7. Biomolecules
The molecules of life are organic chemistry at work: carbohydrates are polyhydroxy aldehydes and ketones, proteins are polyamides, lipids are esters, and nucleic acids are phosphate esters of sugars. The functional-group chemistry you already know explains them all.
Carbohydrates
Glucose cyclizes to a six-membered hemiacetal (pyranose). The configuration at the new stereocenter — the anomeric carbon, C1 — defines the anomer: α-D-glucose has the C1–OH down in a Haworth projection, β-D-glucose has it up. Glycosidic bonds link sugars: maltose is α(1→4), cellulose is β(1→4) — one stereochemical flip, and starch becomes indigestible fibre.
Amino Acids and Proteins
Twenty amino acids link by condensation into peptide (amide) bonds, written N-terminus to C-terminus. The sequence is the primary structure; hydrogen bonding folds it into α-helices and β-sheets (secondary structure).
Lipids
Triglycerides are triesters of glycerol with three fatty acids. Base hydrolysis — saponification — gives glycerol plus three soap molecules (fatty acid salts).
Nucleic Acids
DNA's double helix is held by complementary base pairing: adenine–thymine (2 hydrogen bonds) and guanine–cytosine (3 hydrogen bonds), on a sugar–phosphate backbone.
Practice Problems
- What is the difference between α- and β-D-glucose?
- How many hydrogen bonds hold a G–C base pair together?
- What are the products of saponifying tristearin?
- What is meant by a protein's primary structure?
Answers: 1. The configuration at the anomeric carbon C1 — OH down (α) vs up (β) in the Haworth projection 2. Three 3. Glycerol + 3 sodium stearate 4. Its amino acid sequence, N-terminus to C-terminus