Acyl chain interdigitation in saturated mixed-chain phosphatidylcholine bilayer dispersions

Abstract

The molecular packing of various fully hydrated mixed-chain phosphatidylcholines was studied by X-ray diffraction and electron microscopy. All of the mixed-chain phosphatidylcholines under study were shown to adopt a lamellar or bilayer form in aqueous media. The bilayer thickness of these mixed-chain phosphatidylcholines was determined from the lamellar repeat distance in the small-anglé X-ray diffraction region by controlled swelling experiments. At T greater than Tm, the bilayer thickness of C(18):C(12)PC and C(18):C-(10)PC is found to be comparable to that of C(14):C(14)PC. In contrast, the bilayer thickness of these highly asymmetric phosphatidylcholines is considerably less than that of the symmetric C(14):C(14)PC at temperatures below Tm. Moreover, the wide-angle X-ray diffraction patterns taken at T less than Tm consist of at least two sharp reflections at 4.2 and 4.6 A. These X-ray diffraction data suggest that these highly asymmetric mixed-chain phospholipids, in excess water, form mixed interdigitated bilayers in the gel state and that the acyl chain packing in the gel-state bilayer is not hexagonal. The freeze-fracture planes of these mixed-chain phosphatidylcholines are discontinuous at T less than Tm, supporting the conclusion drawn from X-ray diffraction data that these highly asymmetric phosphatidylcholines form interdigitated bilayers at temperatures below Tm. The molecular packing of fully hydrated C(18):C(14)PCs in bilayers is distinctively different from that of C(18):C(10)PCs or C(18):C(10)PCs.(ABSTRACT TRUNCATED AT 250 WORDS)

Very Long-chain Fatty Acid-containing Lipids rather than Sphingolipids per se Are Required for Raft Association and Stable Surface Transport of Newly Synthesized Plasma Membrane ATPase in Yeast

“…The evidence that the yeast plasma membrane is thicker than other membranes, however, is mostly based on electron microscopy and thus susceptible to possible artifacts based on the fixation and staining procedures (38,39). Biophysical studies with pure lipid bilayers indicate that lipids with highly asymmetric acyl chains can interdigitate into the hydrophobic core of the opposite half of the bilayer and thus do not necessarily increase the thickness of the bilayer (40,41). Such studies, however, do not account for the role of sterols, proteins, and an asymmetric lipid composition of the two leaflets of the bilayer on the precise arrangement of the very long acyl chains and their effects on the bilayer.…”

Phase behavior of selected artificial lipids

“…For phospholipids with a large difference between tails (SDPC [C18:0 and C10:0] or SLPC [C18:0 and C12:0]) this leads to a mixed interdigitation [24] in the gel phases at T b T m : in a bilayer, the two shorter chains touch end-to-end. The longer tails fully reach into the opposite bilayer half.…”

Lipid-dependent surface transport of the proton pumping ATPase: A model to study plasma membrane biogenesis in yeast

“…In such a model, the abundance of C26-containing lipids may determine the thickness of membranes along the secretory pathway and thereby affect the sorting of integral membrane proteins. Biophysical studies with pure lipid bilayers, on the other hand, indicate that lipids with highly asymmetric acyl chains can interdigitate into the hydrophobic core of the opposite half of the bilayer and thus do not necessarily increase the thickness of the bilayer [36]. In addition, membrane proteins themselves have recently been shown to modulate bilayer thickness by more than 10% [37], and observation that is consistent with the proposition that the thickness of the lipid component of a biological membrane must not naturally match that of the embedded proteins [38].…”