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Question about calculating net charge from PKA
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Daniela_5535
#1 Posted : Friday, July 10, 2020 2:05:36 AM
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Hello,

I had a question stating : The pyrole side chain of histidine has a pka = 6.04. If the PKA values of the carbonyl and the alpha amino groups are 1.83 and 9.33 respectively what is the next charge of histidine at Ph 5?

I know that when the ph is greater then the pka we deprotonate and when the ph is less then the pka we protonate.

But do we also have to take into account the overall charge of histidine ? Would it be +2 because of the charges on the amino group and the NH group ?

Thanks !
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INSTR_Molly_59
#2 Posted : Sunday, July 12, 2020 7:26:25 PM
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Hey Daniela,

At a pH of 5, both amine groups (including the pyrrole) on histidine would be protonated, and therefore would have a charge of +2
For the carboxylic acid group, the group will be deprotonated and have a charge of -1.

The amino acid will have an overall charnge of +1.

Hope this helps! :)
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