Characterization of conformational states of POPC and DPPCd62 in POPC/DPPCd62/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 (DPPCd62), 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.

Conformational state diagram of DOPC/DPPCd62/cholesterol mixtures.

Raman spectra of aqueous suspensions of vesicles composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), deuterated 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPCd62), and cholesterol (Chol) were studied at room temperature to determine the conformational states of the phospholipid hydrocarbon chains. Deuteration of DPPCd62 allowed us to characterize the conformational states of DOPC and DPPCd62 independently. The parameters of Raman peaks, which are sensitive to the conformational order, were studied in a wide range of compositions. It was found that the DOPC molecules are conformationally disordered for all compositions. The conformational state of the DPPCd62 molecules changes with composition. Their conformational state is influenced by cholesterol-induced partial disordering and DOPC solvation, transforming the DPPC molecules into the disordered state. The conformational state diagram from the Raman experiment was compared with outcomes from the differential scanning calorimetry (DSC) experiment. The Raman spectra also revealed that the DPPC molecules coexist in the disordered and all-trans ordered states for the DOPC/DPPCd62/Chol mixtures except for the pure liquid-disordered phase.

Raman scattering in protonated and deuterated 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC): Indicators of conformational and lateral orders.

The use of deuterocarbons is an effective method in the Raman spectroscopy of multicomponent lipid materials and biological samples. Here, Raman spectra of hydrated multilamellar vesicles of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), its deuterated analog 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPCd62), and DPPC-DPPCd62 mixtures were studied in a wide temperature range to specify the Raman indicators of conformational and lateral orders. The temperature dependence of the 985 cm-1 line in the deuterated phospholipid unequivocally indicates that this line corresponds to the CC stretching vibrations of deuterated hydrocarbon chains in the all-trans conformation. It was also concluded that the ratio of Raman intensities at the maximum of the peak of the symmetric CD2 stretching and at a maximum near 2168 cm-1 reflects the conformational order of the hydrocarbon chain and can be used for an evaluation of the fraction of the all-trans sequences. The frequency of the symmetric CD2 stretching peak is sensitive to the phase state (gel or fluid) but has a low sensitivity to the partial conformational disordering within the gel phase. The Raman study of DPPC-DPPCd62 mixtures reveals that the lateral order contributes to the ratio of intensities of the antisymmetric and symmetric CH2 stretching peaks as a prefactor enhancing the effect of conformational ordering.

Raman study of temperature-induced hydrocarbon chain disorder in saturated phosphatidylcholines.

Raman spectra of hydrated bilayers of 1,2-dilauroyl-sn-glycero-3-phosphocholine (12:0 PC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (14:0 PC), and 1,2-distearoyl-sn-glycero-3-phosphocholine (18:0 PC) were studied in a wide temperature range to characterize the temperature-dependent hydrocarbon chain disorder of the saturated phosphatidylcholines. Temperature dependencies of the Raman line intensities were analyzed for the antisymmetric CH2 stretching and CC stretching vibrations, which are sensitive to the lipid order. It was found that chain disordering processes occur significantly below the gel-fluid transition. The chain conformational order characterized by the CC stretching line intensity can be well described by a model involving the excitation of the ordered conformational state to the kinked and highly disordered, fluid-like state. A relation between the excitation energy to the disordered state and the enthalpy of the gel-fluid transition is discussed, including also data for other phosphatidylcholines studied before. Temperature behavior of the antisymmetric CH2 stretching line indicates that non-conformational degrees of freedom are released above ∼ 200 K. Experimental findings concerning the hydrocarbon chain length dependence of the Raman polarizability of antisymmetric CH2 stretching vibrations and a low-temperature solid-solid phase transition, identified in 12:0 PC at heating, are also discussed in the work.