Peptide Glossary: A-Z Terminology Guide
Peptide science comes with its own language. Between the biochemistry, the pharmacology, the drug names, and the abbreviations, it's easy to get lost in terminology before you even reach the actual science.
Peptide science comes with its own language. Between the biochemistry, the pharmacology, the drug names, and the abbreviations, it's easy to get lost in terminology before you even reach the actual science.
This glossary covers the terms you'll run into when reading about peptides — from basic biochemistry all the way through clinical drug development. Each entry is written to give you a working understanding, not just a textbook definition. Where relevant, we've linked to deeper articles on specific topics.
Use the letter navigation to jump to what you need, or read straight through for a solid foundation in peptide vocabulary.
Table of Contents
A
Acetylation — The addition of an acetyl group (CH₃CO) to a molecule, often at the N-terminus of a peptide. Acetylation can protect a peptide from enzymatic degradation and alter its biological activity. Many synthetic peptides are acetylated to improve stability.
Active Pharmaceutical Ingredient (API) — The biologically active component in a drug product. When people talk about peptide API manufacturing, they mean producing the actual peptide molecule that does the therapeutic work, as opposed to the inactive fillers and delivery components.
Agonist — A molecule that binds to a receptor and activates it, triggering a biological response. Semaglutide is a GLP-1 receptor agonist — it binds to GLP-1 receptors and turns them on. Contrast with antagonist.
Alpha Helix (α-Helix) — A common secondary structure in proteins and peptides where the chain coils into a right-handed spiral. Hydrogen bonds between every fourth amino acid stabilize the helix. Many peptide hormones adopt alpha-helical structures when they bind to their receptors.
Amidation — The modification of a peptide's C-terminus to end with an amide group (-CONH₂) instead of a free carboxyl group (-COOH). About half of all known peptide hormones are amidated. This modification often improves receptor binding and protects against enzymatic breakdown.
Amino Acid — The building block of peptides and proteins. Each amino acid has a central carbon atom bonded to an amino group (NH₂), a carboxyl group (COOH), a hydrogen atom, and a variable side chain (R-group) that determines its properties. Twenty standard amino acids are encoded by DNA and used to build proteins. See our amino acids and peptide bonds guide for a full breakdown.
Amino Acid Residue — What each amino acid is called once it's been incorporated into a peptide chain. During peptide bond formation, a water molecule is released, so what remains in the chain is a "residue" of the original amino acid.
Amphipathic — Describes a molecule with both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions. Many antimicrobial peptides like LL-37 are amphipathic, which allows them to insert into bacterial cell membranes.
Antagonist — A molecule that binds to a receptor but does not activate it. Instead, it blocks the receptor and prevents agonists from binding. Some peptide drugs work as antagonists — for example, degarelix is a GnRH antagonist used in prostate cancer treatment.
Antimicrobial Peptide (AMP) — A peptide that can kill or inhibit the growth of bacteria, fungi, viruses, or parasites. AMPs are part of the innate immune system in virtually all multicellular organisms. LL-37 is one of the most studied human AMPs. Also called host defense peptides.
B
Bacteriostatic Water (BAC Water) — Sterile water containing 0.9% benzyl alcohol as a preservative. Used to reconstitute lyophilized peptides for injection. The preservative prevents bacterial growth, allowing multiple uses from the same vial.
Beta Sheet (β-Sheet) — A secondary structure in proteins and peptides formed when segments of the chain line up side by side, connected by hydrogen bonds. Beta sheets can be parallel (strands run the same direction) or antiparallel (strands run opposite directions).
Bioavailability — The fraction of an administered dose that reaches systemic circulation in active form. Intravenous injection provides 100% bioavailability. Subcutaneous injection typically delivers 20-100%. Oral peptides face severe bioavailability challenges — oral semaglutide (Rybelsus) achieves only about 1% bioavailability, but its high potency makes this clinically sufficient.
Bioequivalence — When two formulations of the same drug produce equivalent bioavailability and similar concentrations in the blood. Relevant when comparing brand-name peptide drugs to compounded or generic versions.
Biologically Active Peptide (Bioactive Peptide) — Any peptide that exerts a measurable effect on a biological system. This includes peptide hormones, neuropeptides, antimicrobial peptides, and peptides with antioxidant, anti-inflammatory, or other physiological activity.
