Surface structures of dl-valine and l-alanine crystals observed by atomic force microscopy at a molecular resolution

Molecular structures at the surfaces of dl-valine and l-alanine single crystals are observed by atomic force microscopy (AFM). The AFM image of the (0 0 1) surface of dl-valine crystal reveals highly ordered lattices with twofold symmetry. And for the l-alanine crystal, the parallel molecular chains and a periodic structure in the (1 2 0) plane are clearly resolved. The AFM images with molecular resolution are quite consistent with the bulk termination of our crystal models. No reconstruction is evident in the AFM images. The present results also demonstrate that AFM has the ability to probe the surface structures of biological crystals at a molecular resolution in both the lateral and longitudinal dimensions.