Final answer:
Valine's presence in an α-helix depends on the surrounding amino acids. It is a nonpolar amino acid that can be found in the interior of proteins and can also be present in β-pleated sheets due to its short side chain that minimizes steric strain. Option a.
Step-by-step explanation:
The presence of valine in an α-helix depends on the surrounding amino acids. Valine is a nonpolar amino acid and tends to be found in the interior of protein structures due to its hydrophobic nature.
As the secondary structure of proteins, the α-helix is stabilized by hydrogen bonds between the backbone atoms, with the R groups (side chains) of amino acids like valine protruding out from the helix.
However, valine can also be found in β-pleated sheets, where the side chains alternate above and below the plane of the sheet. In such structures, amino acids with short side chains, including valine, reduce steric strain and thus are often preferred. Option a.