Question

At which pH would the net electric charge of tyrosine be zero, given that its -COOH group has a pKa of 2.20, its –NH3+ group has a pKa of 9.11, and its R group has a pKa of 10.07?

Answer 1

We need to make a table in which we need to put charges for the amino acids depending on the pK values. For tyrosine, the charge will be zero below and -1 above its corresponding pK values. For -NH3+, charge will be +1 below and zero above the pK values. For COOH, charge will be zero below and -1 above pK value. Then we need to do total of all the charges at corresponding pH values.

pH	tyr	NH3	COOH	total
1	0	1	0	1
2	0	1	0	1
3	0	1	-1	0
4	0	1	-1	0
5	0	1	-1	0
6	0	1	-1	0
7	0	1	-1	0
8	0	1	-1	0
9	0	0	-1	-1
10	0	0	-1	-1
11	-1	0	-1	-2
12	-1	0	-1	-2
13	-1	0	-1	-2

The charge is zero at pH = 3-8. After this, we need to take pK close to 3 and 8 from values of pK given. The closest values are 2.20 and 9.11. We need to take average of the closest pKa values, which is the pI.

So, average of 2.20 and 9.11 is 5.66.

So, pI = 5.66.

The pH at which net electric charge of tyrosine be zero is 5.66.