PEG-induced
morphologically unstable growth of tetragonal hen egg-white lysozyme
crystals
Heijna, M. C. R., van Wamel, F. F. M., van Enckevort, W. J. P., Vlieg,
E.
CRYSTAL GROWTH & DESIGN 7 (10): 1999-2008 OCT 2007
Abstract: Poly(ethylene
glycol) (PEG) is an often used crystallizing agent in screening
experiments for protein crystallization 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 were 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 stabilizes the
crystal morphology. The results are interpreted in terms of a
competition between two-dimensional nucleation at the edges of the
crystal and step flow on its surface. In this, surface protein
concentration profiles derived from observed crystal shapes are used.