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 alpha phase (T = 38 degrees C) to T = 20 degrees C or below, a subgel phase forms; this formation 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 alpha phase to T = 24 degrees C or above, a ripple phase forms with coexisting ripple wavelengths of 240 A 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 degrees C, or into the ripple phase above 24 degrees 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 degrees C or transforms into a multi-peaked subgel wide-angle pattern below 20 degrees 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.
Tristram-Nagle
S
S
Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213, USA. stn+@andrew.cmu.edu
Isaacson
Y
Y
Lyatskaya
Y
Y
Liu
Y
Y
Brummond
K
K
Katsaras
J
J
Nagle
J F
JF
D002152:Calorimetry, Differential Scanning; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D005782:Gels; D010713:Phosphatidylcholines / Q000138:chemical synthesis / Q000737:chemistry*; D013816:Thermodynamics; D014961:X-Ray Diffraction
D005782:Gels; D010713:Phosphatidylcholines; C086029:1-myristoyl-2-palmitoyl-sn-glycero-3-phosphocholine