Deuterated phospholipids as raman spectroscopic probes of membrane structure. Phase diagrams for the dipalmitoyl phosphatidylcholine(and its d₆₂ derivative)-dipalmitoyl phosphatidylethanolamine system

Raman spectroscopic techniques have been used to construct phase diagrams for the binary phospholipid systems, DPPC-d₆₂/DPPE and DPPC/DPPE (DPPC, dipalmitoyl phosphatidylcholine; DPPE, dipalmitoyl phosphatidylethanolamine). For the former, the half-width of the C-²H stretching modes of the deuterated component near 2100 cm^-1 serves as an indicator of phospholipid fluidity. The phase behavior is described semi-quantitatively using regular solution theory with the following non-ideality parameters: ρ₀⁽¹⁾=0.75kcal/mol and ρ₀^(s)=1.05 kcal/mol. The use of deuterated phospholipids as one component of a binary mixture permits direct evaluation of the conformation of a particular component in the mixture throughout the phase separation region. The approach is demonstrated with the help of a simple model correlating the half-width of the symmetric C-²H stretching mode with the fraction of DPPC-d₆₂ hydrocarbon chains in the liquid crystalline state. The effect of chain perdeuteration on the phase behavior of DPPC with DPPE is evaluated by comparison of the phase diagram of the DPPC-d₆₂/DPPE system with that of DPPC-DPPE. The latter has been constructed previously from both probe and calorimetric techniques, and is created from the Raman spectroscopic data using the I( 1130 1100) ratio to characterize the transgauche population ratio in non-deuterated hydrocarbon chains. A reasonable fit to the phase behavior is obtained using: ρ₀⁽¹⁾=0.85kcal/mol and ρ₀^(s)=0.90 kcal/mol. The similarities of the non-ideality parameters in the two phase diagrams indicate that the effect of perdeuteration on the phase behavior of DPPC is not extensive. The use of deuterated phospholipids as essentially unperturbed components of a model membrane system is justified.