Unsaturated triacylglycerols

The melting points of even-numbered-carbon fatty acids increase with chain length and decrease according to unsaturation. A triacylglycerol containing three sam-rated fatty acids of 12 carbons or more is solid at body temperature, whereas if the fatty acid residues are 18 2, it is liquid to below 0 °C. In practice, natural acylglyc-erols contain a mixture of fatty acids tailored to suit their functional roles. The membrane lipids, which must be fluid at all environmental temperatures, are... 

Figure 11.4 Total ion current (TIC) chromatogram of lipid residue extracted from a potsherd of Early Neolithic date (Ecsegfalva, Hungary). Cx y F refer to fatty acids with carbon number (x) and number of unsaturations (y). Cx T refer to triacylglycerols with number of carbon atoms (x). P = plasticizer contamination. (Craig et al., 2007, 354 Figure 18.1, by permission.)...

Gas chromatography (GC or, less commonly, GLC) is the most widely used separation technique for volatile samples. The resolution is sufficient to routinely separate components, such as homologous series, saturated from unsaturated fatty acids, terpenoids, triacylglycerols, etc. The use of a mass spectrometer to identify the separated components (GC-MS) is discussed in Section 8.4. 

Saturated hydrocarbons Unsaturated hydrocarbons Wax esters Steryl esters Long chain aldehydes Triacylglycerols Long chain alcohols Free fatty acids Quinones Sterols... 

Silver nitrate may be incorporated in the adsorbent slurry (25 g l-1) giving a final concentration of about 5% in the dry plate. The silver ions bind reversibly with the double bonds in the unsaturated compounds, resulting in selective retardation, and the lipids are separated according to the number and configuration (cis or trans) of their double bonds. This technique is extremely useful in fatty acids, mono-, di- and particularly triacylglycerol analyses when even positional isomers may be resolved. Borate ions may also be incorporated in the silica gel and these plates are used to separate compounds with adjacent free hydroxyl groups. 

Fats are solid triacylglycerols, oils are liquid triacylglycerols it is the fatty acid composition of the triacylglycerol that determines its physical properties. Triacylglycerols with a high proportion of unsaturated fatty acids have lower melting points than those rich in sam-rated fatty acids, so the former are oils - that is, they are liquid at room temperature. The unsaturated fatty acids are identified by the number of carbon atoms and the position of the double bonds (see below). 

Figure 11.7 Synthesis of triaq/lglyceroL The precursors are glycerol 3-phosphate and long-chain acyl-CoA. R, is a saturated fatty acid, R2 is an unsaturated fatty acid (one or two doubte bonds) and R3 is either saturated or unsaturated. The activity of GPAT-1 regulates triacylglycerol synthesis. In all reactions involving RCO.SCoA, the CoASH is released but is not shown in this diagram. P,- - phosphate.

Saturated fatty acids do not contain double bonds in the hydrocarbon chain. Unsaturated fatty acids contain from one to hve double bonds. Those with one double bond are known as monounsaturated, those with two as diunsatu-rated and those with more than two as polyunsaturated fatty acids. A brief summary of the roles of saturated and unsaturated fatty acids is given in Table 11.1. The proportion of these fatty acids in triacylglycerol in human adipose tissue is presented in Table 11.2. 

The three acyl residues of a fat molecule may differ in terms of their chain length and the number of double bonds they contain. This results in a large number of possible combinations of individual fat molecules. When extracted from biological materials, fats always represent mixtures of very similar compounds, which differ in their fatty acid residues. A chiral center can arise at the middle C atom (sn -C-2) of a triacylglycerol if the two external fatty acids are different. The monoacylglycerols and diacylglycerols shown here are also chiral compounds. Nutritional fats contain palmitic, stearic, oleic acid, and linoleic acid particularly often. Unsaturated fatty acids are usually found at the central C atom of glycerol. 

The first steps of glycerophospholipid synthesis are shared with the pathway to triacylglycerols (Fig. 21-17) two fatty acyl groups are esterified to C-l and C-2 of L-glycerol 3-phosphate to form phosphatidic acid. Commonly but not invariably, the fatty acid at C-l is saturated and that at C-2 is unsaturated. A second route to phosphatidic acid is the phosphorylation of a diacyl-glycerol by a specific kinase. 

Structure of triacylglycerols (TAG) The three fatty acids esterified] to a glycerol molecule are usually not of the same type. The fefy I acid on carbon 1 is typically saturated, that on carbon 2 is typi-j cally unsaturated, and that on carbon 3 can be either. Recall thal the presence of the unsaturated fatty acid(s) decrease(s) thl I melting temperature of the lipid. An example of a TAG molecule H shown in Figure 16.12. 

Triacylglycerols are quantitatively the most important class of dietary fats. Their biologic properties are determined by the chemical nature of the constituent fatty acids, in particular, the presence or absence of double bonds, the number and location of the double bonds, and the cis-trans configuration of the unsaturated fatty acids. 

Monounsaturated fats Triacylglycerols containing primarily fatty acids with one double bond are referred to as monounsaturated fat. Unsaturated fatty acids are generally derived from vegetables and fish. When substituted for saturated fatty acids in the diet, monounsaturated fats lower both total plasma cholesterol and LDL cholesterol, but increase HDLs. This ability of monounsaturated fats to favorably modify lipoprotein levels may explain, in part, the observation that Mediterranean cultures, with diets rich in olive oil (high in monounsaturated oleic acid), show a low incidence of coronary heart disease. 

Prepare a table listing the retention time for each standard FAME. Use this table to identify the fatty acids present in each triacylglycerol you analyzed. Alternatively, plot the log of the retention time against the chain length of each saturated FAME (see Figure E6.5). Unknown saturated fatty acids can be identified from experimental retention times using this plot. Unsaturated fatty acids cannot be identified from the plot of saturated FAMEs. A separate plot of log retention time vs. the chain length must be prepared for each level of saturation (saturated, monounsaturation, diunsaturation, etc.). 

Lipids are susceptible to oxidation and, therefore, analytical protocols are required to measure their quality. Not all lipids have the same degree of susceptibility to oxidation. Many factors are responsible for a lipid s tendency to oxidize, including the presence of catalysts, oxidative enzymes, radiation, and a lipid-air interface, as well as the oxygen partial pressure, the incorporation of oxygen into the product, and the presence of metal ions. The most important factor is the degree of unsaturation of the lipid itself. The majority of a food product s polyunsaturated fatty acids (PUFAs) are generally contained in phospholipids, which are consequently more prone to autoxi-dation than the triacylglycerol fraction. 

Fig. 29 Separation of triacylglycerols from sunflower seed oil by HPLC with a silver ion column and mass detection. For conditions see text. S = saturated fatty acid M = monounsaturated fatty acid D = di-unsaturated fatty acid.

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