Water Content in Lyophilised Peptides and Why It Is Measured
Freeze-drying removes most of the water from a peptide preparation. It does not remove all of it, and the fraction left behind is both a mass you are paying for and a variable that affects stability.
Bound and free water
Lyophilisation removes bulk water by sublimation, but a proportion remains associated with the peptide through hydrogen bonding. Typical residual figures for a well-lyophilised peptide sit in the low single-digit percentages by mass, and the exact value depends on the sequence, the excipients and how the cycle was run.
Why it matters
There are two distinct consequences, and they pull in opposite directions.
For quantification, residual water is part of gross vial mass but not part of peptide mass. Together with counter-ions it explains why a vial labelled 10mg contains less than 10mg of peptide — the point covered in net peptide content explained and TFA and acetate counter-ions.
For stability, water is the medium most degradation chemistry needs. Hydrolysis, deamidation of asparagine and glutamine, and several oxidation routes all proceed faster with more water present. A drier cake is generally a more stable one, which is the reason lyophilised material outlasts solutions by so much.
How it is measured
Karl Fischer titration is the standard method. It is specific to water — unlike loss-on-drying, which measures everything volatile — and it works at the low percentages relevant here. Coulometric Karl Fischer is used for small sample sizes, which suits peptide quantities.
Some certificates report water content, some report loss on drying, and many report neither. Where a figure is present, note which method produced it: the two are not interchangeable, and loss on drying will read higher if residual solvent is present.
What this means in the laboratory
The practical implication is the same as for counter-ions: do not treat the label figure as peptide mass when precision matters. Where a certificate gives net peptide content, use it. Where it does not, and the experiment depends on accurate molarity, quantify the reconstituted solution directly — absorbance at 280 nm works for sequences containing tryptophan or tyrosine, and amino acid analysis works for any sequence.
Hygroscopicity also means the figure drifts once a vial is opened; see weighing lyophilised peptides. Literature: PubMed.
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