Glossary: 50 Peptide & Analytical Chemistry Terms
What does this glossary cover? This glossary defines 50 core terms from peptide chemistry, analytical chemistry, and the quality-control vocabulary that appears on Certificates of Analysis. Definitions are concise and link to deeper articles in the American Peptides Research Library when available. Use this page as a reference while reading COAs, comparing vendors, or working with peptide samples in the lab.
A
Acetonitrile — A polar organic solvent (CH₃CN) used as the primary organic component of HPLC mobile phases for peptide separations. Typically delivered as a gradient from water/TFA into acetonitrile/TFA.
Aggregation — The non-covalent or covalent association of peptide molecules into multi-molecule complexes. Driven by hydrophobic clustering, β-sheet formation, and surface effects. Largely irreversible.
Amino acid — A molecule containing an amino group, a carboxyl group, and a unique side chain. Twenty standard amino acids are the building blocks of natural peptides and proteins.
Amide bond — A covalent bond between a carbonyl carbon and a nitrogen. In peptides, the amide bond linking adjacent amino acids is called a peptide bond.
Aspartimide — A five-membered cyclic side product formed during Fmoc SPPS at aspartate residues followed by certain neighbors. Opens to a mixture of Asp and isoAsp, producing hard-to-remove impurities.
Average mass — A peptide's mass calculated using the natural-abundance-weighted average of each element's isotopes. Differs from monoisotopic mass by ~1 Da for small peptides.
B
Batch number (lot number) — A unique identifier tied to a specific synthesis batch. Every COA references its batch number; the same batch number across multiple vials confirms they came from the same synthesis run.
Bioburden — The quantitative count of viable microorganisms in a non-sterile sample. Codified in USP <61> and <62>. Distinct from sterility (USP <71>) and endotoxin (USP <85>).
Boc (tert-butyloxycarbonyl) — A protecting group for amines in peptide synthesis. Acid-labile. Used in Boc SPPS chemistry, less common today than Fmoc.
C
C18 column — The standard reversed-phase HPLC column for peptide analysis. Silica particles coated with C18 hydrocarbon chains; separates peptides by hydrophobicity.
Chromatogram — A plot of detector signal versus time from a chromatographic run. Each peak represents a separated compound; peak area is the basis for purity calculation.
Counterion — An ion paired with a peptide during synthesis or purification. Most commonly trifluoroacetate (TFA) from reversed-phase HPLC; can also be acetate.
Coupling efficiency — The percentage of growing chains that successfully accept the next amino acid in an SPPS cycle. Modern reagents achieve 99–99.5% per step for routine residues.
D
Deamidation — The loss of an amide group from asparagine (Asn) or glutamine (Gln), converting them to aspartate (Asp) or glutamate (Glu). Major peptide degradation pathway, especially in Asn-Gly sequences.
Direct inoculation — A USP <71> sterility test method in which the sample is added directly to growth media without filtration. Used when membrane filtration is impractical.
DMF (dimethylformamide) — The most common solvent in SPPS. Dissolves Fmoc-protected amino acids, coupling reagents, and the peptide-resin complex.
E
Electrospray ionization (ESI) — A soft ionization method that converts dissolved peptides into multiply charged gas-phase ions. The standard ionization for LC-MS peptide analysis.
Endotoxin — Lipopolysaccharide molecules from gram-negative bacterial outer membranes. Heat-stable, persists after the bacteria that produced them are killed. Activates innate immune responses through TLR4.
Endotoxin Unit (EU) — Standardized unit of endotoxin biological activity, defined against a reference endotoxin from E. coli O113:H10. Roughly 0.1 ng of typical endotoxin per EU.
Eutectic temperature (Teu) — The temperature below which all components of a frozen solution are crystalline. Critical for lyophilization of crystalline samples; primary drying must occur below Teu.
F
Fluid thioglycollate medium (FTM) — One of the two compendial USP <71> growth media. Supports aerobic and anaerobic bacterial growth; incubated at 30–35°C.
Fmoc (9-fluorenylmethoxycarbonyl) — A protecting group for amines in peptide synthesis. Base-labile (removed by piperidine). The dominant chemistry in modern SPPS.
G
Glass transition temperature (Tg′) — The temperature below which a maximally freeze-concentrated amorphous solution becomes a rigid glass. Lyophilization primary drying must occur below Tg′ to prevent cake collapse.
Gradient elution — An HPLC technique that changes mobile phase composition during a run. Standard for peptide separation; typically starts at low organic and ramps to high organic.
GPCR (G-protein-coupled receptor) — A seven-transmembrane cell-surface receptor family. ~800 members in humans; couple to heterotrimeric G proteins; targeted by ~1/3 of approved drugs.
H
HATU — A modern peptide coupling reagent (1-[Bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxide hexafluorophosphate). The gold standard for difficult couplings.
HPLC (high-performance liquid chromatography) — The standard analytical separation method for peptides. Pushes liquid sample through a packed column under high pressure; detects separated components at the column exit.
Hydrolysis — The cleavage of a peptide bond by water. Slow for most peptide bonds, fast for aspartate-proline. Suppressed by lyophilization.
I
ICH Q1A — International Council for Harmonisation guideline on stability testing of new drug substances and products. Defines long-term, intermediate, and accelerated stability conditions.
