Wl. Edwards et al., RAMAN-SPECTROSCOPIC STUDY OF BOUNDARY LIPID IN 1,2-DIPALMITOYLPHOSPHATIDYLCHOLINE APOLIPOPROTEIN A-I RECOMBINANTS/, Spectrochimica acta. Part A: Molecular spectroscopy, 49(13-14), 1993, pp. 2027-2038
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.