Amino acid and PI

[holdmystethoscope]

Hello all, So looks like most Amino acids have a PI of around 6. Does it mean after 6 it will be slightly negatively charged? So at regular body PH (7.35-7.45) what would be the net charge? I guess what I am asking is if the net charge is determined by PI or pka of each entity in a particular amino acid!

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[theonlytycrane]

Hello all, So looks like all Amino acids have a PI of around 6. Does it mean after 6 it will be slightly negatively charged? So at regular body PH (7.35-7.45) what would be the net charge?

Only amino acids with non-ionizable R groups have PI's around 5.5. Acidic amino acids will have lower PIs and basic amino acids will have higher PIs. At pH = 7, the charge depends on which specific amino acid you're talking about.

Glycine, for example, has a PI ~ 5.5. So at pH = 7, pH > pI and the net charge will be slightly negative.

 

[aldol16]

Hello all, So looks like most Amino acids have a PI of around 6. Does it mean after 6 it will be slightly negatively charged? So at regular body PH (7.35-7.45) what would be the net charge? I guess what I am asking is if the net charge is determined by PI or pka of each entity in a particular amino acid!

The net charge of your amino acid population depends on pI, not the pKa of each ionizable group. I guess you could say it indirectly depends on the pKas of the ionizable groups because if you have a pKa of 1 and 9 versus 2 and 14, you'll obviously have different pI's, but in the case of amino acids, their amino and carboxy termini have almost uniform pKas throughout (in the free form anyway).

And the important thing to note here is that pI does not tell you how much a single molecule of an amino acid is charged. It tells you what to expect the overall average charge of the population to be.