The solubility of peptides depends to a large extent on the polarity of peptides. Acidic proteins are dissolved in alkaline solutions, while alkaline proteins can be dissolved in acidic solutions. Hydrophobic 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.
Polypeptide dissolution test: take a small part of the polypeptide dissolution test before it dissolves. You need to test several different solvents until you find the most appropriate one. Ultrasonic treatment helps to break particles and increase solubility. (note: ultrasonic treatment can cause solution heating and polypeptide degradation. )
Each acidic amino acid was assigned to-1, including aspartic acid (D), glutamic acid (E), and carboxy terminal-COOH. Each basic amino acid is assigned to + 1, including arginine (R), lysine (K), histidine (H) and amino terminal NH2. Then the charge number of the whole polypeptide was calculated.
If the charge of the whole peptide is positive, it indicates that the peptide is alkaline. Try to dissolve with distilled water first; if insoluble in water, then try to dissolve with a small amount of 10% ≤ 25% acetic acid, add some TFA (10-50 microliters) if it still fails, and then dilute it to the ideal concentration with water.
If the charge of the whole peptide is negative, the peptide is acidic. Acidic peptides can be dissolved with PBS (PH 7.4). If insoluble, a small amount of alkaline solvent, such as 0.1 M ammonium bicarbonate, can be added and diluted to the ideal concentration with water. Peptides containing free cysteine should be dissolved in deaerated acid buffers because sulfhydryl groups are rapidly oxidized to disulfide when PH value is greater than 7.
If the charge of the whole peptide is zero, the peptide is neutral. Neutral peptides are usually soluble in organic solvents. First, try adding a small amount of acetonitrile, methanol, or isopropanol. For highly hydrophobic peptides, a small amount of dimethyl sulfoxide can be used to dissolve and then dilute to the ideal concentration with water. For peptides containing free cysteine, DMF instead of DMSO should be used. For peptides with aggregation tendency, 6m guanidine Hydrochloride or 8m urea can be added and diluted as necessary.
In order to prevent or minimize the degradation of peptides, it is better to keep the peptides in the form of lyophilized powder at-20 °C and-80 °C. If the solution peptide needs to be preserved, it is best to store it in small samples to avoid repeated freeze-thaw. A sample should be thrown away if it is not used up after thawing. Bacterial degradation can sometimes be a problem for solution peptides, so please dissolve the peptides in aseptic water or peptide solution and filter them to remove bacteria.
Basic amino acids: K, R, H, N-terminus
Acid amino acids: d, E, C-terminus
Polar neutral amino acids: F, I, L, M, V, W, Y
Non-polar hydrophobic amino acids: G, A, S, T, C, N, Q, P, acetylgroup, amide group.
