Chapter 27: Biomolecules: Lipids
A class defined by what dissolves it
Every other biomolecule class in this course is named for a structure. Carbohydrates are polyhydroxy aldehydes and ketones; amino acids carry an amine and a carboxylic acid; nucleic acids are chains of nucleotides. Lipids are the exception. A lipid is defined by how it behaves in a separating funnel: it is a naturally occurring compound that dissolves in nonpolar organic solvents - diethyl ether, chloroform, hexane - and does not dissolve appreciably in water. Nothing in that sentence names a functional group, and the consequence is that the class contains molecules with almost nothing…
Reading a fatty acid from its shorthand
A fatty acid is a long-chain carboxylic acid. The biologically important ones recur so constantly that they are described with a compact shorthand: the number of carbons, a colon, then the number of carbon-carbon double bonds. Stearic acid is 18:0 - eighteen carbons, no double bonds. Oleic acid is 18:1. Arachidonic acid is 20:4. The carboxyl carbon is counted in the chain length and is numbered C1, so the eighteen of stearic acid includes the COOH carbon. The two-number shorthand does not locate the double bonds, so positions are appended as Δ locants counted from C1. Written out in full,…
Shape, not bond strength, sets the melting point
Melting a crystal means supplying enough thermal energy to pull molecules away from one another against their intermolecular attractions. For hydrocarbon chains those attractions are London dispersion forces: individually weak, but additive along the length of a chain and very sensitive to distance, since they fall off steeply as two surfaces separate. The practical consequence is that a melting point reports how efficiently a molecule packs against its neighbors far more than it reports anything about the strength of the bonds inside the molecule. Holding this straight is the difference…
Three esters on glycerol, and the one-ester case
Glycerol is propane-1,2,3-triol: three carbons, a hydroxyl on each. Esterify all three hydroxyls with fatty acids and the product is a triacylglycerol, still widely called a triglyceride. These are the fats and oils. They are the primary energy store of animals and plants, and they store more energy per gram than carbohydrate does - roughly nine kilocalories per gram against four - because the carbons of a hydrocarbon chain begin far more reduced than the carbons of a sugar and therefore have further to fall on oxidation. Fat and oil are not different classes of compound. Both are…
Boiling a fat with base, and why the product cleans
Boil a fat with aqueous sodium hydroxide and every ester linkage is cleaved. The products are glycerol and the sodium salts of the three fatty acids. This is saponification - literally, soap-making - and it is the oldest industrial organic reaction still practiced, run for at least four thousand years before anyone could write a mechanism for it. The stoichiometry falls straight out of the structure. A triacylglycerol contains three ester groups, each hydrolyzed independently, so three moles of NaOH are consumed per mole of fat, and the products are one mole of glycerol and three moles of…
Hardening an oil, and the isomer it leaves behind
An unsaturated triacylglycerol can be converted into a saturated one by catalytic hydrogenation: treatment with hydrogen gas in the presence of a finely divided metal catalyst, typically nickel in industrial practice and palladium on carbon in the laboratory. The reaction is the same alkene hydrogenation met much earlier in the course, now run on three chains at once. One mole of H₂ is consumed per carbon-carbon double bond, so triolein - three oleic chains, one double bond each - takes up three moles of H₂ and becomes tristearin. A triacylglycerol carrying one oleic, one linoleic, and one…
Two tails, one charged head, and a sheet that closes on itself
Take a triacylglycerol and replace one of its three fatty acids with a phosphate group carrying a small polar alcohol. The result is a glycerophospholipid - the principal structural material of every cell membrane. The glycerol backbone is still there, fatty acids are still esterified at C1 and C2, and C3 now bears the phosphate. Where a triacylglycerol is entirely nonpolar and makes an excellent energy store, a phospholipid has a strongly polar end grafted onto a nonpolar body, and that asymmetry is what builds membranes. The phosphate is a diester - the detail most often missed. Phosphoric…
Twenty carbons, one precursor, two branches
Eicosanoids are hormone-like regulators of inflammation, pain, fever, blood clotting, blood-vessel diameter, and smooth-muscle contraction. They differ from true hormones in range and lifetime: they are not stored, not carried in the bloodstream to a distant organ, and not long-lived. They are made on demand, act on cells within a very short distance of where they were produced, and are degraded within seconds to minutes. This is why the same molecule can do opposite things in different tissues, and why eicosanoid-based drugs tend to have wide-ranging side effects. One precursor fixes the…
Counting a skeleton in fives
Terpenoids are the largest family of natural products known, running to tens of thousands of compounds: the fragrant constituents of plant essential oils, many pigments, several vitamins, and - critically for this chapter - the precursors of every steroid. What holds this sprawling class together is a counting rule. The isoprene rule. Every terpenoid carbon skeleton can be dissected into five-carbon units whose branching pattern matches isoprene, 2-methyl-1,3-butadiene - a four-carbon chain carrying a methyl branch on C2. Two cautions matter here. First, this is a rule about cutting a…
Four fused rings that cannot flip
A steroid is built on a nucleus of four fused rings: three six-membered rings, labeled A, B, and C, and one five-membered ring, labeled D. The full name for the saturated parent skeleton is perhydrocyclopentanophenanthrene, which is why nobody uses it. The skeleton is strikingly conserved across the whole class - steroids differ chiefly in what is attached to it, the oxidation state at a handful of positions, and the stereochemistry at the ring junctions. Cholesterol is the parent compound of the class and worth knowing in detail. Its molecular formula is C₂₇H₄₆O, and the carbons account for…
Part of Organic Chemistry, a free and open textbook licensed CC BY-SA 4.0.