Light scattering measurement and Avrami analysis of the lamellar to inverse hexagonal phase transition kinetics of the lipid DEPE.

The lamellar to inverse hexagonal phase transition of lipids is much studied as a model for understanding cellular processes such as membrane fusion and pore formation. Much remains unknown, including a theoretical understanding and a definitive value of the phase transition temperature for DEPE, as literature values vary over 10°C. Avrami theory has been commonly used to analyze phase transition kinetics. However, to the best of our knowledge, Avrami theory has not been used to analyze the lamellar to inverse hexagonal transition in lipids until now. We used laser light scattering to measure phase transition temperature of the lipid DEPE (1,2-dielaidoyl-sn-phosphatidylethanolamine) and found it to be 61.0 ± 0.5°C. We found the hysteresis, |T(measured)-T(equilibrium)|, scaled as r(β), where r is the ramp rate and β=0.29 ± 0.02. This is the same power law behavior found by others for an isomer of DEPE known as DOPE (1,2-dioleoyl-sn-glycero-3 ethanolamine); however, DEPE exhibits roughly half the hysteresis of DOPE. An analysis of DEPE kinetics yields Avrami exponents ranging from 1 to 7, suggesting the transition propagates one dimensionally and is initiated by a widely varying nucleation rate.