I work quite often with Life Technology's Bolt 4-12% gradient Bis-Tris Plus gels in order to separate smaller proteins or protein fragments. Using the appropriate NuPAGE transfer buffer (25 mM Bicine, 25 mM Bis-Tris, 1,025 mM EDTA with a pH of 7.2 and 10% methanol) and PVDF membranes with 0.2 or 0.45 µM pore size I get a nice transfer of these small proteins when I perform a wet blot at 250 mA for 2 h (Hoefer TE22 Transfer Tank). Guess with a 0.2 µM pore-size NC you can get similar results.
When I was looking to detect 15kDa protein, I would run (10%) large gel very slowly over night so that the loading buffer does not run off the gel to ensure that no proteins or MW markers are lost. You can also do the same thing with mini gels (single % gels are ok, gradient gels are more expensive so not recommended unless you want to detect other MW targets at the same time!). I do semi dry transfer using nitrocellulose membrane (10 ug/min at 18v) . A short transfer, since you are not loading much protein, will happen very fast.
I would recommend PVDF 2µM pores and a semi-dry transfer. The duration of the transfer is crucial for NC membranes (because small proteins pass trough the membrane more readily), but you gain more freedom on that with the PVDF membrane. I use 0.4 mA for 30 to 40 min for small proteins, but the smallest protein I needed to see was 18 kDa. You can optimize your transfer parameters by checking the transfer efficiency with Ponceau Red.
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