What may cause streaking on the 2nd dimension gel after IEF?
There are several reasons why streaking may occur.
(1) Sample is not completely solubilized prior to application.
(2) Sample is poorly soluble in rehydration solution.
(3) Non-protein impurities in the sample can interfere with IEF, causing horizontal streaking in the final 2-D result, particularly toward the acidic side of the gel.
(4) Ionic impurities are present in sample.
(5) Ionic detergent is present in sample.
(6) Sample load is too high.
(7) Underfocusing. Focusing time was not long enough to achieve steady state focusing.
(8) Overfocusing. Extended focusing times (over 100,000 Vh) may result in electroendosmotic water and protein movement, which can produce horizontal smearing.
What should be done?
(1) Be sure that the sample is completely and stably solubilized. Note: Repeated precipitation-resolubilization cycles produce or increase horizontal streaking.
(2) Increase the concentration of the solubilizing components in the rehydration solution.
(3) Modify sample preparation to limit these contaminants or dialyze protein.
(4) Reduce salt concentration to below 10 mM by dilution or desalt the sample by dialysis. Precipitation with TCA and acetone and subsequent resuspension is another effective desalting technique that removes lipids, nucleotides and other small molecules.
Note: Specific and non-specific losses of proteins can occur with dialysis, gel chromatography, and precipitation/resuspension of samples. If the sample preparation cannot be modified, the effect of ionic impurities can be reduced by modifying the IEF protocol. Limit the voltage to 100-150 V for 2 hours, then resume a normal voltage step program. This pre-step allows the ions in the sample to move to the ends of the IPG strip.
(5) If the ionic detergent SDS is used in sample preparation, the final concentration must not exceed 0.25% after dilution into the rehydration solution. Additionally, the concentration of the non-ionic detergent present must be at least 8 times higher than the concentration of any ionic detergent to ensure complete removal of SDS from the proteins.
(6) Extend focusing time. Load less sample.
(7) Prolong focusing time.
(8) Reduce focusing time.
Find additional tips, troubleshooting help, and resources within our Protein Assays and Analysis Support Center.
What does it mean when bands appear to be getting narrower (or "funneling") as they progress down a protein gel?
There may be too much beta-mercaptoethanol (BME), sample buffer salts, or dithiothreitol (DTT) in your samples. If the proteins are over-reduced, they can be negatively charged and actually repel each other across the lanes causing the bands to get narrower as they progress down the gel.
Find additional tips, troubleshooting help, and resources within our Protein Electrophoresis and Western Blotting Support Center.
Will NP-40 affect the migration of the samples in the SDS-PAGE gel?
Yes. All detergents and even phospholipids in cell extracts will form mixed micelles with SDS and migrate down into the gel.
They can also interfere with the SDS:protein binding equilibrium. Most of the nonionic detergents significantly interfere with SDS-PAGE.
We recommend that you keep the ratio of SDS to lipid or other detergent at 10:1 (or greater) to minimize these effects.
Find additional tips, troubleshooting help, and resources within our Protein Electrophoresis and Western Blotting Support Center.
When staining IEF gels with the Colloidal Blue Staining Kit, is it necessary to use the fixing solution?
Yes, the fix serves two purposes: it fixes the sample in the IEF gel and it helps to remove gel background.
If you do not use the fixing solution, the background on the gels will be high and detection will be less sensitive.
High background is caused by ampholytes remaining in the gel.
Find additional tips, troubleshooting help, and resources within our Protein Assays and Analysis Support Center.
After staining an IEF gel, what are reasons why the protein samples do not appear but the IEF protein standard (marker) does?
1) If the protein has a pI greater that 8.5, it may be able to be resolved on a pH 3-10 gel.
2) The protein may be insoluble.
3) There may be protein loss if the fixative is too weak in the fixation step prior to staining.
4) Your sample load may be too low.
Find additional tips, troubleshooting help, and resources within our Protein Electrophoresis and Western Blotting Support Center.
