Differential scanning calorimetry was applied to study the phase diagrams of the following binary lipid systems: myristic acid (C13COOH) / pentadecanoic acid (C14COOH); palmitic acid methyl ester (C15COOMe) / heptadecanoic acid methyl ester (C16COOMe); palmitic acid methyl ester (C15COOMe) / stearic acid methyl ester (C17COOMe); palmitic acid methyl ester (CIsCOOMe) / arachidic acid methyl ester (C19COOMe). A distinct succession in the phase diagram types and phase regions was observed, according to the chemical structure of the mixing components. In the systems C13COOH/C14COOH; C15COOMe/C16COOMe and C16COOMe/ C17COOMe, both components are completely miscible in the low-and high-temperature phase. Contrasting with these three binary lipid systems, the system C15COOMe/ C14COOMe shows complete miscibility only in the high-temperature phase, but almost complete demixing in the solid state. In the phase diagram an incongruent melting "1:1 complex" is built up. This "complex" forms an eutectic mixture with the phase of C15COOMe and a peritectic system with C19COOMe.

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Phosphatidylcholines with branched fatty acid chains were investigated by differential scanning calorimetry, X-ray diffraction, and ESR-spectroscopy. Depending on both the length of the branches in 2-position of the acyl chains and the position of the methyl branches in the acyl chains, the main-transition temperatures and enthalpies pass through a minimum. The systematic change of the phase transition parameters is connected with a modified structural polymorphism.In the gel phase the miscibility properties depend on the phase structures of the components. For one system a phase diagram with a critical point has been found.Branched fatty acids widely occur as components of the lipids of microorganisms, plants, and animals. Despite their widespread occurrence in nature only little attention has been paid to study the influence of branching on biophysical and biochemical properties of the lipids.To continue our works on the phase behaviour and miscibility properties of phospholipid-water model-systems in dependence on the chemical structure of the phospholipids [1][2][3][4][5], the thermotropic phase behaviour of partially synthesized 1,2-diacyl-sn-glycero-phosphocholines with different series of alkyl branched fatty acid residues was investigated.

Abstract:The influence of chain branching on the phase transition parameters and structures of the homologous series of 1-(x-methylpalmitoyl)-2-hexadecyllecithins in the water-saturated two-phase region (50 wt.% water) were studied by differentialscanning-calorimetry as a function of the position x of methyl branching. With increasing x a linear decrease in the enthalpies and alternating temperatures of main transition were observed. The phase diagram of the ternary system 1-(3-methyl-palmitoyl)-2-hexadecyllecithin/1,2-dipalmitoyllecithin/water (50 wt.% water) showed that both components are completely miscible within the high-temperature phase (La-phase). However, in the low-temperature phase (gel phase), the components are partially miscible only. It follows that gel phases with interdigitated chains and those with tilted chains are not completely miscible. The phase diagram of the ternary system 1,2-di-(8-methylpalmitoyl)-lecithin/1,2-dipalmitoyllecithin/water (50 wt.% water) showed that both components are completely miscible within the high-temperature phase and lowtemperature phase. Within the concentration range between the mole fraction x = 0.91 and x = 0.97, a "drop-like" course in the phase diagram was obtained.