How to make protein crystals
A schematic phase diagram of a protein solution. The strength of the short-range attraction (vertical axis) that is induced between protein molecules is quantified by the so-called second virial coefficient, B2 - a quantity that can be measured directly using light scattering.
Crystallization occurs when the interparticle attraction (and therefore B2) is sufficiently negative due to the presence of additives, such as salts or polymers. The horizontal axis measures the concentration of protein in the solution.
Experiments have shown that over two dozen globular proteins crystallize within a narrow band (green). B2 can also be used to quantify the attraction induced between hard spheres by an inert polymer. Such a colloid-polymer mixture crystallizes at the points marked in purple. The model shows biologists the conditions under which protein crystals are likely to form.