Peptide chemistry
How Are Peptides Made?
A laboratory-focused guide to solid-phase peptide synthesis, protecting-group strategy, chain assembly, cleavage, purification, and the analytical evidence needed to characterize a finished material.
The short answer
Peptides can be made by chemical synthesis, recombinant expression, or isolation from a biological source. A foundational chemical method is solid-phase peptide synthesis (SPPS): a protected amino acid is attached to a solid resin, then additional protected amino acids are coupled one at a time until the intended sequence has been assembled.
Synthesis produces a crude mixture, not automatic proof of identity or purity. Cleavage, purification, and method-appropriate analysis remain separate parts of the workflow.
01 · The chemical objective
Peptide synthesis creates a defined amino-acid sequence.
The IUPAC Gold Book defines peptides as amides derived from two or more amino carboxylic acid molecules through a covalent bond from the carbonyl carbon of one unit to the nitrogen of another, with formal loss of water. In practical synthesis, the challenge is to create those bonds in the intended order while controlling all the other reactive groups present in the amino acids.
A sequence is conventionally assembled and written from its N-terminus to its C-terminus, but common SPPS workflows grow the chain from the resin-bound C-terminal residue toward the N-terminus. Each completed cycle adds one residue. The sequence, terminal chemistry, side-chain modifications, disulfide pattern, counterion, and physical format all contribute to the identity of the final material.
02 · Why a solid phase
The resin keeps the growing chain anchored.
In R. B. Merrifield's 1963 solid-phase synthesis paper, the growing peptide remained attached to an insoluble polymer while soluble reagents were added and then washed away. That physical separation is the core operational advantage: coupling and deprotection reagents can be used in excess, while filtration and washing prepare the supported chain for the next reaction cycle.
The resin and the linker connecting it to the first residue are not interchangeable background materials. As summarized in a Methods in Molecular Biology SPPS chapter, their selection affects loading, swelling, reaction access, and whether the cleaved product ends with a C-terminal acid, amide, or another designed functionality.
03 · One SPPS cycle
Expose, couple, wash, and repeat.
Remove the temporary N-terminal protecting group from the resin-bound chain without releasing it from the support.
Convert the incoming amino acid's carboxyl group into a form that can react efficiently under the selected conditions.
Form the next amide bond between the exposed chain terminus and the protected incoming residue.
Remove soluble reagents and by-products, then assess whether the cycle is ready to advance.
This cycle is repeated for every residue in the planned sequence. A missed or incomplete coupling can create a deletion sequence; unintended chemistry can create other closely related products. Small per-cycle losses compound as chain length increases, which is one reason difficult or long sequences require additional strategy.
04 · Protecting groups
Temporary masks control which group reacts.
Amino acids may contain reactive amines, carboxylic acids, alcohols, thiols, and other side-chain functions. A review of protecting groups in peptide synthesis explains why temporarily masking these functions is essential for limiting uncontrolled polymerization and unwanted side reactions.
Fmoc and Boc are two established temporary N-terminal protecting strategies. Fmoc is commonly removed under basic conditions, while Boc is acid-labile. Side-chain protecting groups and the resin linker must be orthogonal enough that one operation can expose the intended reactive group without prematurely disturbing the others. “Fmoc peptide” or “Boc peptide” therefore describes a synthesis strategy, not a distinct class of final peptide.
05 · Cleavage and deprotection
The completed chain must leave the solid support.
After the final coupling cycle, the peptide is cleaved from the resin and its remaining side-chain protecting groups are removed under conditions compatible with the chosen resin, linker, sequence, and protection scheme. The reaction mixture can contain the target peptide, protecting-group fragments, scavenger-derived material, deletion sequences, and other synthesis by-products.
These operations can also reveal sequence-specific difficulties. Acid-sensitive motifs, oxidation-prone residues, aggregation, incomplete deprotection, and side reactions can affect recovery and composition. A complete process description therefore distinguishes chain assembly, cleavage, isolation, purification, and final analysis.
