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 α-cyclodextrin (a-CD). Chemical shift data of protons of DTAB and α-CD were used to determine reliable 1:1 and 1:2 binding constants and chemical shift variations Δδ c o m p l e x at full binding. The intensity of intermolecular crosspeaks in ROESY spectra of aqueous solutions containing α-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 Δδ 1 : 1 c o m p l e x values for the DTAB protons, plotted against the position located in the α-CD cavity, had a peak near the proton H3 of α-CD. The peak became lower and broader, as the alkyl chain is longer. These relations of Δδ 1 : 1 c o m p l e x and ROE intensities with the penetration depth suggest that DTAB shuttles at a wider distance in the α-CD cavity than shorter-chain surfactants. In the 1:2 complex two α-CD molecules adopt the head-to-head structure.