Structural Evidence for an Independent Inclusion of Hexyl and Octyl Chains of Hexyldimethyloctylammonium Bromide in an α-Cyclodextrin Cavity

Proton NMR spectroscopy was applied to determine the macroscopic binding constants and the solution structures of the 1:1 and 1:2 complexes between N,N,N-hexyldimethyloctylammonium bromide (HDOAB) and α-cyclodextrin (a-CD). The terminal methyls of the hexyl and octyl groups of HDOAB are distinguishable from each other in the NMR spectrum. The chemical-shift variations of these methyls, compared with the α-CD complexes of hexyltrimethylammonium (HTAB) and octyltrimetylammonium (OTAB) bromides, indicated that this system formed two 1:1 complexes (10% hexyl-in complex and 90% octyl-in complex). The chemical-shift variations of HDOAB and α-CD protons with the 1:1 and 1:2 complexations were quantitatively explained based on the assumption that the geometry of the hexyl and octyl groups of the HDOAB complexes with α-CD is identical to that of the HTAB and OTAB. This finding indicated that the hexyl and octyl groups bind to α-CD independently. Based on the assumption that the microscopic binding constants of the hexyl and octyl groups of HDOAB are identical to the macroscopic 1:1 binding constants of HTAB and OTAB with α-CD, the mole fraction of the hexyl-in complex in the 1:1 complex was estimated to be 0.11. This is very close to the above-mentioned estimation from the chemical-shift variations of the terminal methyls. The intensities of intermolecular cross-peaks on the ROESY spectrum of a 3 mM HDOAB and 3 mM α-CD solution indicated that the octyl-in complex is the predominant 1:1 complex. The present method and result will serve to estimate noncovalent interactions, binding equilibria, and solution structures of other multiple supramolecular complexes.