PAGE
If the proteins are denatured and put into an electric field, they will all move towards the positive pole at the same rate, with no separation by size. So we need to put the proteins into an environment that will allow different sized proteins to move at different rates. The environment of choice is polyacrylamide, which is a polymer of acrylamide monomers. When this polymer is formed, it turns into a gel and we will use electricity to pull the proteins through the gel so the entire process is called polyacrylamide gel electrophoresis (PAGE). A polyacrylamide gel is not solid but is made of a laberynth of tunnels through a meshwork of fibers (figure 2 and figure 3).
Figure 2. This cartoon shows a slab of polyacrylamide (dark gray) with tunnels (different sized red rings with shading to depict depth) exposed on the edge. Notice that there are many different sizes of tunnels scattered randomly throughout the gel.
Figure 3. This is a top view of two selected tunnels (only two are shown for clarity of the diagram). These tunnels extend all the way through the gel, but they meander through the gel and do not go in straight lines. Notice the difference in diameter of the two tunnels.
Now we are ready to apply the mixture of denatured proteins to the gel and turn on the current (figure 4). If all the proteins enter the gel at the same time and have the same force pulling them towards the other end, which ones will be able to move through the gel faster? Think of the gel as a tiny forrest with many branches and twigs througout the forest but they form tunnels of different sizes. If we let children and adults run through this forest at the same time, who will be able to get through faster? The children of course. Why? Because of their small size, they are more easily able to move through the forest. Likewise, small molecules can manuver through the polyacrylamide forest faster than big molecules.
Figure 4. Cartoon showing a mixutre of denatured proteins (pink lines of differen lengths) beginning their journey through a polyacrylamide gel (gray slab with tunnels). An electric filed is established with the positive pole (red plus) at the far end and the negative pole (black minus) at the closer end. Since all the proteins have strong negative charges, they will all move in the direction the arrow is pointing (run to red).
