Carbon-13 nuclear magnetic resonance studies of cholesterol-egg yolk phosphatidylcholine vesicles

Proton-decoupled natural-abundance (13)C nuclear magnetic resonance spectra at 63 KG were obtained for isolated single-bilayer egg yolk phosphatidylcholine-cholesterol vesicles containing a variable phospholipid/cholesterol ratio. Numerous well-resolved singly carbon resonances of phospholipid and cholesterol carbons were observed. Carbon resonances from different parts of the phospholipid show markedly different behavior as a function of cholesterol content of the vesicles. The line widths of resonances for carbon atoms in the head-group region and the sn-3 carbon of the phospholipid glycerol backbone are relatively independent of cholesterol content. In contrast, resonances from the sn-1 and sn-2 carbon atoms of the glycerol backbone and the envelopes containing the olefinic and aliphatic carbon resonances of the fatty acyl chains of the phospholipids broaden markedly with increasing content of cholesterol. The most prominent cholesterol ring resonance is that for C6. This is, in part, due to its location in a clear window of the spectrum, where it is unobscured by interfering phospholipid resonances. However, resonances for cholesterol ring carbons C9 and C14, 17, which should also appear in clear regions of the spectrum, are not observable. It is suggested that these resonances are broadened by dipolar interactions with neighboring protons. Anisotropic rotation of the cholesterol molecule about its long axis is suggested to be the major mechanism responsible for decreased dipolar interactions for the C6 carbon, while retaining the large dipolar coupling of the C9 and C14, 17 carbons with their neighboring protons. The temperature dependence of spectra of single-bilayer phosphatidylcholine vesicles containing epicholesterol (alpha-cholesterol) is different from that of corresponding cholesterol-containing vesicles. Resonances from the C9 and C14, 17 carbons of epicholesterol in EYPC vesicles are detectable at 35 degrees C whereas the corresponding resonances from beta-cholesterol are not. These data suggest that in vesicles the rotation of cholesterol is more anisotropic than that of epicholesterol and that the stereochemistry of the C3 hydroxyl group of cholesterol is at least partly responsible for the highly anisotropic rotation of the steroid ring within the bilayer.