A MULTINUCLEAR SOLID-STATE NMR-STUDY OF PHOSPHOLIPID-CHOLESTEROL INTERACTIONS - DIPALMITOYLPHOSPHATIDYLCHOLINE-CHOLESTEROL BINARY-SYSTEM

Multinuclear(H-1, C-13, (31)p) MASNMR and static solid-state P-31 NMR were used to study the molecular interactions between dipalmitoylphosphatidylcholine (DPPC) and free cholesterol (CHOL) in multilayers of DPPC containing 0-65 mol% CHOL with respect to total lipid at temperatures between 25 and 55 degrees C. C-13 chemical shifts and line shapes for DPPC and CHOL resonances were measured in C-13 MASNMR spectre. The apparent chemical shift anisotropy (CSA) of the DPPC acyl methylene resonances [(CH2)(n)] was calculated from the H-1 MASNMR spectra. CSA and line shape changes were recorded as a function of CHOL content by P-31 static solids and MASNMR, The presence of CHOL significantly changed the C-13 chemical shifts and line shapes of DPPC carbonyl carbons below or above the main transition temperature of pure DPPC. Chemical shift changes were also observed for CHOL carbons as a function of the mixing ratio, signifying a changing local environment of CHOL. For mixtures with CHOL >50 mol%, C-13 MASNMR detected crystalline CHOL in the monohydrate form. When the excess CHOL was in a submicroscopic crystalline form that was not readily detected by differential scanning calorimetry, or optical microscopy (but readily observed by C-13 MASNMR), the P-31 powder pattern was affected, suggesting interaction of the excess CHOL with the aqueous interface of the bilayer. These results suggest the potential of C-13 MASNMR for detection of crystalline CHOL in biological samples.