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

Abstract

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 Δδcomplex 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 Δδcomplex 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.