You can freeze protein powder
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The stability of proteins: practical aspects
Proteins are stable because the amino acids interact with each other and with the solvent water. Cysteine residues in the protein can form disulfide bridges with neighboring cysteine residues (which considerably stabilize the tertiary structure of a protein), charged amino acids can interact with oppositely charged amino acids, hydrogen bridges form and uncharged amino acids form hydrophobic areas that To a large extent exclude molecules (hydrophobic effect).
- Despite all these effects, the stably folded state of a protein is often only slightly more energetic than the unfolded state (in which the amino acids interact with the surrounding water molecules), and numerous factors must be taken into account when isolating these proteins in their native structure should.
Proteins lose their native structure and denature if they are exposed to extreme temperatures or extreme pH values, for example (see the following pages). However, a whole range of other factors, which have to be checked empirically for each individual protein, can impair the stability of a protein during purification. While some proteins fail on contact with certain surfaces (glass, plastic), others are very sensitive to foam formation during isolation, the presence of heavy metal ions that bind to the protein, or the dilution below a certain concentration in solution. In general:
- Large proteins are often more stable than small proteins
- Proteins with disulfide bridges unfold less easily and are therefore less likely to bind to surfaces.
- The binding of the protein to surfaces can be reduced by adding phospholipids and surface-active substances. Often albumin is also added when a protein is purified. Albumin is very soluble and stabilizes other proteins in aqueous solution.
- Since proteins can also aggregate at the water / air interface, foam formation or vigorous shaking should be avoided when a protein is purified (because this increases this interface considerably!).
The storage of proteins
While some proteins can be stored with or even for years without loss of activity, other proteins can only be stored for a short time (days or hours) before they irreversibly precipitate out of solution or lose their activity. In general, the half-life of a protein in solution is extended at low temperatures. Fast freezing is important here, so that no phase separation can occur within the protein suspension. A phase separation can lead to very high or low buffer concentrations occurring locally and the pH value in these areas fluctuates by +/- three pH units. Storage options are e.g.
- Addition of glycerol. In the case of many proteins, the addition of glycerol (up to 50% w / v) leads to a longer shelf life at low temperatures, especially because glycerol / protein mixtures do not freeze even at low temperatures. Glucose, sucrose, fructose and sorbitol appear to have a similar effect on the stability of a protein in solution
- Storage of a protein in ammonium sulfate (i.e. as a precipitate)
- Precipitation of a protein with ammonium sulfate, centrifugation, freezing the pellet in liquid nitrogen () and storage at
- Storage as a lyophilisate
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