Conformation-dependent binding of diheptanoylphosphatidylcholine by cyclodextrins as revealed by proton nuclear magnetic resonance

The complex formation of 1 mmol kg(-1) diheptanoylphosphatidylcholine (DHPC) with alpha- and gamma-cyclodextrins (CD) in deuterium oxide solutions has been investigated by proton nuclear magnetic resonance. With the addition of CD, the variations in proton chemical shifts of DHPC and in vicinal coupling constants of the glycerol ClH2-C2H protons allow us to estimate the equilibrium constant and stoichiometry of complexation and to image the three-dimensional structures of their complexes. The addition of alpha-CD causes an increase of the trans conformer (T) of DHPC and a decrease of the gauche(+) conformer (G(+)), whereas the addition of gamma-CD results in the reverse changes. From the dependence of the chemical shift of a DHPC proton on the CD concentration, we estimate the equilibrium formation macroconstants K-1 and K-2 of 1:1 and 1:2 complexations of DHPC and CD. In contrast with the case for single-chain surfactants, the K-1 value for alpha-CD is smaller than that for gamma-CD and the K-2 value for gamma-CD is smaller than K-1 for gamma-CD. The equilibrium microconstants of CD complexation of the three rotamers G(+), G(-)(gauche(-)), and T of DHPC are also estimated from the concentration dependence of vicinal coupling constants. Predominant binary complexes of alpha-CD and gamma-CD are composed of the G(-) form and the G(+) form, respectively. The T of the three conformers of DHPC tends most easily to form the ternary complex with alpha-CD and gamma-CD. There seems to be almost no distinction between the 1- and 2-heptanoyl chains of DHPC for binding to the alpha-CD cavity. The inclusion of DHPC into the gamma-CD cavity induces magnetic nonequivalence of the terminal methyl protons of chains 1 and 2. It is likely that DHPC is bound from the side of secondary hydroxyl groups of gamma-CD. The three-dimensional structures of major complexes are proposed on the basis of the magnitude of chemical-shift variation of each proton of DHPC and CD. The intermediate methylene groups of DHPC cue located near the center of an alpha-CD cavity, whereas the two terminal methyl groups of DHPC are deeply penetrated into a gamma-CD cavity. The above results are discussed in relation to the hemolysis of erythrocytes, one of the serious toxicities of CD caused when it was added in foods and pharmaceuticals.