Medicinal Chemistry: Drug-Target Interactions, Solubility, and Acid-Base Principles

Comprehensive vocabulary flashcards covering drug-target interactions, covalent binding modes, non-covalent forces, pH-dependent solubility, salt forms, and partitioning metrics from Medicinal Chemistry practice materials.

Intrinsic Solubility ($S_0$)

The equilibrium aqueous solubility of the unionized (non-dissociated) form of a weak acid or weak base drug, which remains constant regardless of solution $\text{pH}$.

pH-Solubility Relationships

Mathematical formulations defining drug solubility ($S$) relative to intrinsic solubility ($S_0$), $\text{p}K_a$, and $\text{pH}$, defined as $\text{pH} = \text{p}K_a + \log\left(\frac{S - S_0}{S_0}\right)$ for weak acids and $\text{pH} = \text{p}K_a + \log\left(\frac{S_0}{S - S_0}\right)$ for weak bases.

Common Ion Effect

The reduction in aqueous solubility of a sparingly soluble salt when an ion shared with the salt is introduced to the solution, triggering precipitation when the solubility product ($SP$) exceeds the solubility product constant ($K_{sp}$).

Cosolvent Dilution Precipitation

A formulation phenomenon occurring when a cosolvent system (such as Taxol with PEG) is diluted with water, causing drug concentration to decline linearly while drug solubility drops exponentially, precipitating the drug as solubility drops below concentration.

Inorganic Salt

A salt formed with a counterion that contains no carbon (e.g., $Na^+$, $K^+$, $Ca^{2+}$, $Cl^-$, $SO_4^{2-}$), which generally increases dissolution rate, solvation, and aqueous solubility relative to the parent drug.

Organic Salt

A salt formed with a carbon-containing counterion (e.g., maleate, tartrate, stearate, tromethamine), which can be selected to impart enhanced lipid solubility for depot injections or provide local buffering.

Acid-Base Precipitation Incompatibility

A physical incompatibility resulting when an ionized acidic drug (e.g., Penicillin G) is mixed with an ionized basic drug (e.g., Prochlorperazine), leading to mutual charge neutralization and rapid salt precipitation.

Partition Coefficient ($\log(P)$)

The logarithm of the ratio of concentrations of unionized drug partitioned between octanol and water at equilibrium, representing a $\text{pH}$-independent measure of intrinsic lipophilicity.

Distribution Coefficient ($\log(D)$)

The logarithm of the ratio of the sum of the concentrations of all drug forms (both ionized and unionized) in octanol to the sum of all forms in water at a specific $\text{pH}$, making it inherently $\text{pH}$-dependent.

Lipinski's Rule of 5

A set of structural guidelines predicting poor oral absorption and permeability when a candidate drug possesses: molecular weight $> 500\,\text{Da}$, calculated $\log(P) > 5$, more than $5$ hydrogen bond donors, or more than $10$ hydrogen bond acceptors.

Zwitterion

A dipolar molecule containing distinct positively and negatively charged ionic groups that carries a net formal electrical charge of zero at its isoelectric point ($pI$).

Isoelectric Point ($pI$)

The exact solution $\text{pH}$ at which an amphoteric molecule (such as an amino acid or peptide) carries an equal number of positive and negative charges, resulting in a net charge of zero.

Proline (Secondary Structure Conformational Turn)

An amino acid with an aliphatic pyrrolidine ring incorporating its backbone amine, introducing severe steric constraints that frequently generate beta-turns in protein backbones and impede protease cleavage.

Disulfide Bond

A stable covalent bond formed by the oxidation and linkage of the sulfhydryl (thiol) side-chain groups of two cysteine residues, critically maintaining tertiary and quaternary protein structure.

Tyrosine Kinase

An enzyme that catalyzes the transfer of a gamma-phosphate group from ATP to specific tyrosine hydroxyl groups of target proteins; targeted and blocked by the inhibitor gefitinib.