Body-Protective Compound (BPC) — Refers to BPC-157, a synthetic peptide derived from a protein found in human gastric juice. Studied for potential wound-healing and tissue-repair properties, primarily in animal models.
Bond (Peptide Bond) — See Peptide Bond.
C
C-terminus (Carboxyl Terminus) — The end of a peptide chain that has a free carboxyl group (-COOH). By convention, peptide sequences are written from the N-terminus (left) to the C-terminus (right). Also called the carboxy-terminal end.
cAMP (Cyclic Adenosine Monophosphate) — A second messenger molecule involved in many peptide signaling pathways. When a peptide binds to a G protein-coupled receptor (GPCR), it can trigger adenylyl cyclase to produce cAMP, which in turn activates protein kinase A (PKA) and other downstream effectors. See how peptides work for more on signal transduction.
Cell-Penetrating Peptide (CPP) — A short peptide (usually 5-30 amino acids) that can cross cell membranes. CPPs are being researched as tools for drug delivery — they can carry cargo molecules (drugs, nucleic acids, proteins) into cells that would otherwise be inaccessible. TAT peptide, derived from HIV, is one of the most studied CPPs.
Certificate of Analysis (COA) — A document from a manufacturer or testing lab that reports the results of quality testing on a specific batch of peptide. A COA typically includes purity (by HPLC), molecular weight (by mass spectrometry), appearance, solubility, and sometimes endotoxin levels. Learn how to read a COA.
Chiral — Describes a molecule that exists in two mirror-image forms (like left and right hands). All amino acids except glycine are chiral, existing as L (left) or D (right) forms. Natural proteins use almost exclusively L-amino acids. D-amino acid substitution is a strategy to make peptides resistant to enzymatic degradation.
Compounding Pharmacy — A pharmacy that customizes medications for individual patients, including mixing peptide preparations. Peptide compounding has been under increased regulatory scrutiny, particularly after the FDA's actions regarding GLP-1 agonists.
Conjugation — Attaching another molecule to a peptide to modify its properties. Common conjugations include PEGylation (attaching polyethylene glycol to extend half-life), lipidation (attaching a fatty acid), and fluorescent labeling (for research imaging). Semaglutide uses fatty acid conjugation to bind albumin and extend its half-life.
Cyclic Peptide — A peptide in which the chain forms a ring structure, connected by bonds between the backbone or side chains. Cyclization often improves stability, receptor binding, and membrane permeability. As of 2024, 66 cyclic peptide drugs have been approved globally.
Cyclization — The chemical process of connecting the ends or internal parts of a linear peptide to form a ring. Methods include disulfide bonds, lactam bridges, and head-to-tail amide bonds. Cyclization protects peptides from exopeptidases (enzymes that chew from the ends).
Cysteine — An amino acid with a sulfhydryl (-SH) side chain that can form disulfide bonds with other cysteine residues. These bonds are critical for stabilizing the three-dimensional structure of many peptides and proteins, including insulin.
D
D-Amino Acid — The mirror-image form of a naturally occurring L-amino acid. Proteins in living organisms use almost exclusively L-amino acids, so enzymes that break down peptides (proteases) are optimized for L-forms. Substituting D-amino acids into a peptide can dramatically increase its resistance to enzymatic degradation.
Dalton (Da) — A unit of molecular mass, equivalent to one atomic mass unit. Peptides are typically measured in daltons. Most therapeutic peptides range from about 500 Da (small peptides) to about 10,000 Da. Insulin is about 5,800 Da.
Deamidation — A chemical degradation reaction where asparagine or glutamine residues in a peptide lose their amide group, converting to aspartic acid or glutamic acid. Deamidation is a common cause of peptide instability during storage.
Dipeptide — A peptide consisting of exactly two amino acids linked by one peptide bond. Carnosine (β-alanine + histidine) is a well-known naturally occurring dipeptide found in muscle tissue.
Disulfide Bond — A covalent bond formed between the sulfur atoms of two cysteine residues. Disulfide bonds act like molecular staples, holding parts of a peptide or protein together and stabilizing its three-dimensional shape. Insulin has three disulfide bonds — two connecting its A and B chains and one internal to the A chain.
Dose-Response Curve — A graph showing how a biological response changes as the dose of a peptide increases. This relationship is fundamental to understanding peptide pharmacology and determining therapeutic dosing ranges.
