This puzzle starts with several voids in the middle of the protein. Voids appear as translucent red spheres.
The idea is to drag the ends of the protein toward each other. This puzzle can be a little tricky compared to the earlier ones. If you drag the ends too close together, you'll see the voids turn into clashes. Using the shake tool may help clear the clashes, but you may need to reset and try again a few times.
You don't need to totally eliminate the voids, but they should be much smaller. Using shake at the end may help finish the puzzle.
The two curly helixes should be parallel to each other when you're done.
Technical stuff: voids happen when a protein is not "tightly packed" enough. They're bad because water could get into the empty space, which would tend to break the protein apart.