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Shortcut to finding pI of an amino acid

Isoelectric point or pI is a pH at which an amino acid or protein has a net charge of zero.

Biochemistry classes always ask students to find pI for an amino acid and here is the short and sweet way to do it.

For neutral amino acids, two pka's will be given, and pI = (pKa1 + pKa2)/2

For acidic amino acids ( aspartic acid and glutamic acid), we average out the two lowest pka, so we do not take into account the amino terminal pka

For basic amino acids such as arginine, we average out the two highest pka, so we do not take into account the carboxylic acid pka.

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