Solubility

The solubility in water for different amino acids varies greatly. Cystine is the most sparingly soluble, followed by tyrosine. Aspartic acid, glutamic acid, and leucine belong to the category of comparatively insoluble amino acids. The easily soluble amino acids are proline and hydroxyproline, and other amino acids belong to the relatively soluble category. The optical isomers for each amino acid—l,d, and d,l—demonstrate slight difference in solubility.

In general, the solubility increases when the temperature increases. Acidic amino acids especially show a remarkable dependence on temperature, for example, aspartic acid: 2.09 g/L at 0°C and 68.87 g/L at 100°C.

The solubility is expressed using a function of temperature as follows:

The empirical coefficients a, b, and c for various amino acids are summarized in Table 8 (32,34,35).

The solubility of amino acids varies with the change in pH and is minimal at the isoelectric point. The solubility increases when the isoelectric point is deviated toward alkaline or acid in solution. The change in solubility is governed by the ratio of the ionic form corresponding to the pH in solution. Therefore, the monochloride salts of both neutral amino acids and basic amino acids are more solu-

Table 8. Parameter of Solubility for Various Amino Acids

Amino acid

a

b X 102

c X 105

Glycine

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