The liquid state of lipids

As discussed in the preceding section, the lipid molecules are disordered in the liquid-crystalline phases and the hydrocarbon chains give an X-ray scattering pattern characteristic for liquid paraffins (Luzzati et aL, 1960). The structure in the hydrocarbon chain regions of liquid-crystalline phases and in the liquid state of lipids in general will be further considered below. 

The frequency of the X-irradiation is several orders of magnitude higher than that of molecular motions which means that chains in the liquid state are observed in static positions. The significance of the 4.5 A halo in the X-ray scattering of the lamellar phase will first be discussed here, as no geometrical analysis has been reported earlier as far as this author knows. 

Another support for a tetragonal co-ordination is the value of the cross-section per chain in the plane of the hydrocarbon chain layer. Using the value of the cross-section area per CH2-group of 20.25 A and the average direction of the zig-zag units in the L -phase calculated above, a value of 30 A is obtained. This is in good agreement with observed values in L -phases. 

This result, indicating a tetragonal co-ordination in the L -phase, can be generalized to the liquid state of lipids, according to the preferred arrangement in layers, which is treated in the next paragraph. 

It should finally be mentioned that the chain conformation in lipids has been examined by different spectroscopic techniques. 

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The primary product of the hot homogenization is a nanoemulsion resulting from the liquid state of the lipid. Solid particles are expected to be formed by the cooling of the sample to room temperature or below. Because of the small particle size and the presence of the emulsifiers, lipid crystallization may be highly retarded, and the sample may remain as a supercooled melt (nanoemulsion) for several months [28], Westesen and Bunjes found that purported SLN data published by another group were, in fact, measurements from supercooled melts [29],... 

It has been shown by FM that the phase state of the lipid exerted a marked influence on S-layer protein crystallization [138]. When the l,2-dimyristoyl-OT-glycero-3-phospho-ethanolamine (DMPE) surface monolayer was in the phase-separated state between hquid-expanded and ordered, liquid-condensed phase, the S-layer protein of B. coagulans E38/vl was preferentially adsorbed at the boundary line between the two coexisting phases. The adsorption was dominated by hydrophobic and van der Waals interactions. The two-dimensional crystallization proceeded predominately underneath the liquid-condensed phase. Crystal growth was much slower under the liquid-expanded monolayer, and the entire interface was overgrown only after prolonged protein incubation. 

As mentioned earlier, a great deal of literature has dealt with the properties of heterogeneous liquid systems such as microemulsions, micelles, vesicles, and lipid bilayers in photosynthetic processes [114,115,119]. At externally polarizable ITIES, the control on the Galvani potential difference offers an extra variable, which allows tuning reaction paths and rates. For instance, the rather high interfacial reactivity of photoexcited porphyrin species has proved to be able to promote processes such as the one shown in Fig. 3(b). The inhibition of back ET upon addition of hexacyanoferrate in the photoreaction of Fig. 17 is an example of a photosynthetic reaction at polarizable ITIES [87,166]. At Galvani potential differences close to 0 V, a direct redox reaction involving an equimolar ratio of the hexacyanoferrate couple and TCNQ features an uphill ET of approximately 0.10 eV (see Fig. 4). However, the excited state of the porphyrin heterodimer can readily inject an electron into TCNQ and subsequently receive an electron from ferrocyanide. For illumination at 543 nm (2.3 eV), the overall photoprocess corresponds to a 4% conversion efficiency. 

Monte Carlo may be used to study the lateral distribution of lipid molecules in mixed bilayers. This of course is a very challenging problem, and, to date, the only way to obtain relevant information for this is to reduce the problem to a very simplistic two-dimensional lattice model. In this case, the lipid molecules occupy a given site and can be in one of the predefined number of different states. These pre-assigned states (usually about 10 are taken), are representative conformations of lipids in the gel or in the liquid state. Each state interacts in its own way with the neighbouring molecules (sitting on neighbouring sites). Typically, one is interested in the lateral phase behaviour near the gel-to-liquid phase transition of the bilayer [69,70]. For some recent simulations of mixtures of DMPC and DSPC, see the work of Sugar [71]. 

Bitumen is the geological equivalent of lipids, consisting in the widest sense of any sedimentary hydrocarbon ranging in state from tarry (asphalt) through viscous to liquid (petroleum). 

The three fundamental lyotropic liquid crystal structures are depicted in Figure 1. The lamellar structure with bimolecular lipid layers separated by water layers (Figure 1, center) is a relevant model for many biological interfaces. Despite the disorder in the polar region and in the hydrocarbon chain layers, which spectroscopy reveals are close to the liquid states, there is a perfect repetition in the direction perpendicular to the layers. Because of this one-dimensional periodicity, the thicknesses of the lipid and water layers and the cross-section area per lipid molecule can be derived directly from x-ray diffraction data. 

