For amino acid, chirality is the most important feature as enantiomers have vastly different biomedicine effects and applications.Thus the research of chiral distinction is of great significance.In this paper, self-assembled monolayers (SAMs) were prepared by D-cysteine attaching to gold (111) surface and D-cysteine SAMs were used as the chiral template to induce the crystallization of D- or L-aspartic acid.Metallographic Microscope, X-ray diffraction (XRD) and High Performance Liquid Chromatography (HPLC) were carried out to characterize the crystals grown on the SAMs.The experimental results revealed that SAMs have significant effects on the crystallization habit and nucleation rate of aspartic acid enantiomers.When the crystals were grown on SAMs, the predominant nucleating planes were (100) for both D- and L-aspartic acid.Hydrogen bond between cysteine and aspartic acid (100) plane was the main force.Moreover, XRD results of D- and L-aspartic acid crystals show great difference in the peak strength.Micrographic images and HPLC curves demonstrate that a large quantity of L-aspartic acid crystals were obtained while only a small quantity of D-aspartic acid appeared.L-aspartic acids have much more nucleation sites on chiral SAMs.The above results indicated that the nucleation rate of D-aspartic acid was low.In contrast, the nucleation rate of L-aspartric acid on D-cysteine SAMs was rather effective.D- and L-aspartic acid have the nearly identical solubility, therefore, the chiral environment introduced by D-cysteine SAMs leads to the different crystallization rate of aspartic acid enantiomers.The space hinder between D-aspartic acid and D-cysteine molecules may be the key point, which lowers the nucleation rate of D-aspartic acid.

Key words: aspartic acid, chirality, crystallization, cysteine, self-assembled monolayers