Glossary of Peptide Research Terms

A quick reference to terms that appear on Certificates of Analysis, product specifications, and in the peptide research literature.

Acetate salt

A peptide form in which acetate is the counter-ion balancing the peptide’s positive charges. Often preferred over TFA salts for biological research.

Aggregation

Self-association of peptide molecules into larger complexes, which can reduce solubility and alter behavior in assays.

Amidation (C-terminal)

Replacement of the C-terminal carboxylic acid with an amide (-NH₂), common in natural and synthetic peptides.

Amino acid analysis (AAA)

A method that hydrolyzes a peptide into its amino acids and quantifies them. Used to confirm composition and determine net peptide content.

CAS Registry Number

A unique identifier assigned to a specific chemical substance by the Chemical Abstracts Service. Different salt or complex forms receive different numbers.

Certificate of Analysis (COA)

A lot-specific document reporting the test results (identity, purity, and other attributes) for a batch of material. See How to Read a Peptide COA.

Chromatogram

The graph produced by HPLC, showing detector response over time. Each peak represents a separated component.

Counter-ion

The ion paired with a charged peptide in its salt form, commonly acetate or trifluoroacetate. Counter-ions add weight that is not peptide.

Cyclic peptide

A peptide containing a ring structure, formed by a disulfide, lactam, or head-to-tail bond.

D-amino acid

The mirror-image form of a natural (L) amino acid, often used to increase resistance to enzymatic breakdown.

Deamidation

Conversion of asparagine or glutamine side chains into acidic forms, a common degradation pathway.

Deletion sequence

A synthesis impurity missing one or more residues from the intended sequence.

Desiccant

A drying agent (such as silica gel) used to keep stored materials free of moisture.

Disulfide bond

A covalent link between the sulfur atoms of two cysteine residues.

Endotoxin

Lipopolysaccharide from the outer membrane of Gram-negative bacteria, measured in some materials because it can interfere with cell-based experiments.

ESI-MS

Electrospray ionization mass spectrometry, a common technique for confirming peptide identity by molecular mass.

HPLC (RP-HPLC)

(Reversed-phase) high-performance liquid chromatography, the standard method for measuring peptide purity. See What “≥99% by HPLC” Means.

Hygroscopic

Readily absorbing moisture from the air. Many lyophilized peptides are hygroscopic.

Karl Fischer titration

A method for measuring the water content of a sample.

Lot (batch) number

The identifier linking a specific production batch to its documentation. It should appear on both the vial and the COA.

Lyophilization

Freeze-drying: removing water from a frozen material by sublimation under vacuum, producing a stable dry powder. See our storage and stability guide.

MALDI-TOF

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, another technique for measuring molecular mass.

Molecular weight (average vs. monoisotopic)

Average mass reflects natural isotope abundance and is used for weighing. Monoisotopic mass uses the most abundant isotopes and is used to interpret mass spectra.

Net peptide content

The percentage of a powder’s total weight that is the peptide itself, excluding water and counter-ions.

N-terminus / C-terminus

The amine end and carboxyl end of a peptide chain. Sequences are written N- to C-terminus.

Oxidation

A chemical change, especially at methionine, cysteine, and tryptophan residues, that can occur on exposure to air or light.

Peptide bond

The amide bond linking one amino acid to the next.

Purity (area %)

The target peak’s share of total detected peak area in an HPLC chromatogram.

Research use only (RUO)

A designation meaning a material is intended solely for laboratory research, not for human or veterinary use, diagnosis, or treatment.

Retention time

The time a component takes to elute from an HPLC column under a given method, used to identify peaks.

Solid-phase peptide synthesis (SPPS)

The standard method for making synthetic peptides, building the chain one residue at a time on a solid resin support.

TFA (trifluoroacetic acid)

A reagent used in peptide synthesis and purification. Peptides may be supplied as TFA salts.

Truncated sequence

A synthesis impurity in which chain growth stopped before the full sequence was completed.

The Pure Vial Co. supplies research materials for laboratory, analytical, and scientific research use only. Nothing in this guide is medical advice or an indication that any material is suitable for human or veterinary use. See our Research Disclaimer.