Question

12. Isoelectric point of following zwitter ion is
9.6
NH-CH-COOH.
pk = 9.6 I pK, = 4.2
I-2
CH,
(1) 5.9
(3) 4.3
(2) 4.9
(4) 6.9​

Answer 1

2 answer hsuhshsshnshsbdgzvxnhxjsgsjs

Answer 2

The isoelectric point is the pH of which the number of positive and negative charges on a population of molecules is equal.

Explanation:

This amino acid is alanine. It has one amino group(NH2) and one carboxyl group(COOH).

• The pK1 of carboxyl acid group Is 4.2
• And the pK2 of amino group is 9.6.

1. In 4,2 pK1 the amino acid is fully protonated (in figure A of the attached image), that means amino group has positive charge.
2. When the pH pass 4.2, carboxyl group loss a hydrogen and form COO-(has a negative charge). Now the amino acid contains one positive charge(NH3+) and one negative charge(COO-).(figure B)
3. When pH pass 9.6, it loss one hydrogen from the amino group. So now the amino acid has one negative charge (COO-) (figure C)

• So the net charge of figure A is (+1)  

• The net charge of figure B is (0).

• The net charge of figure C is (-1)

The zwitter ion is figure B, because it’s electrically neutral (equal number of positive and negative charge).

So to calculate the isoelectric point(PI) of this amino acid calculate the average of 2to pK values before and after of this zwitter ion.

• pK1=4.2
• pK2= 9.6
• PI=(pK1+pK2)/2= 6.9

So the correct answer is 4)6.9