Final answer:
The pI (isoelectric point) of aspartic acid is approximately 3.9. Option c.
Step-by-step explanation:
The pI (isoelectric point) of an amino acid is the pH at which it exists as a neutral molecule, with equal positive and negative charges. To calculate the pI of aspartic acid, we need to consider its pKa values. Aspartic acid has two pKa values relevant to its side chain, which are 2.1 and 3.9.
We can start by finding the average of these two values, which is (2.1 + 3.9) / 2 = 3.0.
Next, we need to consider the pKa value of the carboxylic acid group (-COOH), which is 2.1. Since this pKa value is lower than the average of the side chain pKa values, it means that at pH 2.1 and below, the carboxylic acid group will be protonated (positive charge) and the side chain will be deprotonated (negative charge).
On the other hand, at pH 3.0 (the average of the side chain pKa values), the carboxylic acid group will be deprotonated (neutral charge) and the side chain will be protonated (positive charge). This is the point where aspartic acid exists as a zwitterion, or a neutral molecule. Therefore, the pI of aspartic acid is approximately 3.9. Option c,