Introduction to Lipids

Abstract

Biological fatty acids usually contain 14 to 20 carbons, although small amounts of 22 and 24 carbon compounds are found in some tissues. The fatty acid acyl chain has a tendency to adopt an extended structure, because this results in the least steric hindrance; however, the chain is very flexible, and will adopt a large variety of conformations. The reason for this flexibility is that each carbon-carbon bond can (more or less) freely rotate, and all fatty acids have many carbon-carbon bonds. Saturated fat and unsaturated fat Many fatty acids contain double bonds. Fatty acids that do not contain carbon-carbon double bonds are considered to be saturated. When producing unsaturated fatty acids, the biosynthetic machinery incorporates nearly exclusively cis configuration double bonds. Some food products, such as margarine, may contain trans double bonds because of manipulations during food processing; some evidence suggests that fatty acids containing trans double bond cannot be metabolized properly, and that food products without trans fatty acids are therefore healthier. Monounsaturated fatty acids contain a single double bond, while polyunsaturated fatty acids contain more than one site of unsaturation. The cis bond causes a kink in the fatty acid acyl chain (recall that, unlike single bonds, double bonds do not allow free rotation). " Partially hydrogenated vegetable oil " is a term often found on food ingredient labels. It means that some of the fatty acids in the oil were " hydrogenated " (reduced, so that the double bonds were converted to single bonds). Saturated fat and monounsaturated fat form solids at room temperature. Polyunsaturated fat is (usually) liquid at room temperature. Mixing fatty acids of varying saturation is a way of regulating consistency of food, and also of regulating the consistency of biological systems. This is because, in membranes or in bulk lipid, cis double bonds alter packing density, and therefore decrease the number of favorable van der Waals contacts. The presence of cis double bonds therefore results in lower melting temperature because the double bonds result in less regular and less stable structures. The table below shows the effect of chain length and number of cis double bonds on melting temperature. Longer chains result in higher melting temperatures; increasing numbers of double bonds decreases melting temperature for fatty acids of a given length.