What is a Peptide Bond?
A peptide bond is a covalent bond formed between two amino acids. This bond forms when the carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH₂) of another amino acid, releasing a molecule of water. This type of reaction is called a condensation reaction (or dehydration synthesis).
The resulting bond is a CO-NH linkage, and the molecule formed is called an amide. This is the fundamental bond that links all amino acids together in peptides and proteins.
Peptide Bond Formation
To form a peptide bond, the amino acid molecules must be oriented so that the carboxylic acid group of one amino acid can react with the amine group of another. At its simplest level:
- Two amino acids combine through peptide bond formation to create a dipeptide
- Additional amino acids can be added to form longer chains
- Chains of 2-50 amino acids are generally called peptides
- Chains of 50-100 amino acids are termed polypeptides
- Chains with more than 100 amino acids are typically called proteins
Bond Stability and Hydrolysis
Peptide bonds can be broken down through hydrolysis—a reaction with water. Although the hydrolysis reaction releases energy (about 10 kJ/mol of free energy), the reaction itself is quite slow under normal conditions. This makes peptide bonds relatively stable, though they are considered metastable because they can break when exposed to water over time.
In biological systems, enzymes called proteases and peptidases catalyze the hydrolysis of peptide bonds, allowing cells to break down proteins when needed.
Structure of the Peptide Bond
X-ray diffraction studies have revealed important characteristics of peptide bonds:
- Rigid and planar: The bond does not freely rotate
- Resonance stabilization: The amide nitrogen can delocalize electrons into the carbonyl oxygen
- Bond lengths: The N–C bond is shorter than typical single bonds, while the C=O bond is longer than typical double bonds
- Trans configuration: The carbonyl oxygen and amide hydrogen are typically in trans arrangement, which is more energetically favorable
The Polarity of Peptide Bonds
The peptide bond exhibits partial double-bond character due to resonance. The nitrogen's lone pair of electrons can delocalize toward the carbonyl carbon, creating a resonance structure with:
- A partial double bond between carbon and nitrogen
- A partial negative charge on the oxygen (-0.28)
- A partial positive charge on the nitrogen (+0.28)
This resonance gives the peptide bond approximately 40% double-bond character, making it rigid and restricting rotation. The resulting permanent dipole is important for protein structure and interactions.
Spectroscopic Properties
Peptide bonds absorb UV light at wavelengths of 190-230 nm. This property is useful for:
- Detecting peptides during HPLC purification (typically at 214-220 nm)
- Quantifying peptide concentration
- Studying protein structure through circular dichroism
Biological Importance
Peptide bonds are fundamental to life. The specific properties of peptide bonds—their partial double-bond character, polarity, and ability to form hydrogen bonds—directly influence:
- Protein secondary structure (α-helices and β-sheets)
- Protein folding and stability
- Enzyme function and specificity
- Molecular recognition and signaling
Key Takeaways
- Peptide bonds form through condensation reactions between amino acids
- The bond is rigid and planar due to resonance
- Approximately 40% double-bond character restricts rotation
- The bond has a permanent dipole affecting protein properties
- UV absorbance at 190-230 nm enables detection