Chemical shifts as a novel measure of interactions between two binding sites of symmetric dialkyldimethylammonium bromides to α-cyclodextrin Original Research Article

Complex formation of α-cyclodextrin (α-CD) with decyltrimethylammonium (DeTAB), N,N-dioctyldimethylammonium (DOAB), and N,N-didecyldimethylammonium bromides (DDeAB) was investigated by proton NMR spectroscopy. Analysis of chemical shifts yielded macroscopic 1:1 and 1:2 binding constants (K1 and K2) and chemical shift differences (ΔδSD and ΔδSD2) for the 1:1 and 1:2 complexes of DeTAB, DOAB, and DDeAB with α-CD. The K1 and K2 values of DDeAB were quantitatively explained on the basis of the assumption that the microscopic 1:1 binding constant of DDeAB is identical to the observed K1 value of DeTAB. The K2 value of DDeAB was also explained in terms of its observed K1 value and the independent binding of two alkyl chains. Furthermore, the ΔδSD and ΔδSD2 values for protons of DDeAB and α-CD were quantitatively explained on the basis of the assumption that the geometry of the decyl group of DDeAB in an α-CD cavity is identical to that of DeTAB. The ΔδSD value was also explicable on the basis of the same geometric assumption and the observed ΔδSD2 value for this system. Similar results were obtained for the 1:1 and 1:2 DOAB–α-CD complexes.