Estimation of cyclodextrin affinity to steroids.

A nonlinear spectrometric method for determination of the stability constant (Ks) for cyclodextrin complex with steroid was developed. The method is based on calculation of the parameters of competitive cyclodextrin complexation by simultaneous fitting of two types of curves. Those of the first type are the dependencies of absorbance of methyl orange solution on the cyclodextrin concentration, the second type being the absorption curves of displacement of the dye, by steroid, from the cyclodextrin complex. With the method proposed, Ks values were calculated with standard deviation less than 10%. This method is validated by determination of Ks values using the phase-solubility technique. For neutral steroid molecules, the effect of pH on Ks was found to be insignificant. Ks values for the cyclodextrin-dye complex were determined for randomly methylated beta-cyclodextrin, 2-hydroxypropyl-beta-cyclodextrin, carboxymethyl-beta-cyclodextrin and sulfobutylether-beta-cyclodextrin. More hydrophobic steroids were characterised by higher Ks values. Anionic beta-cyclodextrins showed high affinity for the steroids studied. Simple equipment and sufficient computing allowed recommendation of the method for express estimation of cyclodextrin's affinity for hydrophobic substrates.