What is really in the vial: peptide content

The label shows a round number. The vial holds a mixture of peptide, salt and residual moisture, and in some lots a bulking agent as well. This article explains which figure answers which question and when the difference matters.

The figure on the label is a nominal amount

Ten milligrams means that the fill was set to this amount of peptide. What is actually in the vial is a total mass, and the peptide is only part of it. The other components are not a deception but chemistry; they are simply rarely shown on the label.

Three components are regularly added to it: the counterion from purification, residual moisture from freeze-drying and, in some lots, a bulking agent.

The counterion adds weight

After preparative separation a peptide is not present as a free base but as a salt. The associated counterion is usually trifluoroacetate, less often acetate or hydrochloride. It sits on the basic side chains, that is on arginine, lysine and histidine.

The more of these sites a sequence carries, the more counterion is attached to the molecule. For sequences with few basic residues about five per cent of the mass is typical; for strongly basic ones it can reach twenty-five. This figure is a property of the sequence, not a mark of quality.

Purity and peptide content answer different questions

Purity from HPLC says: of everything the column shows as a peak, this percentage is the target molecule. It compares the peptide with the other peptides in the same sample.

Peptide content says something else: of the mass weighed out, this percentage is actually peptide. It takes account of everything HPLC does not see at all, namely salt and water. A lot with ninety-nine per cent purity and eighty per cent peptide content is not a contradiction but the normal case.

Peptide content is determined by amino acid analysis or from the nitrogen content. Both take more work than an HPLC run, which is why the figure does not appear on every test report.

Residual moisture

No lyophilisate is completely dry. Loss on drying states how much mass is lost when the material is dried further; a few per cent is typical. The water adds to the weight, and it is also the component that limits how long the dry material keeps.

Mannitol

Some lots contain mannitol as a bulking agent. It keeps the cake stable and makes it easier to dissolve again, especially for small fill weights that would otherwise sit as a barely visible film on the glass. If mannitol is listed on the report, a substantial part of the mass in the vial is mannitol. In the certificate library such lots are specifically marked.

What this means for your figures

Working from the nominal amount means working with an upper limit: the actual quantity of peptide is lower. Within one lot this does not matter, because the gap is the same in every preparation. When you change to another lot or another manufacturer, it is not.

If you need to work more precisely, take the peptide content from the test report and correct the amount weighed out. Dividing the corrected amount by the volume of solvent gives the concentration; the steps of dissolving are described under reconstitution.

An example

A vial with a nominal amount of ten milligrams and eighty per cent peptide content contains eight milligrams of peptide. If two millilitres of solvent are added, that gives four milligrams of peptide per millilitre, not five as the nominal amount suggests. The difference is one fifth, which is larger than the variation most measurements allow.

Which other details a test report lists and how they relate to each other is covered in how to read a certificate of analysis; where the counterion comes from in the first place is explained in the article on how a peptide is made.

Questions

Why is peptide content not shown on every certificate of analysis?

Because it needs a separate determination, usually an amino acid analysis. Many reports list only purity and mass, because both come from the same run.

Is a high trifluoroacetate content a defect?

No. It is a consequence of purification and depends on the sequence. It only becomes relevant if trifluoroacetate interferes with the planned experiment; in that case there are lots with acetate as the counterion.

How can I tell whether a lot contains mannitol?

From the certificate of analysis for that lot. In the certificate library the lots concerned are marked, because the detail is otherwise easy to overlook.