H-2-NMR AND DSC STUDY OF SEPC-CHOLESTEROL MIXTURES

Deuterium nuclear magnetic resonance (H-2-NMR) and differential scanning calorimetry (DSC) have been used to investigate the phase behaviour of mixtures of 1-stearoyl(d35)-2-elaidoyl-sn-glycero-3-phosphocholine (d35-SEPC) with varying amounts x(C) of cholesterol at temperatures between 10 and 55-degrees-C. The analysis of the specific heat traces C(x(C), T) and the first moments of the H-2-NMR spectra M1(x(C), T) gave the characteristics of the chain melting transition T(C) as a function of cholesterol concentration. The location of the 'solid ordered'-'liquid disordered' coexistence region was obtained by the use of the H-2-NMR spectral subtraction technique. The form of the resulting partial phase diagram for the SEPC/cholesterol mixtures is similar to the one obtained for DPPC/cholesterol mixtures (M. Vist and J. Davis (1990) Biochemistry 29, 451-464.). The hydrophobic bilayer thickness expansivity alpha(h)(x(C), T) was calculated from the first moments M1(x(C), T). Using general thermodynamic relations it was possible to explain the close agreement between the specific heat traces C(x(C), T) and the hydrophobic bilayer-thickness expansivity curves alpha(h)(x(C), T).