How to Find pI of Histidine?

The pI (isoelectric point) of histidine is found by averaging the two pKa values that flank the neutral (zwitterionic) form of the amino acid.

Here's a breakdown:

Understanding Histidine's Ionizable Groups

Histidine has three ionizable groups:

• α-carboxyl group (-COOH): pKa₁ ≈ 1.8
• α-amino group (-NH₃⁺): pKa₂ ≈ 9.2
• Imidazole side chain: pKa_R ≈ 6.0

Determining the Dominant Form and Relevant pKa Values

The neutral, zwitterionic form of histidine is dominant between pH 6.0 (pKa_R) and 9.2 (pKa₂). Therefore, to calculate the pI, you need to average these two pKa values. As the reference states, "the neutral form is dominant between pH 6.00 and 9.17, pI is halfway between these two values, i.e. pI = 1/2 (pKa2 + pKa3)". Keep in mind that different sources might list slightly different pKa values.

Calculating the pI

The pI of histidine is calculated as follows:

pI = (pKa_R + pKa₂) / 2
pI = (6.0 + 9.2) / 2
pI = 15.2 / 2
pI = 7.6

Therefore, the pI of histidine is approximately 7.6. The example given in the referenced material gives a pI of 7.59 using slightly different pKa values.