An NMR study of the inclusion geometrics of steroid-cyclodextrin complexes

Abstract

The inclusion geometries formed by the three water insoluble steroids (cyproterone acetate, ethynyl oestradiol and danazol) with β- and y-cyclodextrin were determined with the use of NMR experiments and molecular modelling. Observed intermolecular nuclear Overhauser effects (NOE's) were used to obtain qualitative distance constraints between the protons of the steroid and cyclodextrin. The inclusion geometries of the complexes were modelled using these distance constraints. The steroid-cyclodextrin complexes were prepared in D20 by a kneading and a sonication method. Both methods afforded equivalent concentrations of complex. The concentration of steroid-cyclodextrin complex is independent of the method of preparation. The concentration is only dependant on the stability constant K which is a measure of the tendency of a complex to dissociate in the presence of water. The concentrations of the cyproterone acetate- and danazolp-β-cyclodextrin and the cyproterone acetate- and ethynyl oestradiol-y-cyclodextrin complexes facilitated an NMR study. The ethynyl oestradiol-β-cyclodextrin complex was poorly water soluble and this complex with a stoichiometry of 1:2 (steroid:cyclodextrin) was isolated. The poor solubility of the ethynyl oestradiol-β-cyclodextrin complex in D20 prevented a thorough NMR investigation. The concentration of this complex was increased by preparing it in a 1:2 (v/v) mixture of DMSO-d6 and D20. The observation of NOE's between the proton atoms of ethynyl oestradiol and β-cyclodextrin proved that ethynyl oestradiol-β-cyclodextrin complex exists in the 1:2 mixture despite the fact that pure DMSO-d6 prevents complex formation. The danazol-y-cyclodextrin complex was water insoluble. The concentration of this complex is increased when it is prepared in a 1:2 mixture of DMSO-d6 and D20. Two different inclusion geometries were identified for all the steroid-cyclodextrin complexes. One geometry has the A-ring of the steroid inserted into the cyclodextrin cavity and the other has the D-ring inserted into the cavity. The inclusion of the A-ring results in the formation of a binary complex. The formation of this binary complex is postulated as the solubilising step. The inclusion geometry where the D-ring is inserted into the cavity is most likely obtained from a ternary complex where the A- and D-rings are both capped by a cyclodextrin. y-Cyclodextrin solubilised a larger amount of cyproterone acetate than β-cyclodextrin. This increase in solubility occured alongside an increase in the depth of insertion of the A-ring of cyproterone acetate into the y-cyclodextrin cavity relative to the insertion into β-cyclodextrin cavity. The fact that y-cyclodextrin solubilised ethynyl oestradiol better and that the danazoly- cyclodextrin complex exists in a mixture which contains the destabilising DMSO-d6 solvent proves that both the ethynyl oestradiol- and danazol-y-cyclodextrin complexes are stable. This stability is attributed to favourable entropic contributions towards complex formation which arise as a result of the high mobilites which ethynyl oestradiol and danazol have within the y-cyclodextrin cavity.