Medicinal Chemistry Basics
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Direct answer
Medicinal chemistry explains drug action from molecular structure: whether a molecule reaches and binds its target is written in its physicochemical properties — size, lipophilicity (log P), ionisation governed by pKa, and hydrogen-bonding capacity — and small structural changes reorganise all of them. Structure-activity relationships (SAR) record how each portion of a drug contributes, guiding rational optimisation; Lipinski's rule of five summarises oral druglikeness (molecular weight under 500, log P under 5, no more than 5 hydrogen-bond donors and 10 acceptors, one violation tolerated); prodrugs are inactive derivatives activated by metabolism (enalapril hydrolysed to enalaprilat, cyclophosphamide activated in the liver); and chirality matters because enantiomers differ in biological action — S-warfarin is the more potent anticoagulant, and thalidomide's history justifies single-enantiomer development. Functional groups set solubility, metabolic susceptibility and binding interactions — not decoration.
What you must remember
- Physicochemical quartet: molecular weight, lipophilicity (log P, log D), ionisation (pKa and the unionised fraction by Henderson-Hasselbalch) and polar surface area — jointly deciding absorption, distribution and binding.
- Lipinski's rule of five: molecular weight ≤ 500, log P ≤ 5, donors ≤ 5, acceptors ≤ 10; two or more violations predict poor oral permeability — the classic first filter in drug design.
- SAR reading: electronic effects, steric fit and hydrogen bonding at the receptor.
- Ionisation and transport: only the unionised fraction crosses membranes passively; a weak acid at pH below its pKa is mostly unionised — why aspirin absorbs from the acidic stomach and bases from the alkalinised intestine.
- Prodrug logic: mask a polar group for membrane crossing, then unmask by esterases (enalapril to enalaprilat, valaciclovir to aciclovir); target activation (levodopa decarboxylated in brain); condition-selective activation in tumours or infections.
- Chirality: enantiomers share properties in achiral media but differ at chiral biological targets — S-warfarin several-fold more potent than R; levodopa used as the L-enantiomer; thalidomide's teratogenic history underpins chiral-switch regulation.
- Metabolism-aware design: fluorine blocking labile positions, soft drugs designed for rapid clearance, avoidance of toxicophores such as anilines forming reactive metabolites.
Following one molecule's design decisions
Enalapril shows the whole toolkit inside one structure. The active pharmacophore, enalaprilat, inhibits angiotensin-converting enzyme powerfully but carries a carboxylic acid that leaves it too polar to cross the gut wall — a bioavailability problem written directly in its ionisation. The answer is a prodrug: esterify that acid to the ethyl ester, log P rises, oral absorption succeeds, and esterases hydrolyse the mask to release the active acid in the blood. One design move, one clinical consequence: enalapril dosed orally, enalaprilat only intravenously. Apply the same reading elsewhere — why levodopa rather than dopamine (dopamine cannot cross the blood-brain barrier; levodopa rides an amino-acid transporter and decarboxylates centrally, with carbidopa kept outside the brain to prevent peripheral wastage) — and the pattern repeats: transport first, activation second, target third. When a molecule fails as a drug, the failure is usually at one of these stations, and the medicinal chemist's job is to name which.
How the exam separates answers
The predictable short notes are rule of five, prodrugs, SAR and chirality, each with a standard half-answer. For Lipinski, quoting the four thresholds passes; adding that it is a permeability guideline, not a potency law, and that antibiotics and transporter substrates routinely violate it, is the distinction. For prodrugs, the definition alone is not enough — name the activation mechanism and its purpose, because "inactive derivative activated in vivo" without enalapril or cyclophosphamide has no content. For chirality, the thalidomide story is expected, but the stronger script adds that enantiomers also differ in metabolism (S-warfarin's potency) and that single-enantiomer "chiral switches" are regulatory-commercial as well as scientific events. Ionisation questions reward direction: for a weak acid, alkalinisation ionises and traps (the ion-trapping logic behind salicylate management), acidification unionises — memorising Henderson-Hasselbalch without its absorption consequences stops at half marks.
Frequently asked questions
State Lipinski's rule of five and its purpose.
Oral druglikeness is favoured when molecular weight ≤ 500, log P ≤ 5, hydrogen-bond donors ≤ 5 and acceptors ≤ 10; exceeding two parameters predicts poor absorption, making it an early filter in candidate selection.
What is a prodrug and why design one?
An inactive derivative converted to the active drug in the body — designed to improve absorption (enalapril), reduce toxicity, target a tissue (levodopa in the brain) or extend action.
Why do enantiomers differ in pharmacological action?
Biological targets are chiral proteins, so mirror-image forms bind with different affinities and may act differently — S-warfarin is the more potent anticoagulant, and thalidomide made enantiomeric purity a regulatory concern.
How does pKa govern drug absorption?
Only the unionised fraction crosses lipid membranes passively, and the ratio follows Henderson-Hasselbalch with ambient pH — weak acids absorb best from acidic compartments, weak bases from alkaline ones.
What is a bioisosteric replacement?
Substituting a group with another of similar steric and electronic shape — oxygen by sulphur, CH by nitrogen — to retain activity while improving potency, selectivity, stability or patent position.