Characterization of conformational states of POPC and DPPC in POPC/DPPC/cholesterol mixtures using Raman spectroscopy.

Conformational states of phospholipid chains in ternary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), deuterated 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPC), and cholesterol (Chol) were studied by Raman spectroscopy. Parameters of Raman peaks sensitive to conformational order have been used to determine chain order for mixtures over a wide range of compositions. A ternary diagram of fractions of phospholipid chains in conformationally ordered and disordered states has been constructed. It was found that the addition of POPC and cholesterol increases the fraction of DPPC chains in disordered conformations. The so-called liquid-ordered phase includes DPPC molecules in both ordered and disordered states in comparable proportions. It was found that POPC chains are partially ordered in mixtures with DPPC and cholesterol, in contrast to the case of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). This maybe the underlying reason why ternary mixtures with POPC have different miscibility behavior compared to DOPC.