DOPC-DOPE composition dependent Lα-HII thermotropic phase transition: SAXD study

•DOPC + DOPE system exhibits at XDOPE ≥ 65 mol% a sequence of Lα − HII − QII phases.•Lα to HII phase transition temperature, onset of the QII phase formation and lattice spacings of Lα and HII phases decrease with rising XDOPE.•Structural rearrangement during Lα to HII phase transition is present.•Transition mechanism changes from martensitic to nucleation-and-growth with rising XDOPE.

The structural polymorphism and parameters of lyotropic phases formed in the mixed dioleoylphosphatidylcholine-dioleoylphosphatidylethanolamine (DOPC-DOPE) system upon heating and varying DOPC:DOPE composition were studied by means of small-angle X-ray diffraction (SAXD). In the temperature range 5–80 °C a sequence of fluid lamellar Lα – inverse hexagonal HII – inverse cubic QII phases was detected at DOPE mole fractions XDOPE ≥ 0.65. A superposition of two bicontinuous cubic QII phases of Pn3m and Ia3d space groups was identified. The Lα to HII phase transition temperature, the onset of the QII phase formation, as well as the lattice spacings of the Lα and HII phases were found to decrease with rising DOPE content. Moreover, evidence of structural rearrangement during the Lα to HII phase transition is given and change of transition mechanism with varying XDOPE is suggested.

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