Which of the following amino acids has the lowest iso-electric point?

Aspartic acid

The correct answer is option 3. Aspartic acid.

The isoelectric point \((pI)\) of an amino acid is the pH at which the amino acid has no net charge. The pI depends on the ionizable groups in the amino acid's structure. Here’s how the pI values for the given amino acids compare:

1. Glycine:  Glycine is a simple amino acid with a neutral side chain. It has only two ionizable groups: the amino group and the carboxyl group. Its pI is around 6.1.

2. Alanine: Alanine is another simple amino acid with a neutral side chain similar to glycine. Its pI is also around 6.0.

3. Aspartic Acid: Aspartic acid has an acidic side chain with a carboxyl group, in addition to the amino group and the alpha-carboxyl group. This extra carboxyl group gives it a lower pI because the molecule is more negatively charged at lower pH. Its pI is around 2.8-3.0.

4. Lysine: Lysine has a basic side chain with an additional amino group. This gives it a higher pI because the molecule is more positively charged at higher pH. Its pI is around 9.7-10.5.

Conclusion

Among the given amino acids, aspartic acid has the lowest isoelectric point due to its extra carboxyl group, which makes the molecule more negatively charged and lowers the pI. The correct answer is: Aspartic acid.