Thermodynamic properties, lipid/water

Pouliquen et al. (1997) used NMR technique to study water and lipid reserves in seeds. The temperature dependence of relaxation time was used to identify differences in the thermodynamic properties of water between dry seeds and during germination (Figure 8.3). 

Lipid-water gel phases were previously regarded as metastable structures that are formed before separation of water and lipid crystals when the corresponding lamellar liquid crystal is cooled. New information on gel phases (see below) reveals that they can form thermodynamically stable phases with very special structural properties. This characteristic makes them as interesting as the lamellar liquid crystals from a biological point of view. 

Water is a key factor affecting lipid oxidation in foods during storage by a complex series of events. The water activity of a food is a more appropriate physicochemical property than water content, because in heterogeneous systems it determines the distribution of water between different components. Water activity (Aw) or the thermodynamic availability of water is defined as ... 

The results discussed in this section indicate that the hydrophilic properties of polyphenols facilitate their localization and accumulation at the interface of biomembranes and lipoproteins, thereby suggesting two advantages as antioxidants (a) inhibition of attack by flee radicals in the aqueous phase and (b) efiective repair of lipophilic radicals (such as a-tocopheroxyl radicals). Thus, in addition to thermodynamic (redox potentials) and kinetic (rate constants, stability of the antioxidant-derived radical) properties, solubility properties may determine antioxidant effectiveness in recycling mechanisms at lipid-water interfaces. Also, they may explain that low amounts of polyphenols, concentrated at the lipid-water interfaces, may achieve a concentration high enough to act as antioxidants in biological systems. 

More generally, all solvent-free CG methods have intrinsic limitations when it comes to describing explicitly processes that ultimately depend on the partitioning of molecules between water and oil, a key characteristic of lipids and surfactants. Thus, even though solvent-free CG methods have shown the capability to reproduce structural properties of membranes, more accurate thermodynamic properties require the explicit treatment of water-lipids interactions. 

Lipophilicity is a molecular property expressing the relative affinity of solutes for an aqueous phase and an organic, water-immiscible solvent. As such, lipophilicity encodes most of the intermolecular forces that can take place between a solute and a solvent, and represents the affinity of a molecule for a lipophilic environment. This parameter is commonly measured by its distribution behavior in a biphasic system, described by the partition coefficient of the species X, P. Thermodynamically, is defined as a constant relating the activity of a solute in two immiscible phases at equilibrium [111,112]. By convention, P is given with the organic phase as numerator, so that a positive value for log P reflects a preference for the lipid phase ... 

Spectroscopy will also be vital if and when we can search the atmospheres of potentially habitable planets for the presence of molecules that indicate the existence of life, a topic discussed in Chap. 5. The discussion of intermolecular forces, especially hydrogen bonding, in Sect. 1.4 serves as an introductiOTi to Chap. 6, which is devoted to the role in biochemical systems of a molecule, water, whose universality on Earth might blind us to its remarkable properties. Quite a lot of this introductory chapter has been devoted to thermodynamics. The role and importance of thermodynamics when we consider what conditions might lead to and sustain life are particularly brought out in some of the later chapters of this book. The forces between dissolved species and their solvent and between molecules that are at the boundary of solubility and therefore can form micelles and lipid bilayers were introduced in Sect. 1.4. These species and their properties re-emerge in Chap. 9 as does the topic of enzyme catalysis introduced in Sect. 1.6. 

At the physical interface between the living and nonliving, cell membranes are essentially composed of phospholipids, which globally exhibit a structure with a polar hydrophilic end and a nonpolar hydrophobic end. The assembly constitutes a kind of bi-dimensional liquid, sometimes called a fluid mosaic model. The hydrophilic heads are immersed in the aqueous medium (water) on each side of the double layer, and the hydrophobic tails congregate inside the membrane. The thermodynamic stability of such a liquid is maximal for a thickness of 40 A (4nm), and the mechanical properties of the membranes result from the combination of the lipids with stabilizing elements, which maintain the cohesion of the assembly. The chemical study of these stabilizing elements allows a clear distinction between the cell membranes of the three fundamental types of organism. 

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