1.2 Acid–Base Chemistry of Amino Acids

Question 1

What happens to amphoteric species under low and high pH?

Answer 1

at low pH, ionizable groups tend to be protonated; at high pH, they tend to be deprotonated.

Question 2

How do pH and pKa affect protonation of an AA?

Answer 2

If the pH is less than the pKa, a
majority of the species will be protonated. If the pH is higher than the pKa, a majority of the species will be deprotonated

Question 3

How many pKa values are there for AAs?

Answer 3

2, one for the carboxyl (COOH) and one for amino group (NH2)

Question 4

What are the 1st and 2nd pKa’s?

Answer 4

• pKa1 for the carboxyl group is around 2
• pKa2 for amino group is usually between 9 and 10

Question 5

What happens to AA’s under very acidic (pH 1) conditions?

Answer 5

• AA’s tend to be positively charged
• NH2 -> NH3+
• COOH becomes neutral

Question 6

What happens to AA’s under intermediate (pH 7.4) conditions?

Answer 6

• AA’s are zwitterions
• NH2 -> NH3+
• COOH -> COO-

Question 7

What happens to AA’s under Basic (pH 10.5) conditions?

Answer 7

• AA’s are negative
• NH3+ -> NH2
• Remains COO-

Question 8

What does titration curve of AA look like?

Answer 8

• like a combination of two monoprotic acid titration curves (or three curves, if the side chain is charged)

Question 9

When does the solution act as a buffer?

Answer 9

• When the pH of a solution is approximately equal to the pKa of the solute.
• The titration curve is relatively flat
• As we add more base, the carboxylate group goes from half-deprotonated to fully deprotonated. The amino acid stops acting like a buffer, and pH starts to increase rapidly during this phase

Question 10

Isoelectric point (pI)

Answer 10

the pH at which the molecule is electrically neutral

Question 11

What does the titration curve of a neutral molecule look like?

Answer 11

• When the molecule is neutral, it is especially sensitive to pH changes, and the titration curve is nearly vertical
• As we continue adding base, glycine passes through a second buffering phase as the amino group deprotonates; again, the pH remains relatively constant.

Question 12

What’s glutamic acids charge when fully protonated?

Answer 12

+1, the amino group gets protonated (NH3+)

Question 13

What’s lysines charge when fully protonated?

Answer 13

+2, amino groups get protonated

Question 14

How do basic and acidic side chains affect isoelectric points?

Answer 14

amino acids with acidic side chains (COOH) have relatively
low isoelectric points, while those with basic side chains (NH2) have relatively high
ones.

Question 15

How to find Isoelectric point of AA’s?

Answer 15

• Add the pKas of side chains and divide by 2