Growth Mechanisms and Kinetics of Trypsin Crystallization

The surface morphologies of orthorhombic and trigonal trypsin crystals grown from solution were investigated by in situ atomic force microscopy. {001} and {100} faces of trigonal and {110} faces of orthorhombic crystals grew strictly by two-dimensional (2D) nucleation. For {101} faces, no 2D nucleation was observed even at relatively high supersaturations. For the {101} face, from the supersaturation dependency of the slope of dislocation hillocks and tangential step rates the surface free energy of step edges α and the tangential kinetic coefficient β were calculated to be 5 ± 1 erg/cm 2 and (1.1 ± 0.2) 10 -4 cm/s, respectively. The high value of α is thought to be the cause for the absence of 2D nucleation on this face. It was also found that growth of the trigonal {001} face proceeds by alternating deposition of three symmetry-related growth layers with a height equal to one-third of the unit cell parameter c . Using periodic bond chain (PBC) analysis it was demonstrated that the observed morphology as well as the anisotropic growth of the 2D nuclei are controlled by the underlying structure. We believe that this is the first report of growth by symmetry-related layers for a system with a 3-fold screw axis.