Peptide content (revision 10)
Old revision·20:39, 26 Dec 2024·LipidLedgerLou
| Peptide contentMass fraction | |
|---|---|
Content is a mass balance: what is in the vial that is not peptide has to be measured before what is can be stated. | |
| Definition | |
| Quantity | Mass of peptide ÷ mass of preparation |
| Units | Per cent by mass |
| Typical range | 70–90% for a lyophilised trifluoroacetate salt |
| Analytical method infobox · conventions | |
Peptide content, sometimes called peptide net content, is the proportion of the mass of a preparation that is the peptide itself. Everything else in the vial — water taken up during and after lyophilisation, the counterion left by the purification step, residual solvent, and any excipient — is excluded from the numerator and included in the denominator.[1]
Content is the determination that makes a mass statement possible. A purchaser reconstituting a nominally 5 mg vial and calculating a concentration is implicitly assuming a content figure, and in the ordinary case the assumption made is 100%, which is never correct for a lyophilised peptide salt. Typical content for such material falls between roughly 70% and 90%, so the assumption is wrong by an amount that matters.[1]
The relationship between content and area percent purity is the single most-misread pairing in research-peptide documentation. They are independent quantities: purity is a ratio among the species that eluted, and content is a fraction of the vial. A certificate reporting 99% purity and 83% content is internally consistent, and the two figures together say considerably more than either says alone.
What occupies the rest of the mass
[edit]For a lyophilised peptide purified by reverse-phase chromatography and freeze-dried from an aqueous acetonitrile mobile phase, four components account for essentially all of the non-peptide mass.
| Component | Typical share | Determined by |
|---|---|---|
| Water | 3–10% | Karl Fischer titration or Loss on drying |
| Counterion (usually trifluoroacetate) | 5–20% | Ion chromatography; inferred from the synthesis route only as a last resort |
| Residual solvent | below 1% | Headspace gas chromatography |
| Peptide-related impurities | 0.5–3% | The complement of Area percent purity |
The acetonitrile carried over from preparative purification is the residual solvent that dominates in practice; it is classified for toxicity and given a concentration limit under the harmonised residual-solvent guideline, and it is determined by headspace gas chromatography rather than inferred.[2]
The counterion share is the term that surprises people. Trifluoroacetic acid is used in the mobile phase of the purification, and a basic peptide leaves that step as its trifluoroacetate salt with one counterion per basic site. For a peptide with several basic residues and a modest molecular mass, the counterion can be a fifth of the dry weight.[1]
Water is the term that varies most between vials of the same lot, because a lyophilised cake takes up moisture from the atmosphere on any exposure. That is what the desiccant sachet in a well-packed consignment is for, and it is why a content figure carries a date.
References
- ^ a b c United States Pharmacopeia, General Chapter <1503>, "Quality Attributes of Synthetic Peptide Drug Substances" (informational). USP–NF, current revision. Sets out peptide content as an attribute distinct from chromatographic purity.
- ^ International Council for Harmonisation, Q3C: Impurities — Guideline for Residual Solvents. Classifies solvents by toxicity and sets concentration limits; the acetonitrile used in preparative purification falls in the second class.