E
Endocrine — Relating to hormones and the glands that produce them. Many therapeutic peptides are endocrine agents — for example, semaglutide mimics the incretin hormone GLP-1, and gonadorelin mimics GnRH (gonadotropin-releasing hormone).
Endogenous — Originating from within the body. Endogenous peptides include hormones your body naturally produces (insulin, oxytocin, GLP-1). Contrast with exogenous (from outside the body).
Endopeptidase — An enzyme that cleaves peptide bonds in the interior of a peptide chain (as opposed to exopeptidases, which cut from the ends). Pepsin and trypsin are endopeptidases. Peptide drug designers must account for endopeptidase susceptibility.
Exogenous — Originating from outside the body. Synthetic or administered peptides are exogenous. When you inject BPC-157 or take oral semaglutide, you're introducing exogenous peptides.
Exopeptidase — An enzyme that removes amino acids from either the N-terminus (aminopeptidase) or C-terminus (carboxypeptidase) of a peptide chain. Cyclization and end-capping modifications protect peptides from exopeptidases.
F
Fmoc (Fluorenylmethyloxycarbonyl) — A protecting group widely used in solid-phase peptide synthesis (SPPS). Fmoc chemistry is the dominant method for synthesizing research and therapeutic peptides. It protects the amino group during synthesis and is removed with a mild base (piperidine) after each coupling step.
Folding — The process by which a linear peptide or protein chain assumes its three-dimensional structure. Proper folding is essential for biological activity. Misfolded proteins and peptides are linked to diseases like Alzheimer's (amyloid-beta) and Parkinson's.
G
GH (Growth Hormone) — See Human Growth Hormone.
GHRH (Growth Hormone-Releasing Hormone) — A peptide hormone produced by the hypothalamus that stimulates the pituitary gland to release growth hormone. CJC-1295 and sermorelin are synthetic GHRH analogs.
GHRP (Growth Hormone-Releasing Peptide) — A class of synthetic peptides that stimulate growth hormone release by acting on the ghrelin receptor (GHS-R). Examples include ipamorelin, GHRP-2, and GHRP-6. Unlike GHRH analogs, GHRPs act through a different receptor pathway.
GIP (Glucose-Dependent Insulinotropic Polypeptide) — An incretin hormone released from the gut after eating. Tirzepatide is a dual GIP/GLP-1 receptor agonist, targeting both incretin pathways simultaneously.
GLP-1 (Glucagon-Like Peptide-1) — An incretin hormone released from intestinal L-cells after eating. It stimulates insulin secretion, suppresses glucagon, slows gastric emptying, and reduces appetite. GLP-1 receptor agonists like semaglutide and tirzepatide are among the most commercially successful peptide drugs.
Glucagon — A 29-amino-acid peptide hormone produced by alpha cells of the pancreas. It raises blood glucose levels by promoting glycogen breakdown in the liver. Works in opposition to insulin.
G Protein-Coupled Receptor (GPCR) — A large family of cell-surface receptors that work through G proteins. GPCRs represent about 80% of all known receptors and are the target of most peptide hormones and neuropeptides. When a peptide binds a GPCR, it triggers intracellular signaling cascades involving second messengers like cAMP and calcium. See how peptides work.
H
Half-Life — The time it takes for the concentration of a peptide in the blood to decrease by half. Natural peptides often have very short half-lives (minutes) because proteases break them down quickly. Drug designers extend half-life through albumin binding (semaglutide), PEGylation, or structural modifications.
HPLC (High-Performance Liquid Chromatography) — The standard analytical method for determining peptide purity. HPLC separates a sample's components and measures their relative amounts. Research-grade peptides are typically >95% pure by HPLC; pharmaceutical-grade peptides meet even stricter standards.
Human Growth Hormone (HGH) — A 191-amino-acid protein (technically at the upper limit of peptide size) secreted by the anterior pituitary gland. HGH stimulates growth, cell reproduction, and regeneration. Growth hormone secretagogues like ipamorelin and CJC-1295 stimulate the body's own HGH production.
Hydrolysis — The breaking of a chemical bond by adding water. Peptide bond hydrolysis is how peptides and proteins are broken down — an enzyme inserts a water molecule across the peptide bond, splitting it into two pieces. In the absence of enzymes, peptide bond hydrolysis is extremely slow (half-life of 350-600 years per bond at 25°C).
