peptide & analytical chemistry glossary (40 terms)

the terms

Acetate salt.

A common counter-ion form of a peptide, where acetate ions balance the peptide's positive charges. Contributes to the vial's mass without being peptide. See net peptide content.

Aggregation.

A physical degradation pathway in which peptide molecules unfold and clump together, sometimes precipitating from solution. Driven by freeze-thaw, agitation, and temperature swings.

Amino acid.

The molecular building block of peptides and proteins. Twenty standard amino acids share a common backbone and differ by their side chains. See what is a peptide.

Area percent.

The way HPLC purity is expressed: the area of the target peptide's peak divided by the total area of all peaks, as a percentage.

Bacteriostatic water.

Water containing a small amount of benzyl alcohol that inhibits bacterial growth in a standing solution. See reconstitution solvent chemistry.

Bioburden.

The number of microorganisms present in a sample, reported as a count rather than a pass/fail. Measured by microbial enumeration (USP <61>).

Certificate of Analysis (COA).

A document reporting the analytical test results for a specific batch of material at a specific time. See reading a Certificate of Analysis.

Chromatogram.

The output trace of an HPLC run: peaks plotted against the time each component took to travel through the column.

Counter-ion.

An ion (commonly acetate or trifluoroacetate) that balances a peptide's charge. Part of the vial's mass, distinct from the peptide itself.

Deamidation.

A sequence-specific degradation reaction converting asparagine or glutamine residues to acidic forms. Driven by pH, temperature, and time.

Deletion sequence.

A synthesis impurity in which a chain is missing one amino acid because a coupling step was incomplete. Detectable as a mass lower than expected.

Endotoxin.

Lipopolysaccharide (LPS) residue from gram-negative bacteria. Persists after the bacteria die and can confound in vitro assays. See endotoxin testing.

EU (endotoxin unit).

The unit for reporting endotoxin, defined against a reference standard, usually expressed per milligram (EU/mg).

Fmoc.

A base-removable protecting group and the dominant chemistry in modern peptide synthesis. See solid-phase peptide synthesis.

Freeze-thaw.

A cycle of freezing and thawing that physically stresses a peptide and can drive aggregation, especially in solution. Aliquoting limits it.

Gravimetric analysis.

Measurement by mass, used in determining net peptide content.

Heavy metals.

Trace metallic contaminants (such as arsenic, cadmium, lead, mercury) measured on some COAs, typically by ICP-MS.

HPLC.

High-performance liquid chromatography, the standard method for measuring peptide purity. See what is HPLC.

Hydrophilic / hydrophobic.

Water-attracting versus water-repelling. A peptide's balance of these residues governs its solubility and its solvent choice.

ICP-MS.

Inductively coupled plasma mass spectrometry, an analytical method for measuring trace heavy metals.

Identity.

Confirmation that the compound in a batch is the one the label claims, established by mass spectrometry. See LC-MS and peptide identity.

LAL (Limulus amebocyte lysate).

A horseshoe-crab-derived reagent used to detect endotoxin. Available in gel-clot, turbidimetric, and chromogenic formats.

LC-MS.

Liquid chromatography coupled to mass spectrometry; separates a sample and measures the mass of its components in one workflow.

Lot / batch number.

The identifier tying material and its COA to one specific production run. The basis of traceability.

Lyophilization.

Freeze-drying: removing water from a frozen peptide under vacuum to yield a stable dry powder.

MALDI-TOF.

Matrix-assisted laser desorption/ionization time-of-flight, a mass-spectrometry method that gives mainly singly charged ions and a simple spectrum.

Mass-to-charge ratio (m/z).

The axis a mass spectrometer measures along; combined with charge state, it yields molecular mass.

Molecular weight.

The mass of a molecule, calculable from a peptide's sequence and compared against the measured value to confirm identity.

Monoisotopic mass.

The mass calculated using the principal isotope of each element, used for precise identity confirmation of smaller peptides.

N-terminus / C-terminus.

The two ends of a peptide chain (free amino group and free carboxyl group). Sequences are written N-terminus to C-terminus.

Net peptide content.

The fraction of a vial's dry mass that is actually peptide, versus water and counter-ion salts. Usually lower than the HPLC purity figure. See what "99% purity" means.

Peptide bond.

The covalent link between two amino acids, formed with the loss of a water molecule.

Primary structure.

The linear sequence of amino acids in a peptide, which is the main determinant of its identity.

Purity.

The proportion of the target peptide among detected organic components, measured by HPLC and expressed as an area percentage.

Recombinant Factor C (rFC).

An animal-free, manufactured reagent for endotoxin testing, recognized in USP <86>.

Reconstitution.

Dissolving a lyophilized peptide in an appropriate solvent for laboratory work. See reconstitution solvent chemistry.

Related substances.

Impurities structurally close to the target peptide, arising from synthesis side reactions, seen as minor HPLC peaks.

Retention time.

How long a component takes to travel through an HPLC column; reproducible under fixed conditions.

Reverse-phase (RP-HPLC).

The HPLC mode used for most peptides, using a hydrophobic stationary phase and a water/acetonitrile gradient.

Solid-phase peptide synthesis (SPPS).

The standard method for chemically making peptides on a resin support. See SPPS explained.

Sterility.

The absence of viable microorganisms, tested under USP <71>. See sterility testing.

TFA salt.

Trifluoroacetate, a counter-ion left from synthesis and purification. Like acetate, it contributes to vial mass and affects net peptide content.

USP.

The United States Pharmacopeia, which publishes the compendial standards (numbered General Chapters) many of these tests follow.