"Chemistry is in the News"
To Accompany Wade Organic Chemistry 4/e.
Chapter 25. Lipids.
For each of the following questions, please refer to the following article:
Cholesterol is an important structural component in the lipid bilayer of cells. In the body, cholesterol travels via lipoproteins. There are several different types of lipoproteins, and they are classified according to their density. Their density is directly related to their lipid (more lipids = less density). High-density lipoproteins (HDL), low-density lipoproteins (LDL), and very low-density lipoproteins (VLDL) are important lipoproteins for cholesterol transport. Very low density lipoproteins (VLDL) carry cholesterol and cholesteryl esters from the liver to body tissues. LDL form from breakdown of VLDL. HDL carries cholesterol and cholesteryl esters from the tissues to the liver, where they are metabolized and eventually excreted. A schematic of this is shown here.
Research has shown that high HDL levels are correlated with lower rates of heart disease and high levels of LDL are correlated with increased rates of heart disease. Therefore, HDL and LDL are known as “good” and “bad” cholesterol, respectively. The exact reasons for the elevated heart disease with LDL and decreased heart disease with HDL is not known, but it appears that because HDL returns cholesterol to the liver (where it is metabolized, and eventually secreted), less cholesterol is available for oxidation. The oxidation occurs through oxygen, which exists as a di-radical.
Peroxides and free radicals are formed when oxygen reacts with the double bonds in unsaturated fatty acids. These peroxides and free radicals can damage cell components such as lipids, nucleic acids, and proteins. In addition, oxidation of LDL cholesterol can lead to plaque deposits in the arteries, which are a risk factor for heart disease. Thus anti-oxidants, compounds that take up or react with these free radicals, are extremely beneficial.
Chocolate contains many kinds of anti-oxidants, including polyphenols and flavonoids. Some studies report that flavonoids inhibit LDL cholesterol oxidation. In addition to the reduction of LDL cholesterol, chocolate contains a fat called stearic acid that presents another health benefit. Research published in the American Journal of Clinical Nutrition demonstrates that stearic acid can boost HDL levels. Other research indicates that oleic acid may also be good for the heart.
This heart-healthy news about chocolate will come as good news to many people that claim to crave it. While research on the health benefits of chocolate increases, chocolate “addiction” has been receiving some attention also. There are over 400 chemicals in chocolate, many of which are known to affect mood. Studies published by the ADA indicate that the addiction is real, but there are arguments as to whether they are physiological or psychological in nature. For example, chocolate consumption can affect levels of endorphins, serotonin, and phenylethylamine. It appears that women may be drawn to chocolate to boost certain hormonal levels that produce a desired mood.
Clearly, chocolate is a more complex delicacy than anyone realized and
it seems that its benefits are only beginning to be uncovered. As
with anything, the saying “too much of a good thing” holds for chocolate
as well. One cannot ignore the disadvantages of eating too much fat
in favor of the moderate advantages of lower cholesterol and anti-oxidants.
Pertinent Text References
- cholesterol: pg 633-634
- phenylethylamine: pg 819
Chapter 25: Lipids
- oleic acid: pg 1166
- cholesterol: pg 1174 & 1180
Question 1: What is the structure of oleic acid and stearic
acid, and give the IUPAC name?
Question 2: How is cholesterol oxidized?
Question 3: Show a mechanism of how the O2 di-radical
reacts with double bonds in unsaturated fatty acids. (Hint:
Allyl stablized radicals)
Question 4: Phenylethylamine is a hormone and is the component
in chocolate that make one feel "in love." Taken as a drug, it is
regarded as highly addictive. Synthesize phenylethylamine from benzene.
Use any reagents necessary.
Question 5: If we advertise chocolate as a health benefit, will people ignore the other negative health effects?