Nucleation Of Aspartic Acid On Self-assembled Monolayers

Controlled crystallization has been a key goal for scientists for a long time, due toits important in crystal morphological engineering and biominerlization. Generally, thereare several crystal forms in one compound which are different in physical or even inchemical properties, and only one crystal form can satisfy the specific require in industry.Since Landau et al. had reported a monolayer was capable to induce highly specificcrystallization at air-water surface, the surface modification method likeLangmuir-Blodget (LB) films and Self-assembled Monolayer (SAMs) have became anefficient choice to control the morphology, orientation and structure of crystal.Self-assembled monolayers (SAMs) can turn itself into a organized monolyer, throughthe modification of the surface, which was capable of becoming as a chiral center for thelater crystallization. L-Cysteine SAMs can act as a chiral surfaces to control the organicmaterial’ s nucleation.Aspartic acid (Asp) is a fundamental chiral amino acid in nature and an essentialbuilding block for the higher organized bioorganic material. Furthermore, it is one of the20amino acids which can be commonly found in proteins and it was one of the mostabundant amino acid in the primitive earth and detectable in the Murchison aerolit. Thesimple molecular structure, polymorphism and chirality provide us multiple directions inresearching.DL-Asp has5known crystal structure, in this paper, we successfully control thecrystal cell parameters by L-Cysteine SAMs at normal temperature and pressure toobtain a DL-Asp crystal form, which G.M Wang crystallized by hydrothermal method inZn2+mixing solution. Moreover, we also carried out a theoretical modeling andexperiment study to examine the physical and chemical reaction that cause the structurechange on the SAMs. Self-assembled films of L-Asp have been used as templates forcrystallization of DL-Asp acids. DL-Asp acids changed crystal structure on SAMsdependent on the functional of L-Cys. The selectivity of absorbed faces are rationalizedby the electrostatic interaction between SAMs and the crystal interface. Hence, molecular dynamic calculation has proved that (001) and (011) has more strongerinteraction with L-Cys. The changing of the crystal form is caused by the shorten C3-H4and narrowed O4-C4-O3angle. These results proved that SAMs is an efficient way tocontrol crystal form and producing specific crystal structure.Chiral Self-assembled monolayers were used as a template to control thecrystallization processes of Aspartic acid. It has been shown that D-Aspartic acid grewon a fast rate on the L-Cysteine SAMs than the L-Aspartic acid did. The results havebeen proved by XRD and HPLC. Moreover, the mechanism was realized throughmolecular recognition between Asp unit crystals and L-Cysteine SAMs in two possiblemodels which illustrates the steric hindrance is the main reason to prevent the growing ofL-Aspartic acid. However, further experiments need to be carried out to investigate theinfluence of SAMs in the nucleation of racemic Aspartic acid. Such research may offer asupportable evidence for the solid-liquid hypothesis of the homochiral of original life.At last we investigated the effect of SAMs in DMF/water solution using3-MPA asthe substrate of SAMs. Under the influence of3-MPA, morphological features havechanged from platelike to monoclinal pyramid, which might be reduced by the H-bondinteraction between DMF-Water-Asp.