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

Raman spectroscopy has been used to obtain thermal phase transition pr ofiles for recombinant particles composed of 1,2-dipalmitoylphosphatid ylcholine (DPPC) and apolipoprotein A-I. Comparison of these profiles with unilamellar vesicles of DPPC indicates that lateral packing of DP PC acyl chains is tighter in recombinant DPPC/apolipoporotein A-I part icles than in uncomplexed lipid of unilamellar vesicles. Consequently, the magnitude of the entropy change associated with acyl chain meltin g in the recombinants at the main lipid phase transition is almost twi ce that of unilamellar DPPC. In addition, a second phase transition ha s been observed for the DPPC/apolipoprotein A-I complex and has been a ssigned to the acyl chain melting of DPPC molecules which are bound to the apolipoprotein annulus on the periphery of the discoidal complexe s. A combination of results from Raman spectroscopy, electron microgra ph measurements and chemical analysis leads to the conclusion that the se 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 apolipoprot ein A-I molecule in the DPPC/apolipoprotein A-I discoidal complexes pr epared for this study.