Protein-induced morphological transitions in KCl crystal growth

We investigated the formation of KCl crystals on a glass surface by phase contrast, fluorescent, and atomic force microscopy on the micrometer scale and observed interesting morphological transitions as a function of the experimental conditions. The presence of proteins in the solution from which the salt crystals grow during the drying up leads to complex microscopic patterns of crystals, some of which are analogous to those commonly observed on the macroscopic scale. We tested the effect of tubulin, FITC-labeled albumin, and IgG on the morphology of crystals grown either slowly or fast. A rich variety of protein-specific and concentration-dependent morphologies was found and described by a morphological diagram. We give a phenomenological interpretation, which can explain the growth of complex patterns. Fluorescent images prove that a protein layer covers the surface of the KCl structures. We propose that this layer reduces the anisotropy of the effective surface tension during growth. The tip splitting fractal regime is attributed to the decrease of anisotropy. Further possible mechanisms, which can cause a morphological transition, are also discussed. We found elongated saw-toothed crystals induced by proteins, especially IgG, and identified their structure.