Final answer:
In the α-helix structure of a protein, the amino acid side chains project outward from the helix axis, while in the β-pleated sheet, the side chains extend above and below the pleat folds. These structures help stabilize the protein and allow different amino acids to fit without steric strain.
Step-by-step explanation:
In the α-helix, the side chains of the amino acids project outward from the axis of the helix. This structural feature is due to the polypeptide chain acquiring a coil shape that spirals clockwise. The helix is stabilized by hydrogen bonds between the carbonyl oxygen atoms and amide hydrogen atoms, which occur every 3.6 amino acid residues. These hydrogen bonds contribute to the helix's structural rigidity, while the side chains (-R groups) protrude outwards, enabling different side chain geometries to be accommodated within the helix without steric strain.
In contrast, the β-pleated sheet is another secondary structure found in proteins where the R groups extend above and below the folds of the pleat. These are stabilized by hydrogen bonds between aligned amino acid chains, forming a sheet-like arrangement. The side chains in a β-pleated sheet often have short side chains to minimize steric strain.