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PVDF membrane should be washed with deionized water to remove any gel debris. The blot can then be incubated with the blocking solution.
After the transfer is complete, PVDF membranes can be dried before continuing on to staining or immunodetection procedures. Drying enhances the adsorption of the proteins to the PVDF polymer, helping to minimize desorption during subsequent analyses. As the blotted membrane dries, it becomes opaque. This optical change is a surface phenomenon that can mask retention of water within the depth of the pores. The membrane should be dried for the recommended period to ensure that all liquid has evaporated from within the membrane’s pore structure (refer to the membrane Drying Methods Protocol, later in this section).
Membranes can be stored dry for long periods of time after proteins have been transferred with no ill effects to the membrane or the proteins (up to two weeks at 4°C; up to two months at -20°C; for longer periods at -70°C). Some proteins, however, may be sensitive to chemical changes (e.g. oxidation, deamidation, hydrolysis) upon prolonged storage in uncontrolled environments. Long term storage at low temperature is recommended. Prior to further analysis, dried membrane must be wet by soaking in 100% methanol for PVDF membranes or Milli-Q water for nitrocellulose membranes.
Transillumination (see “Calf Liver Protein Stains” figure on the following page) is a visualization technique unique to PVDF membranes and was first described for Immobilon-P transfer membrane (Reig and Klein, 1988). This technique takes advantage of a characteristic unique to PVDF membranes; areas of PVDF coated with transferred protein are capable of wetting out in 20% methanol while the surrounding areas of PVDF are not. In the areas where the PVDF wets, it becomes optically transparent, allowing visualization of protein bands using backlighting and photographic archiving. The process is fully reversible by evaporation. Further denaturation of the proteins is unlikely as the proteins had been previously exposed to methanol during blotting. Even though this technique does not allow for visualization of low abundance proteins, it can be used to assess the overall transfer efficiency and the suitability of the blot for further analysis.Staining
Staining (see “Calf Liver Protein Stains” figure below) is a simple technique to make proteins visible on a blot. Staining can be used to:
Reversible stains allow assessment of the blot and then can be washed from the membrane. These will not interfere with subsequent immunodetection or other analysis of the proteins on the blot. The most commonly used reversible protein stain is Ponceau-S red. The major drawback of reversible stains is that they are less sensitive than irreversible stains. Since the staining pattern of more abundant proteins in a blot is generally a good indicator of how well low abundance proteins transferred, this drawback can be minimized in most cases.
Fluorescent blot stains are highly sensitive and compatible with downstream immunodetection, Edmanbased sequencing and mass spectrometry (Berggren et al., 1999). Sypro Ruby and Sypro Rose protein blot stains (Invitrogen) can be used prior to chromogenic, fluorogenic or chemiluminescent immunostaining procedures and provide sensitivity of about 1-2 ng/band (Haugland, 2002).
Irreversible stains generally exhibit the best sensitivity but can interfere with or prevent further analysis of the proteins. Examples of irreversible stains are amido black and Coomassie Brilliant Blue.
|Detection Reagent||Approximate Sensitivity (protein per spot)||Reference|
|Reversible||Ponceau-S||5pμ||Dunn et al., 1999|
|Fast Green FC||5pμ||Dunn et al., 1999|
|CPTS||1pμ||Bickar et al., 1992|
|Sypro Ruby||1-2 ng||Haugland, 2002|
|Sypro Rose||1-2 ng||Haugland, 2002|
|Irreversible||Amino black 10B||1μg||Dunn et al., 1999|
|Coomassie Brilliant Blue 250||500ng||Dunn et al., 1999|
|India Ink||100ng||Dunn et al., 1999|
|Colloidal gold||4ng||Dunn et al., 1999|