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.