Macroripple-structures induced by different branched-chain phosphatidylcholines in bilayers of dipalmitoylphosphatidylcholine

Abstract

Branched-chain phosphatidylcholines with four chains of the length C16, C18 and C20 have been synthesized by dibranching positions C2 or C4 of the acyl chains. Macroripples have been observed by freeze-fracture electron microscopy at concentrations of 5–30 mol% branched-chain phospholipids in DPPC. Macroripples differ from pretransition ripples in structure and dimension but also in their behaviour during incubation at low temperatures for a longer time. Macroripples may survive incubation at 4°C for 14 weeks. Smaller macroripples with initial periodicity of 30–50 nm may widen to twice the primary spacing during this incubation. Macroripples are commonly accompanied by parallel stripes. The stripes, which are most prominent at the crests of the macroripples, are seen on the bilayer fracture-face but not at the bilayer surface revealed by etching. Parallel stripes can also exist without tilting of the bilayer into macroripples. We suggest that phase separation or at least partial demixing of lipids may be a prerequisite for formation of macroripples. The macroripples have a tendency for a second tilting perpendicular to the main tilting. This effect was characteristic for the C2-branched phosphatidylcholines with C18- and C20-chains. A dramatic change from normal straight macroripples into those with second tilt was induced by high-pressure freezing of multilamellar vesicles with C4-branched phosphatidylcholines. The appearance of macroripples was more dependent on chain length than on the location of branching. Branched C16-chains having the same chain length as DPPC were most effective in inducing macroripples. An elongation of the chains by two C-atoms reduced macroripples more than an elongation by four C-atoms. Both types of elongation resulted in a restriction because macroripples are present only in parts of the lamellae. A speciality of the elongated branched-chain phosphatidylcholines is the predominance of macroripples with large periodicities (100 nm and more).