Crystallization of leucine on a self-assembled monolayer with covalently attached enantiomeric leucine molecules

Crystallization of chiral amino acid, leucine, on the gold substrate modified with a self-assembled monolayer (SAM) was investigated with X-ray diffraction (XRD) and atomic force microscopy (AFM). It was found that the formation of enantiomeric crystalline phase occurred enantioselectively when the specimen modified with the SAM, to which one enantiomeric form of leucine molecules was covalently attached, was immersed in a pure enantiomeric solution of leucine. On the contrary, a racemic crystalline phase was formed on both d- and l-leucine-attached SAMs after soaking in a racemic leucine solution. In non-racemic solutions, both enantiomeric and racemic crystalline phases were formed on the enantiomer-attached SAM when the one enantiomeric form with the same chirality as the attached enantiomer was contained in excess, while neither of the crystals was formed with the other enantiomeric form in excess. Changes in the surface morphology of the specimens accompanied with the growth of crystalline phases on the SAM, reflecting the crystallinity suggested from the XRD method, were observed by using AFM.