Multiple complexation of didecyldimethylammonium bromide and cyclodextrinsdeduced from electromotive force measurements

The 1:1 and 1:2 macroscopic binding constants of didecyldimethylammonium br omide (DDAB) and alpha-, beta-, and gamma-cyclodextrins (CD) are determined from the electromotive force measurements with a DDAB-selective electrode, and the magnitude of these constants is interpreted in terms of the molecu lar structures of complexes. The 1:1 macroscopic binding constants of a dou ble chain surfactant is 2-fold larger than that of a single chain, because the former has two chains. The 1:1 macroscopic binding constants of DDAB ar e in the increasing order alpha-CD < beta-CD approximate to alpha-CD, consi stent with those of a single chain surfactant. In the 1:1 complex of DDAB a nd gamma-CD, one or two decyl chains are incorporated into the CD cavity. T he 1:2 macroscopic binding constants of DDAB are in increasing order gamma- CD << beta-CD alpha-CD. This order is interpreted in terms of the structure of complexes: the first larger ligated CD inhibits the second ligation mor e strongly. The present result serves for the quantitative understanding of the interaction between membrane phospholipid and CD as well as the struct ure-complexation relationship.