Answer
The 2 possible reasons are
1. Pro lacks a backbone N-H group and cannot participate in H bonding, however since the four most N terminal residues of an alpha helix cannot donate hydrogen bonds within the alpha helix, Pro can occupy the N terminal turn.
2. Pro normally sterically clashes with the residue one turn towards the N terminus, but it has no steric clashes in the N-terminal turn itself.
Work Step by Step
The 2 possible reasons are
1. Pro lacks a backbone N-H group and cannot participate in H bonding, however since the four most N terminal residues of an alpha helix cannot donate hydrogen bonds within the alpha helix, Pro can occupy the N terminal turn.
2. Pro normally sterically clashes with the residue one turn towards the N terminus, but it has no steric clashes in the N-terminal turn itself.