Hydrophilic — "Water-loving." Describes amino acids or molecular regions that interact favorably with water. Polar and charged amino acids (serine, lysine, glutamic acid) are hydrophilic and tend to face outward on protein surfaces.
Hydrophobic — "Water-fearing." Describes amino acids or molecular regions that avoid water. Nonpolar amino acids (leucine, valine, phenylalanine) are hydrophobic and tend to pack into protein interiors. Hydrophobic interactions are a major driving force in protein folding.
I
Incretin — A hormone released from the gut in response to food that stimulates insulin secretion. The two main incretins are GLP-1 and GIP. Incretin-based peptide drugs have transformed the treatment of type 2 diabetes and obesity.
Insulin — A 51-amino-acid peptide hormone produced by beta cells of the pancreas. Insulin regulates blood glucose by promoting glucose uptake into cells. It consists of an A chain (21 amino acids) and a B chain (30 amino acids) connected by two disulfide bonds. Insulin was the first protein to have its amino acid sequence determined (Frederick Sanger, 1951) and the first to be synthesized chemically.
Intramuscular (IM) — A route of injection into muscle tissue. Some peptides are administered intramuscularly, though subcutaneous injection is more common for most therapeutic peptides.
Intranasal — Delivery through the nasal mucosa. Several peptides are administered intranasally, including oxytocin and desmopressin. The nasal route avoids first-pass liver metabolism and can provide brain-targeted delivery for certain peptides.
In Vitro — "In glass." Refers to experiments performed outside a living organism, typically in test tubes, petri dishes, or cell cultures. Many peptide studies begin with in vitro work before progressing to animal models.
In Vivo — "In the living." Refers to experiments or effects occurring within a living organism. In vivo studies in animal models or human clinical trials are necessary to confirm results observed in vitro.
K
Kilodalton (kDa) — A unit of molecular mass equal to 1,000 daltons. Larger peptides and small proteins are often described in kilodaltons. Most therapeutic peptides fall between 0.5 and 10 kDa.
L
Ligand — Any molecule that binds to a receptor. Peptide hormones, neuropeptides, and peptide drugs are all ligands for their respective receptors. The term is neutral — it doesn't specify whether the ligand activates (agonist) or blocks (antagonist) the receptor.
Linear Peptide — A peptide chain with free N-terminal and C-terminal ends, as opposed to a cyclic peptide that forms a ring. Most naturally occurring peptide hormones are linear.
Lipidation — The attachment of a fatty acid or lipid chain to a peptide. Lipidation can improve a peptide's ability to bind to albumin (extending its half-life in the blood) and can improve membrane permeability. Semaglutide uses a C18 fatty acid chain to achieve once-weekly dosing.
Lyophilization (Freeze-Drying) — The process of removing water from a peptide solution by freezing it and then reducing pressure to allow the ice to sublimate directly into vapor. Lyophilized peptides are stored as a dry powder, which is more stable than liquid formulations. They must be reconstituted with bacteriostatic water or saline before use.
M
Mass Spectrometry (MS) — An analytical technique that measures the mass-to-charge ratio of ions. Used to confirm the molecular weight and identity of synthesized peptides. MALDI-TOF and ESI-MS are common mass spectrometry methods for peptide analysis.
Melanocortin — A family of peptide hormones derived from pro-opiomelanocortin (POMC) that act on melanocortin receptors (MC1R through MC5R). Alpha-MSH (alpha-melanocyte-stimulating hormone) regulates skin pigmentation. Bremelanotide (PT-141) targets MC4R for sexual dysfunction.
Metabolism (Peptide) — The enzymatic breakdown of peptides in the body. Peptides are metabolized primarily by proteases and peptidases in the blood, liver, kidneys, and gut. This rapid metabolism is why most peptides have short half-lives and why oral delivery is so challenging.
Motif — A short, conserved sequence of amino acids that recurs across different peptides or proteins and often has a specific biological function. The RGD motif (arginine-glycine-aspartic acid), for example, is found in many peptides that bind to integrin receptors.
N
N-terminus (Amino Terminus) — The end of a peptide chain that has a free amino group (-NH₂). By convention, peptide sequences are written starting from the N-terminus. Also called the amino-terminal end.
Neuropeptide — A peptide that acts as a signaling molecule in the nervous system. Neuropeptides include endorphins, enkephalins, substance P, oxytocin, and many others. They typically function as neurotransmitters or neuromodulators, often at much lower concentrations than classical neurotransmitters.
