Week 7: Amino Acid Ionisation Practical

Question 1

What is the structure of glycine?

Answer 1

H2N-CH2-COOH

Glycine is the simplest amino acid with an amino group (NH2), a carboxyl group (COOH), and a hydrogen atom as its side chain.

Question 2

How many pKa values does glycine have?

Answer 2

Two

Glycine has two ionizable groups, leading to two pKa values.

Question 3

What is the pKa for the carboxylic acid group of glycine?

Answer 3

2.34

This value corresponds to the deprotonation of the carboxylic acid group (-COOH).

Question 4

What is the pKa for the ammonium group of glycine?

Answer 4

9.89

This value corresponds to the deprotonation of the ammonium group (-NH3+).

Question 5

What is the formula to calculate pKa from Ka?

Answer 5

pKa = -log(Ka)

This relationship allows the determination of pKa values based on the ionization constant (Ka).

Question 6

What is a buffer?

Answer 6

A solution that resists changes in pH when small amounts of acid or base are added

Buffers maintain a stable pH in a solution, typically within one pH unit of the pKa.

Question 7

How can the buffer region be determined?

Answer 7

pKa +/- 1 pH unit

This range indicates the effective buffering capacity of the solution.

Question 8

What is the isoelectric point (pI)?

Answer 8

The pH at which a molecule has a net charge of zero

The isoelectric point is calculated using pKa values.

Question 9

How can we calculate the isoelectric point (pI) of glycine?

Answer 9

pI = (pKa1 + pKa2) / 2 = (2.34 + 9.89) / 2 = 6.12

This calculation provides the pH at which glycine exists predominantly as a zwitterion.

Question 10

What is the pKa for the C-terminus of glycyl glycine?

Answer 10

3.00

This value reflects the acidity of the carboxylic acid terminal group.

Question 11

What is the pKa for the N-terminus of glycyl glycine?

Answer 11

8.34

This value corresponds to the ammonium group at the N-terminus.

Question 12

What is the buffer range for glycyl glycine?

Answer 12

pKa +/- 1 pH unit

For glycyl glycine, the buffer range is from 2.34-4.34 for the C-terminus and 7.34-9.34 for the N-terminus.

Question 13

What is the isoelectric point of glycyl glycine?

Answer 13

5.67

This value is derived from the average of the pKa values for the two ionizable groups.

Question 14

What are the pKa values for glutamic acid?

Answer 14

2.12, 4.09, 9.67

Glutamic acid has three pKa values due to its additional carboxylic acid group.

Question 15

What is the isoelectric point of glutamic acid?

Answer 15

3.11

This is calculated using the pKa values for glutamic acid.

Question 16

What is the net charge of glycine at pH 7?

Answer 16

0

At the isoelectric point, glycine has no net charge.

Question 17

What is the net charge of glutamic acid at pH 7?

Answer 17

-1

Glutamic acid carries a negative charge due to its additional carboxylic acid group.

Question 18

What is the significance of pKa values in amino acids?

Answer 18

They determine the ionization state of the amino acid at different pH levels

pKa values influence the behavior of amino acids in biological systems.

Question 19

Fill in the blank: The titration curve of an amino acid typically shows the relationship between _______ and pH.

Answer 19

Volume of titrant

This relationship is essential for determining pKa values.