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Differential scanning calorimetry, x-ray diffraction, and infrared and (31)P-nuclear magnetic resonance ((31)P-NMR) spectroscopy were used to examine the thermotropic phase behavior and organization of cationic model membranes composed of the P-O-ethyl esters of a homologous series of n-saturated 1,2-diacyl phosphatidylcholines (Et-PCs). Differential scanning calorimetry studies indicate that on heating, these lipids exhibit single highly energetic and cooperative endothermic transitions whose temperatures and enthalpies are higher than those of the corresponding phosphatidylcholines (PCs). Upon cooling, these Et-PCs exhibit two exothermic transitions at temperatures slightly below the single endotherm observed upon heating. These cooling exotherms have both been assigned to transitions between the liquid-crystalline and gel phases of these lipids by x-ray diffraction. The x-ray diffraction data also show that unlike the parent PCs, the chain-melting phase transition of these Et-PCs involves a direct transformation of a chain-interdigitated gel phase to the lamellar liquid-crystalline phase for the homologous series of n > or = 14. Our (31)P-NMR spectroscopic studies indicate that the rates of phosphate headgroup reorientation in both gel and liquid-crystalline phases of these lipids are comparable to those of the corresponding PC bilayers. However, the shape of the (31)P-NMR spectra observed in the interdigitated gel phase indicates that phosphate headgroup reorientation is subject to constraints that are not encountered in the non-interdigitated gel phases of parent PCs. The infrared spectroscopic data indicate that the Et-PCs adopt a very compact form of hydrocarbon chain packing in the interdigitated gel phase and that the polar/apolar interfacial regions of these bilayers are less hydrated than those of corresponding PC bilayers in both the gel and liquid-crystalline phases. Our results indicate that esterification of PC phosphate headgroups results in many alterations of bilayer physical properties aside from the endowment of a positively charged surface. This fact should be considered in assessing the interactions of these compounds with naturally occurring lipids and with other biological materials.

Lactylated FA esters of glycerol and propylene glycol (LFEGPG) are a lipid blend that is commercially available and used in the food industry as an emulsifying agent. Because the mutual impact of the two different backbones on the lipid phase behavior was of particular interest, we fractionated the commercial lipid blend by column LC. Fractions with varying ratios of glycerol and propylene glycol esters were collected and characterized by MS. DSC and X-ray diffraction were applied to study the thermotropic phase behavior of the dry emulsifier and the derived lipid fractions. LFEGPG exhibited rich polymorphic behavior, adopting a sub-α-crystalline phase that converted to an α-crystalline phase. Concomitantly, a β-crystalline phase was formed by some components of this lipid mixture. We found that the fractions with the highest amounts of lipids bearing the less-polar propylene glycol as their backbone tended to form a β-crystalline phase. Also, a higher number of self-polymerized lactic acid molecules in the head group of the propylene glycol esters favored the formation of a β-crystalline phase.A range of synthetic nonphospholipids (NPL), prevalently MAG, DAG, and derivatives thereof, are used in the food industry as legally approved surfactants for the preparation of emulsions and microemulsions (1,2). Furthermore, the use of synthetic NPL as alternatives to conventional phospholipids (PL) in various applications is a promising strategy because of the exceptional advantages of NLP, namely, their ease of manufacture, stability, versatility (3), stability against phospholipases in vivo (4), and affordability. At present, the phase behavior of most of these food emulsifiers is poorly characterized. To fill this gap and to extend our understanding of the phase behavior of NPL derived from a food-grade source, we studied a mixture of lactylated FA esters of glycerol and propylene glycol (LFEGPG) that is commercially available, e.g., under the name Durlac 300. In this lipid blend, the esterified FA are mainly palmitic and stearic acids. In addition to examining the thermotropic phase behavior of the solid LFEGPG sample, it was of particular interest to understand how the different lipid backbones, glycerol and propylene glycol, mutually influence the lipid phase behavior. Therefore, we fractionated the commercial lipid blend by column LC and collected fractions with increased amounts of lactylated propylene glycol esters.No literature is available on the phase behavior of the LFEGPG blend. However, one of its basic structural constituents, MAG, is known to exhibit rich polymorphic behavior. When solidified from the melted state, a thermodynamically unstable α-crystalline phase with hexagonally packed hydrocarbon chains is formed, but it converts within a short time interval to a stable β-crystalline phase with triclinically packed chains (2,5). Systematic studies of the polymorphic phase behavior of pure palmitic and stearic acid esters of propylene glycol were performed by Martin and Lutton (6) and Lutton et al. (7)....

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