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Organic chemistry for medicine and biology students Chem 2311 Chapter 15 Lipids and Detergents By Prof. Dr. Adel M. Awadallah Islamic University of Gaza.
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Organic chemistry for medicine and biology students Chem 2311 Chapter 15 Lipids and Detergents By Prof. Dr. Adel M. Awadallah Islamic University of Gaza
The lipids are a large and diverse group of naturally occuring organic compounds that are related by their solubility in nonpolar organic solvents (e.g. ether, chloroform, acetone & benzene) and general insolubility in water. There is great structural variety among the lipids, Fatty AcidsSoaps and DetergentsFats and OilsWaxesPhospholipidsEicosonoidsTerpenesSteroidsLipid Soluble Vitamins
Fats and Oils are made from two kinds of molecules: glycerol and three fatty acids. Since there are three fatty acids attached, these are known as triglycerides.
Most fatty acids are unbranched and contain an even number of carbon atoms. Unsaturated acids usually have the Cis (Z) configuration. They have lower boiling points than the corresponding saturated ones.
Saturated triglycerides are usually solids because they can back together fairly regularly, as in a crystal. • Space filling and schematic models of glyceryl tripalmitate
Oils, however, are liquids because they contain higher percentage of unsaturated fatty acids • Space filling and schematic models of glyceryl dipalmitooeleate
Naturally-occurring unsaturated vegetable oils have almost all cis bonds, but using oil for frying causes some of the cis bonds to convert to trans bonds. If oil is used only once like when you fry an egg, only a few of the bonds do this so it’s not too bad. However, if oil is constantly reused, like in fast food French fry machines, more and more of the cis bonds are changed to trans until significant numbers of fatty acids with trans bonds build up. The reason this is of concern is that fatty acids with trans bonds are carcinogenic, or cancer-causing. The levels of trans fatty acids in highly-processed, lipid-containing products such as margarine are quite high, and I have heard that the government is considering requiring that the amounts of trans fatty acids in such products be listed on the labels.
Since fats are valued over oils by some Northern European and North American populations, vegetable oils are extensively converted to solid triglycerides (e.g. Crisco) by partial hydrogenation of their unsaturated components. Some of the remaining double bonds are isomerized (to trans) in this operation. These saturated and trans-fatty acid glycerides in the diet have been linked to long-term health issues such as atherosclerosis.
Phospholipids 40 % of cell membrane (60 % proteins) Phospholipids are the main constituents of cell membranes. They resemble the triglycerides in being ester or amide derivatives of glycerol or sphingosine with fatty acids and phosphoric acid. The phosphate moiety of the resulting phosphatidic acid is further esterified with ethanolamine, choline or serine in the phospholipid itself.
This bilayer membrane structure is also found in aggregate structures called liposomes. Liposomes are microscopic vesicles consisting of an aqueous core enclosed in one or more phospholipid layers. They are formed when phospholipids are vigorously mixed with water. Unlike micelles, liposomes have both aqueous interiors and exteriors.
Prostaglandins Thromboxanes & Leukotrienes • Named Prostaglandines in the assumption that these substances came from the prostate gland • The members of this group of structurally related natural hormones have an extraordinary range of biological effects. • They can lower gastric secretions, • stimulate uterine contractions, • lower blood pressure, • influence blood clotting • and induce asthma-like allergic responses. • Because their genesis in body tissues is tied to the metabolism of the essential fatty acid • arachadonic acid (5,8,11,14-eicosatetraenoic acid) • they are classified as eicosanoids. • Many properties of the common drug asprin result from its effect on the cascade of reactions associated with these hormones.The metabolic pathways by which • arachidonic acid is converted to the various eicosanoids • are complex and will not be discussed here. A rough outline of some of the transformations that take place is provided below. It is helpful to view arachadonic acid in the coiled conformation shown in the shaded box.
4. Waxes • Waxes are esters of fatty acids with long chain monohydric alcohols (one hydroxyl group). Natural waxes are often mixtures of such esters, and may also contain hydrocarbons. • Waxes are widely distributed in nature. The leaves and fruits of many plants have waxy coatings, which may protect them from dehydration and small predators. The feathers of birds and the fur of some animals have similar coatings which serve as a water repellent. Carnuba wax is valued for its toughness and water resistance.
Terpenes and Steroids Terpenes constitute the largest and most diverse class of natural products. A majority of these compounds are found only in plants, but some of the larger and more complex terpenes ( e.g. squalene & lanosterol ) occur in animals. Terpenes incorporating most of the common functional groups are known, so this does not provide a useful means of classification. Instead, the number and structural organization of carbons is a definitive characteristic. Terpenes may be considered to be made up of isoprene ( more accurately isopentane ) units, an empirical feature known as the isoprene rule. Because of this, terpenes usually have 5n carbon atoms ( n is an integer ), and are subdivided as follows:
Examples of Terpenes Isoprene itself, a C5H8 gaseous hydrocarbon, is emitted by the leaves of various plants as a natural byproduct of plant metabolism. Next to methane it is the most common volatile organic compound found in the armosphere.
Steroids The important class of lipids called steroids are actually metabolic derivatives of terpenes, but they are customarily treated as a separate group. Steroids may be recognized by their tetracyclic skeleton, consisting of three fused six-membered and one five-membered ring, as shown in the diagram to the right. The four rings are designated A, B, C & D as noted, and the peculiar numbering of the ring carbon atoms (shown in red) is the result of an earlier misassignment of the structure. The substituents designated by R are often alkyl groups, but may also have functionality. The R group at the A:B ring fusion is most commonly methyl or hydrogen, that at the C:D fusion is usually methyl. The substituent at C-17 varies considerably, and is usually larger than methyl if it is not a functional group. The most common locations of functional groups are C-3, C-4, C-7, C-11, C-12 & C-17. Ring A is sometimes aromatic. Since a number of tetracyclic triterpenes also have this tetracyclic structure, it cannot be considered a unique identifier.
Cholesterol 27 carbon atoms Biosynthesized from Lanosterol Present in all animal cells Concentrated in the brain and spinal cord The chief constituent of gallstones Total amount in the average human body = ½ bound < 200 mg / dl (desirable) > 280 mg / dl ((coronary heart disease
Sex Hormones: compounds produced in the ovaries and testes (pl of. testis) that control reproductive physiology and secondary sex characteristics
Cholesterol is the precursor to our sex hormones and Vitamin D. Vitamin D is formed by the action of UV light in sunlight on cholesterol molecules that have “risen” to near the surface of the skin. At least one source I read suggested that people not shower immediately after being in the sun, but wait at least ½ hour for the new Vitamin D to be absorbed deeper into the skin. Our cell membranes contain a lot of cholesterol (in between the phospholipids) to help keep them “fluid” even when our cells are exposed to cooler temperatures. • Many people have hear the claims that egg yolk contains too much cholesterol, thus should not be eaten. An interesting study was done at Purdue University a number of years ago to test this. Men in one group each ate an egg a day, while men in another group were not allowed to eat eggs. Each of these groups was further subdivided such that half the men got “lots” of exercise while the other half were “couch potatoes.” The results of this experiment showed no significant difference in blood cholesterol levels between egg-eaters and non-egg-eaters while there was a very significant difference between the men who got exercise and those who didn’t.
Synthesis mechanism (form 3) • Vitamin D3 is synthesized from 7-dehydrocholesterol, a derivative of cholesterol,
