Peptides — A Complete Guide

A peptide is a short chain of amino acids linked together by covalent peptide bonds. The term peptide is conventionally applied to molecules containing roughly 2 to 50 amino acid residues, though the boundary with proteins is fluid and depends on the source. Above approximately 50 residues, the molecule is generally classified as a polypeptide or, when it folds into a stable functional structure, a protein.

Peptide bonds form via a condensation reaction between the carboxyl group of one amino acid and the amino group of another, releasing a single water molecule. The resulting bond is planar and partially rigid due to resonance, which constrains the conformations a peptide can adopt and is fundamental to its biological activity.

In the human body, peptides function as hormones, neurotransmitters, neuromodulators, growth factors, and signalling molecules. Outside the body, synthetic peptides are produced for pharmaceutical research, biochemical assays, and analytical reference standards.

The distinction between peptides and proteins is a matter of convention rather than chemistry. Both are chains of amino acids joined by peptide bonds. The accepted heuristics are:

• Length. Peptides are typically under 50 amino acids; proteins are longer. Insulin (51 residues) sits at the boundary and is treated as a small protein in some sources and a large peptide in others.
• Folding. Proteins have stable tertiary structures held together by hydrogen bonds, disulphide bridges, and hydrophobic interactions. Peptides may be unstructured in solution or adopt secondary structures only when bound to a target.
• Function. Proteins typically perform structural, catalytic, or transport roles. Peptides typically perform signalling roles.

The peptides supplied for research at Pillar Research range from tripeptides like GHK-Cu (3 residues) to 39-residue compounds like Retatrutide. All sit firmly within the conventional peptide range.

Research peptides are typically grouped by the receptor system they act upon or the biological pathway they investigate. The most commonly encountered classes include:

Every peptide is described by its amino acid sequence, conventionally written from the N-terminus (free amine) to the C-terminus (free carboxyl). The sequence determines mass, charge, hydrophobicity, secondary structure propensity, and ultimately receptor binding behaviour.

Modifications are common. Acetylation of the N-terminus, amidation of the C-terminus, the introduction of unnatural amino acids (such as Aib or D-amino acids), and the conjugation of fatty acid chains to extend half-life are all routine. Semaglutide, for instance, carries a C18 fatty acid via a γGlu-2xOEG linker; Retatrutide carries a C20 fatty diacid moiety. These modifications protect the peptide from proteolytic degradation and dramatically extend circulating half-life.

Cyclisation is another important modification. Linear peptides can be constrained into cyclic structures by disulphide bridges, head-to-tail amide bonds, or lactam bridges between side chains. MT-2 is a cyclic lactam analogue of α-MSH; the cyclisation locks the peptide into a bioactive conformation and resists enzymatic cleavage.

The vast majority of research peptides are produced by solid-phase peptide synthesis (SPPS), a method developed by Bruce Merrifield in the 1960s that earned him the Nobel Prize in Chemistry in 1984. In SPPS the C-terminal amino acid is anchored to an insoluble polymer resin, and successive amino acids are coupled stepwise toward the N-terminus.

Each cycle involves three core steps: deprotection of the N-terminal amine, coupling of the next protected amino acid using a coupling reagent (HBTU, HATU, DIC), and washing to remove excess reagents. Once the full sequence is assembled, the peptide is cleaved from the resin and side-chain protecting groups are removed, typically with trifluoroacetic acid.

The crude peptide is then purified — almost always by reversed-phase high-performance liquid chromatography (RP-HPLC) — to research-grade purity. Identity is confirmed by mass spectrometry, and final purity is reported as the percentage area under the main HPLC peak. Anything you would call research-grade should reach a minimum of 98% purity by HPLC; Pillar Research compounds average above 99%.

Larger peptides and peptides with complex modifications may also be produced via recombinant expression in E. coli or yeast, particularly when fatty-acid conjugation or specific glycosylation patterns are required. Hybrid approaches — recombinant expression followed by chemical modification — are common for the GLP-1 class.

Peptides are central to several active research fields. Metabolic research has been transformed by GLP-1 receptor agonist work, with single, dual, and triple agonists driving Phase 2 and Phase 3 clinical programmes globally — Australia included. Tissue remodelling research uses peptides such as BPC-157 and TB-500 in preclinical models of musculoskeletal injury, gastrointestinal repair, and angiogenic signalling.

Receptor pharmacology and binding-affinity studies use peptide ligands to map G-protein coupled receptor systems. The melanocortin (MC1R–MC5R), ghrelin (GHS-R), GHRH, and GLP-1 receptor families are all routinely interrogated using synthetic peptide tools. Neuropeptide research uses compounds like Semax to investigate BDNF expression, dopaminergic signalling, and serotonergic pathway modulation in animal and cell-culture models.

Beyond receptor work, peptides serve as analytical reference standards for mass spectrometry calibration, as positive controls in immunoassay development, and as tool compounds in structure-activity relationship (SAR) studies aimed at developing the next generation of peptide therapeutics. The published literature on these applications is extensive and accessible through PubMed, ChEMBL, and DrugBank.

In Australia, most peptides are classified as Schedule 4 (prescription-only) substances under the TGA Poisons Standard. They can be lawfully supplied for legitimate research purposes under the Research Use Only (RUO) framework. Some peptides — BPC-157 is the prominent example — are additionally listed in Appendix D of the Poisons Standard, which carries specific record-keeping and supply restrictions.

Research peptides supplied under the RUO framework are not therapeutic goods. They are not listed on the Australian Register of Therapeutic Goods (ARTG), are not approved or marketed for human or animal therapeutic use, and are intended exclusively for in vitro laboratory and educational research. Pillar Research operates strictly within this framework. For a full breakdown, see our guide to the Australian peptide regulatory landscape.

Reproducible research requires verifiable starting materials. Every research peptide should ship with a batch-specific Certificate of Analysis (COA) that documents three things: purity (typically by HPLC, expressed as % peak area), identity (typically by mass spectrometry, comparing observed mass to theoretical mass), and endotoxin level (by LAL assay, in EU/mg).

At Pillar Research every batch is tested by an independent third-party Australian laboratory. We do not rely on manufacturer COAs. Each compound page links the batch-specific COA for download. For a walkthrough of how to read these documents and the red flags that mark sub-standard supply, see our guide on how to read a peptide Certificate of Analysis.

Eleven research-grade peptides currently in stock, all independently HPLC and mass spectrometry verified, all dispatched same-day from Australian stock with batch-specific COAs.