Residual Water Is Part of What You Weighed
Most of the water leaves a peptide preparation during freeze-drying. All of it does not. Whatever remains is simultaneously a mass appearing on your invoice and a variable acting on stability.
Putting a number on it
Karl Fischer titration is the standard technique. Its advantage over loss-on-drying, which registers everything volatile, is specificity to water alone, and it performs at the low percentages that matter here. Small sample sizes suit the coulometric variant, which in turn suits the quantities peptides come in.
Certificates vary. Water content appears on some, loss on drying on others, and neither figure on many. Wherever a number does appear, establish which method generated it, because the two do not substitute for one another and loss on drying reads higher whenever residual solvent is in the sample.
Two consequences pulling opposite ways
| Consequence | What follows |
|---|---|
| Quantification | Gross vial mass includes residual water; peptide mass does not. Alongside counter-ion, this is why a vial labeled 10mg contains less than 10mg of peptide, a point developed in net peptide content explained and TFA and acetate counter-ions. |
| Stability | Water is the medium that most degradation chemistry needs. Hydrolysis, the deamidation of asparagine and glutamine, and several oxidation routes all accelerate as water increases. Drier cakes are generally more stable cakes, which accounts for lyophilized material outlasting solutions by such a wide margin. |
What survives sublimation
Bulk water goes off by sublimation during lyophilization, yet a proportion remains associated with the peptide through hydrogen bonding. For a well-lyophilized peptide, typical residual figures fall in the low single-digit percentages by mass, the precise value turning on sequence, excipients and the way the cycle was run.
Acting on it
What applies to counter-ions applies equally here: where precision counts, the label figure is not peptide mass. Use net peptide content whenever a certificate supplies it. Where no such figure exists and the experiment hinges on accurate molarity, quantify the reconstituted solution directly instead, by absorbance at 280 nm for sequences containing tryptophan or tyrosine, or by amino acid analysis for any sequence at all.
Because the material is hygroscopic, that figure also drifts once a vial has been opened; see weighing lyophilized peptides. Literature: PubMed.
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