Question

_______ amino acid is never found in $\alpha$ -helix .

Answer 1

$\alpha$- helix is a right-hand spiral confirmation in which every backbone $N-H$ group donates a hydrogen bond to the to the backbone $C=O$ group located three or four residues earlier along the protein sequence. This can be easily observed from the figure.

Proline cannot be found in $\alpha$- helix since it does not have an amide hydrogen to donate. Also, steric hindrance in proline inhibits the formation of $\alpha$- helix