Non-Proteinogenic Amino Acid — An amino acid that is not among the 20 standard amino acids encoded by the genetic code. Examples include ornithine, citrulline, and 2-aminoisobutyric acid (Aib). Non-proteinogenic amino acids are sometimes incorporated into synthetic peptides to improve stability or alter activity. Semaglutide contains Aib at position 8.
O
Oligopeptide — A short peptide, typically fewer than 20 amino acid residues. Dipeptides (2), tripeptides (3), tetrapeptides (4), and pentapeptides (5) are all oligopeptides. Many skincare peptides like Argireline (a hexapeptide) and Matrixyl (a pentapeptide) fall into this category.
Oral Bioavailability — The percentage of an orally administered peptide that reaches systemic circulation. For unmodified peptides, this is typically less than 1% due to proteolytic degradation in the gut and poor membrane permeability. Oral semaglutide (Rybelsus) achieves about 1% bioavailability using SNAC, a permeation enhancer.
Oxidation — A chemical reaction involving the loss of electrons. In peptide chemistry, oxidation is important for forming disulfide bonds between cysteine residues. Unwanted oxidation of methionine residues is a common degradation pathway during peptide storage.
P
PEGylation — The covalent attachment of polyethylene glycol (PEG) chains to a peptide or protein. PEGylation increases the molecule's size (reducing kidney clearance), shields it from proteases, and extends its half-life. Peginesatide was a PEGylated peptide used for anemia, though it was later withdrawn from market.
Pentapeptide — A peptide consisting of five amino acids. Matrixyl (palmitoyl pentapeptide-4) is a well-known skincare pentapeptide that signals collagen production.
Peptidase — An enzyme that breaks peptide bonds. Peptidases (also called proteases or proteolytic enzymes) are found throughout the body — in the stomach (pepsin), small intestine (trypsin, chymotrypsin), blood, and cell surfaces. They are the primary reason peptides have short half-lives.
Peptide — A chain of 2 to approximately 50 amino acids linked by peptide bonds. The boundary between "peptide" and "protein" is somewhat arbitrary — some definitions put it at 50 amino acids, others at 100. Functionally, peptides tend to be smaller, more flexible, and less structurally complex than proteins. Learn more in our what are peptides guide.
Peptide Bond — The covalent bond formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH₂) of another, with the release of a water molecule. Peptide bonds have partial double-bond character, making them rigid and planar. This rigidity shapes the backbone geometry of all peptides and proteins. See amino acids and peptide bonds.
Peptide Hormone — A peptide that is secreted into the blood and acts on distant target tissues by binding to specific receptors. Examples include insulin, glucagon, GLP-1, GnRH, oxytocin, and ACTH.
Peptide Library — A collection of peptides with systematically varied sequences, used for screening and drug discovery. Combinatorial peptide libraries can contain millions of different sequences, allowing researchers to identify peptides with specific binding or activity profiles.
Peptidomimetic — A molecule designed to mimic the structure and function of a natural peptide but with improved drug-like properties (better stability, bioavailability, or selectivity). Peptidomimetics can be small molecules, modified peptides, or hybrid structures.
Pharmacokinetics (PK) — The study of how the body processes a drug — absorption, distribution, metabolism, and excretion (ADME). Peptide pharmacokinetics typically feature rapid absorption after subcutaneous injection, limited distribution due to size, enzymatic metabolism, and renal excretion of fragments.
Polypeptide — A single chain of amino acids, typically more than 20-50 residues long. Proteins are made of one or more polypeptide chains. The distinction between a large peptide and a small polypeptide is fuzzy and context-dependent.
Post-Translational Modification (PTM) — Chemical modifications to a peptide or protein after it has been synthesized by the ribosome. Common PTMs include phosphorylation, glycosylation, acetylation, and amidation. PTMs can alter a peptide's activity, stability, localization, or interactions.
Protease — See Peptidase.
Proteinogenic — "Protein-creating." Refers to the 20 standard amino acids that are encoded by the genetic code and incorporated into proteins during translation. Also sometimes called "coded" amino acids.
Purity — In peptide chemistry, the percentage of the target peptide in a sample, as measured by HPLC. Research-grade peptides are typically >95% pure. Pharmaceutical-grade peptides must meet even higher standards, often >98%.
