Mixed-chain phosphatidylcholine bilayers: structure and properties

Abstract

Calorimetric and X-ray diffraction data are reported for two series of saturated mixed-chain phosphatidylcholines (PCs), 18:0/n:0-PC and n:0/18:0-PC, where the sn-1 and sn-2 fatty acyl chains on the glycerol backbone are systematically varied by two methylene groups from 18:0 to 10:0 (n = 18, 16, 14, 12, or 10). Fully hydrated PCs were annealed at -4 degrees C and their multilamellar dispersions characterized by differential scanning calorimetry and X-ray diffraction. All mixed-chain PCs form low-temperature "crystalline" bilayer phases following low-temperature incubation, except 18:0/10:0-PC. The subtransition temperature (Ts) shifts toward the main (chain melting) transition temperature (Tm) as the sn-1 or sn-2 fatty acyl chain is reduced in length; for the shorter chain PCs (18:0/12:0-PC, 12:0/18:0-PC, and 10:0/18:0-PC), Ts is 1-2 degrees C greater than Tm, and the subtransition enthalpy (delta Hs) is much greater than for the longer acyl chain PCs. Tm decreases with acyl chain length for both series of PCs except 18:0/10:0-PC, while for the positional isomers, n:0/18:0-PC and 18:0/n:0-PC, Tm is higher for the isomer with the longer acyl chain in the sn-2 position of the glycerol backbone. The conversion from the crystalline bilayer Lc phase to the liquid-crystalline L alpha phase with melted hydrocarbon chains occurs through a series of phase changes which are chain length dependent. For example, 18:0/18:0-PC undergoes the phase changes Lc----L beta'----P beta'----L alpha, while the shorter chain PC, 10:0/18:0-PC, is directly transformed from the Lc phase to the L alpha phase. However, normalized enthalpy and entropy data suggest that the overall thermodynamic change, Lc----L alpha, is essentially chain length independent. On cooling, the conversion to the Lc phases occurs via bilayer gel phases, L beta', for the longer chain PCs or through triple-chain interdigitated bilayer gel phases, L beta, for the shorter chain PC 18:0/12:0-PC and possibly 10:0/18:0-PC. Molecular models indicate that the bilayer gel phases for the more asymmetric PC series, 18:0/n:0-PC, must undergo progressive interdigitation with chain length reduction to maintain maximum chain-chain interaction. The L beta phase of 18:0/10:0-PC is the most stable structure for this PC below Tm. The formation and stability of the triple-chain structures can be rationalized from molecular models.

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.…”

Alkyl Chain Ordering of Asymmetric Phosphatidyicholines Adsorbed at a Liquid-Liquid Interface

“…As previously mentioned, subtransitions have been observed typically in enantomerically pure saturated PCs after low temperature incubation near 0°C for several days to months (Finegold and Singer, 1984;Mattai et al, 1987). It is therefore quite intriguing to have obtained crystalline monolayer formation in less than 24 hours at 22°C.…”

“…Many saturated, mixed chain PCs have also been reported to undergo subtransitions. In a series of PCs having a C-|8 chain in the sn-1 or sn-2 position and the chain in the other position varying between 10 and 18 (even numbers) carbons in length, PSPC, DSPC and SPPC had the highest subtransition temperatures (Mattai, et al, 1987). It is possible that the longer chain PC monolayers adsorbed at the CCI4-D2O interface undergo a similar transition into a crystalline state below their respective subtransition temperatures, which would acount for the large increase in hydrocarbon chain ordering observed at room temperature for DSPC as compared to DPPC.…”

Thermal behavior of liposomes containing PCs with long and very long chain PUFAs isolated from retinal rod outer segment membranes

“…Regarding the degree of unsaturation, it is also known that the introduction of just a single double bond in the hydrocarbon chain of a fatty acid has dramatic effects on the chainmelting transition (Tt may be decreased between 1ЊC and Ϫ60ЊC, depending on the position of the double bonds in the chains) (36)(37)(38). The third factor, unequal length of the two acyl chains at the sn-1 and sn-2 positions, is known to affect interchain attraction and to decrease the chain packing density at the region of the hydrocarbon terminus (13,39). Thus, the large total number of carbons in dipolyunsaturated PCs is the dominating factor, shifting the curves to the right with respect to that of 16:0/22:6-PC.…”

“…In the case of dipolyunsaturated species, such a chain-chain matching is likely to be possible for 22:6/22:6-PC, but not for VLCPUFA/22:6-PCs. In order to maximize van der Waals contacts in the gel phase, and prevent the occurrence of voids toward the region of the terminal methyl groups (39), phospholipids with such a dissimilar length of their acyl chains like the ones studied here may be expected to undergo some degree of interdigitation of the supernumerary carbons across the central region of the bilayer. This possibility has been proposed even in the case of phospholipids with only one or two carbons of difference between chains (43).…”