Organic Chemistry · Biomolecules: Amino Acids, Peptides, and Proteins
Peptides and Proteins
On this page 9 sections
In 30 seconds
A peptide is a chain of amino acids joined by peptide (amide) bonds, formed when the α-carboxyl group of one amino acid condenses with the α-amino group of the next, releasing water. The chain has direction: it begins at the free N-terminus (the amino end) and ends at the free C-terminus (the carboxyl end). The peptide bond Amide link between the α-CO of one residue and the α-NH of the next Full entry → is special — resonance gives it partial double-bond character, making it planar and restricting rotation. Long chains are called polypeptides; when they fold into functional molecules they are proteins. This topic covers peptide bond formation and geometry, chain terminology, disulfide cross-links, and the functions of famous peptides and proteins.
Why this matters
Proteins are the workhorses of the cell: enzymes catalyze reactions, antibodies defend against infection, hemoglobin transports oxygen, keratin and collagen provide structure, and hormones such as insulin and oxytocin carry signals. Insulin — a 51-residue, two-chain protein with three disulfide bonds — was the first protein whose sequence was determined (Sanger, 1958 Nobel Prize) and remains a life-saving therapy for diabetes. Oxytocin, a nine-residue peptide, is used clinically to induce labor. Understanding how amino acids link into chains is the prerequisite for everything that follows in this chapter: sequencing (Topics 5–6), synthesis (Topics 7–8), and folding into three-dimensional structure (Topic 9).
The college version
Core Concepts
The peptide (amide) bond
A peptide bond forms by condensation Reaction that joins molecules while releasing water Full entry → — loss of water between two amino acids:
H2N–CH(R)–COOH + H2N–CH(R')–COOH ⇌ H2N–CH(R)–CO–NH–CH(R')–COOH + H2O
The product is a dipeptide with one peptide bond. In water this equilibrium favors hydrolysis, so cells do not simply mix amino acids: the ribosome joins amino acids that are activated as tRNA esters, and laboratory synthesis (Topics 7–8) uses coupling reagents. The portion of an amino acid remaining inside the chain is called a residue Amino acid unit inside a chain (minus the elements of water) Full entry →. A chain of n residues has n − 1 peptide bonds, one free N-terminus, and one free C-terminus. Naming runs N→C: Gly–Ala–Ser is glycylalanylserine — every residue except the last gets the suffix "-yl."
Planarity and restricted rotation
The peptide bond is not a simple single bond. The nitrogen's lone pair is delocalized into the carbonyl π system (amide resonance Delocalization of the N lone pair into the C=O Full entry →), giving the C–N bond roughly 40% double-bond character. Consequences:
- The six atoms of the amide unit (Cα–C–O–N–H–Cα) lie in one plane.
- Rotation about the C–N bond is strongly restricted.
- The trans arrangement (R groups on opposite sides) is strongly favored, because it minimizes steric clash — except at X–Pro linkages, where the proline ring makes cis isomers unusually common.
The protein backbone is therefore a series of rigid, planar amide units connected by rotatable single bonds at the α-carbons — the geometry that later produces α-helices and β-sheets (Topic 9).
Disulfide bonds
Two cysteine side chains (–CH₂–SH) can be oxidized to form a disulfide (–S–S–) linkage, converting the pair into cystine. Disulfides are covalent cross-links that staple parts of a chain together and stabilize folded proteins. They must be reduced (and the thiols alkylated) before sequencing, because a disulfide-linked chain behaves like a single molecule — a practical point that returns in Topic 6.
From peptides to proteins
Terminology is a size convention, not a sharp chemical boundary: dipeptide (2 residues), tripeptide (3), oligopeptide (typically up to a few dozen), polypeptide Long chain of amino acid residues Full entry → (long chain), and protein (a functional polypeptide, usually 50+ residues, that folds into a defined structure). The primary structure Amino acid sequence plus disulfide positions Full entry → of a protein is its amino acid sequence plus the positions of any disulfide bonds.
Representative peptides and proteins
| Molecule | Size and features | Role |
|---|---|---|
| Glutathione | Tripeptide γ-Glu–Cys–Gly | Antioxidant; protects cells from oxidative damage |
| Oxytocin | 9 residues, one disulfide | Hormone: uterine contraction, milk letdown |
| Insulin | 51 residues: A chain (21) + B chain (30), three disulfides | Hormone: blood-glucose regulation |
| Hemoglobin | Four polypeptide subunits | Oxygen transport in blood |
| Keratin, collagen | Long fibrous chains | Structural proteins (hair, skin, tendons) |
Glutathione's unusual γ-linkage joins the side-chain carboxyl of glutamate rather than the α-carboxyl — a detail worth remembering, because it makes the tripeptide resistant to ordinary peptidases.
