Question

The pKa of the side chain group and the α-carboxyl group of glutamate are 4.1 and 2.1, respectively. Which statement accounts for this difference? The side chain is a different functional group than the α-carboxyl group. The α-carboxyl group has less steric hindrance and is therefore ionized more easily. The side chain has more possible resonance structures. The α-carboxyl group is closer to the α-amino group than the side chain is.

Answer 1

The
`\( \alpha \)`-carboxyl group is closer to the
`\( \alpha \)`-amino group than the side chain is.

Step-by-step explanation:

The
`\( pK_a \)` value gives an indication of the acidity of a group. A lower
`\( pK_a \)` value indicates a stronger acid, meaning it donates its proton more easily.

In glutamate, the
`\( \alpha \)`-carboxyl group has a
`\( pK_a \)` of 2.1, making it more acidic than the side chain carboxyl group with a
`\( pK_a \)` of 4.1.

The difference in
`\( pK_a \)` values between the
`\( \alpha \)`-carboxyl group and the side chain carboxyl group can be attributed to the proximity of the
`\( \alpha \)`-carboxyl group to the
`\( \alpha \)`-amino group. The positive charge on the
`\( \alpha \)`-amino group can stabilize the negative charge on the
`\( \alpha \)`-carboxyl group when it ionizes, making it more acidic.

Therefore, the correct statement is:

The
`\( \alpha \)`-carboxyl group is closer to the
`\( \alpha \)`-amino group than the side chain is.