QSAR basics: physicochemical parameters and Hammett/Taft analysis provides M.Pharm students with a focused review of key quantitative structure–activity relationship concepts. This blog covers how molecular descriptors — such as lipophilicity (logP/π), electronic substituent constants (Hammett σ), Taft steric (Es) and polar (σ*) terms, pKa, hydrogen-bonding, and polar surface area — influence biological activity. Emphasis is placed on interpreting Hansch and Hammett/Taft linear free-energy relationships, use of descriptors in multivariate models, and validation considerations like cross-validation and applicability domain. The aim is to prepare students for applying physicochemical knowledge in computer-aided drug design and critical evaluation of QSAR models.
Q1. What is the primary goal of QSAR in drug design?
- To synthesize new chemical compounds
- To correlate chemical structure or descriptors with biological activity
- To determine crystal structures of drug-target complexes
- To measure in vivo pharmacokinetics exclusively
Correct Answer: To correlate chemical structure or descriptors with biological activity
Q2. Which descriptor is represented by the Hansch π constant?
- Electronic donating/withdrawing power of a substituent
- Steric hindrance relative to hydrogen
- Hydrophobicity (lipophilicity) of a substituent relative to hydrogen
- Polar surface area contribution of a substituent
Correct Answer: Hydrophobicity (lipophilicity) of a substituent relative to hydrogen
Q3. In the Hammett equation, what does a positive ρ (rho) value indicate about the reaction?
- The reaction is accelerated by electron-donating substituents
- The reaction is insensitive to substituent electronic effects
- The reaction is accelerated by electron-withdrawing substituents
- The reaction is dominated by steric effects only
Correct Answer: The reaction is accelerated by electron-withdrawing substituents
Q4. Hammett σ constants are experimentally derived from which reference reaction?
- Hydrolysis rates of aliphatic esters
- Ionization (dissociation) equilibrium of substituted benzoic acids
- NMR chemical shifts of substituted benzene rings
- Partitioning between octanol and water of substituted toluenes
Correct Answer: Ionization (dissociation) equilibrium of substituted benzoic acids
Q5. Taft analysis separates substituent effects into which two parameters?
- π (hydrophobic) and σ (electronic)
- Es (steric) and σ* (polar)
- HBD (hydrogen-bond donors) and HBA (hydrogen-bond acceptors)
- PSA (polar surface area) and MW (molecular weight)
Correct Answer: Es (steric) and σ* (polar)
Q6. Which statement best describes the Taft steric parameter (Es)?
- It quantifies electronic donating power in aromatic systems
- It is derived from rates of ester hydrolysis and measures steric hindrance
- It measures hydrophobicity relative to hydrogen
- It represents polar surface area in Ų
Correct Answer: It is derived from rates of ester hydrolysis and measures steric hindrance
Q7. In a Hansch-type QSAR model, a significant positive coefficient for π indicates:
- Activity increases with increasing hydrophilicity
- Activity increases with increasing lipophilicity
- Activity is independent of lipophilicity
- Hydrogen bond donors are critical for activity
Correct Answer: Activity increases with increasing lipophilicity
Q8. Which descriptor is commonly used to predict passive intestinal absorption and blood–brain barrier penetration?
- Topological polar surface area (TPSA)
- Index of refraction
- Atomic weight of halogens
- Number of aromatic rings only
Correct Answer: Topological polar surface area (TPSA)
Q9. What is the commonly cited TPSA threshold below which molecules are more likely to penetrate the BBB?
- Greater than 200 Ų
- Between 140–180 Ų
- Less than about 90 Ų
- Exactly 120 Ų
Correct Answer: Less than about 90 Ų
Q10. Which Lipinski rule helps flag potential problems with oral bioavailability?
- Number of rotatable bonds should be >20
- LogP should be ≤5 and molecular weight ≤500 Da
- PSA should be greater than 200 Ų
- pKa must be between 7 and 9 for all drugs
Correct Answer: LogP should be ≤5 and molecular weight ≤500 Da
Q11. How does ionization (pKa) influence the measured logD at physiological pH?
- Ionization has no effect on logD
- Ionization increases logD for acids at physiological pH
- Ionization reduces apparent lipophilicity (logD) for ionized species
- Ionization only affects solubility, not logD
Correct Answer: Ionization reduces apparent lipophilicity (logD) for ionized species
Q12. Which validation metric represents cross-validated R-squared (internal predictive ability) in QSAR?
- R
- Q² (Q-squared)
- Mean molecular weight
- LogP value
Correct Answer: Q² (Q-squared)
Q13. What is the primary practical purpose of defining an applicability domain for a QSAR model?
- To increase the number of descriptors used in the model
- To identify the chemical space where model predictions are reliable
- To automatically synthesize new compounds
- To eliminate the need for experimental validation
Correct Answer: To identify the chemical space where model predictions are reliable
Q14. Which of the following best explains why Hammett σ values at the meta position mainly reflect inductive effects?
- Resonance effects are strongest at meta position
- Resonance contribution to meta position is negligible, so inductive effects dominate
- Meta position is closer to solvent and solvent effects dominate
- Steric effects completely mask electronic effects at meta
Correct Answer: Resonance contribution to meta position is negligible, so inductive effects dominate
Q15. Multicollinearity between descriptors in a QSAR model is problematic because:
- It always improves model predictivity
- It makes parameter estimates unstable and inflates variance
- It reduces the number of required training compounds
- It increases the chemical diversity of the dataset
Correct Answer: It makes parameter estimates unstable and inflates variance
Q16. A Taft σ* (sigma star) value is intended to quantify:
- Hydrophobicity of aromatic substituents
- Polar (electronic) effects of aliphatic substituents separate from steric effects
- Topological complexity of a molecule
- Hydrogen-bonding capacity of a substituent
Correct Answer: Polar (electronic) effects of aliphatic substituents separate from steric effects
Q17. Which of the following is a common consequence of increasing lipophilicity (logP) beyond optimal ranges in drug leads?
- Improved aqueous solubility
- Higher clearance and potential off-target toxicity due to non-specific binding
- Absolute increase in polar surface area
- Uniform improvement in oral bioavailability for all compounds
Correct Answer: Higher clearance and potential off-target toxicity due to non-specific binding
Q18. Which descriptor type would you choose to explicitly account for hydrogen-bond donor/acceptor counts in a QSAR model?
- Topological indices only
- Counts of HBD (hydrogen-bond donors) and HBA (hydrogen-bond acceptors)
- Taft Es values
- Van der Waals volume exclusively
Correct Answer: Counts of HBD (hydrogen-bond donors) and HBA (hydrogen-bond acceptors)
Q19. The Hansch approach is best described as which type of QSAR methodology?
- Mechanistic docking-only method
- Linear free-energy regression correlating biological activity with hydrophobic, electronic and steric descriptors
- Unsupervised clustering of compounds without activity data
- Only 3D pharmacophore mapping
Correct Answer: Linear free-energy regression correlating biological activity with hydrophobic, electronic and steric descriptors
Q20. Which experimental source is typically used to determine Taft Es steric parameters?
- Partition coefficients between octanol and water
- Relative rate constants for acid- or base-catalyzed hydrolysis of esters
- UV–Vis absorption maxima of substituted benzenes
- Equilibrium constants for substituted phenol dissociation
Correct Answer: Relative rate constants for acid- or base-catalyzed hydrolysis of esters
