Cystine Disodium Salt
As a salt of a dibasic amino acid, Cystine Disodium Salt exhibits typical amphoteric characteristics, with its ionization state closely dependent on the pH of the solution.
- In acidic conditions (pH < 3.0): The carboxylate anions (−COO−) accept protons and are converted back to carboxyl groups (−COOH). As the pH decreases, the solubility of the compound drops sharply, and free L-cystine precipitates out of the solution. This reaction is the basis for the industrial preparation of L-cystine from Cystine Disodium Salt—acidification with dilute hydrochloric acid or sulfuric acid triggers crystallization of L-cystine with high purity.
- In neutral to weakly alkaline conditions (pH 7.0–9.0): The compound exists stably as a zwitterion, with carboxylate groups in the anionic form and amino groups protonated to −NH3+. This is the most stable state for Cystine Disodium Salt in aqueous solutions, and it remains fully dissolved, making it suitable for formulating oral or injectable preparations.
- In strongly alkaline conditions (pH > 12.0, with heating): The disulfide bond is prone to cleavage under the action of strong bases (e.g., 40% NaOH solution), producing sodium cysteinate. Prolonged heating accelerates this hydrolysis reaction, which is a common method for preparing L-cysteine from Cystine Disodium Salt in the laboratory.
The isoelectric point (pI) of the corresponding free L-cystine is approximately 5.05, but Cystine Disodium Salt itself does not have a distinct isoelectric point due to its pre-ionized carboxylate groups; its solubility minimum occurs at pH close to the pI of L-cystine.
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