R
Receptor — A protein on the cell surface (or sometimes inside the cell) that binds a specific ligand and triggers a cellular response. Most peptide receptors are G protein-coupled receptors (GPCRs), though some are receptor tyrosine kinases or ion channels.
Reconstitution — The process of dissolving a lyophilized (freeze-dried) peptide powder in a solvent, typically bacteriostatic water, to create an injectable solution. Proper reconstitution technique — gentle swirling, correct solvent volume, sterile handling — is critical for maintaining peptide integrity. See our step-by-step reconstitution guide.
Residue — See Amino Acid Residue.
S
Second Messenger — A small intracellular molecule that relays and amplifies signals from cell-surface receptors. When a peptide binds a GPCR, the receptor doesn't directly produce the cellular response — it triggers the production of second messengers like cAMP, IP3, DAG, or calcium ions, which then activate downstream effectors. This cascade allows a single peptide binding event to produce a large cellular response.
Secretagogue — A substance that stimulates secretion. Growth hormone secretagogues (GHS) are compounds that stimulate growth hormone release from the pituitary. MK-677 (ibutamoren) is technically not a peptide but is often discussed alongside peptide GHS like ipamorelin.
Selectivity — The degree to which a peptide binds to its target receptor over other receptors. High selectivity means fewer off-target effects and side effects. Ipamorelin is considered highly selective for the growth hormone secretagogue receptor compared to earlier GHRPs.
Sequence — The specific order of amino acids in a peptide chain, read from N-terminus to C-terminus. The sequence determines the peptide's structure, function, and biological activity. Even a single amino acid change can dramatically alter a peptide's properties.
Side Chain (R-Group) — The variable chemical group attached to the alpha carbon of each amino acid. The side chain determines whether an amino acid is hydrophobic, hydrophilic, charged, aromatic, or has special properties (like cysteine's ability to form disulfide bonds).
Signal Peptide — A short peptide sequence (typically 16-30 amino acids) at the N-terminus of a newly synthesized protein that directs it to the proper cellular compartment. Signal peptides are cleaved off after the protein reaches its destination. Not the same as a signaling peptide.
SNAC (Sodium N-[8-(2-Hydroxybenzoyl) Amino] Caprylate) — A permeation enhancer used in the oral formulation of semaglutide (Rybelsus). SNAC raises the local pH in the stomach, protects semaglutide from pepsin, and facilitates absorption across the gastric lining.
Solid-Phase Peptide Synthesis (SPPS) — The dominant method for manufacturing synthetic peptides. In SPPS, the first amino acid is anchored to an insoluble resin bead, and subsequent amino acids are added one at a time in a cycle of deprotection and coupling reactions. After the sequence is complete, the peptide is cleaved from the resin. Developed by Robert Bruce Merrifield in 1963, which earned him the Nobel Prize in Chemistry in 1984.
Subcutaneous (SC or SubQ) — A route of injection into the fatty tissue just beneath the skin. Most therapeutic peptides are administered subcutaneously because it provides good bioavailability (typically 70-100%), is self-administrable, and allows for slow, sustained absorption. See our injection technique guide.
T
Tertiary Structure — The overall three-dimensional shape of a single polypeptide chain, determined by interactions between amino acid side chains (hydrophobic packing, hydrogen bonds, ionic bonds, disulfide bridges). Tertiary structure determines biological activity — a misfolded peptide often cannot bind its receptor.
Tetrapeptide — A peptide consisting of four amino acids. Several cosmetic peptides are tetrapeptides, including acetyl tetrapeptide-5 (Eyeseryl), used in under-eye products.
Thymosin — A family of peptides originally isolated from the thymus gland. Thymosin beta-4 (TB-500) is studied for tissue repair, while thymosin alpha-1 is used clinically as an immune modulator in some countries.
Topical — Applied to the skin surface. Topical peptides are common in skincare — copper peptides (GHK-Cu), Matrixyl, and Argireline are all used in topical formulations. Topical delivery avoids systemic effects but limits peptide penetration to the upper skin layers unless penetration enhancers are used.
Tripeptide — A peptide consisting of three amino acids. Glutathione (glutamic acid-cysteine-glycine) is the body's primary intracellular antioxidant and is a tripeptide. Several cosmetic peptides are also tripeptides.
U
Unnatural Amino Acid — See Non-Proteinogenic Amino Acid. Also called non-natural or non-coded amino acids. Used in synthetic peptide design to improve properties like stability, selectivity, and bioavailability.
