Which of the following amino acids has the highest pI?
A) arginine
B) cysteine
C) glycine
D) glutamic acid

Question

Quizplus · Accepted Answer

pI (isoelectric point) is the pH at which the amino acid has no net charge. Arginine has three ionizable groups: the amino group, the guanidino group, and the carboxylate group. At low pH, the amino and guanidino groups are protonated, giving the molecule a net positive charge. As the pH is increased, these groups progressively lose their protons, while the carboxylate group remains deprotonated. At the pI, all three groups are present in their zwitterionic form, and the molecule has no net charge. Arginine has a high pKa value for its guanidino group, giving it a high pI value of around 10.8. The other amino acids listed have lower pI values (cysteine has a pI of 5.1, glycine has a pI of 6.0, and glutamic acid has a pI of 3.2).

Which of the Following Amino Acids Has the Highest PI