Troubleshooting Common Issues in SDS-PAGE-FAQ

Q1：Why are my bands distorted, or why do they curve upward at the edges like a smile？

A1：Smiling Bands (U-shaped):

(1) Uneven heat dissipation during electrophoresis: This is the most common cause. The center of the gel generates more heat, has lower resistance, and runs faster, while the edges dissipate heat more efficiently and run slower. Solution: Reduce the electrophoresis voltage (recommended not to exceed 150 V) or use an electrophoresis tank equipped with a cooling function.

(2) Gel preparation issues: Uneven polymerization of the gel. Ensure thorough mixing of the gel solution during casting and a stable polymerization environment at room temperature.

Frowning Bands:

These are typically caused by improper stacking gel preparation or excessively high voltage, resulting in samples entering the resolving gel without being compressed into a sharp line. Check that the stacking gel buffer pH is correct, or consider extending the running time for the stacking gel.

Q2：What causes band tailing (vertical streaking)？

A2：

(1) Sample degradation: The target protein has been degraded by proteases.

Solution: Add protease inhibitors to the sample lysis buffer; perform all steps on ice; use samples promptly after preparation or store at -80°C, avoiding repeated freeze-thaw cycles.

(2) Sample impurities: The sample is viscous (e.g., containing genomic DNA) or contains excessive insoluble material.

Solution: Sonicate the sample before loading to shear DNA; centrifuge thoroughly (4°C, 12,000 rpm, ≥10 min) and collect the supernatant.

(3) Overloading: The amount of sample loaded exceeds the binding capacity of the gel.

Solution: Reduce the protein loading amount (typically 0.5–2 μg for purified proteins, 30–50 μg total protein for complex samples is sufficient).

Q3：Why does my protein migrate at a different molecular weight than calculated?

A3：

(1) Glycosylation/Phosphorylation modifications: Glycosylated proteins (such as those expressed in mammalian systems) often migrate more slowly on SDS-PAGE, appearing larger than their calculated molecular weight. Phosphorylation may also cause slight retardation.

(2) Hydrophobicity/Charge abnormalities: Highly hydrophobic proteins or those with extreme charge characteristics may bind SDS abnormally, leading to altered migration rates.

(3) Incomplete reduction: If disulfide bonds are not fully broken, proteins may retain partial folding, causing them to migrate faster (appearing smaller).

Q4： Why do I see multiple bands？

A4：

(1) Degradation products: Partial degradation of the protein. Try using protease inhibitors as mentioned previously.

(2) Post-translational modification isoforms: Variations in glycosylation or phosphorylation levels can result in different apparent molecular weights for the same protein.

(3) Incomplete disulfide bond reduction: Under non-reducing conditions (without DTT), both monomers and dimers may coexist.

(4) Aberrant initiation codons: In prokaryotic expression systems, translation may occasionally initiate from internal methionine residues, producing truncated forms.

Q5：Why is the bromophenol blue dye front uneven?

A5：

(1) Foreign matter at the bottom of the glass plate or air bubbles at the gel bottom, causing an uneven electric field.

(2) Gaps between the gel and the glass plate, allowing current to flow directly through the gaps.