Study by infrared spectroscopy of the interdigitation of C26:0 cerebroside sulfate into phosphatidylcholine bilayers.

The insertion mode of the long fatty acid chain of the asymmetric glycosphingolipid C26:0-cerebroside sulfate (C26-CBS) in symmetric matrices of phosphatidylcholines of different acyl chain length has been investigated by transmission and attenuated total reflectance (ATR) infrared spectroscopy. The concentration of C26-CBS in myelin is increased in the demyelinating disease adrenoleukodystrophy. The conformational order and the orientation of the chains of the asymmetric glycosphingolipid have been evaluated for C26-CBS incorporated at 8 mol % in perdeuterated dimyristoylphosphatidylcholine (DMPC-d54) and perdeuterated dipalmitoylphosphatidylcholine (DPPC-d62). The results, for the gel phase, are consistent with interdigitation of the C26-CBS long acyl chain across the bilayer center of an all-trans-DMPC bilayer in which DMPC is less tilted than in the absence of CBS. In contrast, in DPPC the results suggest that although the CBS long chain interdigitates across the center of the bilayer, it does not change the tilt angle of the DPPC molecules in the gel phase. Furthermore, in DPPC, C26-CBS is less well oriented than the host DPPC molecules and it increases the gauche content of the DPPC acyl chains. The observation of the amide spectral region indicates that exposure of the sphingosine amide moiety to buffer is greater in the longer chain length DPPC bilayer than in the shorter chain length DMPC bilayer. The thermotropic behavior of the lipid mixtures of C26-CBS at 8 mol % in DMPC or DPPC shows that the glycosphingolipid stabilizes the gel phase of the short chain length bilayer while it destabilizes the long chain length one. Our results further demonstrate that, at this concentration, C26-CBS is completely miscible in DMPC and DPPC in the gel and the liquid crystalline phases. The difference in behavior of C26-CBS in DMPC and DPPC is a consequence of the greater mismatch between the C26 chain length and the bilayer thickness of DPPC relative to DMPC. They may help to understand the deleterious effects of glycosphingolipids with very long chain fatty acids in adrenoleukodystrophy.

Study by infrared spectroscopy of the interaction of bovine myelin basic protein with phosphatidic acid.

The effect of bovine myelin basic protein (MBP) on dimyristoylphosphatidic acid (DMPA) and phosphatidic acid prepared from egg yolk phosphatidylcholine (EPA) has been investigated by transmission and attenuated total reflectance (ATR) Fourier transform infrared spectroscopy. Interaction of MBP with DMPA and EPA dispersions decreases the lipid acyl chain conformational disorder as a consequence of hydrophobic interactions of the protein with the lipids. Since these effects are more important for EPA dispersions than for DMPA, MBP is believed to penetrate more into EPA bilayers. This could be due to the fact that the hydrogen bond network formed by the charged polar headgroups of EPA is weaker than that of DMPA. This is supported by the spectra of the phosphate region showing that the phosphate groups of EPA are less hydrogen bonded than DMPA. In the presence of MBP, the hydrogen bond network is replaced by electrostatic interactions of the protein with the polar headgroups of the lipid. Infrared spectra of the polar headgroup region also show evidence that MBP enhanced the second ionization state of the phosphate group at neutral pH, this effect being more important for EPA than for DMPA bilayers. Also, infrared spectra of the lipid carbonyl stretching region show evidence that MBP limits the accessibility of water molecules to the interfacial part of the lipid bilayer. Finally, ATR measurements on oriented films of lipid/protein complexes indicate that the penetration of the protein into the lipid bilayer is followed by a reorientation of the lipid acyl chains toward the normal to the bilayer in the case of EPA.(ABSTRACT TRUNCATED AT 250 WORDS)