Analysis of protein crystal growth at molecular resolution by atomic force microscopy

High-resolution atomic force microscopy (AFM) studies have been performed to analyze the molecularity of growth steps of the (1 1 0) face of tetragonal lysozyme crystals. Besides a major population of step heights of about 5.5 nm also step heights of about half this size were observed. The latter steps always appeared pairwise. Both surfaces the 1 1 0) face and the (1 0 1) face could be imaged at molecular level. Comparison of the height pattern of the corresponding surface structure indicates that the (1 1 0) face is relatively smooth of less than 0.2 nm compared to the (1 0 1) face of about 1.5 nm. AFM linescan images of the (1 0 1) face indicate the insertion of lysozyme aggregates in solution to the crystal surface rather than lysozyme monomers. This study suggests that insertion of lysozyme aggregates in the solution yields growth steps of the (1 1 0) face of monomolecular as well as of bimolecular unit height.