IEF (Isoelectric Focusing) Calculators

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Isoelectric focusing (IEF) is an electrophoretic technique that separates proteins based on their isoelectric point (pI) — the pH at which a protein carries no net charge. In IEF, a pH gradient is established across a gel or strip, and proteins migrate under an electric field until each reaches the zone where pH equals its pI, at which point it stops moving. IEF offers extremely high resolution — separating proteins differing by as little as 0.01 pH units — and is the first dimension of two-dimensional gel electrophoresis (2D-PAGE), the classic method for proteome-wide protein profiling.

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How Isoelectric Focusing Works

Every protein has a characteristic isoelectric point (pI) — the pH at which its positive and negative charges balance to give zero net charge. In IEF, proteins are placed in a pH gradient under an electric field. A protein with a net positive charge (pH below its pI) migrates toward the cathode; a protein with net negative charge migrates toward the anode. As it moves into the zone where pH equals its pI, its net charge reaches zero and migration stops. This produces sharp, focused bands at each protein's pI.

IPG Strips vs. Carrier Ampholytes

Two methods establish the pH gradient:

  • Immobilized pH gradient (IPG) strips: The pH gradient is covalently built into the polyacrylamide matrix using immobiline compounds during casting. Pre-cast, reproducible, and available in defined pH ranges (e.g., 3–10, 4–7). The modern standard for 2D-PAGE.
  • Carrier ampholytes: Small synthetic amphoteric molecules added to a gel that self-organize into a gradient under voltage. Less reproducible than IPG strips; used in traditional tube gel IEF.

IEF Protocol (IPG Strip)

  1. Rehydrate IPG strip with protein sample in 8 M urea, CHAPS detergent, DTT, and ampholytes
  2. Run multi-step voltage program (e.g., 300 V → 1,000 V → 8,000 V; total 50,000–80,000 Vhr)
  3. Equilibrate focused strip in SDS buffer (DTT then iodoacetamide steps)
  4. Place strip on SDS-PAGE gel for second-dimension separation by molecular weight

Applications

  • Proteomics and global protein expression profiling (2D-PAGE)
  • Protein isoform and post-translational modification analysis
  • Hemoglobin variant identification (sickle cell, thalassemia diagnosis)
  • Antibody charge variant analysis in biopharmaceutical quality control

Glossary

Isoelectric Point (pI)
The pH at which a protein carries no net electrical charge. The point at which a protein stops migrating in IEF — it focuses into a sharp band at its pI in the pH gradient.
IPG Strip (Immobilized pH Gradient)
A pre-cast polyacrylamide gel strip with a covalently immobilized pH gradient. Used as the first dimension in 2D-PAGE. More reproducible and stable than carrier ampholyte-based gradients.
2D-PAGE
Two-dimensional polyacrylamide gel electrophoresis — combines IEF (first dimension, separating by pI) with SDS-PAGE (second dimension, by molecular weight) to resolve hundreds to thousands of proteins simultaneously.

Frequently Asked Questions

The isoelectric point (pI) is the pH at which a protein has no net electrical charge. In IEF, proteins migrate in a pH gradient under an electric field until they reach the zone where pH = pI, at which point they stop moving and focus into a sharp band. This allows separation of proteins with very similar properties — IEF can resolve proteins differing by as little as 0.01 pH unit in pI.

An IPG (Immobilized pH Gradient) strip is a pre-cast polyacrylamide gel strip in which the pH gradient is covalently built into the matrix using immobiline compounds. Unlike carrier ampholyte gels, IPG strips produce highly reproducible, stable gradients that do not drift during focusing. They are available in a range of pH spans (narrow or broad) and lengths, and are the standard first dimension for 2D-PAGE proteomics.

2D-PAGE uses IEF as the first dimension — proteins are separated by pI along an IPG strip. The strip is then equilibrated in SDS buffer (which denatures and uniformly charges proteins) and placed on top of an SDS-PAGE gel. The second dimension separates by molecular weight. Each protein spot's position on the resulting 2D gel encodes both its pI (x-axis) and molecular weight (y-axis), enabling simultaneous resolution of hundreds to thousands of proteins.

IEF requires complete protein denaturation and solubilization. Standard IEF buffer contains 8 M urea (or 7 M urea + 2 M thiourea) to unfold proteins; CHAPS detergent to solubilize hydrophobic proteins; DTT to reduce disulfide bonds; and carrier ampholytes. Ionic species (salts, SDS) must be minimized as they generate current that interferes with focusing. Samples are typically desalted using precipitation or cleanup columns before IEF.