Peptide Bond Calculators
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Peptide Bond Formation
H₂N−CHR₁−COOH + H₂N−CHR₂−COOH → H₂N−CHR₁−CO−NH−CHR₂−COOH + H₂O
The reaction is endergonic (+ΔG) in aqueous solution — in cells, it is driven forward by ribosomal coupling with GTP hydrolysis (4 high-energy bonds consumed per peptide bond). Peptide bonds can be hydrolyzed by acid, base, or proteases.
Planarity and Resonance
The C−N bond in a peptide bond has ~40% double-bond character due to resonance between: C=O ↔ C⁻−O⁻ with C⁺=N structure. This partial double bond restricts rotation around the C−N bond — the four atoms of the peptide bond (Cα−C=O−N−Cα) are coplanar. The peptide bond adopts preferentially the trans configuration (opposing Cα atoms on opposite sides of the C−N bond) due to lower steric strain; cis peptide bonds occur primarily before proline (~0.1% of bonds in proteins).
Ramachandran Plot
Protein backbone conformation is defined by the dihedral angles φ (phi, rotation around N−Cα bond) and ψ (psi, rotation around Cα−C bond). The ω angle (peptide bond rotation) is fixed at ~180° (trans). Allowed φ/ψ combinations form the Ramachandran plot: α-helix region (φ ≈ −60°, ψ ≈ −45°); β-sheet region (φ ≈ −120°, ψ ≈ +120°); most combinations are sterically excluded.
Peptide Bond Properties
- Length: C−N = ~1.33 Å (between single bond 1.47 Å and double bond 1.27 Å)
- Polarity: C=O is a strong H-bond acceptor; N−H is an H-bond donor → critical for α-helix and β-sheet formation
- UV absorption: peptide bonds absorb at ~214 nm — used to quantify protein concentration by absorbance
Glossary
Frequently Asked Questions
A peptide bond is an amide bond formed between the carboxyl group (−COOH) of one amino acid and the amino group (−NH₂) of the next, with release of water (condensation reaction). The ribosome catalyzes this reaction using GTP hydrolysis and peptidyl transferase activity (actually a ribozyme — the 23S/28S rRNA catalyzes the reaction). The resulting C−N bond links the two amino acid residues. A chain of amino acids linked by peptide bonds is a polypeptide. The N-terminus (free NH₂) and C-terminus (free COOH) are the chain ends; ribosomes synthesize chains N→C.
The peptide bond has partial double-bond character due to resonance: lone-pair electrons on nitrogen are delocalized into the adjacent carbonyl group (C=O ↔ C−O⁻ with C⁺−N). This electron delocalization gives the C−N bond ~40% double-bond character, making it shorter and more rigid than a pure single bond. As a result, the four atoms surrounding the peptide bond (Cα−C=O−N−Cα) are constrained to a single plane (coplanar). Rotation around the peptide C−N bond is restricted (energy barrier ~80 kJ/mol), though free rotation occurs around the Cα−N (φ angle) and Cα−C (ψ angle) bonds.
The Ramachandran plot maps the allowed backbone dihedral angles of polypeptide chains: φ (phi) = rotation around the N−Cα bond; ψ (psi) = rotation around the Cα−C bond. The ω angle (peptide bond itself) is fixed at ≈180° (trans). Most φ/ψ combinations are sterically forbidden due to clashing side chains and backbone atoms. Allowed regions correspond to: α-helix (φ ≈ −60°, ψ ≈ −45°); β-sheet (φ ≈ −120°, ψ ≈ +120°); left-handed helix region. The Ramachandran plot is used to validate protein structure quality — high-quality crystal structures have >98% of residues in allowed regions.
The C=O and N−H groups of every peptide bond are capable of forming hydrogen bonds. In α-helices: C=O at position i forms an H-bond with N−H at position i+4 (within the same chain); this stabilizes the right-handed helix with 3.6 residues per turn. In β-sheets: C=O and N−H groups of different strands form H-bonds between parallel or antiparallel chains, stabilizing the extended sheet structure. All backbone H-bonding capacity must be satisfied in protein interiors — unsatisfied H-bond donors and acceptors represent a thermodynamic penalty that drives chain folding into structures that maximize H-bond formation.