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Step 4 continued: Separate and Purify

Paper Chromatography and Gel Electrophoresis
- Method 3

Another simple way to separate proteins is to use paper chromatography. What happens to a piece of paper if you turn it on edge and dip it into some water? The water migrates up the paper, right?  Scientists use this same principle to get a solution of proteins and water to migrate up a piece of paper. This will separate proteins! 

We have a special Power Point demonstration of paper chromatography. Click here.

Why does this separate the proteins?

The proteins separate on the paper because some proteins are bigger and heavier than others. They are not dragged as far by the water.  Then, if you let the paper dry, the separated proteins are just sitting there, and you can spray a stain on them. The stain makes them appear like spots, each at a different location along the paper.

gelectrophoresis.gif (11482 bytes)Still, this is a little crude, and better results can be had if you use an electric current to help pull proteins apart.  While the proteins are migrating, an electric current can separate the proteins based on their electrical charge.  A more protein that has a negative charge would be pulled toward the positive source of current. An electropositive protein would be pulled toward the negative source of current. If a semi-solid substrate, instead of water, is used, the substrate can be dried and stained after the proteins are separated to reveal where each one is concentrated. This is exactly the technique commonly used today.  It is called Gel Electrophoresis, because the substrate is a Jell-O-like gel (semi-solid) that has microscopic pores that let proteins migrate through.