Peptide Bond
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, with the elimination of a water molecule in a condensation reaction. The resulting –CO–NH– bond has partial double bond character due to resonance delocalisation of the nitrogen lone pair into the carbonyl, making it planar and restricting rotation around the C–N bond. Peptide bonds are the primary covalent linkages in all protein chains.
Key Formula
R1-COOH + H2N-R2 → R1-CO-NH-R2 + H2O (peptide bond formation with loss of water)
LaTeX: R_1\text{-COOH} + H_2\text{N-}R_2 \rightarrow R_1\text{-CO-NH-}R_2 + H_2O
| Symbol | Meaning | Unit |
|---|---|---|
| R1 | Side chain of first amino acid | — |
| R2 | Side chain of second amino acid | — |
| –CO–NH– | Peptide bond (amide linkage) | — |
| H2O | Water released during condensation | — |
Worked Example
Problem
A pentapeptide is formed from 5 amino acids by sequential peptide bond formation. How many water molecules are released, and what is the formula for the number of peptide bonds in a chain of n amino acids?
Solution
Step 1: Each peptide bond formation releases one water molecule. Step 2: Joining 2 amino acids → 1 peptide bond, 1 H2O released. Step 3: Joining 3 amino acids → 2 peptide bonds, 2 H2O released. Step 4: General rule: n amino acids → (n–1) peptide bonds → (n–1) H2O released. Step 5: For a pentapeptide (n=5): 5–1 = 4 peptide bonds; 4 H2O molecules released.
Answer
A pentapeptide has 4 peptide bonds and releases 4 water molecules during synthesis.
Properties of the Peptide Bond
| Property | Value/Description | Significance | Consequence in Protein |
|---|---|---|---|
| Bond length (C–N) | 1.32 Å (between single 1.47 Å and double 1.27 Å) | Partial double bond character | Planar, rigid geometry |
| Rotation | Restricted (ω ≈ 180° trans) | Resonance delocalisation | Constrains backbone folding |
| Bond energy | ~200–270 kJ/mol | Stronger than C–N single bond | Requires acid/base for hydrolysis |
| Conformation | Planar unit | Due to resonance | Limits secondary structure options |
| Hydrolysis | H2O + acid/base or protease | Cleavable under harsh conditions | Digestion of proteins |
Interactive Tools
Khan Academy — Peptide Bonds
Video explaining peptide bond formation and resonance character.
Open ToolNCBI — Protein Structure Basics
Authoritative text on peptide bonds and protein primary structure.
Open ToolBrilliant.org — Peptide Bonds
Detailed notes on peptide bond geometry, resonance, and protein synthesis.
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Amino Acid
An amino acid is an organic molecule that contains both an amino group (–NH2) and a carboxyl group (–COOH) attached to the same central (alpha) carbon, along with a variable side chain (R group) that determines the identity and properties of each amino acid. There are 20 standard amino acids encoded by the genetic code that serve as the building blocks of proteins. Amino acids differ in polarity, charge, size, and chemical reactivity, which directly determines protein structure and function.
Protein Structure
Protein structure refers to the hierarchical three-dimensional arrangement of a protein, described at four levels: primary (sequence of amino acids), secondary (local folding into alpha helices and beta sheets), tertiary (overall 3D fold of a single polypeptide), and quaternary (assembly of multiple polypeptide subunits). The structure of a protein is intimately linked to its function, and alterations in structure — through mutation, denaturation, or misfolding — can lead to loss of function or disease. Determining and predicting protein structure is one of the central challenges of biochemistry and structural biology.
Condensation Reaction
A condensation reaction is a type of chemical reaction in which two molecules combine to form a larger molecule with the simultaneous loss of a small molecule, most commonly water (H2O) but sometimes methanol, HCl, or ammonia. Condensation reactions are fundamental in the synthesis of polymers, esters, amides, and biological macromolecules including proteins, nucleic acids, and polysaccharides. The reverse process, in which the small molecule is reincorporated to break the bond, is called hydrolysis.
From Greek 'peptos' (digested, cooked), related to 'peptein' (to digest). Peptides were first distinguished from proteins by molecular weight; Emil Fischer, who determined the structure of peptide bonds in the early 1900s, introduced the term 'peptide' in 1902 to describe small chains of amino acids joined by amide linkages.