The complexation of flurbiprofen with β-cyclodextrin: a NMR study in aqueous solution

The complexation process between racemic flurbiprofen and beta-cyclodextrin in solution was investigated by 1D and 2D proton NMR spectroscopy. In the presence of beta-cyclodextrin, the aromatic protons of flurbiprofen were the most affected, suggesting a strong involvement of the phenyl groups in the inclusion mechanism. The stoichiometry of the complex was determined by the method of continuous variation, using the chemical induced shifts of both host and guest protons. The association constant, K-a of the obtained complex was calculated and found to be 2483.8 M-1. On the other hand, signals belonging to the protons associated with the carboxyl group are split in the presence of beta-cyclodextrin indicating enantiomeric differentiation. Rotating frame NOE spectroscopy, (ROESY), was used to ascertain the solution geometry of the host-guest complex. The result suggested that the flurbiprofen molecule fully penetrates the beta-cyclodextrin cavity with the carboxyl group protruding from the primary hydroxyl side and the phenyl group close to the secondary rim.