Spectroscopic characterization of the inclusion complex of a luteinizing hormone-releasing hormone agonist, buserelin acetate, with dimethyl-β- cyclodextrin

Abstract

Inclusion complexation of buserelin acetate, an agonist of luteinizing hormone-releasing hormone, with dimethyl-β-cyclodextrin (DM-β-CyD) in aqueous solution was studied spectroscopically and its mode of interaction was assessed. Ultraviolet absorption and circular dichroism (CD) spectroscopies indicate that the aromatic side chains of buserelin acetate, L-tryptophan and L-tyrosine residues, are incorporated into the hydrophobic environment of the DM-β-CyD cavity. Furthermore, proton and carbon-13 nuclear magnetic resonance spectroscopies suggest that in addition to the two aromatic side chains, a tertiary butyl D-serine residue is inserted into the DM-β-CyD cavity from the secondary hydroxyl side. On the other hand, the continuous variation plots for the buserelin acetate: DM-β-CyD system showed a 1:1 stoichiometry of the complex. Therefore, the complexation should be initiated by the inclusion of one of the three binding sites on the buserelin molecule into DM-β-CyD, which may in turn prevent the further access of the second cyclodextrin to the other binding sites, probably due to steric hindrance and/or conformational changes of the peptide. These structural features of the complex would account for the stabilizing effect of DM-β- CyD on the enzymatic degradation of buserelin acetate.