pI. vs. pka

[bozz]

If a question is comparing two amino acids, for example, and asks which one has the higher isoelectric point, can I think of the question in terms of pKa values?

If pH> pka ... compound's deprotonated
If pH< pka ... compound's protonated

Similarly

If pH> pI ... compound's deprotonated
If pH< pI ... compound's protonated

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[BerkReviewTeach]

If a question is comparing two amino acids, for example, and asks which one has the higher isoelectric point, can I think of the question in terms of pKa values?

If pH> pka ... compound's deprotonated
If pH< pka ... compound's protonated

Similarly

If pH> pI ... compound's deprotonated
If pH< pI ... compound's protonated

Sort of!

The pKa is found on a titration curve where midway through the buffer region. A pI is found at the equivalence point where the molecule has no net charge. So you can definitely draw a conclusion when comparing pI to pH, but in terms of cationic versus anionic, rather than protonated versus deprotonated.

When pH > pI, the solution is basic relative to the zwitterion, so the amino acid (or protein) will exist as a mixture of the zwitterion and anion, meaning that we can think of the average charge for all of the amino acid species in solution as being slightly negative. As such, it will migrate to the anode in electrophoresis or bind a cationic column in ion exchange chromatography.

The reverse conclusion can be drawn when the pH < pI.