What is a distinguishing feature of parallel β-sheets compared to anti-parallel ones?

In parallel β-sheets, the strands run in the same direction, meaning that the N-terminus and C-terminus of adjacent strands align. This alignment results in hydrogen bonds that are not optimal; they tend to be angled rather than straight. The angle introduced by this alignment leads to less stable interactions compared to anti-parallel β-sheets, which have straight hydrogen bonds due to the opposite orientation of their strands.

The structure and stability of β-sheets are critically dependent on how the hydrogen bonds form between the nitrogen and oxygen atoms of the polypeptide backbone. In anti-parallel β-sheets, the adjacent strands run in opposite directions, facilitating the formation of straight hydrogen bonds that enhance the stability of the sheet. Therefore, the distinction between angled hydrogen bonds in parallel sheets and straight hydrogen bonds in anti-parallel sheets is a clear defining characteristic, supporting the correctness of the answer.