Compared to antiparallel β-sheets, parallel β-sheets are weaker due to:

The correct answer highlights the nature of hydrogen bonding in parallel β-sheets. In parallel β-sheets, hydrogen bonds form between the amide groups of adjacent strands that run in the same direction. This orientation leads to the formation of bent hydrogen bonds, which are less stable compared to the linear hydrogen bonds found in antiparallel β-sheets.

In antiparallel β-sheets, the strands run in opposite directions, allowing the hydrogen bonds to align linearly. Linear hydrogen bonds are more favorable geometrically, leading to stronger interactions between the strands. This difference in bond alignment contributes to the overall stability of the β-sheet structure; hence, the presence of bent hydrogen bonds in parallel β-sheets renders them weaker than their antiparallel counterparts.

Understanding the orientation and nature of hydrogen bonds in these two structural formations is key to appreciating their differing stabilities in proteins.