Question

Calculate the pI of ASN (α-carboxyl group pK a= 2.1; : α-amino group pK a=9.0

Answer 1

first of all let me clear something

the question is not complete but I will solve this by both the ways

incomplete and complete...

firstly by the statement is given above

ASN stands for asparagine which is one of the amino acids

and pI stands for Isoelectric point which is the value of pH at which net charge accumulated by molecule is zero

and is calculated by the average values of given pKa of the molecule

as two value are given here

so simply by mean of these values

pI = ( pKa 1 + pKa 2 ) / 2

= (2.1 + 9.0 )  /2 = 5.55

second method when question is assumed to be complete

as ASN is basic amino acid so there will be another value

pK ( R-group) which will be approximately 13.2

so now

to calculate pI

there are two conditions

1- if value of pK1 and pK(R- group) < 7

then pI = (pK1 + pK(R-group ) ) /2

2- if value of pK2 and pK(R- group) > 7

then pI = ( pK2 + pK (R-group)) /2

as second condition fulfils

then pI = ( 9.0 + 13.2) /2 = 10.6