Polyhedral instability of glucose isomerase crystals as revealed by confocal scanning fluorescence microscopy

The polyhedral stability of a crystal during its growth plays a crucial role in determining its final quality. The polyhedral instability of glucose isomerase crystals in the presence of poly(ethylene glycol) (PEG) was observed using confocal scanning fluorescence microscopy, which enabled in situ visualization and real-time determination of the variation of protein concentration around the growing crystal. The presence of a zone depleted in protein around the crystal shows that the observed polyhedral instability occurred under diffusion-limited conditions and so was governed mainly by the supersaturation, the crystal size, and the solution viscosity. The polyhedral instability of protein crystals therefore shares some common features with the instability of crystals of inorganic or small organic molecules.