`{:(HOOC-CH_(2)-underset(NH_(2))underset("| ")("C ")H-COOH,underset((-COOH))(pK_(a_(1))),underset((-NH_(2)))(pK_(a_(2))),underset(("Acid or base of "-R))(pK_(a_(3)))),(" Aspartic acid"," 2.1"," 9.8"," 3.9"),(HOOC-underset(NH_(2))underset("| ")("C ")H-(CH_(2))_(4)-NH_(2),underset((-COOH))(pK_(a_(1))),underset((-NH_(2)))(pK_(a_(2))),underset(("Acid or base of "-R))(pK_(a_(3)))),(" Lysine"," 2.2"," 9.0"," 10.5"),(HOOC-underset(NH_(2))underset("| ")("C ")H-CH-OH,,,),(" Threonine"," 2.6"," 10.4",),(HOOC-underset(NH_(2))underset("| ")("C ")H-CH_(2)CH_(2)CONH_(2),,,),(" Glutamine"," 2.2"," 9.1",):}`
At pH = 3 which of the following of aspartic acid is predominant ?

Options

A. `""^(-)OOC-CH_(2)-underset(NH_(3)^(+))underset("| ")("C ")H-COO^(-)`
B. `""^(-)OOC-CH_(2)-underset(NH_(3)^(+))underset("| ")("C ")H-COO^(-)`
C. `HOOC-CH_(2)-underset(NH_(3)^(+))underset("| ")("C ")H-COO^(-)`
D. `HOOC-CH_(2)-underset(NH_(2))underset("| ")("C ")H-COO^(-)`

Explanation

To determine which form of aspartic acid is predominant at pH 3, we will follow these steps:

### Step 1: Understand the pKa values
The pKa values for aspartic acid are given as:
- pKa1 = 2.1 (for the carboxyl group -COOH)
- pKa2 = 3.9 (for the side chain carboxyl group -COOH)
- pKa3 = 9.8 (for the amino group -NH2)

### Step 2: Determine the isoelectric point (pI)
The isoelectric point (pI) is calculated using the pKa values of the acidic groups. For aspartic acid, we consider the two acidic groups:
\[ \text{pI} = \frac{\text{pKa1} + \text{pKa2}}{2} = \frac{2.1 + 3.9}{2} = \frac{6.0}{2} = 3.0 \]

### Step 3: Analyze the charge at pH 3
At pH 3:
- The carboxyl group (pKa1 = 2.1) will be deprotonated and negatively charged (-COO⁻).
- The side chain carboxyl group (pKa2 = 3.9) will still be protonated and neutral (-COOH).
- The amino group (pKa3 = 9.8) will be protonated and positively charged (-NH₃⁺).

### Step 4: Determine the overall charge
At pH 3:
- The overall charge of aspartic acid can be calculated as follows:
- One -COO⁻ (negative charge)
- One -COOH (neutral)
- One -NH₃⁺ (positive charge)

Thus, the overall charge is:
\[ -1 + 0 + +1 = 0 \]
This means that aspartic acid is neutral at this pH.

### Step 5: Identify the predominant form
Since the overall charge is zero, the predominant form of aspartic acid at pH 3 is the zwitterionic form, where it has one positive charge and one negative charge.

### Conclusion
Thus, at pH 3, the predominant form of aspartic acid is the zwitterionic form.

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