Proline introduces a kink or steric disruption into an $\alpha$-helix and lacks a backbone amide proton ($-NH-$) for hydrogen bonding once in a peptide bond, making it a classic 'helix breaker'.

Helical geometry and axial dimensions of Pauling's $\alpha$-helix:

• $200\text{ Å}$ assumes an incorrect rise of $1.0\text{ Å}$ per residue.
• $240\text{ Å}$ is an incorrect computation.
• $260\text{ Å}$ is an incorrect computation.
• In an ideal right-handed $\alpha$-helix, there are $3.6$ amino acid residues per turn with a helical pitch of $5.4\text{ Å}$ ($0.54\text{ nm}$). The axial translation (rise per residue along the helical axis) is: $\Delta z = \frac{5.4\text{ Å}}{3.6} = 1.5\text{ Å}$ ($0.15\text{ nm}$). For a helix consisting of $N = 200$ residues, the total length is: $L = 200 \times 1.5\text{ Å} = 300\text{ Å}$ ($30\text{ nm}$).

Electrostatic constraints on $\alpha$-helix stability:

• Assertion (A) is true: Consecutive runs of charged residues prevent adoption of the compact $\alpha$-helical conformation, causing the polypeptide to adopt an extended random coil.
• Reason (R) is true: The guanidinium side chain of arginine has a high $pK_a \approx 12.5$ and is fully protonated with a net positive charge ($+1$) at physiological pH 7.0. In an $\alpha$-helix, side chains project outward and successive turns place residues separated by 3–4 positions in close spatial proximity, generating severe mutual electrostatic repulsion that destabilizes and disrupts the helix.
• Reason (R) provides the exact biophysical reason why poly-arginine fails to form an $\alpha$-helix at neutral pH.