pI for acidic vs basic amino acids

[pm1]

Argh.. I thought I had this done. But may be not.

I thought that when:

pI > pH -- the amino acid is protonated and receives a partial or full positive charge --> attracted to cathode (-)

pI < pH -- the amino acid is deprotonated and receives a partial or full negative charge --> attracted to anode (+)



but I don't get why basic amino acids have a higher pI. Wouldn't they be a zwiterion when they are in a lower pH???

or is it because basic amino acids would be so willing to receive a H+ that they would do so even when H+ is more scarce (higher pH)??

Sorry if the question is convoluted.. got really confused

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[monkeyvokes]

Argh.. I thought I had this done. But may be not.

I thought that when:

pI > pH -- the amino acid is protonated and receives a partial or full positive charge --> attracted to cathode (-)

pI < pH -- the amino acid is deprotonated and receives a partial or full negative charge --> attracted to anode (+)



but I don't get why basic amino acids have a higher pI. Wouldn't they be a zwiterion when they are in a lower pH???

or is it because basic amino acids would be so willing to receive a H+ that they would do so even when H+ is more scarce (higher pH)??

Sorry if the question is convoluted.. got really confused

when they're in a lower pH (but still above the carboxyl pKa), the basic amino acids protonate and take on a +1 net charge.

To be at pI for basic amino acids, you need to be at a pH above their side chain's pKa but below the amino groups pKa. This way, the side chain is uncharged and the main chain's carboxyl and amino groups remain charged (net = zero, isoelectric)
Maybe try drawing these stages on a titration graph of a basic amino acid to see how it changes with pH?

 

[BerkReviewTeach]

Argh.. I thought I had this done. But may be not.

I thought that when:

pI > pH -- the amino acid is protonated and receives a partial or full positive charge --> attracted to cathode (-)

pI < pH -- the amino acid is deprotonated and receives a partial or full negative charge --> attracted to anode (+)



but I don't get why basic amino acids have a higher pI. Wouldn't they be a zwiterion when they are in a lower pH???

or is it because basic amino acids would be so willing to receive a H+ that they would do so even when H+ is more scarce (higher pH)??

Sorry if the question is convoluted.. got really confused

Basic amino acids have sidechains that get protonated and carry a (+)-charge, so to form a zwitterion, they must be deprotonated twice from their fully protonated state. This is why you must average pK_a2 and pK_a3 to determine their pI.

As far as electrophoresis goes, check out the last part of this post. It does a good job at explaining the migration process for proteins and AAs. It also has a couple other cool math tricks too.

 

[ashtonjam]

This picture of Lysine titration (basic amino acid) should help you a lot. Nitrogens on amines are known to be + charged or neutral, never negative (in context of amino acids). Basic amino acids have two nitrogen containing groups, so at their most protonated state, the alpha amino group is +1, the side chain is +1, and the acid group is 0. After you raise pH, you slowly deprotonate one at a time.

It goes +2 -> pKa1 (the acid) -> +1 -> pKa2 (the alpha amino group) -> +0 -> pKa3 (side chain) -> -1.

Look where +0 is; it's between pKa2 and pKa3. You average those to find pI. pKa2 and pKa3 are both values above 7. That's why basic amino acids have pI above 7. This works for not only Lysine and Arginine, but also Histidine. For Histidine, the ring deprotonates before the alpha amino group, but pKa2 and pKa3 are still averaged.