Polymorphism in myristoylpalmitoylphosphatidylcholine

This study focuses on the mixed-chain lipid myristoylpalmitoylphosphatidylcholine (MPPC) near full hydration. The lipid, synthesized according to the procedure of (Mason et al., 1981a, has a low degree of acyl chain migration. When MPPC is temperature-jumped (T-jumped) from the L(α) phase (T=38°C) to T=20°C or below, a subgel phase forms; this formation takes less than 1 h at a temperature below T=12°C. The subgel remains stable up to T=29°C. When MPPC is T-jumped from the L(α) phase to T=24°C or above, a ripple phase forms with coexisting ripple wavelengths of 240 and 130 A. In contrast, when MPPC is melted from the subgel phase, the ripple phase is characterized by bilayers having a single ripple wavelength of 130 A. In agreement with earlier studies (Stumpel et al., 1983; Serrallach et al., 1984. Structure and thermotropic properties of mixed-chain phosphatidylcholine bilayer membranes. Biochemistry 23:713-720.), no stable gel phase was observed. Instead, an ill-defined low-angle X-ray pattern is initially observed, which gradually transforms into the subgel phase below 20°C, or into the ripple phase above 24°C. In the wide-angle X-ray diffraction, a single peak is observed, similar to the ripple phase wide-angle pattern, that either persists above 24°C or transforms into a multi-peaked subgel wide-angle pattern below 20°C. The absence of a gel phase can be understood phenomenologically as the relative dominance of the subgel phase in mixed-chain PCs compared to same-chain PCs. The subgel structure and molecular interactions responsible for this comparative behavior are interesting open issues. Copyright (C) 1999 Elsevier Science Ireland Ltd.