The dissociation constant of protonated form of phenylalanine and neutral form of phenylalanine are `10^(-4) and 10^(-6)` respectively. Hence isoelectric point of phenyl alanine is

Options

A. 4
B. 6
C. 5
D. 7

Answer

To find the isoelectric point (pI) of phenylalanine using the given dissociation constants, we can follow these steps:

### Step 1: Identify the dissociation constants
We are given:
- \( K_{a1} = 10^{-4} \) (for the protonated form of phenylalanine)
- \( K_{a2} = 10^{-6} \) (for the neutral form of phenylalanine)

### Step 2: Calculate pKa values
The pKa values can be calculated using the formula:
\[ pK_a = -\log(K_a) \]

#### For \( K_{a1} \):
\[ pK_{a1} = -\log(10^{-4}) = 4 \]

#### For \( K_{a2} \):
\[ pK_{a2} = -\log(10^{-6}) = 6 \]

### Step 3: Calculate the isoelectric point (pI)
The isoelectric point is calculated using the formula:
\[ pI = \frac{pK_{a1} + pK_{a2}}{2} \]

Substituting the values we calculated:
\[ pI = \frac{4 + 6}{2} = \frac{10}{2} = 5 \]

### Conclusion
The isoelectric point (pI) of phenylalanine is **5**.

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