Peptides: What Are Peptides?

Peptides are short chains of amino acids — typically 2 to 50 residues — linked by covalent peptide bonds (–CO–NH–). This places them in a molecular weight range of roughly 500 to 5000 Daltons, occupying a pharmacological middle ground between small-molecule drugs and full proteins.

The Three-Tier Molecular Hierarchy

Biochemistry divides biologically active molecules into three tiers based on size and structure:

• Small molecules: MW <500 Da, typically single-ring or short-chain organic compounds. Orally bioavailable via passive diffusion. Examples: aspirin (180 Da), caffeine (194 Da), testosterone (288 Da).
• Peptides: MW 500–5000 Da, 2–50 amino acids. Too large for simple passive diffusion; degraded by peptidases in the gut and blood. Examples: oxytocin (1007 Da, 9 AA), BPC-157 (~1419 Da, 15 AA), semaglutide (~4114 Da, 31 AA).
• Proteins: MW >5000 Da, >50 amino acids. Complex tertiary structure; cannot cross gut wall intact; require injectable delivery. Examples: insulin (~5808 Da, 51 AA — borderline), growth hormone (~22,000 Da, 191 AA).

Class	MW Range	Amino Acids	Oral BA Route	Degradation	Examples
Small molecule	<500 Da	N/A	Passive diffusion	Hepatic CYP450	Aspirin, caffeine, testosterone
Peptide	500–5,000 Da	2–50	Requires enhancer	Gut peptidases, serum proteases	BPC-157, ipamorelin, oxytocin
Protein	>5,000 Da	>50	Not orally available	Proteolysis, denaturation	Insulin, GH, albumin
Antibody	>140,000 Da	>1000	Not orally available	Proteolysis	Adalimumab, trastuzumab
Amino acid	<200 Da	1	Passive + transporter	Hepatic metabolism	Leucine, glycine
Cyclic peptide	500–2,000 Da	4–12	Variable (some oral)	Partial resistance	Cyclosporin A

Why Peptide Bonds Matter

The peptide bond forms when the carboxyl group (–COOH) of one amino acid condenses with the amino group (–NH₂) of the next, releasing water. This creates a partial double bond (bond length 1.33 Å) that restricts rotation, giving peptide chains their characteristic planarity and secondary structure potential (alpha helices, beta sheets).

Every peptide has a free N-terminus and a free C-terminus. Modifications at these termini — acetylation, amidation — can dramatically alter stability and half-life by blocking aminopeptidase and carboxypeptidase attack respectively.

Why Size Determines Delivery Route

Peptides in the 500–5000 Da range face a dual barrier problem: they are too large to passively diffuse through lipid bilayers (Lipinski’s Rule of Five breaks down above ~500 Da), yet too small to survive the GI tract intact without formulation protection. Gut peptidases (trypsin, chymotrypsin, pepsin, brush-border peptidases) cleave most unmodified peptides within minutes of GI exposure [PMID 25450112].

This is why virtually all therapeutic and research peptides are administered by injection — subcutaneous, intramuscular, or intravenous — bypassing the GI barrier entirely.

Market Context

Approximately 80 peptide-based drugs had received global regulatory approval by the mid-2010s, with roughly 150 more in clinical trials [PMID 25450112]. The market has grown substantially since: GLP-1 receptor agonists (semaglutide, tirzepatide) became among the highest-revenue drugs globally by 2024, demonstrating that the delivery challenge can be overcome with the right formulation or receptor pharmacology.