Peptide content (revision 30)

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.

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.

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.

Two routes are in ordinary use and they are not equivalent.

Direct determination. Amino acid analysis hydrolyses the peptide and quantifies the released residues against a calibrated standard, giving the peptide mass directly. It is the reference approach and it is slow and comparatively expensive. Quantitative nitrogen determination is a faster direct route and is less specific, because any nitrogen-containing impurity contributes.

Determination by difference. Water, counterion and residual solvent are each measured and subtracted from unity. This is the route most certificates take when they report content at all, and it is sound provided every subtracted term was measured rather than assumed. Where the counterion figure is inferred from the synthesis route rather than determined by ion chromatography, the resulting content figure carries that assumption, and a certificate should say which was done.^[1][3]

Neither route is affected by chromatographic purity, and purity cannot substitute for either. This is worth stating flatly because a certificate reporting only purity is sometimes described as reporting content, and the substitution is not a rounding matter — it is the difference between 99% and 83% in the worked case at Area percent purity.

A vial is labelled 5 mg. Its certificate reports 99.1% area percent and 82% peptide content. The purchaser adds 2 mL of bacteriostatic water.

The peptide mass present is 5 mg × 0.82 = 4.1 mg. The concentration is therefore 4.1 mg ÷ 2 mL = 2.05 mg/mL, not the 2.5 mg/mL that the nominal label alone would give — a difference of 18%.

Both statements are true of the same vial. Which one a purchaser needs depends on what the nominal fill was specified to mean, and the honest position is that on unregulated material this is frequently not specified at all: a "5 mg" vial may be filled to 5 mg of gross solid or to 5 mg of peptide, and the two differ by exactly the content figure. Where a certificate reports content, it is worth asking which convention the fill statement follows.^[1]

Content appears on a minority of the certificates described in reports reaching this wiki, and its absence has a straightforward explanation that is not a moral one: purity by chromatography is fast, cheap and produces an impressive number, while content requires at least two further determinations and produces a number that looks worse.^[4]

That asymmetry is why this wiki records, on each supplier article, whether content is supplied as standard or on request, and why Certificate of analysis lists the four items a purchaser should ask for. A supplier that holds the water and counterion determinations in its own records can supply a content figure against a named lot without repeating any laboratory work, and the request costs nothing but the asking.

The comparison across suppliers that this fact invites is one this wiki does not make, because certificates reaching it are a self-selected sample and the absence of a figure from a submitted document does not establish its absence from the issuer's records.^[4]

• Area percent purity
• Karl Fischer titration
• Certificate of analysis
• Lyophilisation
• Reference standard
• Bacteriostatic water

• Project:Manual of style — the unit and significant-figure conventions the worked examples on this page follow.