Final answer:
In peptides, the peptide bond between amino acids has limited rotation due to its partial double bond character. Other single bonds in the structure allow for free rotation, contributing to the protein's flexibility. The peptide bond's rigidity plays a crucial role in protein folding and stability.
Step-by-step explanation:
In amino acids and peptides, the rotation around different bonds is not the same. The main bond in question is the peptide bond, which is the amide bond that connects the carboxyl group of one amino acid to the amine group of another. Due to resonance stabilization, the peptide bond has partial double bond character, which limits the rotation around it. This characteristic means that the peptide bond is planar, much like the structure of alkenes, and the four groups around it exist in the same plane.
Rotation can freely occur around other σ-bonds (single bonds) in the amino acid structures, such as those connected to the alpha carbon, which is the central carbon in the amino acid. This allows for some flexibility in the overall three-dimensional structure of peptides and proteins. However, the rigidity of the peptide bond itself plays a key role in determining the secondary and tertiary structures of proteins, including the formation of alpha helices and beta sheets, which are stabilized by hydrogen bonds between the N-H groups and C=O groups.
Understanding the rotation capabilities and constraints of peptide bonds is important for studying protein structure and, potentially, for making modifications to peptides through isosteric replacement to affect their stability and biological activity.