Question 1

At pH 12, what is the correct structure of the amino acid leucine? Consider the ionization states of both the amino and carboxyl groups at this pH.

Accepted Answer

The amino group is $$\mathrm{NH_2$}$$ and the carboxyl group is $$\mathrm{COO^-}$$:

$$\mathrm{H_2N{-}CH(CO_2$^-)CH_2CH(CH_3$)_2}$$

Question 2

Which of the following best explains why the carboxyl group of leucine is deprotonated at pH 12?

Accepted Answer

Because the pH is much higher than the pKa of the carboxyl group, so it loses a proton and becomes negatively charged.

Question 3

At pH 12, what is the net charge of leucine?

Accepted Answer

-1

Question 4

Which of the following statements about amino acid ionization is correct at pH 12?

Accepted Answer

The amino group is deprotonated and the carboxyl group is deprotonated.

Question 5

Given the following pKa values: $$pK_a($$carboxyl) $$\approx 2.3$$, $$pK_a($$amino) $$\approx 9.7$$, which groups are deprotonated in leucine at pH 12?

Accepted Answer

Both the carboxyl and amino groups

Question 6

Which of the following best describes the effect of increasing pH on the ionization state of the amino group in leucine?

Accepted Answer

The amino group loses a proton and becomes neutral ($$\mathrm{NH_2$}) as pH $$increases above its $$pK_a.$$

Question 7

If a solution of leucine is titrated from pH 2 to pH 12, which of the following changes occurs first?

Accepted Answer

The carboxyl group loses a proton.

Question 8

Which of the following is the correct Henderson-Hasselbalch equation for calculating the ratio of deprotonated to protonated amino groups in leucine at pH 12?

Accepted Answer

$$pH = pK_a + \log_{10} \left( \frac{[\mathrm{NH_2$}]}{[\mathrm{NH_3$^+}]} \right)$$

Question 9

A patient’s blood pH rises to 12 due to poisoning. Which form of leucine would predominate in their blood?

Accepted Answer

Leucine with a deprotonated amino group ($$\mathrm{NH_2$}) $$and a deprotonated carboxyl group ($$\mathrm{COO^-})$$

Question 10

Which of the following best describes the isoelectric point ($$pI) of $$leucine?

Accepted Answer

$$pI = \frac{pK_{a,\,\mathrm{COOH}} + pK_{a,\,\mathrm{NH_3$^+}}}{2}$$

Amino Acid Structure of Leucine at High pH

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