Diacylglycerols crystallization

The spatial and steric requirements for high affinity binding to protein kinase C (PKC), a macromolecule that has not yet been crystallized, were determined. Protein kinase C plays a critical role in cellular signal transduction and is in part responsible for cell differentiation. PKC was identified as the macromolecular target for the potent tumor-promoting phorbol esters (25). The natural agonists for PKC are diacylglycerols (DAG) (26). The arrows denote possible sites of interaction. 

Studies on the addition of standard diacylglycerols demonstrated that their racemic purity had a significant influence on crystallisation behavior of milk fat triacylglycerols. Differences in the observed effects on crystallization... 

The use of bicelles has been reported to have several advantages compared to micelles. For instance, the integral membrane protein diacylglycerol kinase retains its activity in bicelles, in contrast to micelles (31). Bicelles also have been reported to form stable crystals with the membrane protein bacteriorhodopsin, allowing X-ray crystallographic studies (32). In addition, the interaction of the HIV-1 envelope peptide with micelles induces a strong curvature in this model membrane, which is not observed in the case of bicelles (17). 

In many food products and even some processing operations, it is important to be able to control lipid crystallization to obtain the desired number, size distribution, polymorph, and dispersion of the crystaHine phase. In most foods, it is crystallization of triacylglycerols (TAG) that is most important, although, at times, crystallization of other lipids (i.e., monoacylglycerols, diacylglycerols, phospholipids, etc.) may also be important to product quality. 

Parameters that affect crystallization may influence either the thermodynamic behavior or the crystallization kinetics (or both). Parameters that influence lipid crystallization include chemical composition, subcooling, cooling rate, agitation, minor components of fats (mono- and diacylglycerols, polar lipids, etc.), and scale of operation. The effects of these parameters on lipid crystallization will be reviewed briefly in this section. More detailed information about the effects of these parameters on lipid nucleation and crystal growth may be found elsewhere (4, 24, 28, 54). 

At International Food Science Center, we have measured diacylglycerols levels varying between 1.5% and 2.8%. Higher diacylglycerol levels affect the crystallization of cocoa butters remarkably, and thus all efforts should be made to reduce these levels in good quality cocoa butters. 

From reported literature values, it can be seen that the Avrami exponent for crystallizing oils and fats is usually about three or four with the exceptions of anhydrous milk fat, its triacylglycerols, and its mixtures with diacylglycerol at temperatures below 20°C, where an exponent of three is more common. 

Minor components (non-triacylglycerol species) were removed from anhydrous milk-fat (AMF) to obtain purified milk-fat TAG (MF-TAG) by column chromatography using Florisil as the stationary phase (2). As previously described, the crystallization behavior of the original AMF, the MF-TAG, and MF-TAG to which 0.1% milk-fat diacylglycerol was added (MF-DAG) was studied by pNMR and turbidimetry (2). Although crystallization was studied between 5.0 and 27.5°C, we will concentrate only on data collected at 22.5°C. 

Fig. 4. Fractional crystallization of anhydrous milk-fat (AMF) (A), MF-TAC (B), and milk-fat MF-TAC with 0.1% milk-fat diacylglycerol (MF-DAC) (Q determined by pNMR measurements of solid fat content, turbidity measurements, and polarized light microscopy coupled to image analysis at 22.5°C. Symbols in (A) and (B) represent the average and standard errors of three replicates. See Figures 2 and 3 for other abbreviations.

Two PLAs have been purified from Escherichia coli based on their differential sensitivity to treatment with detergents [2]. A detergent-insensitive enzyme is localized in the outer membrane, whereas a detergent-sensitive enzyme is found on the cytoplasmic membrane and in soluble fractions. The outer membrane enzyme, known as outer membrane phospholipase A, has broad substrate specificity and demonstrates PLA, PLAj, lysophospholipase A, and lysophospholipase Aj activity as well as activity for hydrolyzing monoacylglycerols and diacylglycerols. The crystal structure allows a more detailed discussion of an integral membrane phospholipase [12]. 

N.L. Nguyen, J. Dedier, H.T. Nguyen, G. Siegaud, Synthesis and characterization of thermotropic amphiphilic liquid crystals Semiperfluoroalkyl- 8-D-glu-copyranosides, Liq. Cryst., 27, 1451(2000) H.M. Von Minden, M. Morr, G. Milkereit, E. Heinz, V. Vill, Synthesis and mesogenic properties of glycosyl diacylglycerols, Chem. Phys. Lipids, 114, 55 (2002). 

Figure 10 Variations in crystallization temperature of w-hexadecane-water emulsions at different concentrations of sucrose oligoester (S-170), polyglycerine ester (DAS-750), and diacylglycerol (DS). 

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