V
Vasoactive Intestinal Peptide (VIP) — A 28-amino-acid neuropeptide that acts as a vasodilator, stimulates water and electrolyte secretion in the gut, and has immunomodulatory properties. Studied for potential applications in inflammatory and respiratory conditions.
FAQ
What's the difference between a peptide and a protein?
Size and complexity. Peptides are generally shorter chains (2-50 amino acids), while proteins are larger (50+ amino acids) and fold into complex three-dimensional structures. The boundary is fuzzy — insulin (51 amino acids) is sometimes called a peptide, sometimes a small protein. Functionally, peptides tend to act as signaling molecules, while proteins have broader structural and enzymatic roles. Read more in what are peptides.
What does "reconstitution" mean?
Reconstitution is dissolving a freeze-dried (lyophilized) peptide powder in a liquid, usually bacteriostatic water, to create an injectable solution. Most research and therapeutic peptides are shipped as lyophilized powder because it's more stable. Our reconstitution guide walks through the process step by step.
What's the difference between a GHRH analog and a GHRP?
Both stimulate growth hormone release, but through different receptor pathways. GHRH analogs (like CJC-1295 and sermorelin) mimic the body's natural growth hormone-releasing hormone and act on GHRH receptors. GHRPs (like ipamorelin and GHRP-6) act on the ghrelin/GHS receptor. They are sometimes used in combination for a synergistic effect.
What does "peptide purity" mean?
Purity refers to the percentage of the target peptide in a sample, measured by HPLC. A purity of 98% means 98% of the sample is the intended peptide, and 2% is impurities (incomplete sequences, deletion sequences, or chemical byproducts). Higher purity is important for both research accuracy and safety.
Why do most peptides need to be injected?
Because the digestive system destroys them. Stomach acid and digestive enzymes (proteases) break peptide bonds efficiently — that's their job with dietary protein. Oral bioavailability of unmodified peptides is typically less than 1%. Some peptides use special formulations (like oral semaglutide with SNAC) or structural modifications (cyclization, D-amino acids) to survive the gut, but injection remains the standard for most therapeutic peptides.
The Bottom Line
This glossary covers the terms you're most likely to encounter when reading about peptide science, therapy, and skincare. Bookmark it and come back when you hit an unfamiliar term. As the peptide field grows — and it is growing fast, with a market projected to exceed $100 billion by 2033 — the vocabulary will grow with it.
For a deeper understanding of the science behind these terms, start with what are peptides, then move to how peptides work for the mechanistic details, and amino acids and peptide bonds for the chemistry.
References
- Muttenthaler M, et al. "Trends in peptide drug discovery." Nature Reviews Drug Discovery. 2021;20(4):309-325. https://pubmed.ncbi.nlm.nih.gov/33536635/
- Wang L, et al. "Therapeutic peptides: current applications and future directions." Signal Transduction and Targeted Therapy. 2022;7:48. https://www.nature.com/articles/s41392-022-00904-4
- IUPAC-IUB Joint Commission on Biochemical Nomenclature. "Nomenclature and Symbolism for Amino Acids and Peptides." https://iupac.qmul.ac.uk/AminoAcid/
- National Center for Biotechnology Information. "Biochemistry, Peptide." StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK562260/
- Merrifield RB. "Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide." Journal of the American Chemical Society. 1963;85(14):2149-2154. https://pubmed.ncbi.nlm.nih.gov/14044142/
- Al Musaimi O. "2024 FDA TIDES (Peptides and Oligonucleotides) Harvest." Pharmaceuticals. 2025;18(3):291. https://pmc.ncbi.nlm.nih.gov/articles/PMC11945313/
- Bachem. "Peptides & Amino Acids for Beginners." https://www.bachem.com/knowledge-center/peptide-guide/peptides-and-amino-acids-for-beginners/
- Tocris Bioscience. "Peptide Nomenclature Guide." https://www.tocris.com/resources/peptide-nomenclature-guide
- National Human Genome Research Institute. "Peptide." https://www.genome.gov/genetics-glossary/Peptide
- Henninot A, et al. "The Current State of Peptide Drug Discovery: Back to the Future?" Journal of Medicinal Chemistry. 2018;61(4):1382-1414. https://pubmed.ncbi.nlm.nih.gov/28737935/