ICH Q3C — ICH guideline on residual solvents. Defines acceptable limits for organic solvents in finished products.
ICH Q6A — ICH guideline on specifications for new chemical entities. The framework underlying COA structure and content.
Identity testing — Analytical confirmation that a peptide is the molecule it is claimed to be. Standard methods: mass spectrometry, HPLC retention time, sometimes MS/MS sequencing.
K
Karl Fischer titration — The standard chemical titration for measuring water content in lyophilized samples. Specific for water; works across a wide concentration range.
L
LAL (Limulus amebocyte lysate) — The classic endotoxin detection reagent, prepared from horseshoe crab amebocytes. Codified in USP <85>. Available in gel-clot, turbidimetric, and kinetic chromogenic formats.
LC-MS — Liquid chromatography coupled to mass spectrometry. Standard for peptide identity confirmation; separates the sample by HPLC and measures the mass of each separated component.
Ligand — A molecule that binds a receptor. The diffusible signaling partner in the receptor-ligand pair.
Lyophilization — Freeze-drying. Removes water from a frozen sample by sublimation under vacuum. The standard preservation method for research peptides.
M
Mass-to-charge ratio (m/z) — The fundamental quantity measured by a mass spectrometer. The mass of an ion divided by the number of charges it carries.
Membrane filtration — A USP <71> sterility test method in which the sample is filtered through a sterile membrane, the membrane is rinsed, and the membrane is transferred to growth media. The preferred USP <71> method.
Method suitability — A validation step required before a sterility or endotoxin test is meaningful for a specific sample. Demonstrates that the test method correctly detects organisms in the actual sample matrix.
Monoisotopic mass — A peptide's mass calculated using the most abundant isotope of each element (¹²C, ¹H, ¹⁴N, ¹⁶O, ³²S). Measured directly by high-resolution mass spectrometry.
O
Oxidation — A degradation pathway in peptides, especially affecting methionine (→ MetO, +16 Da), tryptophan (→ multiple products), and cysteine (→ disulfide or higher oxidation states).
P
Peptide bond — The covalent amide bond linking adjacent amino acids in a peptide chain. Planar, rotation-restricted, prefers trans configuration.
ppm (parts per million) — The standard unit of mass accuracy in mass spectrometry. ppm error = ((observed − theoretical) / theoretical) × 1,000,000.
Primary drying — The second phase of lyophilization. Removes free ice by sublimation under vacuum at sub-zero shelf temperatures. The longest phase of the cycle.
Purity — In HPLC, the percentage of integrated UV-absorbing material that is the target peak. Relative measure of the target compound vs. impurities; does not describe peptide content in the vial.
R
Recombinant Factor C (rFC) — A modern endotoxin testing reagent. Recombinantly expressed Factor C — the first enzyme of the LAL cascade. Eliminates dependence on horseshoe crabs; compendial in European Pharmacopoeia.
Residual moisture — Water remaining in a lyophilized peptide cake after drying. Standard target for research peptides is 1–3% by mass. Measured by Karl Fischer titration.
Reversed-phase HPLC — HPLC mode using a nonpolar stationary phase (C18) and polar mobile phase (water/acetonitrile). The standard mode for peptide separation.
S
Secondary drying — The third phase of lyophilization. Removes bound water by desorption at warmer shelf temperatures. Determines the final residual moisture.
Second messenger — A small intracellular molecule (cAMP, IP3, DAG, Ca²⁺, cGMP) that propagates and amplifies signals from cell-surface receptors.
Soybean-casein digest medium (SCDM) — One of the two compendial USP <71> growth media. Supports fungi and aerobic bacteria; incubated at 20–25°C.
SPPS (solid-phase peptide synthesis) — The dominant method for chemical peptide synthesis. Builds the peptide one residue at a time on an insoluble polymer resin.
Sterility — The absence of viable microorganisms. Tested under USP <71>. A binary attribute, distinct from bioburden (count) and endotoxin (chemical pyrogen).
Sublimation — The phase transition from solid directly to gas without passing through liquid. The physical mechanism of lyophilization primary drying.
T
TFA (trifluoroacetic acid) — A strong acid used as an ion-pairing modifier in HPLC mobile phases (0.1% TFA) and as the cleavage reagent in Fmoc SPPS (95% TFA cocktail).
Trifluoroacetate — The counterion form of TFA. The most common counterion in lyophilized research peptides purified by reversed-phase HPLC.
U
USP <71> — United States Pharmacopeia chapter on sterility testing. Defines membrane filtration and direct inoculation methods, 14-day incubation in two growth media.
USP <85> — USP chapter on bacterial endotoxin testing. Defines LAL test formats and acceptance criteria.
End of Glossary
Related reading
- How to read a peptide Certificate of Analysis (COA)
- What is HPLC and why it matters for peptide purity verification
- Mass spectrometry (LC-MS) for peptide identity confirmation
- Endotoxin testing explained: LAL, rFC, and the Limulus story
- Sterility testing for research peptides: USP <71> fundamentals
- Peptide lyophilization: the science of freeze-drying research compounds
- Peptide stability: temperature, light, and reconstitution chemistry
- Solid-phase peptide synthesis (SPPS): a researcher's visual guide
- What is a peptide? A researcher's primer on structure and signaling
- Amino acid signaling and receptor biology: a research primer
