Question 1

Which term describes an amino acid that has both a positively charged and a negatively charged group, but is overall electrically neutral at physiological pH?

Accepted Answer

Zwitterion

Question 2

At physiological pH (approximately 7.4), what is the predominant form of the backbone of a free amino acid?

Accepted Answer

The amino group is protonated ($$\mathrm{NH_3$^+}) $$and the carboxyl group is deprotonated ($$\mathrm{COO^-})$$

Question 3

Which of the following best explains why the zwitterion form of an amino acid predominates at physiological pH?

Accepted Answer

The pH is between the $$pK_a$$ values of the amino and carboxyl groups, causing one to be protonated and the other deprotonated

Question 4

Given the $$pK_a$$ values for the carboxyl group ($$pK_a \approx 2) $$and the amino group ($$pK_a \approx 9$$), at what pH will the amino acid have a net zero charge?

Accepted Answer

At a pH between the two $$pK_a$$ values, typically around pH 7

Question 5

Which of the following equations correctly represents the Henderson-Hasselbalch equation used to determine the ratio of protonated to deprotonated forms of an amino acid group?

Accepted Answer

$$pH = pK_a + \log_{10} \left( \frac{[A^-]}{[HA]} \right)$$

Question 6

If the $$pK_a of $$the carboxyl group is 2.2 and the $$pK_a of $$the amino group is 9.6, what is the predominant form of valine at pH 7?

Accepted Answer

Carboxyl group is deprotonated ($$\mathrm{COO^-}$$), amino group is protonated ($$\mathrm{NH_3$^+})$$

Question 7

Amino acids can act as buffers in solution. Which property allows them to resist changes in pH?

Accepted Answer

They have ionizable groups with distinct $$pK_a$$ values

Question 8

At what pH would an amino acid have both a neutral $$-COOH$$ and a neutral $$-NH_2$ group?

Accepted Answer

Not likely to occur at any pH

Question 9

Which of the following best describes the process of determining the predominant structure of an amino acid at a given pH?

Accepted Answer

Compare the pH to the $$pK_a$$ values of the ionizable groups to determine their protonation states

Question 10

If the pH of a solution is much lower than the $$pK_a of an $$amino acid group, what is the expected protonation state of that group?

Accepted Answer

The group will be predominantly protonated

Question 11

Arginine has three ionizable groups with $$pK_a$$ values of 2.17, 9.04, and 12.48. At pH 8.5, which group is most likely to be deprotonated?

Accepted Answer

The carboxyl group ($$pK_a = 2.17)$$

Question 12

Which of the following statements about the isoelectric point ($$pI) of an $$amino acid is correct?

Accepted Answer

It is the pH at which the amino acid has no net electrical charge

Question 13

Which of the following best describes the effect of increasing pH on the charge of the backbone of a free amino acid?

Accepted Answer

The backbone becomes more negatively charged as the amino group loses a proton

Question 14

A patient’s blood pH drops from 7.4 to 6.8 due to acidosis. What change would you expect in the predominant form of amino acids in their blood?

Accepted Answer

More amino groups will be protonated ($$\mathrm{NH_3$^+}$$), and more carboxyl groups will be protonated ($$\mathrm{COOH})$$

Question 15

Which of the following is the correct order of protonation states for the carboxyl group of an amino acid as pH increases from 1 to 10?

Accepted Answer

$$\mathrm{COOH} \rightarrow \mathrm{COO^-}$$

Question 16

Which of the following amino acid forms would predominate at a pH equal to the $$pK_a of $$its carboxyl group?

Accepted Answer

A 1:1 ratio of protonated ($$\mathrm{COOH}) $$and deprotonated ($$\mathrm{COO^-}) $$forms

Question 17

Which of the following best describes the relationship between $$pK_a$$ and acid strength?

Accepted Answer

Lower $$pK_a$$ indicates a stronger acid

Question 18

If alanine has a $$pK_a of 2.3 $$for its carboxyl group and 9.7 for its amino group, what is the predominant structure at pH 13?

Accepted Answer

Both the carboxyl and amino groups are deprotonated ($$\mathrm{COO^-}$$ and $$\mathrm{NH_2$})$$

Question 19

Which of the following is a correct application of the concept of $$pK_a in $$predicting the charge of an amino acid at a given pH?

Accepted Answer

If pH \u003e $$pK_a$$, the group is deprotonated; if pH \u003c $$pK_a$$, the group is protonated

Amino Acid Zwitterions and Predominant Structures at Different pH

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