The pKa of the carboxyl group in an amino acid valine, `(CH_3)_2 CHCH(NH_2)(COOH)` is 2.31 and the pKa for amino group of the same amino acid is 9.69. Compute the isolectric point (pI) for valine and draw the structure of this amino acid when the pH of the solution equals top 1. Also draw the structures of valine that predominate at pH =2 and pOH=2.

Explanation

The isoelectric point (pI) is the pH at which the amino acid exists only as a dipolar ion with net charge zero. At isoelectric point, for a neutral amino acid, `pI=(pK_(a_1)+ pK_(a_2))/(2)`
The dissociation of cationic form of valine can be represented as


The species with zero net charge exists between species with (+1) and (-1) net charges.
`pI= (pK_(a_1)+ pK_(a_2))/(2)= (9.69+2.31)/(2)=6`
When the pI-I of the solution equals to pI, the structive of valine is

When the pH of the solution is two, the structure of valine is

When the pH of the solution is 12, the structure of valine is