Absolutely Small - Michael D. Fayer [108]
FIGURE 16.2. Ball-and-stick model of oleic acid. Oleic acid has 18 carbon atoms like stearic acid in Figure 16.1, but it has one carbon-carbon double bond between carbons 9 and 10 counting from the acid group.
THE SHAPES OF FAT MOLECULES
Stearic acid, as shown in Figure 16.1, is in the all-trans conformation. Figure 14.13 shows butane in the all-trans configuration, with rotation around one of the bounds to give a gauche conformation. Stearic acid can assume many conformations besides the all-trans conformation shown. The all-trans conformation is the most linear, which is the lowest energy conformation for a saturated hydrocarbon or a saturated fat. Because saturated fats only have carbon-carbon single bonds, they are constantly interconverting from one conformer to another. Figure 15.7 displays heptadecaneacetate not in an all-trans configuration. In contrast to stearic acid, oleic acid (Figure 16.2) is not naturally produced in an all-trans configuration. In Figure 16.2, the angles made by carbons 8, 9, and 10 and by carbons 9, 10, and 11 are 120° (trigonal) and not 109.5° (tetrahedral). Therefore, under normal biological conditions adding one double bond locks in a particular shape around the double bond.
SATURATED, MONOUNSATURATED, AND POLYUNSATURATED FATS
Figure 16.3 is a ball-and-stick model of α-linolenic acid. Like stearic acid and oleic acid, α-linolenic acid has 18 carbons, but it has three double bonds. α-linolenic acid is said to be polyunsaturated in that it has more than one double bond. In linolenic acid, six of the carbon atoms, specifically 9, 10, 12, 13, 15, and 16, are trigonal, with 120° angles formed by three carbon atoms, such as 8, 9, and 10, rather than with 109.5° tetrahedral angles. Under normal conditions, these additional double bonds force the shape further away from an all-trans configuration. While α-linolenic acid has three double bonds, linolenic acid, which is similar, has only two double bonds. A fat with no double bonds is saturated. A fat with one double bond is monounsaturated, and a fat with two or more double bonds is polyunsaturated.
FIGURE 16.3. Ball-and-stick model of α-linolenic acid, which has 18 carbon atoms and three carbon-carbon double bonds.
DOUBLE BONDS IN FATS MATTER
Why does it matter whether or not a fat has double bonds? This is a question that divides into two types of issues, those for chemically modified fats and those for nonchemically modified fats contained in oils and other sources of fat like butter. First, let’s look at fats that have not been chemically modified. Fats have been correlated with either increasing or decreasing cholesterol levels. Myristic acid, which is a saturated fat with 14 carbons, is believed to increase cholesterol significantly in a deleterious manner. Palmitic acid, which is a saturated fat with 16 carbons, is thought to increase cholesterol to some extent. In contrast, linolenic acid (18 carbons with two double bonds) and other polyunsaturated fats decrease cholesterol levels. However, some saturated fats, such as stearic acid (Figure 16.1), do not appear to have much influence on cholesterol levels, which is also true of monounsaturated fats such as oleic acid (Figure 16.2).
In common oils used in food, the fractions of important unsaturated and polyunsaturated fats vary widely. Butter fat and coconut oil contain large amounts of myristic acid and palmitic acid and very little linolenic acid. Olive oil contains no myristic acid, but it has a significant amount palmitic acid. It also has some linolenic acid. Canola oil has no myristic acid and almost no palmitic acid. It has a substantial amount of linolenic acid. Grape seed oil, safflower oil, and sunflower oil (the later two when not processed for high-temperature cooking, see below) have no myristic, small amounts of palmitic, but very large amounts of linolenic acid. The chemical composition of these fats indicate that butter fat and coconut oil will have a deleterious effect on blood cholesterol, olive oil is approximately