Research

Peptide & Analytical Chemistry Glossary: 50 Terms, Defined

Fifty terms that recur across peptide chemistry, analytical documentation, and the research literature — defined in one or two sentences each, in plain language, with links into the research library where a full article exists. Organized in five groups: structure, synthesis, analytics, documentation, and pharmacology.

Peptide structure

Amino acid — The building block of peptides and proteins: a molecule carrying an amine group, a carboxylic acid group, and a variable side chain on a central carbon. Twenty standard amino acids make up natural sequences.
Residue — An amino acid as it exists inside a chain, having lost a water molecule in bond formation. A "39-residue peptide" contains 39 linked amino acids.
Peptide bond — The amide linkage joining one amino acid's carboxyl group to the next one's amine, releasing water. The repeating backbone bond of every peptide and protein.
Peptide — A chain of amino acids joined by peptide bonds — conventionally up to roughly 40–50 residues, beyond which the molecule is usually called a protein. The boundary is convention, not chemistry.
Sequence — The ordered list of residues from N-terminus to C-terminus; the peptide's primary structure and its defining identity.
N-terminus / C-terminus — The two ends of a chain: the end bearing the free amine (N) and the end bearing the free carboxyl (C). Sequences are written N→C by convention.
Side chain — The variable group on each residue that gives it its character — hydrophobic, acidic, basic, aromatic. Side chains determine a peptide's charge, solubility, and chemistry.
Amidation (C-terminal) — Replacement of the C-terminal carboxyl with an amide (–NH₂), a common modification that removes a negative charge and often improves stability; written as "-NH₂" at the end of a sequence.
Cyclic peptide — A peptide whose chain is closed into a ring — head-to-tail or through side chains (a lactam bridge, as in Melanotan II). Cyclization restricts conformation, typically raising stability and receptor-binding preorganization.
D-amino acid — The mirror-image stereoisomer of the natural L-form. Natural synthesis produces L-residues; designed analogues incorporate D-residues to resist enzymatic degradation, as in ipamorelin.
Disulfide bond — A covalent sulfur–sulfur link between two cysteine residues, constraining a peptide's fold.
Analogue — A designed variant of a natural peptide — substitutions, truncations, or modifications engineered to change stability, potency, or selectivity while retaining the parent's essential activity.
Fragment — A peptide corresponding to a portion of a larger parent molecule, studied when a specific region carries a specific activity — as with KPV, the C-terminal tripeptide of α-MSH.
Blend — A fixed-ratio co-formulation of two or more distinct peptides in one vial, as in the Wolverine, GLOW, and KLOW preparations. Chemically the components remain separate molecules.
Molecular weight (Da) — A molecule's mass in daltons, the sum of its atoms' masses. For peptides, calculable directly from sequence — the basis of identity testing.

Synthesis & production

SPPS (solid-phase peptide synthesis) — The standard manufacturing method: the chain is assembled residue-by-residue while anchored to resin beads. Full article: How research peptides are made.
Resin — The insoluble polymer beads to which the growing chain is anchored during SPPS.
Protecting group — A removable chemical cap that blocks a reactive site so only the intended bond can form during each coupling; Fmoc (on the α-amine) defines the dominant modern strategy.
Coupling — The bond-forming step of each synthesis cycle: the incoming residue's activated carboxyl reacting with the chain's free amine.
Deletion sequence — The characteristic SPPS impurity: a chain that missed one coupling and lacks a single residue — nearly identical to the target, differing by one residue's mass.
Truncation — An incomplete chain terminated partway through synthesis, typically by deliberate capping of failed couplings so it can be removed in purification.
Cleavage — The final synthesis step: cutting the finished peptide from the resin (in concentrated TFA for Fmoc chemistry) while stripping all protecting groups.
TFA / trifluoroacetate — Trifluoroacetic acid, the cleavage and purification acid; its anion remains paired to the peptide's basic sites as a counterion, contributing to vial mass — a key reason purity and content differ.
Counterion — The oppositely charged ion accompanying a charged peptide in solid form; commonly trifluoroacetate or acetate.
Preparative HPLC — Chromatography scaled from analysis to production: separating a crude synthesis mixture and collecting only the target fraction.
Lyophilization — Freeze-drying: removing water from a frozen solution by sublimation under vacuum, producing the stable dry cake research peptides ship as. Full article: What is lyophilization.
Lyophilized cake — The porous solid a well-run freeze-drying cycle produces — a cast of the departed ice, holding the peptide in a dry glassy matrix.

