Atomic force microscopy studies on growing surfaces of bovine insulin crystals

The morphology and the growth mechanism of the {100} surfaces of 2Zn-insulin crystals were investigated by in situ and ex situ atomic force microscopy (AFM). The dominant growth mechanism of the insulin crystals is the formation and lateral expansion of two-dimensional (2D) nuclei on the growing surfaces. Several types of dislocation spirals, such as conventional single, double and triple spirals as well as spirals generated by a planar defect, were also observed on the surfaces. Besides those, growth by three-dimensional (3D) insulin islands with mutilayer stacks, which suddenly appeared on the surface, occasionally occurred. Every step was separated from each other with a height of about 3.5 nm, which corresponds to that of a single layer composed of insulin hexamers. An anisotropy of step velocity was revealed from a series of sequential images of growing crystal surfaces. The step velocity of the multilayer stacks was about 50% of that of a single step, which implies that only little of the diffusion fields exist. On the other hand, the degeneration behavior of the multilayer steps provided evidence for the Schwoebel effect, i.e. surface diffusion slightly operates as the mechanism of solute transport towards the steps. Some topics, such as the growth rate controlling factor, aggregation phenomena and the occurrence of point and planar defects, are also discussed on the basis of the observations. (C) 2003 Elsevier Science B.V. All rights reserved.