An Atomic Force
Microscopy Study of the (001) Surface of Triclinic Hen Egg-White
Lysozyme Crystals
Maurits C. R. Heijna, Patrick B. P. van den Dungen, Willem J. P. van
Enckevort and Elias Vlieg
Cryst. Growth & Design 6 (2006) 1206-1213
Abstract:
The (001) surface of triclinic hen egg-white lysozyme-nitrate crystals
has been investigated by in-situ atomic force microscopy (AFM) to
compare its growth mechanisms and properties to those of other lysozyme
pseudopolymorphs. The crystal morphology derived from connected net
analysis using the macrobond concept is in good agreement with the
experimental morphology of the crystals. Surface structures observed by
AFM include rounded steps, growth spirals, three-dimensional (3D)
nucleation, and impurity pinning. The growth spirals are rounded and
highly anisotropic. Both screw and edge dislocations are found,
characterized by hollow core outcrops. The observed hollow core radii
are an order of magnitude larger than theoretical values but do depend
on supersaturation. From the rounded shape of the spirals and the
absence of two-dimensional (2D) nucleation, we find an edge free energy
1.3 < m/kT < 3, which is similar to values found for tetragonal
lysozyme. Serrated step patterns indicate impurity pinning. Complete
blocking by impurities is not found. The step velocity is proportional
to the relative supersaturation. Using the kinetic coefficient, we find
a sticking fraction of 4 × 10-4 for hen egg-white lysozyme (HEWL)
molecules to become attached to a kink, which is significantly higher
compared to the sticking fraction for the orthorhombic lysozyme.