New Ordered Metastable Phases between the Gel and Subgel Phases in Hydrated Phospholipids

Abstract

Formation of low-temperature ordered gel phases in several fully hydrated phosphatidylethanolamines (PEs) and phosphatidylcholines (PCs) with saturated chains as well as in dipalmitoylphosphatidylglycerol (DPPG) was observed by synchrotron x-ray diffraction, microcalorimetry, and densitometry. The diffraction patterns recorded during slow cooling show that the gel-phase chain reflection cooperatively splits into two reflections, signaling a transformation of the usual gel phase into a more ordered phase, with an orthorhombic chain packing (the Y-transition). This transition is associated with a small decrease (2-4 microl/g) or inflection of the partial specific volume. It is fully reversible with the temperature and displays in heating direction as a small (0.1-0.7 kcal/mol) endothermic event. We recorded a Y-transition in distearoyl PE, dipalmitoyl PE (DPPE), mono and dimethylated DPPE, distearoyl PC, dipalmitoyl PC, diC(15)PC, and DPPG. No such transition exists in dimyristoyl PE and dilauroyl PE where the gel L(beta) phase transforms directly into subgel L(c) phase, as well as in the unsaturated dielaidoyl PE. The PE and PC low-temperature phases denoted L(R1) and SGII, respectively, have different hydrocarbon chain packing. The SGII phase is with tilted chains, arranged in an orthorhombic lattice of two-nearest-neighbor type. Except for the PCs, it was also registered in ionized DPPG. In the L(R1) phase, the chains are perpendicular to the bilayer plane and arranged in an orthorhombic lattice of four-nearest-neighbor type. It was observed in PEs and in protonated DPPG. The L(R1) and SGII phases are metastable phases, which may only be formed by cooling the respective gel L(beta) and L(beta') phases, and not by heating the subgel L(c) phase. Whenever present, they appear to represent an indispensable intermediate step in the formation of the latter phase.

Molecular structure of the lecithin ripple phase

“…The phosphatidylethanolamine head group is smaller than the PC head group and it is well known that dipalmitoylphosphatidylethanolamine (DPPE) and dilauroylphosphatidylethanolamine (DLPE) lipids do not form ripple phases. Instead, in simulations of DPPE and DLPE bilayers we find L ␤ structures for these lipids, in accordance with experimental data (29,30).…”

Mesoscopic models of biological membranes

“…For longer tails, the L phase was observed in between the L c and P phase. This phase behavior well reproduces experimental results for PCs [136,137]. The transition temperatures were seen to increase with increasing tail length, in agreement with experimental data, with the exception of the transition temperature from the L c phase to the L , which was almost constant with increasing tail length.…”

Dynamics of Model Membranes by NMR

“…The phase transition between crystalline L C and gel L β phase occurs at T s temperature [4]. In the lamellar L β phase, the lipid molecules take up a larger area and are more hydrated than in the L C phase [21]. The correlation time of the rotational motion of the acyl chains is about 10 À5 s, which is about 100 times slower than the isomerization of the hydrocarbon chains [22].…”