PEG-induced morphologically unstable growth of tetragonal hen egg-white lysozyme crystals

Poly(ethylene glycol) (PEG) is an often used crystallising agent in screening experiments for protein crystallisation conditions. To investigate the influence of poly(ethylene glycol)-4600 on the diffusion process during protein crystal growth, a series of in-situ optical microscopy experiments was performed in which the diffusion coefficient was changed by the addition of PEG. Depending on the balance between diffusion coefficient and driving force, lysozyme crystals were found to grow faceted, kinetically rough, or morphologically unstable. In-situ observations show the development of a lysozyme crystal from polyhedral stability to instability and back to stability again as the balance changes within one experiment. A stability diagram showing the occurrence of morphological instability based on the starting conditions is presented, showing that kinetic roughening stabilises the crystal morphology. The results are interpreted in terms of a competition between 2D nucleation at the edges of the crystal and step flow on its surface. In this, surface protein concentration