Polymorphism in myristoylpalmitoylphosphatidylcholine

This study focuses on the mixed-chain lipid myristoylpalmitoylphosphatidylc holine (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-alpha phase (T = 38 degrees C) to T = 20 degrees C or below, a subgel phase forms; this format ion takes less than 1 h at a temperature below T = 12 degrees C. The subgel remains stable up to T = 29 degrees C. When MPPC is T-jumped from the L-al pha phase to T = 24 degrees C or above, a ripple phase forms with coexistin g ripple wavelengths of 240 Angstrom and 130 Angstrom. In contrast, when MP PC is melted from the subgel phase, the ripple phase is characterized by bi layers having a single ripple wavelength of 130 Angstrom. In agreement with earlier studies (Stumpel et al., 1983; Serrallach et al., 1984. Structure and thermotropic properties of mixed-chain phosphatidylcholine bilayer memb ranes. Biochemistry 23:713-720.), no stable gel phase was observed. Instead , an ill-defined low-angle X-ray pattern is initially observed, which gradu ally transforms into the subgel phase below 20 degrees C, or into the rippl e phase above 24 degrees C. In the wide-angle X-ray diffraction, a single p eak is observed, similar to the ripple phase wide-angle pattern, that eithe r persists above 24 degrees C or transforms into a multi-peaked subgel wide -angle pattern below 20 degrees C. The absence of a gel phase can be unders tood phenomenologically as the relative dominance of the subgel phase in mi xed-chain PCs compared to same-chain PCs. The subgel structure and molecula r interactions responsible for this comparative behavior are interesting op en issues. (C) 1999 published by Elsevier Science Ireland Ltd. All rights r eserved.