Acid-Base Chemistry of Amino Acids

Question 1

What is a zwitterion?

Answer 1

A molecule with both a positive AND negative charge, it is electrically neutral.

Question 2

Under neutral conditions, are amino acids protonated or deprotonated?

Answer 2

Neither!

Amino acids are electrically neutral and are considered zwitterions.

Question 3

What is isoelectric point (pI)?

Answer 3

The pH at which an amino acid is electrically neutral.

Question 4

How to calculate the pI of a neutral amino acid:

Answer 4

(calculate the average of the amino and carboxy terminals)

Question 5

How to calculate the pI of an acidic amino acid:

Answer 5

(calculate the average of the carboxy terminal and side chain)

Question 6

How to calculate the pI of a basic amino acid:

Answer 6

(calculate the average of the amino terminal and side chain)

Question 7

Which molecule is released during the formation of a peptide bond in an amino acid?

Answer 7

Water

Question 8

At physiological pH why is the carboxylic acid group of an amino acid, COO^- instead of COOH?

Answer 8

Because pH (7.4) > pKa (2).

This causes the carboxyl group to become deprotonated.

Question 9

At physiological pH why is the amino group of an amino acid, NH₃⁺ instead of NH₂?

Answer 9

Because pH (7.4)

This causes the amino group to become protonated.

Question 10

Which seven amino acids have ionizable groups?

(these side groups are either basic/acidic or charged)

Answer 10

1. arginine
2. histidine
3. lysine
4. glutamate
5. aspartate
6. tyrosine
7. cysteine