The correct answer is option (C), where
P–iii, Q–iv, R–i, S–ii is the proper match between molecular properties and biochemical separation methods.​

Introduction

Understanding the correct match between molecular properties and biochemical separation methods is essential for mastering protein and nucleic acid purification questions in exams like GATE and CSIR NET. Each technique exploits a specific property—solubility, ionic charge, polarity or molecular size—to resolve complex mixtures with high selectivity.​

The question provides:

• Column I (molecular properties):
  P. Solubility
  Q. Ionic charge
  R. Polarity
  S. Molecular size

• Column II (methods):
  i. Reverse phase chromatography
  ii. Ultracentrifugation
  iii. Salting out
  iv. Isoelectric focusing
  v. Gel electrophoresis

The task is to match each property to the separation method that primarily depends on it.

Correct matching for each method

P. Solubility → iii. Salting out

• Principle: Salting out uses high salt concentration to decrease the solubility of proteins so that they precipitate from solution. Salt ions compete with protein surface groups for water, reducing hydration and driving aggregation, so separation is based on differential solubility at a given ionic strength.​

• Therefore: The key molecular property exploited is solubility, so P matches iii.​

Q. Ionic charge → iv. Isoelectric focusing

• Principle: In isoelectric focusing (IEF), proteins migrate in a pH gradient under an electric field until they reach the pH where their net ionic charge is zero (their isoelectric point, pI); migration stops at this point. Thus proteins are separated solely according to differences in charge/pI, not size.​

• Therefore: The central property is ionic charge (via pI), so Q matches iv.​

R. Polarity → i. Reverse phase chromatography

• Principle: Reverse phase chromatography uses a non‑polar stationary phase and a polar mobile phase, separating analytes according to hydrophobicity/polarity. Less polar (more hydrophobic) molecules interact more strongly with the non‑polar stationary phase and elute later, while more polar molecules elute earlier.​

• Therefore: The dominating molecular property is polarity (hydrophobic vs hydrophilic), so R matches i.​

S. Molecular size → ii. Ultracentrifugation

• Principle: In ultracentrifugation, particles sediment at rates that depend on their mass, shape and density; sedimentation coefficient scales strongly with molecular size/mass. Larger or heavier macromolecules move differently in a centrifugal field than smaller ones, allowing separation on a size basis (especially in rate‑zonal or density‑gradient ultracentrifugation).​

• Therefore: The key exploited property is molecular size, so S matches ii.​

Thus, the correct set is:

• P–iii

• Q–iv

• R–i

• S–ii

Which corresponds to option (C).

Why the other options are wrong (option‑wise analysis)

Option (A): P–i; Q–ii; R–v; S–iii

• P–i (Solubility → Reverse phase chromatography): Incorrect because reverse phase chromatography primarily depends on polarity/hydrophobicity, not plain solubility in water.​

• Q–ii (Ionic charge → Ultracentrifugation): Incorrect since ultracentrifugation separates primarily by size and mass, not charge.​

• R–v (Polarity → Gel electrophoresis): Gel electrophoresis separates molecules mainly by size and charge, not by their polarity relative to a stationary phase.​

• S–iii (Molecular size → Salting out): Salting out is governed by solubility changes with salt, not directly by molecular size; proteins of similar size can behave differently depending on surface properties.​

Option (B): P–iii; Q–iv; R–ii; S–i

• P–iii (Solubility → Salting out): This part is correct, as discussed above.​

• Q–iv (Ionic charge → Isoelectric focusing): Also correct.​

• R–ii (Polarity → Ultracentrifugation): Incorrect because centrifugal separation does not rely on molecular polarity; it depends on mass/size and density.​

• S–i (Molecular size → Reverse phase chromatography): Incorrect since reverse phase chromatography does not primarily separate by size; small and large molecules elute according to hydrophobicity.​

Because two pairings are wrong, option (B) is overall incorrect.

Option (C): P–iii; Q–iv; R–i; S–ii ✔

• All four associations—solubility with salting out, charge with isoelectric focusing, polarity with reverse phase chromatography, and molecular size with ultracentrifugation—are correct for the reasons detailed above.​

Option (D): P–v; Q–iv; R–iii; S–ii

• P–v (Solubility → Gel electrophoresis): Gel electrophoresis requires solubility to load samples but separates primarily by size and charge, not differences in solubility.​

• Q–iv (Ionic charge → Isoelectric focusing): This single pairing is correct.​

• R–iii (Polarity → Salting out): Salting out reflects how salt affects protein solubility, which is influenced by hydration of charged and polar groups, but the method is classified as based on solubility rather than chromatographic polarity.​

• S–ii (Molecular size → Ultracentrifugation): Correct as noted earlier.​

Because P–v and R–iii are mismatched, option (D) is incorrect.

Quick reference table

This conceptual map helps in quickly remembering which molecular property drives each biochemical separation method for exam problems.