Analytical methods

HPLC — High-performance liquid chromatography: separation of a dissolved sample into components by their interaction with a packed column, the method behind peptide purity figures. Full article: What is HPLC.
Reverse-phase (RP-HPLC) — The peptide-standard HPLC configuration: hydrophobic stationary phase (typically C18), polar mobile phase, separation by hydrophobicity.
Chromatogram — The output trace of an HPLC run: detector signal against time, with each separated component appearing as a peak.
Retention time — When a component emerges from the column; reproducible under fixed conditions and characteristic of the species.
Peak area — The integrated size of a chromatographic peak, proportional to the amount of that component — the quantity purity percentages are calculated from.
Gradient — A mobile phase whose composition changes through the run (typically water→acetonitrile), releasing progressively more-retained species in sequence.
Mass spectrometry (MS) — Measurement of molecular mass via ionization and mass-to-charge analysis; the identity method of peptide documentation. Full article: MS for peptide identity.
m/z — Mass-to-charge ratio, the quantity a mass spectrometer actually measures; multiply-charged peptide ions appear at fractions of their true mass.
ESI (electrospray ionization) — The soft ionization method standard for peptides: charged droplets sprayed from a high-voltage needle evaporate down to intact gas-phase peptide ions.
Deconvolution — Computing a single molecular mass from the family of charge-state peaks one peptide produces in ESI.
Monoisotopic mass — Molecular weight computed from each element's most abundant isotope; what high-resolution instruments report. Differs from average mass by up to a few daltons on large peptides.
MS/MS (tandem MS) — Fragmenting a selected ion and reading sequence information from the fragment masses; the method that distinguishes same-mass sequence isomers.
Size-exclusion chromatography (SEC) — Separation by molecular size rather than chemistry; the standard method for detecting aggregates that reverse-phase HPLC can miss.

Documentation & quality

COA (Certificate of Analysis) — The laboratory document reporting measurements on a specific batch: methods, purity, identity, lot reference, date, and sign-off. Full article: How to read a COA.
Lot / batch — A quantity of material produced and tested as one unit; the lot reference is what ties a certificate to physical vials.
Purity (HPLC) — The main peak's share of total detected peak area — a proportion within the peptide population, not a statement about vial mass.
Net peptide content — The fraction of a vial's mass that is peptide, measured by amino acid or nitrogen analysis; typically 60–90% for lyophilizates once counterions and moisture are counted. Full article: Purity vs content.
Identity (analytical) — Confirmation that the material is the intended molecule: measured mass matching the mass calculated from the sequence, within tolerance.
Third-party testing — Analysis performed by a laboratory unaffiliated with the seller — independence being the property that makes reported numbers credible. Full article: Why third-party testing matters.
RUO (Research Use Only) — The intended-use classification under which research compounds are supplied: laboratory research exclusively, not for human or veterinary use. Full article: What RUO means.

Pharmacology (as used in the research literature)

Receptor — The cellular protein a signaling molecule binds to trigger a response; most peptides in this catalog act on cell-surface receptors in published assays.
GPCR — G-protein-coupled receptor, the large receptor family — spanning the melanocortin, incretin, GHRH, and ghrelin receptors — whose activation is read out through second messengers like cAMP and calcium.
Agonist — A ligand that activates its receptor; a selective agonist activates one subtype preferentially, a pan-agonist activates several, and a multi-receptor agonist is engineered to engage defined sets — the design axis of the incretin comparison.
EC50 / Ki — Standard potency measures from in vitro assays: the concentration producing half-maximal effect (EC50) and the binding-affinity constant (Ki). Method-dependent, and comparable chiefly within one assay system.

What Is HPLC? How Peptide Purity Is Actually Measured How to Read a Peptide Certificate of Analysis Solid-Phase Peptide Synthesis: How Research Peptides Are Made The Peptide Bond: The Link That Builds Every Peptide Amino Acid Structure: The 20 Residues, One Reference Chart What Is a Peptide? Structure, Size, and Why Sequence Is Everything GPCRs and Receptor Pathways: How Peptide Signals Become Cell Responses

References

This glossary is a research reference. HEEZ Research products are supplied for in vitro laboratory research only — not for human or veterinary use.

Last reviewed August 2026 by the HEEZ Research team.

↑ Back to top