What Are Peptides?

Peptides are short polymers of amino acids connected by peptide bonds. Depending on the counting convention, a peptide is anything from two amino acids (a dipeptide) up to around fifty residues — beyond that length, the same molecule is conventionally called a protein.

The peptide bond

The peptide bond is the defining structural feature. It is a covalent amide linkage formed when the carboxyl group of one amino acid condenses with the amine group of another, releasing a water molecule. The bond is partially double-bonded because of resonance between the nitrogen lone pair and the carbonyl system — this gives the bond planar geometry and restricts rotation, which in turn dictates how a peptide chain folds and signals.

Classification by length

• Oligopeptides — 2–20 amino acids. Includes most neuropeptides, regulatory hormones like oxytocin (9 residues), and the cosmetic tripeptide GHK.
• Polypeptides — 20–50 amino acids. Includes GHRH (44 residues), calcitonin (32), and insulin A and B chains (21 and 30 residues respectively).
• Proteins — conventionally >50 amino acids with defined tertiary structure.

The boundary is convention rather than chemistry. Insulin is sometimes called a peptide, sometimes a protein.

Endogenous vs synthetic peptides

Most peptides we discuss fall into one of three camps:

1. Endogenous peptides — produced by the body, often as signaling molecules (GLP-1, GHRH, ghrelin, substance P).
2. Analog peptides — synthetic sequences modified from an endogenous template to extend half-life, enhance receptor selectivity, or improve oral bioavailability (semaglutide modifies GLP-1; CJC-1295 modifies GHRH).
3. Novel synthetic peptides — designed sequences that may or may not correspond to anything endogenous (BPC-157 is derived from a gastric-juice protein but the 15-residue active fragment is synthetic).

Why peptides now

The modern peptide-therapy conversation is dominated by a handful of recent inflection points:

• Incretin success — semaglutide's STEP trials and tirzepatide's SURMOUNT trials demonstrated 15–22% weight reduction, reshaping obesity medicine.
• Cardiovascular outcomes — SELECT (2023) showed semaglutide reducing major adverse cardiovascular events by 20% in non-diabetic adults.
• Regulatory flux — FDA's 2023 section 503A additions restricted compounding of BPC-157, CJC-1295, and others in the US, reshaping the research-peptide landscape.

Peptides bridge the gap between small molecules (simple, orally bioavailable, often promiscuous) and biologics (highly specific, expensive, structurally complex). Their advantage is high target specificity at manageable synthesis cost; their disadvantage is typically poor oral bioavailability, driving the dominance of injectable administration.

Related entries

• GHRH — growth-hormone-releasing hormone
• GLP-1 — glucagon-like peptide-1
• Reconstitution — preparing a lyophilized peptide for injection