Aziridinium Ion

A strained, three-membered, electrophilic quaternary ammonium ring intermediate formed during the intramolecular cyclization of nitrogen mustards such as chlorambucil prior to covalent alkylation of DNA.

Chlorambucil Alkylation Pathway

A two-step irreversible covalent pathway in which chlorambucil loses chloride to generate a strained, highly reactive electrophilic aziridinium ion (Step 1) that is subsequently attacked by a nucleophilic macromolecule such as DNA (Step 2).

Acylation (LACE Functional Targets)

A covalent modification mechanism wherein a drug transfers an acyl moiety onto an active-site nucleophile, most commonly targeting lactones, lactams, amides, carbamates, or esters.

Aspirin Acylation of Cyclooxygenase (COX)

An irreversible covalent transesterification reaction wherein the acetyl ester of aspirin transfers an acetyl group to the hydroxyl oxygen of a catalytic serine residue within cyclooxygenase, releasing salicylic acid.

Beta-Lactam Transpeptidase Acylation

The covalent inactivation of bacterial transpeptidase wherein the active-site serine hydroxyl attacks the strained cyclic amide carbonyl of a beta-lactam antibiotic, forming a long-lasting acyl-enzyme complex that prevents peptidoglycan cross-linking.

Neostigmine Carbamylation

A covalent acylation reaction in which neostigmine transfers a dimethylcarbamate group to the catalytic serine of acetylcholinesterase, yielding a carbamylated enzyme intermediate that undergoes very slow hydrolytic regeneration.

Pralidoxime Chloride (2-PAM)

An oxime antidote for organophosphate poisoning that reactivates phosphorylated acetylcholinesterase by nucleophilically displacing the covalently bound phosphate group from the active-site serine before aging occurs.

Reversible Stage in Acetylcholinesterase Phosphorylation

The intact, un-aged organophosphate-serine conjugate (represented in the green square) that can be nucleophilically attacked and displaced by 2-PAM, prior to irreversible loss of an alkyl group (aging).

Chelation

A non-covalent binding interaction in which a polyvalent metal ion binds simultaneously to two or more electron-donating atoms on the same ligand molecule, creating a coordination ring complex.

Complexation

A non-covalent coordinate bonding interaction where a central metal ion (such as $Zn^{2+}$ in ACE) binds to a single electron-donating atom or carboxylate group on a ligand without forming a chelate ring.

Ionic Bond Distance Dependency

An electrostatic non-covalent interaction between fully charged opposite formal ions that falls off with distance as $1/r$, allowing it to act over the greatest distance and guide initial drug approach to a target.

London Dispersion Forces (Van der Waals)

Weak, transient non-covalent attractions ($0.5-1\,\text{kcal/mol}$) formed between synchronized instantaneous and induced dipoles in nonpolar regions, falling off steeply with distance as $1/r^6$.

Hydrophobic Effect

A thermodynamically favorable binding phenomenon driven by an increase in entropy ($+\Delta S$) when structured, ordered water cages surrounding nonpolar drug and receptor surfaces are displaced into the bulk solution.

Meperidine Hydrophobic Binding Site

Carbon A (the terminal methyl carbon of the ethyl ester side chain), which represents the most flexible, accessible aliphatic hydrocarbon group suitable for hydrophobic binding interactions.

Ketorolac Tromethamine

A water-soluble organic salt comprising the NSAID ketorolac and the counterion tromethamine, which buffers the solution to physiological $\text{pH}$ to permit non-irritating ophthalmic administration.

Aliphatic Amine Basicity Hierarchy

The relative order of base strength for aliphatic amines in aqueous medium, established as secondary ($2^\circ$) > primary ($1^\circ$) > tertiary ($3^\circ$), resulting from a balance between inductive electron donation and steric stabilization of the hydrated cation.

Clopidogrel Activation Mechanism

A metabolic bioactivation sequence wherein the inactive thienopyridine prodrug clopidogrel undergoes oxidation and ring opening to expose a reactive thiol that forms an irreversible covalent disulfide bond with the platelet P2Y12 receptor.