Question

Consider the structure of alanine in its fully protonated state with a +1 charge.

Alanine has a methyl side chain. The alpha amino group is protonated and positively charged. The alpha carboxyl group is protonated and is neutral.

Give the p?a value for the carboxyl group of alanine. An answer within ±0.5 is acceptable.
p?a (−COOH)=
Give the p?a value for the amino group of alanine. An answer within ±0.5 is acceptable.
p?a (−NH2)=
Calculate the pI of alanine. Give your answer to two decimal places.
pI =

Answer 1

The pKa value for the carboxyl group of alanine is 2.34

p​​​​​​K​​​​​a (pk_{​​​​​1}) =2.34

The p​​​​​​Ka value for the amino group of alanine is 9.69

p​​​​​​K​​​​​a (pk_{​​​​​2} ) = 9.69

The pI of alanine = (pk_{​​​​​1}+ pk_​​​2 ​​​​​​ )/2

= (2.34 +9.69)/2

=12.03/2

=6.015

= 6.01

Therefore the pI value of alanine is 6.01