Subgel phase

Tenchov B, Koynova R, Rapp G. New ordered metastable phases between the gel and subgel phases in hydrated phospholipids. Biophys. I. 2001 80 1873-1890. 

Kodama, M. Kato, H. Aoki, H. Water molecules in subgel phase of dimyristoylphosphatidylethanolamine/water. Thermochim. Acta 2000, 352, 213-221. 

Fig. 9. Temperature-composition isobaric phase diagram for the fully hydrated dipal-mitoylphosphatidylcholine/dipalmitoyl-phosphatidylethanolamine system constructed using the temperature gradient method. The notation used is that of Luzzati [8] and is as follows Lc, lamellar crystalline (also referred to as the subgel phase) L f, lamellar gel phase with hydrocarbon chains tilted with respect to the bilayer normal P)el, ripple phase L. lamellar liquid crystal phase. Insert bold line in graph as indicated...

Figure 4.4. In this phase diagram we find the Lp-, Pp-, Lp and phases that have already been described, plus a few new ones, namely Iv, Lo and L p. W (denoted c in the figure) is the pure DPPC crystalline or subgel phase, that is only observed in annealed (i.e. kept at 4 °C for several days) aqueous multilamellar dispersions of DPPC. A thermotropic transition, the subtransition , converts the Lc- into the Lp- phase at ca. 18 °C. Lo and Lop are two different hquid ordered (Lo) regions, respectively called liquid-crystalUne-like liquid ordered (Loo) and gel-like liquid ordered (Lop). The latter two phases differ in the orientational order of the hydrocarbon chains, and in the relative position of the cholesterol molecule in the host PC bUayer.

VIII. Analysis of Water Molecules in the Subgel Phases of Lipid-Water Systems... 

What is the role of water molecules in the conversion of the gel to the subgel phase ... 

On the other hand, the present lipids have different chain lengths. This is because when the dimyristoylphosphatidylcholine (DMPC) system is used in place of the DPPC system, its subgel phase exists at temperatures below at least 0°C (i.e., it is impossible to measure the ice-melting curve for the subgel phase). Similarly, when the dipalmitoylphosphatidylethanolamine (DPPE) system is used in place of the DMPE system, the system requires much longer periods (at least 40 days) for completion of the conversion of the gel to the subgel phase by annealing. 

In Fig. 22, three typical types of DSC curves are shown for the DMPE-water system at the same water content. A distinct difference in the thermal behavior is observed not only for the phase transition of the Upid but also for the melting of the ice. Figure 23 shows a schematic diagram of relative enthalpy (AH) versus temperature (t) curves that was constructed on the basis of the transition enthalpies and temperatures associated with the lipid phase transitions shown in Fig. 22. By reference to the diagram, it becomes apparent that the DMPE-water system can be present in two phases, designated the L- and //-subgel phases, other than the gel phase at temperatures where the hydrocarbon chains of the lipids are in a solid-like state and the thermodynamic stability of these phases increases... 

FIG. 23 Schematic diagram of the relative enthalpy (AH) versus temperature (t) for the DMPE-water system, constructed from the transition enthalpies and temperatures associated with the lipid phase transitions shown in Figs. 22a-22c. The system is also present in the H- and L-subgel phases, which are different from the gel phase. 

VIII. ANALYSIS OF WATER MOLECULES IN THE SUBGEL PHASES OF LIPID-WATER SYSTEMS... 

FIG. 26 Comparison of deconvoluted ice-melting curves between (a) L-subgel phase and (b) gel phase of the DPPC-water system at the same water content (IVhjO = 28.0 wt%, = 15.9). The deconvoluted curves (I-V) and their sum (the theoretical curve) are shown by dotted lines and the DSC curves by solid lines. 

FIG. 27 AHb and curves for the L-subgel phase of the DPPC-water system. For comparison, corresponding curves for the gel phase are shown hy dashed lines. 

FIG. 32 Water distribution diagram for the Z subgel phase N > 10) of the DMPE-water system. The cumulative numbers of water molecules (per molecule of lipid) present as nonfreezable and freezable interlameUar water and as bulk water are plotted versus N. ... 

On the other hand, complete conversion of the gel to the H-subgel phase by... 

FIG. 34 AHb and AHm) curves for the //-subgel phase of the DMPE-water system. Corresponding curves for the gel phase are shown by dashed lines. 

On the other hand, as discussed above, the L-subgel phase of the DPPC-water system involves the extra nonfreezable interlamellar water up to one molecule of HjO per molecule of lipid, compared with the gel phase. This nonfreezable interlamellar water comes from the freezable interlamellar water present in the gel phase, indicating the critical role of this freezable water in the conversion of the gel to the L-subgel phase. In fact, as shown in Fig. 25B, the conversion to the L-subgel phase by annealing is not realized for a gel sample at < 5 (see curve a), i.e., when there is no freezable interlamellar water (see Fig. 14). Furthermore, as shown in Fig. 25B, the fixed peak of the L-subgel-to-gel phase transition is observed above 12-13 where the subgel phase is fully hydrated (see... 

Fig. 28), although the N value is lower than the corresponding 15-16 for the gel phase (see Fig. 14). This fact indicates that both the fully hydrated subgel and gel phases are characterized by limiting, fixed peaks of their transitions to the respective high temperature phases. This is because, after the attainment of the fully hydrated gel and subgel phases, lateral packings of lipid molecules in a bilayer are unchangeable, even though the water content is further increased [49].

Finally, the most stable //-subgel phase of the DMPE system, which directly transforms to the liquid crystal phase, is discussed from the standpoint of the interlamellar water. As shown in Fig. 35, the //-subgel phase is characterized by a much smaller amount of interlamellar water compared with the L-subgel and... 

The lowest-temperature thermotropic lipid phases are known as the subgel phases L, . These phases are pseudocrystalline, and the molecules in the bilayer are extremely restricted in both rotation and translational diffusion. In the plane of the bilayer, the head groups have long-range order and a hexagonal packing arrangement. Lipid tails may also be tilted with respect to the layer normal. 

McIntosh TJ, Simon SA. (1993) Contributions of hydration and steric (entropic) pressures to the interactions between phosphatidylcholine bilayers Experiments with the subgel phase. Biochemistry 32 8374-8384. 

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