1:1 and 1:2 complexes between long-chain surfactant and α-cyclodextrin studied by NMR

One- and two-dimensional proton NMR spectroscopy was applied to determine the equilibrium constants and the solution structures of 1: 1 and 1:2 complexes between dodecyltrimethylammonium bromide (DTAB) and alpha-cyclodextrin (alpha-CD). Chemical shift data of protons of DTAB and alpha-CD were used to determine reliable 1: 1 and 1:2 binding constants and chemical shift variations Deltadelta(complex) at full binding. The intensity of intermolecular crosspeaks in ROESY spectra of aqueous solutions containing alpha-CD and DTAB allowed us to determine detailed structures of the complexes. The correlation coefficient for the relation between these ROE intensities and the effective interproton distances had a maximum at a penetration depth, where the time-average structures of the 1: 1 and 1:2 complexes were determined. As the alkyl chain of a surfactant is longer, the maximum correlation coefficient decreased and the peak of the relation between these ROE intensities and the effective interproton distances became broader. Furthermore, the Deltadelta(1:1complex) values for the DTAB protons, plotted against the position located in the alpha-CD cavity, had a peak near the proton H3 of alpha-CD. The peak became lower and broader, as the alkyl chain is longer. These relations of Deltadelta(1:1complex) and ROE intensities with the penetration depth suggest that DTAB shuttles at a wider distance in the alpha-CD cavity than shorter-chain surfactants. In the 1:2 complex two alpha-CD molecules adopt the head-to-head structure.