Binding of Short-Chain Lecithin by β-Cyclodextrin

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Complex formation between diheptanoylphosphatidylcholine (DHPC) and β-cyclodextrin (β-CD) in deuterium oxide solution has been investigated by measurements of proton NMR chemical shifts and the ROESY spectrum and molecular mechanics calculations. The vicinal coupling constants for protons of α-, β-, and γ-CDs in their DHPC complexes have been also determined for comparison among these CDs. From the variations in chemical shifts of DHPC and β-CD with the addition of β-CD, the macroscopic equilibrium constant, K1, of the 1:1 complexation of DHPC and β-CD is estimated. The magnitude of K1 is in the order of β-CD > γ-CD > α-CD for DHPC, although it is in the order of α-CD ≥ β-CD > γ-CD for single-chain surfactants. From the vicinal coupling constants of the glycerol C1H2-C2H protons of DHPC, the populations of three rotamers, gauche+ conformer (G+), gauche- conformer (G-), and trans conformer (T), are estimated. The addition of β-CD causes a decrease in the T conformer and an increase in the G+ conformer. These changes are the same as those of γ-CD but opposite to those of α-CD. The microscopic binding constants of CD complexation for the three rotamers are also estimated from the concentration dependence of vicinal coupling constants. The microscopic binding constant for the T conformer is in the order of α-CD > β-CD > γ-CD, as expected. The presence of DHPC causes large changes in vicinal coupling constants of the β-CD protons, suggesting that the β-CD macrocycle is deformed by incorporation of DHPC. A three-dimensional structure of the major complex β-CD−G+ is proposed on the basis of these variations in chemical shift and vicinal coupling constant, the ROESY spectrum, and molecular mechanics calculations: two heptanoyl chains of DHPC are simultaneously incorporated into a β-CD cavity so that the rim of β-CD is deformed to an ellipse. This tight contact between the hydrophobic groups of DHPC and β-CD leads to a large binding constant.