Peptide research glossary: terms, units and identifiers

What do the terms in peptide research mean?

Agonist

A molecule that binds a receptor and activates it. A full agonist produces the maximal response the receptor can give; a partial agonist produces less.

Amidation

Replacement of the C-terminal carboxylic acid of a peptide with an amide (-NH2). Written -NH2 at the end of a sequence, as in Lys-Pro-Val-NH2.

Acetylation (N-terminal)

Addition of an acetyl group to the N-terminus of a peptide, written Ac- at the start of a sequence.

Amino acid residue

One amino acid as it appears inside a peptide chain, after loss of water in the peptide bond. A peptide of 15 residues has 15 amino acids.

Average molecular weight

The mass of one mole of a substance in g/mol, calculated from the molecular formula with standard atomic weights. It is the figure printed on most certificates and databases.

CAS Registry Number

A numeric identifier assigned by the Chemical Abstracts Service to a specific substance. One compound can have separate numbers for the free form and each salt.

Certificate of analysis (CoA)

A document from a testing laboratory that reports the results of tests on one lot of a material, such as HPLC purity and mass spectrometry identity. It applies to the lot named on it.

Counterion

The ion that balances the charge of a peptide in a solid, for example acetate or trifluoroacetate. It adds to the weight of the solid without being part of the peptide.

Cyclic peptide

A peptide whose chain is closed into a ring, for example by a disulfide bond between two cysteines or a lactam bridge between side chains.

D-amino acid

The mirror-image form of an amino acid. Written with a D- prefix, as in D-Ala. It has the same formula as the L-form but a different three-dimensional arrangement.

DOI

Digital Object Identifier, a persistent link to a published paper. Written as 10.xxxx/..., resolved at doi.org.

Evidence design

The type of study behind a finding: in vitro (cells or isolated molecules), animal study, human study, or a review that summarises other work. This hub labels every cited paper with one of these.

G-protein-coupled receptor (GPCR)

A membrane receptor that signals through intracellular G proteins. The growth hormone secretagogue, GHRH, GLP-1, GIP, glucagon and melanocortin receptors are all GPCRs.

HPLC

High-performance liquid chromatography. A method that separates the components of a sample; for a peptide it is used to report purity as the share of the main peak by area.

Half-life

The time the concentration of a substance needs to fall by half in a stated system, such as blood plasma. It is a property of a measurement, not of the substance alone.

Isotopic mass (monoisotopic)

The mass of a molecule calculated from the most abundant isotope of each element. Mass spectrometers report it; it differs slightly from the average molecular weight.

Lipidation

Attachment of a fatty acid chain to a peptide, used to promote binding to albumin. Semaglutide, tirzepatide and retatrutide carry a fatty diacid chain.

Lot number

The identifier of one production batch. A certificate of analysis is valid for the lot it names, so the number on the vial and the number on the certificate must match.

Lyophilised

Freeze-dried. A lyophilised peptide is a dry solid that is stable for storage and is dissolved in a solvent before laboratory work.

Mass concentration

The mass of a dissolved substance per volume of solution, in mg/mL or ug/mL. It is calculated by dividing the mass by the volume. Peptency's concentration calculator outputs mass concentration only.

Mass spectrometry (LC-MS)

A method that measures the mass of molecules, used to confirm that the main component of a sample has the expected molecular weight, which is an identity check.

Net peptide content

The share of a weighed solid that is peptide, after counterions and bound water. It is lower than 100 percent, and it is different from purity.

PMID

PubMed identifier. A number assigned to each record in the PubMed database of the US National Library of Medicine. Every study on this hub is listed with its PMID and links to its PubMed record.

Purity (HPLC)

The percentage of the HPLC signal that belongs to the main component. It says how much of the peptide in the sample is the expected sequence, not how much of the solid is peptide.

Receptor

A protein, usually in a cell membrane, that a signalling molecule binds. Binding changes the receptor's state and starts a response inside the cell.

Research use only

A designation stating that a material is supplied for laboratory and analytical work and not for use in people or animals. Regulators judge intended use on the totality of what a seller says, shows and ships, not on the label alone.

Selectivity

The degree to which a ligand acts at one receptor and not at related ones. A ligand described as unselective acts at several receptors of a family.

Sequence

The order of amino acids in a peptide, written from the N-terminus to the C-terminus, in three-letter codes joined by hyphens or in one-letter code.

Systematic review

A review that searches for and appraises studies by a stated method. It is distinct from a narrative review, which summarises the literature without a fixed search.

The concentration calculator outputs mass concentration only.