06 · Purification
Crude peptide and purified peptide are different materials.
Preparative chromatography is commonly used to separate the intended product from synthesis-related components. Conditions are selected around the target's size, charge, hydrophobicity, solubility, and the impurity profile. Fractions judged to contain the target can be pooled, concentrated, exchanged into an appropriate counterion or solvent system, and dried or formulated.
A purification step is a process, not a result. The method, fraction decisions, and final analytical measurements determine what can be stated about a particular lot. The word “purified” alone does not establish identity, amount, chromatographic purity, sterility, or fitness for an experiment.
07 · Analytical characterization
Identity, purity, and quantity are separate questions.
Mass spectrometry can provide measured mass-to-charge evidence consistent with an expected molecular mass.
Analytical HPLC can separate detectable components and report a method-specific area percentage.
Net peptide content or concentration requires an appropriate quantitative method; vial fill weight is not automatically peptide amount.
The report, sample, and physical package need a traceable lot relationship for the record to apply to the received material.
Our HPLC vs LC-MS guide explains the complementary analytical roles, while the peptide COA guide shows how to inspect report identity, lot linkage, methods, and limitations.
08 · Why some sequences are difficult
Chain length is only one source of synthetic complexity.
Hydrophobic sequences can aggregate on the resin, sterically hindered residues can couple slowly, and particular motifs can undergo side reactions. A 2025 review of backbone protecting groups describes how aggregation and chain insolubility can reduce yield and create deletion impurities that may be difficult to separate from the intended product.
Sequence-specific strategies can include repeated coupling, altered activation chemistry, different resins or linkers, backbone protection, pseudoproline building blocks, segment condensation, native chemical ligation, or recombinant production. The appropriate route depends on the molecular target and cannot be inferred from residue count alone.
09 · Sources
Primary references and technical reviews.
- IUPAC Gold Book: Peptides
- Merrifield: Solid Phase Peptide Synthesis I
- Shelton and Jensen: Linkers, resins, and general procedures for SPPS
- Conda-Sheridan and Krishnaiah: Protecting Groups in Peptide Synthesis
- Amblard and colleagues: Methods and protocols of modern SPPS
- Behrendt and colleagues: Backbone Protecting Groups for Enhanced Peptide and Protein Synthesis
Peptide synthesis FAQ
Common laboratory questions.
How are peptides made?
Peptides can be produced by chemical synthesis, recombinant expression, or isolation from a biological source. Solid-phase peptide synthesis is a widely used chemical method in which protected amino acids are coupled stepwise to a chain attached to a resin.
What is solid-phase peptide synthesis?
Solid-phase peptide synthesis, or SPPS, is an iterative method that anchors the growing peptide to an insoluble support. Each cycle exposes the terminal amino group, couples the next protected amino acid, and prepares the chain for another cycle before final cleavage and deprotection.
Why are protecting groups used in peptide synthesis?
Amino acids contain several reactive functional groups. Temporary protecting groups control which group reacts during each step, helping limit unwanted polymerization and side reactions while the intended sequence is assembled.
What is the difference between Fmoc and Boc peptide synthesis?
Fmoc and Boc are temporary N-terminal protection strategies with different removal conditions. Fmoc is generally removed under basic conditions, while Boc is acid-labile. The overall protecting-group and resin strategy must be chemically compatible with the intended sequence.
Does peptide synthesis guarantee purity?
No. Coupling failures, deletion sequences, side reactions, aggregation, and cleavage products can create impurities. Crude material normally requires purification, and the resulting lot still requires method-appropriate analytical characterization.
How are synthetic peptides purified and analyzed?
Preparative chromatography is commonly used to separate the target peptide from related components. Analytical HPLC can describe chromatographic separation and area percentage, while mass spectrometry can provide molecular-mass evidence. These methods answer complementary questions.