Final answer:
Some a-helices have alternating hydrophobic and hydrophilic amino acids to enable improved packing within helical bundles, contributing to the stability and function of the protein's tertiary structure.
Step-by-step explanation:
Some a-helices have alternating hydrophobic and hydrophilic amino acids to improve the packing of helical bundles. The a-helix is a secondary structure of proteins that is stabilized by hydrogen bonds between nearby residues in a polypeptide chain. Every helix turn of an alpha helix has 3.6 amino acid residues, with the R groups of the polypeptide protruding outward from the chain, allowing for the spatial accommodation of different-sized side chains without steric strain. A helical twist configuration is often observed as a right-handed spiral, and the effective packing of these helices in protein structures is influenced by the properties of the amino acids within them. Hydrophilic and hydrophobic amino acids are organized to create distinct faces on the helix, optimizing interactions and stability within the protein's tertiary structure. Proteins like keratin consist largely of the a-helix, demonstrating its structural importance in various biological materials.