Glycine: calculating the isoelectric point
Glycine has pKa values of 2.34 (carboxyl group) and 9.60 (amino group). What is its isoelectric point?
Answer C: explanation for every choice
A
This is the carboxyl pKa, where half the carboxyl groups are deprotonated and the average charge is +½.
B
This is half of the amino pKa, not the average of the two pKa values.
C
Correct. For an amino acid without an ionizable side chain, pI = (2.34 + 9.60)/2 = 5.97.
D
This is the sum of the two values, without dividing by 2.
TakeawayWith no ionizable side chain, pI = (pKa1 + pKa2)/2.
Equation usedIsoelectric point
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