An atomic force microscopy investigation of protein crystal surface topography

Tapping mode atomic force microscopy was employed to study the surface stru cture of different protein crystals in a liquid environment. The (101) face of hen egg-white lysozyme crystals and the (111) face of horse spleen ferr itin crystals were studied. On the (101) face of lysozyme crystals we obser ved islands delimitated by micro-steps and elongated in the [010] direction . The elongation direction coincides with the preferential growth direction predicted by a growth model reported in the literature. The islands observ ed on the ferritin (111) face are also delimitated by micro-steps but have circular symmetry. Sectioning of the images allowed us to measure the step heights. The surface free energy was estimated from the growth step morphol ogy. Molecular resolution was achieved for ferritin crystals, showing a hex agonal surface packing, as expected for the molecular lattice of a (111) fa ce in a fcc crystal.