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Using in situ atomic force microscopy (AFM) we have recorded the incorporation, on the nanometer scale, of molecules into growth steps of macromolecular crystals grown from solution. From these investigations we were able to deduce the fine structure of the growth step edges and the surface layer of the {101} faces of tetragonal thaumatin crystals. Although the height of the growth step corresponds to unit cells of eight molecules, step advancement occurs by ordered addition of individual protein molecules rather than molecular clusters. Advancement of growth steps in specific directions, which is anisotropic, occurs by formation of one-dimensional nuclei of varying sizes which generate subkinks on the step edges. Incorporation of molecules into kinks occurs through the same kind of nucleation process. The crystal surface as a whole is created from molecular chains parallel with the surface. Models for the packing of protein molecules comprising the surface layer, the structure of the step edges, and the structure of two-dimensional nuclei were developed which are consistent with the AFM images. |
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