Solubility Tests for Amino Acids and Proteins

The solubility of amino acids and proteins is largely dependent on the pH of the solution. The structural changes that occur at different pH values in an amino acid or protein change the molecule's relative solubility.

Amino and carboxylic groups are both protonated in acidic solutions. Both groups are deprotonated in basic solutions. In water, amino acids are essentially soluble. Depending on the structure of its side chains, their solubility in water, dilute alkali and dilute acid varies from one compound to another. Glycine, tyrosine, glutamic acid and cysteine will be used for this test.

Procedure

The solubility of amino acids in water and alcohol should be noted by placing a small quantity in the test tube, adding a few mL of solvent and if necessary, warming up.

Use the litmus paper to determine whether the amino acid solution is acidic or basic, while testing the solubility in water. Using dilute HCl and dilute NaOH, repeat the solubility test.

Note:

All amino acids are generally soluble in low pH solutions because they exist in their protonated form at these pH values. However, there are some amino acids that are more soluble than others in acidic conditions. These amino acids tend to have polar or charged side chains that can interact well with water molecules.

Examples of amino acids that are relatively more soluble in low pH solutions include:

• Aspartic acid (Asp, D)
• Glutamic acid (Glu, E)
• Histidine (His, H)
• Lysine (Lys, K)
• Arginine (Arg, R)

Amino acids tend to be less soluble in high pH solutions because they exist in their deprotonated form at these pH values, which makes them less soluble in water. However, there are some amino acids that are relatively more soluble in high pH solutions. These amino acids tend to have nonpolar or hydrophobic side chains that can interact well with the hydrophobic environment that is present at high pH values. 

Examples of amino acids that are relatively more soluble in high pH solutions include:

• Alanine (Ala, A)
• Valine (Val, V)
• Leucine (Leu, L)
• Isoleucine (Ile, I)
• Phenylalanine (Phe, F)
• Tryptophan (Trp, W)
• Tyrosine (Tyr, Y)

It is important to note that the solubility of amino acids in high pH solutions can be affected by various factors, such as their concentration, temperature, and the presence of other solutes. Therefore, the solubility of an amino acid in high or low pH solution may vary depending on these factors.