Raman Spectroscopy of the Thermal Properties of Reassembled High-Density Lipoprotein: Apolipoprotein A-I Complexes of Dimyristoylphosphatidylcholine

Isolated complexes of apolipoprotein A-I (apoA-I), the major apoprotein of human plasma high-density lipoproteins, and dimyristoylphosphatidylcholine (DMPC) have been prepared and studied by differential scanning calorimetry (DSC) and Raman spectroscopy. DSC studies establish that complexes having lipid to protein ratios of 200, 100, and 50 to 1 each exhibit a broad reversible thermal transition at T_c = 27°C. The enthalpy of lipid melting for each of the three complexes is about 3 kcal/mol of DMPC. Raman spectroscopy indicates that the physical state of lipid molecules in the complexes is different from that in DMPC multilamellar liposomes. Analysis of the C-H stretching region (2800-3000 cm^-1) of the complexes and of the pure components in water suggests that below 24°C (T_c for DMPC) there is considerably less lateral order among lipid acyl chains in the complexes than in DMPC liposomes. Above 24°C, these types of interactions appear to contribute equally or slightly less to the complex structure than in pure DMPC. The temperature dependence of peaks in the C-C stretching region (1000-1180 cm^-1) reveals a continuous increase in the number of lipid acyl chain C-C gauche isomers over a broad range with increasing temperature. Compared to liposomes, DMPC in the complexes has more acyl chain trans isomers at temperatures above 24 °C; at temperatures above ca. 30°C, trans isomer content is about the same for complexes and liposomes. A large change was observed in a protein vibrational band at 1340 cm^-1 for pure vs. complexed apoA-I, indicating that protein hydrocarbon side chains are immobilized by lipid binding. The Raman data indicate that the reduction in melting enthalpy for complexed DMPC (∼3 kcal/mol) compared to that for free DMPC (∼6 kcal/mol) is due to reduced van der Waals interactions in the low-temperature lipid phase.