Which secondary structure is specifically characterized by a tight turn in polypeptide chains?

The β-turn is specifically characterized by a tight turn in polypeptide chains, allowing the chain to reverse direction. This structural feature is crucial in the overall three-dimensional folding of proteins and is typically found in regions where the protein folds back on itself. The β-turn commonly consists of four amino acid residues, with the second and third amino acids often engaging in hydrogen bonding that stabilizes the turn.

This structure plays a significant role in the compactness of globular proteins and helps in forming the core of protein structures by facilitating close interactions between different parts of the polypeptide chain. It is especially important in the alignment of β-strands in beta sheets and in ensuring that the protein achieves the necessary folds for proper function.

In contrast, other structures like the α-helix are characterized by their right-handed helical configuration without such sharp turns, while disulfide bridges refer to covalent bonds formed between cysteine residues that stabilize protein structure rather than define turns. Loops, while they can also involve turns in polypeptide chains, lack the specific tightness and definition given to β-turns. Thus, the characterization of the β-turn is distinct and crucial for understanding protein architecture.