1. What is the pI of histidine?
For histidine pKa1=2.3, pKa2=6.0 and pKa3=9.6.
At the pI the net charge of the molecule is zero. To find the pI, average the two pKa values on either side of the neutral form of histidine.
Draw out the titration curve for histidine to convince yourself that the uncharged form is predominant at this pH.
For more practice:
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