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Although dissolving peptides is not a complex thing, it is generally difficult to determine the appropriate solvent at once. Usually take a little test first, and do not dissolve it all before determining the appropriate solvent.

The following methods can help you choose the right solvent: 

(1) Determine the specific charge of the peptide, and set the acidic amino acids Asp (D), Glu (E) and C-terminal COOH to - 1; The basic amino acids Lys (K), Arg (R), His (H) and N-terminal NH2 are + 1, and the charge of other amino acids is 0. Roughly calculate the charge of the peptide.

(2) If the net charge number > 0, the peptide is alkaline and dissolved with water: if it is insoluble or has little solubility, add acetic acid (more than 10%); If the peptide cannot be dissolved, add a small amount of TFA (25ul) to dissolve, and then add 500ul of water to dilute.

(3) If the net charge is less than 0, the peptide is acidic and dissolved in water; If it does not dissolve or has little solubility, add ammonia (25ul) to dissolve it, and then add 500ul water to dilute it.

(4) If the net charge number = 0 and the peptide is neutral, it generally needs to be dissolved in organic solvents such as Acetonitrile, methanol or Isopropanol, DMSO, etc. Some researchers suggest that urea is needed to dissolve hydrophobic peptides for reference.

The solubility of peptides depends largely on the polarity of peptides. Acidic proteins are dissolved in alkaline solutions, while alkaline proteins can be dissolved in acidic solutions. Hydrophobic peptides and neutral peptides containing a large number of uncharged polar amino acid residues or hydrophobic amino acids can be dissolved in a small amount of organic solvents, such as DMSO, DMF, acetic acid, Acetonitrile, methanol, Propanol or Isopropanol, and then diluted with water (distilled water). Peptides containing Methionine or Cysteine cannot be dissolved with DMSO because DMSO may cause side chain oxidation.

Peptide dissolution test: take a small part for peptide dissolution test before peptide dissolution. You need to test several different solvents until you find the most appropriate one (determined according to the experimental requirements and solubility experiment). Ultrasonic treatment helps to break up particles and increase solubility. (Note: ultrasonic treatment will cause solution heating and peptide degradation.)

Preservation of lyophilized powder peptide

All products marked "keep freeze-dried" must be stored under freezing conditions, preferably - 20 ℃. If stored below - 80 ℃, most peptides can maintain their activity for several years. (depending on the nature of the peptide, the storage period may change)

When using frozen products, the bottle or test tube shall be raised to room temperature in the drying oven containing fresh desiccant before opening the cover. For products stored at - 20 ℃, this process takes one hour or more, depending on the size of the package. Otherwise, when the bottle is opened, water vapor enters, resulting in peptide condensation and reducing its stability. Once opened, it should be weighed quickly and sealed immediately to  avoid moisture absorption and hydrolysis, especially for hydrophilic peptides.
Preservation of peptide solution

The stability of peptide solution is much worse than that of dry powder. In order to obtain the best results, the following principles shall be followed:

1. Repeated freezing and thawing will damage the activity of peptides, so it is recommended to pack them into small packages for storage. Thaw as much as you need, and discard the rest after use.

2. Dissolve in sterile buffer with pH value of 5- 7 and store at - 20 ℃.

3. Peptides containing Cys, Met, Typ, Glu and Asp are easy to oxidize, so they must be stored in an oxidant free environment.

Because bacteria can degrade peptides, they must be filtered and sterilized before storage.

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