What is the pI of arginine assuming the alpha-amino and guanidinium functional groups have a pKa = 10, and the carboxyl pKa = 4?

pI of arginine = 10

Explanation:

The isoelectric point, pI for amino acids without ionizable side chains is given by

pI = (pKa of acidic group + pKa of basic groups)/2 (arginine has two basic groups)

The carboxyl group for arginine is blocked off, therefore, pI for arginine,

(pKa + pKa)/2 = (0 + 20)/2 = 10.

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c) should be the answer.