amount of CD. This decrease is due to the lower diffusion coefficient (D) of estradiol–cyclodextrin complex compared with the free guest. In absence of CD, an estradiol D value of 8.6 × 10−6 ± 0.1 × 10−6 cm2 s−1 was obtained. The addition of CDs produced a maximum decrease of 20%, 22% and 30% in the diffusion coefficient with CD, SBCD and HPCD, respectively. Using the variation in D, association constants of 132 ± 18, 247 ± 32 and 276 ± 28 M−1 for CD, HPCD and SBCD were determined.