1AG9 - E. coli flavodoxin
Hoover and Ludwig (1997) Protein Sci. 6: 2525-2537

A short stretch of aminoacids can assume a
helicoidal shape.

Such an arrangement is stable because hydrogen bonds can form between C=O of aminoacid n and N-H of aminoacid n+4 (i.e., between the 1st and the 5th, the 2nd and the 6th, etc.). For simplicity, only the peptide N-C_α-C=O backbone is shown in the figure. The helix rises about 5.4 Å per turn. The angular distance between aminoacids is close to 100º, so that about 3.6 aminoacids are present in each turn of the helix:
The sidechains direct themselves outwards
A longer α-helix is stabilized by many hydrogens bonds. Helix-destabilizing residues (e.g. like
proline can break the helix, or bend it (as in this case). Observation of the
complete α-helix (with sidechains) allows one to predict where in the protein it will be. Spin the structure in order to appreciate how one of the faces of the helix contains polar, hydrophylic residues, while the other mostly contains hydrophobic residues.
The depiction of this α-helix in the protein indeed shows that the hydrophylic side faces the solvent, while the hydrophobic side faces the protein core.
The tertiary structure of this protein.