Question

Using the provided $p{K}_a$ values for fully protonated histidine, determine an isoelectric point at which the aminoacid exist mostly in its zwitter ionic form. (upto two decimal places)

Answer 1

A certain pH at which the concentration of the dipolar ion is at its maximum and the concentrations of the anions and cations are equal is called isolelctric point.
The given amino acid is a basic amino acid.
Given:
$p{K}_2=9.2$
$p{K}_3=6.1$
For basic amino acid,
$pI=\frac{p{K}_2+p{K}_3}{2}$

$pI=\frac{9.2+6.3}{2}$
$pI=\frac{15.5}{2}$
$pI=7.75$