You cannot use a single protein to generate a diffraction pattern because how will you know how the protein is orientated? It could be facing forward, or upside down, or on its side, etc. and you will get a different diffraction pattern each time.
2D crystals, I don't know what that means. How can you even make a 2D crystal?
It has to be 3D crystal so that the protein is all orientated in a highly organized manner so that you can generate a diffraction pattern that you know is right for the protein.
