Growth inhibition of protein crystals: A study of lysozyme polymorphs

Crystal morphology is determined by the relative growth rates of the different faces involved. Opposite faces (hkl) and (h̄k̄l̄) can show different rates if the crystal structure does not have inversion symmetry. Protein crystals, being built of asymmetric molecules do not have identical opposite faces, except for those pairs linked by rotational symmetry. Here, we present an in-situ microscopy study on the polar growth of various polymorphs of hen egg-white lysozyme crystals. It was found that in a number of cases the growth of one of the two faces was blocked, whereas the opposite one was not slowed down. To explain our results we propose a self-poisoning mechanism based on solventinduced adsorption of misorientated lysozyme molecules on the inhibited faces. This mechanism can also prevent some proteins from forming crystals at all.