The structure of a basic amino acid, lysine, is given below:
`(HOOC-underset(.^(o+)NH_(3))underset(|)(C)H-(CH_(2))_(3)-CH_(2)overset(o+)(N)H_(3))(A)`
The `pK_(a_1),pK_(a_2),` and `PK_(a_3)` of `(A)`, respectively, are: `2.18,8.95`, and `10.53`.
What is the `pH` at isoelectric points `(pI)`?
A. `(pK_(a_(1))+pK_(a_(2)))/(2)`
B. `(pK_(a_(1))+pK_(a_(3)))/(2)`
C. `(pK_(a_(2))+pK_(a_(3)))/(2)`
D. `(pK_(a_(1))+pK_(a_(2))+pK_(a_(3)))/(3)`
