Raman spectroscopic studies of the dipalmitoylphosphatidylcholine/glucagon and the dipalmitoylphosphatidylcholine/cardiolipin/insulin systems

Abstract The phospholipid hydrocarbon chain conformation has been measured in lipid/protein complexes of dipalmitoylphosphatidylcholine (DPPC)/glucagon and dipalmitoylphosphatidylcholine/cardiolipin/insulin using the conformation‐sensitive CH stretching region of the Raman spectrum as a probe. In the DPPC/glucagon complex, which models hydrophobic interactions between (membrane) proteins and phospholipids, the Raman data show that the lateral interactions between phospholipid hydrocarbon chains have been greatly disrupted in the complex. The primary effect of the protein appears to be to induce a disordering of the phospholipid. The gel–liquid crystal phase transition of DPPC is reduced in temperature and is rendered less cooperative. In the DPPC/cardiolipin/insulin complex, which models the electrostatic interaction between peripheral membrane proteins and phospholipids, the primary effect of complexation is to rigidify and order the hydrocarbon chains of the phospholipids. The main interaction of the protein occurs with the cardiolipin and results in the neutralization of the excess charge. The ternary complex undergoes a broad phase separation not present in the binary phospholipid mixture.