(D) P2, P4, P1, P3

Ammonium sulphate precipitation follows “salting out,” where proteins with more hydrophobic regions precipitate first at lower salt concentrations. For proteins P1 (17%), P2 (10%), P3 (21%), P4 (14%) hydrophobic amino acids, the order is P3, P1, P4, P2.

Correct Answer

(A) P3, P1, P4, P2.

Proteins precipitate by hydrophobicity in decreasing order during stepwise ammonium sulphate addition. Higher hydrophobic content (P3: 21% > P1: 17% > P4: 14% > P2: 10%) means stronger hydrophobic interactions when salt strips hydration layers, causing aggregation at lower concentrations first.

Precipitation Mechanism

Ammonium sulphate increases solution ionic strength, favoring salt ion hydration over protein surfaces. This exposes hydrophobic residues (e.g., Leu, Val, Ile, Phe), promoting protein folding, self-association, and precipitation (“salting out”). Hydrophobic proteins precipitate earliest as their reduced solubility threshold is reached first.

Option Breakdown

(A) P3, P1, P4, P2

Correct. Matches descending hydrophobicity: P3 (21%) first, then P1 (17%), P4 (14%), P2 (10% last). Standard fractionation adds salt incrementally; most hydrophobic pellets first.

(B) P3, P1, P2, P4

Incorrect. Places P2 (10%, least hydrophobic) before P4 (14%), reversing solubility order. Less hydrophobic proteins need higher salt to precipitate.​

(C) P2, P4, P3, P1

Incorrect. Starts with P2 (lowest hydrophobicity), opposite of mechanism—hydrophilic proteins stay soluble longest.​

(D) P2, P4, P1, P3

Incorrect. Again begins with least hydrophobic P2; ignores that P3 (highest) should precipitate first.​

Hydrophobicity Table