Isoelectric point question

[kfcman289]

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How is it that an amino acid would be negatively charged if the pH is above the isoelectric point? The isoelectric point is the average of the two pka's, so wouldn't the amino acid not be negatively charged until the pH was at the higher pkA?

 

[azor ahai]

the pI is just when the zwitterion is the most abundant, that shifts towards -1 as it approaches the pKa of the N-terminal when that is its most abundant.

 

[erythrocyte666]

How is it that an amino acid would be negatively charged if the pH is above the isoelectric point? The isoelectric point is the average of the two pka's, so wouldn't the amino acid not be negatively charged until the pH was at the higher pkA?

for individual amino acid, yes; for population of amino acid, no.
think of the bell curve (in actuality, the Boltzmann distribution) - say pH is at pI, majority of particles will be zwitterions which would be shown by peak of bell curve; BUT there are still going to be a few to the right of the peak that will be negatively charged and a few (an equal amount) to the left of peak that will be positively charged. apply this similarly to pH above and below pI. this is due to Brownian motion