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Peptide Bonds
2.2.1 Definition
A peptide bond is a covalent amide linkage formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of another. This bond connects individual amino acids into a polypeptide chain, forming the backbone of peptides and proteins. Peptide bonds are sometimes referred to as amide bonds in organic chemistry.
2.2.2 Formation Mechanism
Peptide bonds are formed via a condensation (dehydration) reaction, where a molecule of water is released: R1-COOH + R2-NH2 → R1-CONH-R2 + H2O {R1-COOH + R2-NH2 → R1-CONH-R2 + H2O}R1-COOH + R2-NH2 → R1-CONH-R2 + H2O
This reaction is thermodynamically unfavorable under physiological conditions and is catalyzed by ribosomes in cells during protein biosynthesis. In chemical peptide synthesis, activating agents such as carbodiimides, uronium salts, or phosphonium salts facilitate bond formation.
2.2.3 Structural Characteristics
Planarity: The peptide bond has partial double-bond character due to resonance between the carbonyl oxygen and the amide nitrogen, restricting rotation.
Rigidity: The planar nature stabilizes the backbone and contributes to secondary structure formation.
Trans Configuration: Peptide bonds are usually trans, minimizing steric clashes between side chains. Rare cis bonds occur, especially preceding proline residues.
2.2.4 Chemical Properties
Polarity: The peptide bond has a dipole moment, making it capable of hydrogen bonding, which is essential for α-helices and β-sheets.
Stability: Peptide bonds are relatively stable under physiological conditions but can be cleaved enzymatically by proteases or chemically under strong acidic or basic conditions.
Reactivity: The carbonyl carbon is electrophilic, allowing activation during peptide synthesis for coupling reactions.
2.2.5 Role in Peptide Structure
Peptide bonds form the backbone of polypeptides, linking amino acids in a defined sequence.
They provide structural rigidity that allows formation of secondary structures such as α-helices, β-sheets, and turns.
Hydrogen bonding between peptide bond NH and CO groups is critical for protein folding and stability.
2.2.6 Conclusion
Peptide bonds are the fundamental chemical linkages that form peptides and proteins. Their planarity, polarity, and ability to form hydrogen bonds make them central to the three-dimensional structure and biological function of peptides. Understanding peptide bond chemistry is essential for peptide synthesis, protein engineering, and structural biology.
2.2.1 Definition
A peptide bond is a covalent amide linkage formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of another. This bond connects individual amino acids into a polypeptide chain, forming the backbone of peptides and proteins. Peptide bonds are sometimes referred to as amide bonds in organic chemistry.
2.2.2 Formation Mechanism
Peptide bonds are formed via a condensation (dehydration) reaction, where a molecule of water is released: R1-COOH + R2-NH2 → R1-CONH-R2 + H2O {R1-COOH + R2-NH2 → R1-CONH-R2 + H2O}R1-COOH + R2-NH2 → R1-CONH-R2 + H2O
This reaction is thermodynamically unfavorable under physiological conditions and is catalyzed by ribosomes in cells during protein biosynthesis. In chemical peptide synthesis, activating agents such as carbodiimides, uronium salts, or phosphonium salts facilitate bond formation.
2.2.3 Structural Characteristics
Planarity: The peptide bond has partial double-bond character due to resonance between the carbonyl oxygen and the amide nitrogen, restricting rotation.
Rigidity: The planar nature stabilizes the backbone and contributes to secondary structure formation.
Trans Configuration: Peptide bonds are usually trans, minimizing steric clashes between side chains. Rare cis bonds occur, especially preceding proline residues.
2.2.4 Chemical Properties
Polarity: The peptide bond has a dipole moment, making it capable of hydrogen bonding, which is essential for α-helices and β-sheets.
Stability: Peptide bonds are relatively stable under physiological conditions but can be cleaved enzymatically by proteases or chemically under strong acidic or basic conditions.
Reactivity: The carbonyl carbon is electrophilic, allowing activation during peptide synthesis for coupling reactions.
2.2.5 Role in Peptide Structure
Peptide bonds form the backbone of polypeptides, linking amino acids in a defined sequence.
They provide structural rigidity that allows formation of secondary structures such as α-helices, β-sheets, and turns.
Hydrogen bonding between peptide bond NH and CO groups is critical for protein folding and stability.
2.2.6 Conclusion
Peptide bonds are the fundamental chemical linkages that form peptides and proteins. Their planarity, polarity, and ability to form hydrogen bonds make them central to the three-dimensional structure and biological function of peptides. Understanding peptide bond chemistry is essential for peptide synthesis, protein engineering, and structural biology.