Lattice resolution and solution kinetics on surfaces of amino acid crystals: an atomic force microscope study

Abstract

We report atomic force microscopy (AFM) results on six amino acid crystal surfaces: glycine, L-aspartic acid, L-valine, L-isoleucine, L-leucine, and L-phenylalanine. Samples were grown by slow evaporation of concentrated aqueous solutions. All samples contained crystalline areas where the AFM showed extended molecularly flat sheets (up to hundreds of nm in size) separated by steps a single molecule thick. The ordered lattice of each amino acid could be imaged on the sheets. Images revealed periodicities corresponding to bulk terminations in most cases, as well as other periodicities which probably correspond to molecular structure within the unit cell. Step motion kinetics were also imaged in situ during dissolution of L-leucine in flowing propanol. Steps oriented along the 〈010〉 direction traveled with speeds that were independent of both interstep distance and solvent flow rate for flow rates above 20 μl/s, indicating a reaction rate limited process. Orthogonal bends along the 〈001〉 direction moved at speeds one to ten times that of steps, with narrow bends moving faster than wide. We speculate that these speed differences were caused by anisotropy in reaction kinetics coupled with partially saturated boundary layers near wide bends.