Free fatty acids, derived primarily from adipocyte triglycerides, are transported as a physical complex with plasma albumin. Triglycerides and cholesteryl esters are transported in the core of plasma lipoproteins [134], Deliconstantinos observed the physical state of the Na+/K+-ATPase lipid microenvironment as it changed from a liquid-crystalline form to a gel phase [135], The studies concerning the albumin-cholesterol complex, its behavior, and its role in the structure of biomembranes provided important new clues as to the role of this fascinating molecule in normal and pathological states [135]. 

Solid-state NMR has done much to dispel the mysteries of humin compositions, and significant advances have recently been made using proton NMR in the liquid state (see Section 15.3.3 of Chapter 15). Based on solid-state 13C NMR spectra, Hatcher et al. (1980) concluded that a repeating aliphatic structural unit, possibly attributable to branched and cross-linked algal or microbial lipids, is common to both soil and sediment humin samples. Hatcher et al. (1983) viewed the increase in humin relative to the other humic fractions as a selective preservation of the aliphatic compounds of the sediments and did not support condensation theories. 

Another parameter that can have a great influence on the results obtained is the type of the simulation performed. Generally, simulations are carried out at constant particle number (N). The volume (V) and energy (E) of the simulated system can be held constant, leading to a so-called NVE, or microcanonical, ensemble. When the volume and temperature are held constant, this yields a canonical or NVT ensemble. In both cases, the size of the simulated system is chosen in such a way as to represent the desired state of the phospholipid, mostly the liquid crystalline La phase. The surface per lipid and the thickness of the bilayer are set based on experimental values and remain unchanged during the simulation. Therefore, the system is not able to adjust its size and thickness. 

The third class of lipids found in stratum corneum extracts is represented by cholesterol and cholesteryl esters. The actual role of cholesterol remains enigmatic, and no clear reason for its role in the barrier function has been proposed so far. However, it is possible that contrary to what is the role in cell membranes where cholesterol increases close packing of phospholipids, it can act as kind of a detergent in lipid bilayers of long-chain, saturated lipids.30,31 This would allow some fraction of the barrier to be in a liquid crystalline state, hence water permeable in spite of the fact that not only ceramides, but also fatty acids found in the barrier are saturated, long-chain species.28,32... 

The transition from the crystalline to the liquid state is accompanied by absorption of heat, a loss of long-range order, and an increase in molecular volume. However, many long chain lipids show only small volume changes (10-20%) during the transition from solid to liquid, which indicates that some short-range order should remain in the liquid state (Small, 1986, pp. 56-57). 

In the 1970s, the fluid mosaic concept emerged as the most plausible model to account for the known structure and properties of biological membranes [41]. The fact that membranes exist as two-dimensional fluids (liquid disordered) rather than in a gel state (solid ordered) was clearly demonstrated by Frye and Ededin [42], who showed that the lipid and protein components of two separate membranes diffuse into each other when two different cells were fused. Since that time, numerous studies have measured the diffusion coefficient of lipids and proteins in membranes, and the diffusion rates were found to correspond to those expected of a fluid with the viscosity of olive oil rather than a gel phase resembling wax. 

In previous sections of this chapter, as well as in chapter 6, we have discussed several reasons why liquid water is so critical for life. To briefly review the salient points (1) Water is essential for driving the formation of the three-dimensional structures of macromolecules. These structures, on which macromolecular function depends, are encoded in a latent form in the linear primary structures of proteins and nucleic acids, but can be manifested only when liquid water is present to foster hydrophobic interactions. (2) The assembly of bilayer membranes from lipids and proteins likewise is driven in large measure by hydrophobic effects. (3) Water in the liquid state is a requirement for most types of transport of materials between organism and environment and between compartments within the organism. (4) Lastly, the... 

Foht PJ, Quynh MT, Lewis RNAH, McElhaney RN. Quantitation of the phase preferences of the major lipids of the Acholeplasma laidlawii B membrane. Biochemistry 1995 34 13811-13817. Lewis RNAH, McElhaney RN. Acholeplasma laidlawii B membranes contain a lipid (glycxerylphosphoryldiglucosyl diacylgly-cerol) which forms micellar rather than lamellar or reversed phases when dispersed in water. Biochemistry 1995 34 13818-13824. Steim JM, Tonrtellotte ME, Reinert JC, McElhaney RN, Rader RL. Calorimetric evidence for the liquid-crystalline state of lipids in a biomembrane. Proc. Natl. Acad. Sci. U.S.A. 1969 63 104-109. Reinert JC, Steim JM. Calorimetric detection of a membrane lipid phase transition in living cells. Science 1970 168 1580-1582. Melchior DL, Morowitz HJ, Sturtevant JM, Tsong TY. Characterization of the plasma membrane of Mycolplasma laidlawii. Vni. Phase transitions of membrane lipids. Biochim. Biophys. Acta 1970 219 114-122. 

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