Common Confusions
| Common Confusion | Correct Understanding |
|---|---|
| "The peptide bond is a simple C–N single bond with free rotation." | Amide resonance gives it ~40% double-bond character: planar, restricted rotation, trans-favored. |
| Peptides and proteins are chemically different classes. | Same chemistry; the terms are size/function conventions (protein ≈ 50+ residues, folded). |
| A tripeptide has three peptide bonds. | n residues have n − 1 bonds: a tripeptide has two. |
| Sequence can be read in either direction. | Direction matters: N→C. Gly–Ala–Ser and Ser–Ala–Gly are different molecules. |
| Disulfide bonds form between any two residues. | Only cysteine side chains form them, by oxidation of –SH to –S–S–. |
| Glutathione's γ-Glu bond is a normal peptide bond. | It joins the side-chain carboxyl of Glu — an unusual linkage resistant to normal peptidases. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
A protein is like a bead necklace where the beads are amino acids and the knots are peptide bonds. The necklace always has a clasp end (the N-terminus) and a tail end (the C-terminus), and each knot is stiff and flat — it cannot twist. Some necklaces get extra safety clips, called disulfide bonds, that join beads that are far apart on the string.
Worked example
Example 1: Counting bonds and termini
Consider a heptapeptide written H₂N–Gly–Ala–Ser–Val–Leu–Ile–Phe–COOH. With n = 7 residues, the number of peptide bonds is n − 1 = 6. There is exactly one free N-terminus (the glycine amino group) and one free C-terminus (the phenylalanine carboxyl group). The full name is glycylalanylserylvalylleucylisoleucylphenylalanine — every residue except the last contributes "-yl."
Example 2: How many different tripeptides are possible?
With the 20 standard amino acids and repeats allowed, the number of distinct tripeptides is a product rule calculation:
20 × 20 × 20 = 203 = 8000
For a 51-residue chain (the size of insulin), the count is 2051, an astronomically large number — which is why the sequence carries information: the same 20 beads can encode an essentially unlimited variety of proteins.
Example 3: Hydrolysis mass balance with dimensional analysis
A disaccharide-style mass check applies to peptide bonds: breaking one peptide bond consumes one water molecule. Hydrolyze 1.00 g of the dipeptide glycylglycine (Gly–Gly) completely. First compute the dipeptide molar mass from glycine (MW 75.07 g/mol):
M(Gly–Gly) = 2M(Gly) - M(H2O) = 2(75.07) - 18.02 = 132.12 g/mol
Convert the mass to moles, then to moles of glycine produced (2 mol Gly per mol dipeptide):
1.00 g Gly–Gly × 1 mol Gly–Gly132.12 g × 2 mol Gly1 mol Gly–Gly × 75.07 g Gly1 mol Gly = 1.136 g glycine
The product weighs more than the starting dipeptide because each broken bond adds the elements of water — a classic quantitative check in amino acid analysis.
Key takeaways
- Peptide bond = amide bond; formed by condensation with loss of water; n residues → n − 1 peptide bonds.
- The peptide bond is planar with ~40% double-bond character; rotation about C–N is restricted; trans is favored (cis is common at X–Pro).
- Chains are written and named from N-terminus (left) to C-terminus (right); residues except the last get "-yl."
- Primary structure = amino acid sequence + disulfide positions.
- Disulfides form by oxidation of two Cys side chains; reduce them before sequencing.
- Glutathione = γ-Glu–Cys–Gly (antioxidant); oxytocin = 9 residues; insulin = 51 residues, two chains, three disulfides.
- Peptide vs protein is a size/function convention, not a chemical boundary.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
How many peptide bonds are in a decapeptide (10 residues)? How many free termini?
Show answer
n − 1 = 9 peptide bonds; one free N-terminus and one free C-terminus.
Why is rotation about the peptide C–N bond restricted?
Show answer
Amide resonance delocalizes the nitrogen lone pair into the C=O, giving the C–N bond partial double-bond character; double bonds do not rotate freely.
Write the condensation equation for forming a dipeptide from glycine and alanine, and name the product.
Show answer
Gly + Ala → H₂N–Gly–CO–NH–Ala–COOH + H₂O; the product is glycylalanine.
What is the difference between a peptide and a protein?
Show answer
Both are amino acid chains joined by peptide bonds; "protein" conventionally means a longer, folded, functional chain (~50+ residues), while "peptide" covers short chains.
Which amino acid side chains form disulfide bonds, and what oxidation product do they give?
Show answer
Cysteine (–CH₂–SH); two side chains oxidize to a disulfide (–S–S–), forming cystine.
How many distinct dipeptides can be built from the 20 standard amino acids (repeats allowed)?
Show answer
20 × 20 = 400 distinct dipeptides.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- peptide bond
- Amide link between the α-CO of one residue and the α-NH of the next
- condensation
- Reaction that joins molecules while releasing water
- residue
- Amino acid unit inside a chain (minus the elements of water)
- N-terminus / C-terminus
- Free amino end / free carboxyl end
- amide resonance
- Delocalization of the N lone pair into the C=O
- disulfide bond
- S–S cross-link between two cysteine side chains
- primary structure
- Amino acid sequence plus disulfide positions
- polypeptide
- Long chain of amino acid residues
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.