Nonfreezable interlamellar

A characteristic feature of DSC is its ability to distinguish clearly between freezable water, for which ice-melting behavior is observed, and nonfreezable interlamellar water, for which it is not observed even at temperatures low enough to form ice. As is well known, the structure of ice is characterized by networks of hydrogen bonds formed among neighboring water molecules. Therefore, the point to note is that the water molecules present as nonfreezable water cannot participate in the formation of such hydrogen bonds even when cooled to extremely low temperatures. 

As discussed above, the water molecules in lipid-water systems are classified into three types nonfreezable interlamellar, freezable interlamellar, and bulk water. A correlation between the numbers of water molecules of these three types at a desired total water content is given by the equation... 

FIG. 4 Comparison of ice-melting enthalpy curves of AH and for bulk and freezable interlamellar water per mole of hpid. The AHb and AH if curves are determined from the deconvoluted ice-melting curves for the bulk and freezable interlamellar water, respectively. iVw(a) and iV (b) are water/lipid molar ratios at the maximum amounts of freezable and nonfreezable interlamellar water, respectively. The designations 1,11, and 111 represent the three regions < N (b) (i.e., in the presence of only nonfreezable interlamellar water), Al (b) interlamellar water), and > N a.) (i.e., in the presence of nonfreezable and freezable interlamellar and bulk water), respectively. 

Finally, we discuss the role of interlamellar water in lipid phase transitions. As shown in Fig. 36, the phase behavior of the lipid in the DMPE-water system is complex in the absence of freezable interlamellar water [21], Presumably, in a region of such low water content, the lipid bilayers exist as hydrated crystals containing only nonfreezable interlamellar water. However, with the appearance of freezable interlamellar water (curves d-m), the lipid phase transition comes to be characterized by a certain peak that is gradually shifted to lower temperatures with increasing water content and finally converges to a fixed temperature, generally ascribed to the gel-to-liquid crystal phase transition. Such phase behavior suggests that freezable interlamellar water is absolutely necessary for the formation of the gel phase of lipid-water systems. In this respect, another noticeable point is that the fixed peak of the gel-to-liquid crystal transition is obtained above a certain water content where a maximum uptake of the freezable interlamellar... 

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... 

Chapter 7 is an interesting review by Kodama and Aoki (Japan) on the behavior of water in phospholipid bilayer systems. The authors distinguish between nonfreezable interlamellar water and freezable intralamellar and bulk water, and estimate the number of molecules of water in each category. They also examine the relationship between lipid phase transitions and ice-melting behavior in lipid-water systems. The behavior of water is also discussed in the gel phase of systems such as DPPC, DMPE, and DPPG. 

It is more plausible that the seven HjO molecules per lipid constitute real nonfreezable water (which is not detected by DSC) and freezable interlamellar water (to which the above-mentioned melting peak below 0°C may be ascribed). This number is to be compared with the five nonfreezable water molecules per molecule of lipid plus five freezable interlamellar water molecules per molecule of lipid that were evaluated by Kodama and Aoki [134] for the gel phase of the DPPC-water system, using a deconvolution analysis. The 7 A (= 65 - 58) decrease in the interbilayer distance is then due to the loss of five molecules of... 

A//b curve intersects the abscissa and Nb at each Ny, is calculated from A/ b/1-436 (2) the enthalpy difference, AHj - AH, between the theoretical and experimental curves corresponds to 1.436 Ni, where Ni = Nn f) + lVi(f) in Eq- (3), so for each iVw Ni is calculated as equal to (AHj - A//b)/1-436. AH curve a is parallel to the theoretical curve, indicating that the total (nonfreezable plus freezable) amount of interlamellar water reaches a maximum at the N, value of the intersection point, denoted as iVw(a), and so iVw(a) just gives the maximum amount of interlamellar water above iVw(a), all the water added exists as bulk water outside the lamellae. Thus, such a parallel curve proves a limited uptake of the interlamellar water. On the other hand, AH curve b, which is not parallel to the theoretical curve, indicates an infinite uptake of the interlamellar water. Thus, some of the water added beyond the intersection point exists as bulk water, and the remainder increases the amount of interlamellar water. 

FIG. 14 Water distribution diagram for the gel phase of the DPPC-water system. The cumulative numbers of water molecules (per molecule of hpid) present as nonfreezable and freezable interlamellar water and as bulk water are plotted against N . 

The limiting, maximum numbers of water molecules are approximately 5 HjO and 5 (= 10-5) H2O per molecule of lipid for the nonfreezable and freezable interlamellar water, respectively. 

Table 2 Ice-Melting Enthalpies of Freezable Interlamellar and BuUc Water, AHiff, and AH, Respectively, per Mole of Lipid and the Numbers of Nonfreezable and Freezable Interlamellar and Bulk Water Molecules, Afi( t), A i(t), and N-b, Respectively, per Lipid Molecule at Varying Water Contents (Wh o) in the Gel Phase of the DMPE-Water System...

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