What is an IPG strip?

An immobilised pH gradient strip carries a fixed pH gradient cast into the gel. In the first dimension of 2D-PAGE, proteins migrate until they reach the pH equal to their pI, where they stop because their net charge is zero. The strip's pH range sets which proteins focus.

Why does strip range affect resolution?

A narrow strip spreads a small pH window over the same physical length, so proteins with close pI values separate further apart. A broad strip surveys more of the proteome but packs spots closer together, lowering resolution per pH unit.

When should I use a narrow zoom strip?

Use a narrow range (for example pH 4–7 or 6–9, or ultra-narrow 5–6) when your proteins of interest cluster in a pI window and you need to resolve closely spaced spots. Pair multiple overlapping narrow strips to cover the whole proteome at high resolution.

What if my proteins span the whole pH range?

Use a broad-range strip such as pH 3–10 for an initial survey. It captures most cytosolic proteins in one run. Very acidic or very basic proteins may need dedicated low- or high-pH strips, since the extremes of a broad strip resolve poorly.

Does pI depend on conditions?

Yes. IEF is run under denaturing conditions with urea and reductant, so the relevant pI is the denatured value. Post-translational modifications such as phosphorylation lower pI and create characteristic spot trains, which is part of what 2D-PAGE reveals.

IPG Strip pH Range Selector for 2D-PAGE

The first dimension of 2D-PAGE separates proteins by isoelectric point on an immobilised pH gradient strip, and choosing the strip range is the single biggest lever on resolution. This selector matches your protein or proteome pI range to the appropriate broad, medium, or narrow IPG strip.

How it works

A protein focuses at the position in the gradient where the local pH equals its pI, because that is where its net charge is zero and it stops migrating. The rule the selector applies is simple: the strip range must bracket every pI you care about, and the narrower the bracket, the higher the resolution per pH unit:

resolution ∝ strip length / (pH_high − pH_low)

For a survey it recommends a broad strip (pH 3–10) that catches most proteins in one run. For a zoom it recommends the narrowest standard strip that still spans your entered pI range, so closely spaced spots separate as far as possible.

Notes and tips

Standard commercial ranges include broad pH 3–10 and 3–11, medium pH 4–7 and 6–11, and narrow zoom strips such as 4–5, 5–6, and 6–7. Running several overlapping narrow strips is how proteomics labs achieve whole-proteome coverage at high resolution. Remember IEF runs denatured, so use denatured pI values, and expect modified proteins (such as phosphorylated forms) to appear as horizontal spot trains shifted toward lower pI.