Research peptide glossary

Area percent

Also: area %

How HPLC purity is reported: the area of the main peak divided by the total area of all peaks, at a stated wavelength. It is a proxy for quantity, not a direct measure of it, because different molecules absorb differently.

Average molecular weight

Also: molecular weight, MW

The mass averaged across the natural isotope distribution of every atom in the molecule. Used for weighing and molar calculations; not the figure an MS trace is compared against.

Certificate of analysis

Also: COA

The document that connects the label on a vial to measured data for one specific lot. A complete one states identity (usually by mass spectrometry), purity (usually by HPLC, with the chromatogram reproduced), peptide content, appearance and the lot number. A certificate for a different lot is not documentation of yours.

Chromatogram

The plot a chromatography run produces — detector response against time. A purity figure with no chromatogram behind it is an assertion rather than a measurement.

Counter-ion

Also: counterion

The salt form a peptide carries out of purification, typically trifluoroacetate or acetate. Counter-ions add mass to the powder without adding peptide, which is why label mass and peptide mass differ.

Endotoxin

Bacterial cell-wall fragments. A certificate of analysis does not establish endotoxin content unless a specific assay is listed. Purity and sterility are different properties.

HPLC

Also: high-performance liquid chromatography

High-performance liquid chromatography. The separation technique behind almost every peptide purity figure. Components are separated by how strongly they interact with the column, and a detector reports absorbance over time, producing peaks.

Lot

Also: batch

A single production run. Documentation is lot-specific: the lot number on the certificate must match the lot number on the vial in hand.

Mass spectrometry

Also: MS

Measures mass-to-charge ratio, which the instrument converts to molecular weight. Comparing observed against theoretical weight confirms identity. It says nothing about how much of the sample is that molecule.

Monoisotopic mass

The mass calculated using the single most abundant isotope of each element. This is the figure to compare against a mass-spectrometry result. It diverges from average molecular weight as molecules get larger.

Peptide content

Also: net peptide weight

The proportion of a lyophilised vial that is actually peptide, as opposed to residual water and counter-ions. A 10 mg vial at 80% peptide content holds roughly 8 mg of peptide. Where work depends on peptide mass rather than powder mass, this figure is not optional.

Retention time

How long a component takes to travel through the column and reach the detector. Consistent retention time across lots is one signal that a material is what it was last time.

Reverse-phase

Also: reversed-phase, RP-HPLC

The most common HPLC mode for peptides. The column surface is non-polar and the mobile phase is polar, so components separate by hydrophobicity — more hydrophobic molecules are retained longer.

TFA

Also: trifluoroacetate, trifluoroacetic acid

Trifluoroacetic acid. The most common mobile-phase additive in peptide purification, and therefore the most common counter-ion in the finished powder.

Analogue

Also: analog

A molecule structurally related to another, typically differing by a modification such as an added acyl chain or a substituted residue.

CAS number

Also: CAS

A unique registry identifier assigned by the Chemical Abstracts Service. Useful for unambiguous identification across catalogs and literature, where names vary.

Co-lyophilised

Also: blend, blended

Two or more components freeze-dried together as a single cake. They dissolve together at the ratio set by the fill and cannot be separated afterwards.

Hydrogen-bond donor

Also: HBD, hydrogen bond acceptor, HBA

Counts of the sites in a molecule able to donate or accept a hydrogen bond. They describe how a molecule interacts with water and with other molecules.

Lyophilisation

Also: lyophilised, lyophilized, freeze-drying

Freeze-drying. Water is removed from a frozen solution under vacuum, leaving a dry cake that is far more stable than the solution it came from. Nearly every research peptide is supplied this way.

Molecular formula

The count of each element in one molecule, such as C62H98N16O22. Distinct molecules can share a formula, so it supports identity rather than establishing it.

Peptide

A chain of amino acids joined by peptide bonds. Shorter than a protein by convention, though the boundary is not sharp.

PubChem CID

Also: CID

The compound identifier in PubChem, the public chemical database maintained by the US National Library of Medicine. Every computed descriptor on this site's compound pages is sourced from it.

Residue

Also: amino acid residue

A single amino acid unit within a peptide chain. Certain residues carry specific vulnerabilities — methionine and tryptophan oxidise readily, asparagine and glutamine deamidate.

Topological polar surface area

Also: TPSA

A computed measure of the surface contributed by polar atoms, in square angstroms. Higher values indicate a more polar molecule.

XLogP

Also: logP

A computed partition coefficient describing the balance between water-loving and fat-loving character. Strongly negative values indicate a hydrophilic molecule; most peptides sit well below zero.

Aggregation

Peptide molecules associating into larger assemblies, often irreversibly. Driven by freeze-thaw cycles, agitation and the air-liquid interface, which is why vials are swirled rather than shaken.

Aliquot

Also: aliquoting

Dividing a solution into smaller single-use portions. The highest-value habit in peptide handling: it avoids repeated freeze-thaw cycles and repeated needle entries into one vial.

Bacteriostatic water

Sterile water containing approximately 0.9% benzyl alcohol as a preservative. It inhibits bacterial growth rather than killing it, which is what makes a multi-entry vial workable.

Benzyl alcohol

The bacteriostatic agent in bacteriostatic water. A small number of peptides show reduced stability in its presence, and some cell-based assays are sensitive to it.

Deamidation

A degradation route in which asparagine or glutamine residues convert to aspartate or glutamate, changing the molecule's mass and charge. Accelerated at higher pH.

Diluent

The liquid used to reconstitute or dilute. For lyophilised peptides this is usually bacteriostatic water, sometimes sterile water where a preservative would interfere.

Excursion

Also: temperature excursion

A departure from the specified storage temperature, such as time at ambient during shipping. Short excursions for lyophilised material are normal and are not equivalent to prolonged warm storage.

Freeze-thaw cycle

Also: freeze thaw

One round of freezing and thawing a solution. Each cycle costs material to aggregation, which is why aliquoting matters.

Photo-oxidation

Light-driven oxidation, most relevant to sequences containing tryptophan, tyrosine, methionine or cysteine. The reason light-sensitive materials ship in protected vials.

Reconstitution

Also: reconstitute

Dissolving a lyophilised powder in a liquid. Concentration equals vial mass divided by diluent volume, so choosing the volume is choosing the concentration — and it cannot be undone.

Sterile water

Also: water for injection, WFI

Sterile water with no preservative. Suitable for single-entry use; a vial reconstituted with it is generally discarded after first use.

Cost per milligram

Also: cost per mg, unit price

Price divided by total material, the only figure that compares cleanly across different strengths and vial counts. A single 20 mg vial at $45 is $2.25/mg; a ten-vial pack of the same strength at $125 is $0.63/mg.

In vitro

Also: in-vitro

Latin for 'in glass'. Work carried out outside a living organism — in tubes, plates or cell culture. Every material in this catalog is supplied for in-vitro laboratory research.

Pack

Also: kit

A multi-vial unit, typically ten vials of one strength. Substantially lower cost per milligram than a single vial.

Research use only

Also: RUO

A designation meaning a material is supplied for laboratory research and is not a drug, food, cosmetic or medical device. It is not for human or veterinary use, and no therapeutic or dosing guidance accompanies it.

Single vial

One vial sold on its own, for evaluating a compound or a supplier before committing to pack quantities. Higher cost per milligram, lower cost to find out.

SKU

Stock-keeping unit. The catalog code identifying one exact specification, used on order confirmations and when requesting lot documentation.

Specification

Also: spec

A particular strength and vial count of a compound, such as '10 mg x 10 vials'. One compound typically has several.