RAMAN-SPECTROSCOPIC STUDY OF BOUNDARY LIPID IN 1,2-DIPALMITOYLPHOSPHATIDYLCHOLINE/APOLIPOPROTEIN A-I RECOMBINANTS

Raman spectroscopy has been used to obtain thermal phase transition profiles for recombinant particles composed of 1,2-dipalmitoylphosphatidylcholine (DPPC) and apolipoprotein A-I. Comparison of these profiles with unilamellar vesicles of DPPC indicates that lateral packing of DPPC acyl chains is tighter in recombinant DPPC/apolipoporotein A-I particles than in uncomplexed lipid of unilamellar vesicles. Consequently, the magnitude of the entropy change associated with acyl chain melting in the recombinants at the main lipid phase transition is almost twice that of unilamellar DPPC. In addition, a second phase transition has been observed for the DPPC/apolipoprotein A-I complex and has been assigned to the acyl chain melting of DPPC molecules which are bound to the apolipoprotein annulus on the periphery of the discoidal complexes. A combination of results from Raman spectroscopy, electron micrograph measurements and chemical analysis leads to the conclusion that these protein-bound lipids, the ''boundary layer'', account for about 20% of the total lipid in the recombinant material. Calculations indicate that there are about 55 protein-bound lipid molecules per apolipoprotein A-I molecule in the DPPC/apolipoprotein A-I discoidal complexes prepared for this study.