Consider the following sequence for aspartic acid:

The $\mathrm{pI}$ (isoelectric point) of aspartic acid is:

At the isoelectric point of aspartic acid, we have

$\left[{\mathrm{H}}_3{\mathrm{NCH}}_{(}^{+}\left({\mathrm{CH}}_2\mathrm{COOH}\right){\mathrm{CO}}_2\mathrm{H}\right]=\left[{\mathrm{H}}_3{\mathrm{NCH}}^{+}\left({\mathrm{CH}}_2{\mathrm{COO}}^{-}\right){\mathrm{CO}}_2^{-}\right]$

Hence $\mathrm{pI}=\frac{1}{2}\left(\mathrm{p}{K}_1+\mathrm{p}{K}_{\mathrm{R}}\right)=\frac{1}{2}(1.88+3.65)=2.77$