Lyophilisation (revision 10)
Old revision·05:19, 27 Sep 2024·CategoryBot
| Lyophilisation | |
|---|---|
| Also known as | Freeze-drying |
| Class | Dehydration by sublimation |
| Physical requirement | Pressure below the triple point of water |
| Typical cycle length | 20–70 hours |
| Analytical method infobox · conventions | |
Lyophilisation, also called freeze-drying, is a dehydration process in which a solution is frozen and the ice is then removed by sublimation at a pressure below the triple point of water, leaving a porous solid whose volume approximates that of the original fill. It is the standard final step in the isolation of synthetic peptides after preparative purification, and the reason a research peptide is distributed as a dry cake in a sealed vial rather than as a solution.[1]
The process is used because peptides in aqueous solution degrade by hydrolytic routes — deamidation of asparagine and glutamine, backbone cleavage, aggregation — whose rates depend strongly on molecular mobility and on the availability of water. Removing water and holding the residue in an amorphous glass suppresses those routes by orders of magnitude, converting a product with a shelf life of days at ambient temperature into one with a shelf life measured in years.[2][3]
A lyophilisation cycle has three phases: freezing, primary drying in which ice is sublimed, and secondary drying in which water sorbed to the solid is desorbed. Each phase is bounded by a physical constraint. Freezing determines the ice-crystal structure and therefore the resistance the vapour must overcome later; primary drying must be conducted below the temperature at which the freeze-concentrated matrix loses rigidity; secondary drying must reach a residual moisture low enough for stability without over-drying a solid that may require a small amount of water for conformational integrity.[4][5]
Physical basis
[edit]Water can pass directly from solid to vapour only at pressures below its triple point, which lies at 611.657 Pa and 0.01 °C. Above that pressure, warming ice produces liquid; below it, warming ice produces vapour. Every lyophilisation cycle therefore operates at a chamber pressure well beneath 611 Pa, typically between 5 and 20 Pa, so that heat supplied to the product drives sublimation rather than melting.[1]
The driving force for sublimation is the difference between the vapour pressure of ice at the product temperature and the partial pressure of water vapour in the chamber. Because the vapour pressure of ice falls steeply with temperature, small changes in product temperature produce large changes in drying rate.
| !Temperature | Vapour pressure | Ratio to 0 °C |
|---|---|---|
| 0 °C | 611.2 Pa | 1.00 |
| −10 °C | 259.9 Pa | 0.43 |
| −20 °C | 103.2 Pa | 0.17 |
| −30 °C | 38.0 Pa | 0.062 |
| −40 °C | 12.84 Pa | 0.021 |
| −50 °C | 3.94 Pa | 0.0064 |
Values follow the standard formulations reviewed by Murphy and Koop.[6] The table explains a practical asymmetry of the process: a cycle run with the product at −40 °C rather than −30 °C loses roughly two-thirds of its driving force, and a cycle run 5 °C warmer than intended may dry substantially faster while risking the structural failure described below.
References
- ^ a b Nail SL, Jiang S, Chongprasert S, Knopp SA. "Fundamentals of freeze-drying." Pharmaceutical Biotechnology 14:281–360 (2002).
- ^ Wang W. "Lyophilization and development of solid protein pharmaceuticals." International Journal of Pharmaceutics 203(1–2):1–60 (2000).
- ^ Chang LL, Pikal MJ. "Mechanisms of protein stabilization in the solid state." Journal of Pharmaceutical Sciences 98(9):2886–2908 (2009).
- ^ Tang X, Pikal MJ. "Design of freeze-drying processes for pharmaceuticals: practical advice." Pharmaceutical Research 21(2):191–200 (2004).
- ^ Carpenter JF, Pikal MJ, Chang BS, Randolph TW. "Rational design of stable lyophilized protein formulations: some practical advice." Pharmaceutical Research 14(8):969–975 (1997).
- ^ Murphy DM, Koop T. "Review of the vapour pressures of ice and supercooled water for atmospheric applications." Quarterly Journal of the Royal Meteorological Society 131(608):1539–1565 (2005).