肽键

A peptide bond forms through a condensation reaction where the carboxyl group of one amino acid links to the amino group of another, releasing a water molecule. This covalent bond creates an amide group, joining the two amino acids together. The reaction is reversible through hydrolysis, which adds water back to break the bond.

In cis conformation, the alpha carbons of both amino acids are on the same side of the peptide bond, while in trans conformation they are on opposite sides. Peptide bonds predominantly occur in the trans conformation, except when proline contributes its amino group to bond formation, where cis bonds are more common.

The peptide bond exhibits rigidity due to resonance between the carbonyl group's double bond and the carbon-nitrogen single bond. This electron sharing decreases the single bond length and increases the double bond length, preventing rotation around the peptide bond and creating its characteristic planar structure.

The N-terminal is the end of a peptide chain with a free amino group not involved in peptide bond formation, while the C-terminal has a free carboxyl group. Amino acid sequences always begin at the N-terminal and end at the C-terminal, establishing the directional orientation of the polypeptide chain.

Peptides are classified by chain length: dipeptides contain two amino acids, tripeptides contain three, and tetrapeptides contain four. When several amino acids link through peptide bonds, they form polypeptides. Polypeptides that fold into specific three-dimensional structures with distinct functions are called proteins.

Peptide bonds break through hydrolysis, which adds a water molecule to cleave the bond. This can occur rapidly using chemical catalysts like acids or enzymes called proteases, or slowly through spontaneous hydrolysis in the presence of water alone. Both mechanisms reverse the condensation reaction.

Resonance in peptide bonds involves electron sharing between the carbonyl group's double bond and the carbon-nitrogen single bond. This resonance stabilizes the bond structure, reduces rotation around the peptide linkage, and contributes to the rigid planar geometry essential for protein folding and function.