Peptide Glossary

Amino acid

The building blocks of peptides and proteins. Twenty standard amino acids appear in human biology, each with different chemical properties.

Peptide

A chain of amino acids joined end to end. Shorter than a protein — the boundary is fuzzy, but chains under about fifty residues are usually called peptides.

Residue

One amino acid within a chain. A "fifteen-residue peptide" is a chain of fifteen amino acids.

Sequence

The order of amino acids in a peptide, written from one end to the other. The sequence determines everything else about the molecule.

N-terminus / C-terminus

The two ends of a peptide chain. Chemically different from each other, and each can be modified to change how the molecule behaves.

L- and D-amino acids

Mirror-image forms of the same amino acid. Biology uses the L form almost exclusively, so substituting a D form makes a peptide unrecognisable to the enzymes that would otherwise break it down.

Amidation

Capping the C-terminal end with an amide group. Removes a negative charge and slows degradation.

Acetylation

Adding an acetyl group, usually at the N-terminal end. Sometimes a stabilising modification, sometimes part of the natural molecule.

Acylation

Attaching a fatty acid chain. Commonly used to make a peptide bind to albumin in blood, which greatly extends how long it lasts.

Disulphide bridge

A chemical bond between two cysteine residues that ties parts of a peptide together, holding a loop or a fold in place.

Cyclisation

Joining two points of a peptide to close it into a ring. Makes the molecule more rigid and much harder for enzymes to break down.

Analogue

A molecule deliberately modelled on another, usually a natural hormone, with changes intended to improve stability or selectivity.

Fragment

A piece of a larger protein, made as a standalone molecule. Used when one region of a protein carries the activity of interest.

Molecular weight

The mass of one molecule, given in daltons or g/mol. A useful identity check: if the stated weight is wrong, the molecule is wrong.

Molecular formula

A count of each type of atom in the molecule, such as C14H22N4O9. Two different molecules can share a formula, so it confirms composition rather than identity.

CAS number

A unique registry number assigned to a chemical substance. The most reliable way to confirm which compound is being discussed, since names vary.

Receptor

A protein, usually on a cell surface, that recognises a specific signalling molecule and triggers a response inside the cell when it binds.

Agonist

A molecule that binds a receptor and activates it, producing the same kind of response as the natural signal.

Antagonist

A molecule that binds a receptor without activating it, blocking the natural signal from getting through.

Selectivity

How narrowly a compound acts on its intended target. A non-selective compound activates several related receptors, producing several effects at once.

GPCR

G-protein coupled receptor — the largest family of cell-surface receptors, and the target of most peptides in this catalogue.

Second messenger

A molecule inside the cell, such as cyclic AMP or calcium, that carries a signal onward after a receptor is activated.

Secretagogue

A compound that causes a gland to release a hormone it already makes, rather than supplying that hormone directly.

Half-life

The time taken for half of an administered compound to be cleared. Determines how long an effect lasts and how often a study administers it.

Pharmacokinetics

What the body does to a compound — how it is absorbed, distributed, broken down and cleared. Distinct from what the compound does to the body.

DPP-4

An enzyme in blood that cleaves certain peptides near one end, inactivating them within minutes. Several compounds here are modified specifically to resist it.

Albumin binding

Attaching to the most abundant protein in blood. A bound peptide is too large for the kidneys to filter out, so it circulates far longer.

Endogenous

Produced naturally within the body. The opposite, exogenous, means introduced from outside.

Pulsatile

Released in discrete bursts rather than continuously. Growth hormone is normally secreted this way, and whether a compound preserves that pattern matters.

Lyophilised

Freeze-dried. Water is removed under vacuum, leaving a stable powder that is reconstituted before use.

Reconstitution

Dissolving a freeze-dried powder back into liquid.

Bacteriostatic water

Sterile water containing a preservative that prevents bacterial growth, allowing a vial to be used more than once.

Diluent

The liquid used to dissolve a lyophilised powder.

Freeze–thaw cycle

One round of freezing and thawing. Each cycle damages peptides, which is why solutions are aliquoted rather than repeatedly refrozen.

Oxidation

A chemical degradation reaction. Methionine and tryptophan residues are especially prone to it, and each event adds detectable mass.

HPLC

High-performance liquid chromatography — the standard method for separating a sample into its components and measuring how much is the intended compound.

Mass spectrometry

Measures molecular weight very precisely. Confirms that a sample is the molecule it should be, and detects modifications that change mass.

NMR

Nuclear magnetic resonance — resolves how atoms are arranged, not just what they weigh. The definitive identity test for small molecules.

Purity

The proportion of a sample that is the intended compound, usually determined by HPLC and expressed as a percentage.

Deletion sequence

A peptide missing one or more residues, produced when a synthesis step does not go to completion. Chemically similar to the target and the main impurity in long peptides.

Certificate of analysis

A document reporting the analytical results for a specific batch. Useful only when it identifies the batch and states which methods were used.

In vitro

In cell culture or a test tube, outside a living organism.

In vivo

In a living organism.

Preclinical

Research conducted before any human studies — typically cell culture and animal models.

Randomised

Subjects assigned to groups by chance rather than by choice, so the groups differ only in the treatment received.

Placebo-controlled

Compared against an inactive substance, so that the effect measured is the compound rather than the act of administering something.

Double-blind

Neither the subjects nor the researchers know who received which treatment until the study ends.

Knockout model

An animal genetically engineered to lack a specific gene. Used to test whether a compound needs that gene product to work.

Case report

A published description of one individual. Establishes that something happened, not that the compound caused it.

Replication

An independent group repeating a finding and getting the same result. The strongest evidence a result is real.

Research use only

Supplied for laboratory research. Not a medicine, not tested or approved for use in humans, and not to be used as one.