Alpha Helix Facts

5.4 angstroms in each turn.

3.6 residues

About 10 residues on average

Between the atoms of the peptide backbone: between an amide H and the carboxyl O of the 4th amino acid residue to its N-terminal side

3-4

One side of the helix contains hydrophilic amino acids and the other side contains hydrophobic amino acids. They rotate every 3 to 4 residues since the helix makes a turn for every 3.6 residues.

Acidic and basic side chains because of charge repulsion and steric hinderance. Proline is also not accomodated because its N is in a rigid ring and its secondary amine group has no H to make an H-bond. Glycine is also not common because of its conformational flexibility; it is entropically unfavorable.

The last 3-4 amino acids on the ends of the helix cannot hydrogen bond and have their polar carbonyl and amide unpaired. The N-terminus is partially positive because of the unpaired amide H, therefore negatively charged amino acids are often found near it. And vice versa.

Charged amino acids neutralize the charges on the helix ends. Presence of a negatively charged amino acid near the C-terminus, and vice versa, destabilizes the alpha helix.