Final answer:
The unusual feature not commonly observed in α helices is left-handedness, as these structures are predominantly right-handed. They are characterized by a helical turn every 3.6 amino acids, a cylindrical shape, outward-pointing amino acid side chains, and backbone hydrogen bonds. Option 1 is correct answer.
Step-by-step explanation:
The alpha (α) helix is a common motif in the secondary structure of proteins and is characterized by its right-handed coil that holds its shape through hydrogen bonds. These bonds occur between the carbonyl oxygen atom of one amino acid and the amide hydrogen atom four amino acids away. When discussing the features of the α helix structure, it is important to consider various characteristics that define it. An α helix typically has the following features:
- A right-handed coiled structure.
- A helical turn every 3.6 amino acids.
- A generally cylindrical shape.
- Amino acid side chains that point outward from the helix.
- Hydrogen bonds that exist among the polypeptide backbone.
Given these common features, the characteristic that is not commonly observed in α helices is left-handedness. Alpha helices are predominantly right-handed due to the chiral nature of most amino acids.
Therefore, the correct option is:
1) left-handedness.