Investigate drug action through receptor and enzyme binding using intermolecular forces, and apply structure-activity relationships (SAR) to explain why functional-group modifications change biological activity

What this dot point is asking

VCAA wants you to explain HOW drugs work at the molecular level (interactions between drug and biological target via intermolecular forces) and how systematic changes to a drug's structure change its activity. The skill is to predict, from a structural change, whether binding affinity, lipophilicity, or metabolic stability will go up or down.

The answer

A medicine is a small molecule that modifies a biochemical process by binding to a biological target (enzyme, receptor, ion channel, transporter). Strong drug action requires (1) the right shape to fit the target, (2) the right collection of functional groups to make specific intermolecular contacts, and (3) the right balance of polarity to reach the target in vivo.

Drug-target binding via intermolecular forces

Most drugs do not form covalent bonds with their targets (the exceptions, such as aspirin's acetylation of cyclooxygenase, are explicit). Binding is therefore reversible and depends on multiple non-covalent interactions accumulating in the binding site:

• Hydrogen bonds. Strongest non-covalent force per interaction (around 5 to 30 kJ/mol). Hydroxyl, amine, amide and carbonyl groups serve as donors or acceptors. Hydrogen bonds give directionality and contribute substantially to specificity.
• Ionic interactions (salt bridges). Charged groups on the drug interact with oppositely-charged groups on the target. A protonated amine on the drug pairs with a carboxylate on the protein, for example.
• Dispersion (London) forces. Universal but weak per interaction; significant when a large nonpolar surface of the drug contacts the target.
• Dipole-dipole interactions. Between polar groups on the drug and the target.
• Hydrophobic effect. Aromatic and aliphatic groups on the drug occupy hydrophobic pockets in the target; the energetic driver is the displacement of ordered water molecules from the pocket.

A typical drug-target binding pose recruits three to six of these interactions simultaneously. Removing any one substantially weakens binding, which is why small structural changes can large changes in activity.

Structure-activity relationships (SAR)

SAR is the systematic study of how structural changes affect activity. Drug discovery uses SAR to optimise from a lead compound to a clinically useful drug. The relevant questions for VCE:

Does the modification add or remove a binding contact

Adding a hydroxyl that hydrogen-bonds to a target residue increases affinity; removing it decreases affinity.

Does the modification change lipophilicity

Many drugs need to cross cell membranes. More lipophilic groups (alkyl chains, halogens, aromatic rings) raise log P; more polar groups (hydroxyl, amine, carboxylic acid) lower log P. Targets in the central nervous system typically need lipophilic drugs to cross the blood-brain barrier; targets in the bloodstream do not.

Does the modification change metabolic stability

The body metabolises drugs via Phase I (oxidation, hydrolysis) and Phase II (conjugation) reactions. Adding fluorine to a metabolic hot spot (e.g. an aromatic carbon prone to hydroxylation) often slows metabolism. Adding ester groups can speed metabolism (esterases hydrolyse esters readily).

Does the modification change solubility

Adding amino or carboxylic acid groups that ionise at physiological pH increases water solubility (important for oral or injectable formulations). Salt forms (e.g. hydrochloride for amine drugs, sodium salt for acid drugs) are common solubility-enhancing strategies.

Common functional groups in drug structures

• Amines. Often protonated at physiological pH (the protonated form is the active form for many CNS drugs); contribute hydrogen-bond donors when protonated; bind ionically to carboxylate residues; commonly salt-formed as hydrochloride.
• Amides. The peptide-bond functional group. More resistant to hydrolysis than esters; provide both donor and acceptor for hydrogen bonding.
• Carboxylic acids. Often deprotonated at physiological pH; bind ionically to lysine or arginine residues; common in NSAIDs (aspirin, ibuprofen, naproxen) and salicylate-family drugs.
• Esters. Often labile; used as prodrugs that hydrolyse to the active acid in vivo (e.g. aspirin is an ester prodrug of salicylic acid).
• Alcohols and phenols. Hydrogen-bond donors and acceptors; phenols are slightly acidic.
• Aromatic rings. Provide structural rigidity; pi-stacking interactions with aromatic residues in proteins.
• Halogens (Cl, F, Br). Modulate lipophilicity; can block specific metabolic pathways. Fluorine is a common medicinal-chemistry substituent because it is small (similar to hydrogen) but changes electronic and metabolic properties.

Examples in context

Example 1. Beta-lactam antibiotics (the penicillin family). Penicillins all share the four-membered beta-lactam ring that targets bacterial cell-wall synthesis enzymes (transpeptidases). The variations across the family (penicillin G, amoxicillin, ampicillin, methicillin) sit on a side chain attached to the beta-lactam core. Each modification changes spectrum, stability, or resistance to bacterial enzymes (beta-lactamases). Amoxicillin's hydroxyl on the side chain improves oral absorption; methicillin's bulky side chain resists beta-lactamase cleavage. The family is a classic example of SAR-driven optimisation around a privileged scaffold.

Example 2. Paracetamol vs aspirin. Both are analgesics with different mechanisms and functional-group profiles. Paracetamol has an amide and a phenol (without the carboxylic acid that aspirin / salicylic acid carry). Paracetamol is less anti-inflammatory than aspirin and less ulcerogenic; its mechanism is partial COX inhibition primarily in the central nervous system. Comparing the two illustrates how different functional-group combinations produce different therapeutic profiles even within the same general "non-opioid analgesic" category.

Try this

Q1. Identify three intermolecular forces that contribute to drug-target binding and give one functional group that participates in each. [3 marks]

• Cue. Hydrogen bonding (hydroxyl, amine, amide, carbonyl); ionic interactions (protonated amine, deprotonated carboxylic acid); hydrophobic effect (aromatic rings, alkyl chains); dispersion forces (any nonpolar contact); dipole-dipole (polar but non-ionic groups).

Q2. Explain why aspirin (acetylsalicylic acid) is less ulcerogenic than salicylic acid, with reference to the structural difference. [4 marks]

• Cue. Aspirin masks the free phenol of salicylic acid as an acetyl ester. The acetyl ester is less reactive in the stomach and less directly irritating. In the bloodstream, esterases hydrolyse the acetyl group to regenerate active salicylic acid. Aspirin is a prodrug of salicylic acid.

Q3. A medicinal chemist replaces a hydrogen on an aromatic ring of a CNS drug with fluorine. Identify two likely effects of this modification. [4 marks]

• Cue. Fluorine is similar in size to hydrogen but more electronegative; it can block oxidative metabolism at that position (improving metabolic stability and prolonging half-life). It can slightly raise lipophilicity (small log P increase), potentially improving blood-brain barrier penetration. Specific binding affinity changes depend on whether fluorine participates in any direct contact with the target.