(D) Ala-Arg

Ala-Arg shows the highest propensity for α-helix conformation among the given pairs due to their favorable individual helix-forming tendencies. The correct answer is (D) Ala-Arg, key for protein folding in exams like NEET/JEE biotech.

Correct Answer

(D) Ala-Arg.

Alanine (Ala) has the highest α-helix propensity (strong helix former, ΔG ≈ 0 kcal/mol), while Arginine (Arg) is favorable (≈0.21 kcal/mol). Their combination supports stable H-bonding and minimal backbone entropy loss in helices.

Alpha Helix Basics

α-Helices form via backbone H-bonds (i to i+4 residues). Propensity depends on side-chain steric effects, H-bonding, and entropy: Ala excels due to small methyl group; helix breakers like Pro/Gly disrupt via rigidity/flexibility.

Option Breakdown

(A) Gly-Asp

Incorrect. Glycine (Gly, lowest propensity ≈1 kcal/mol worse than Ala) is a helix breaker due to high conformational flexibility. Asp has moderate propensity but can’t compensate Gly’s entropy penalty.

(B) Pro-His

Incorrect. Proline (Pro) is the strongest helix breaker (no backbone amide H-bond donor, rigid ring). His is neutral/moderate; Pro dominates, preventing helix initiation/continuation.

(C) Gly-Pro

Incorrect. Worst pair: Gly’s flexibility + Pro’s rigidity sterically clash and disrupt H-bonding network entirely. Common in turns, not helices.

(D) Ala-Arg

Correct. Ala (top propensity) pairs with Arg (good helix former via guanidinium H-bonding/salt bridges). High combined stability in solvent-exposed helices.

Propensity Ranking Table