Isoelectric Point Calculator (pI Biochemistry

Structural Biochemistry & Isoelectric Point Principles

The Isoelectric Point (pI) defines the exact hydrogen ion concentration (pH) at which an amino acid, peptide, or protein exhibits zero net electrical mobility in an electric field.

Core Calculation Rules by Amino Acid Type

• Neutral Amino Acids (Gly, Ala, Val, Leu, Ile): pI = ½ ×

• Acidic Amino Acids (Aspartate pK_R 3.65, Glutamate pK_R 4.25): pI = ½ ×

• Basic Amino Acids (Lysine pK_R 10.53, Arginine pK_R 12.48, Histidine pK_R 6.00): pI = ½ ×

Isoelectric Focusing (IEF) & Protein Precipitation

At pH = pI, intermolecular electrostatic repulsion between protein molecules drops to zero, making protein solubility reach its absolute minimum and inducing precipitation.

Analytical Application in Ion-Exchange Chromatography

Biochemists select cation or anion exchange resin columns based on whether the mobile phase buffer pH is set above or below the protein target's calculated pI value.

How to Use This Calculator

Enter your pK1 (α-Carboxyl Group pKa) (e.g. e.g. 2.34 in pKa), pK2 (α-Amino Group pKa) (e.g. e.g. 9.60 in pKa), pKR (Ionizable Side Chain pKa, if applicable) (e.g. e.g. 0 in pKa), and Amino Acid Classification — choose from Neutral Amino Acid (e.g. Alanine, Glycine, Leucine), Acidic Amino Acid (e.g. Aspartate pKR 3.65, Glutamate pKR 4.25), Basic Amino Acid (e.g. Lysine pKR 10.53, Arginine pKR 12.48, Histidine pKR 6.00). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Formula Reference: Isoelectric Point Calculator (pI Biochemistry)

Input variables:

pK1_alpha_carboxyl — pK1 (α-Carboxyl Group pKa)

pK2_alpha_amino — pK2 (α-Amino Group pKa)

pKR_side_chain — pKR (Ionizable Side Chain pKa, if applicable)

aminoAcidType — Amino Acid Classification

Computed outputs:

isoelectricPointPi — Isoelectric Point (pI)

netChargeAtPh7 — Net Charge State at pH 7.4

predominantIonFormAtPh7 — Predominant Ionic State

Mathematical relationships extracted from calculation logic:

pI = (pk1 + pk2) / 2

Reference constants:

pI = 5.97

Standard: IAB and MMA digital advertising metric definitions

Worked Example

Calculating the pI of Alanine with pK₁ = 2.34 (carboxyl) and pK₂ = 9.69 (amino): The average is pI = 6.02 pH. At physiological pH 7.4, Alanine carries a net negative charge (-1) and exists in deprotonated anionic form.

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