Proton NMR Study of α-Cyclodextrin Inclusion of Short-Chain Surfactants

Proton NMR spectroscopy was applied to determine the binding constants and the solution structures of 1:1 α-cyclodextrin (α-CD) complexes with hexyltrimethylammonium (HTAB) and octyltrimethylammonium (OTAB) bromides. Chemical shift data of all protons, referred to an internal standard, were used to determine reliable binding constants and chemical shift variations Δ δ complex induced by complex formation. The ROESY spectra of aqueous solutions containing α-CD and HTAB or OTAB provide information about rough structures of the complexes. The alkyl chains of HTAB and OTAB are incorporated from the wide rim of the α-CD cavity. The ROE intensities of intermolecular cross-peaks are plotted against the effective interproton distances for many structures different in the penetration depth of the alkyl chain. Detailed structures of the α-CD complexes with HTAB and OTAB are determined from the best correlations between the ROE intensities and the interproton distances. The Δ δ complex values for the protons of propanol, HTAB, and OTAB depend on the position located in the α-CD cavity. For instance, the proton near the proton H3 of α-CD exhibits the largest variation (ca. 0.2 ppm). The present data on single- and short-chain surfactants will provide the basis for determining the stoichiometry, the binding constants, and the structures of CD complexes with long-chain surfactants and double-chain surfactants.