The Crystallization Of Amino Acids On Self-assembled Films Of Cysteine

The Self-assembled monolayer (SAM) of L-cysteine was employed to induce thecrystallization of L-lysine monohydrochloride dihydrate (LHM) and L-glutamic acid. Herewe investigated the regulative role of cysteine self-assembly membrane in controlling thenucleation, orientation, morphology and polymorphs of the amino acids and hydrochloridecrystals in different conditions. XRD and SEM were used as characterization methods.It was found that the crystals of LMH grew preferentially in solution rather than on theSAM. That the SAM did not promote but inhibit the nucleation of LMH was found firstly inthe research field of inducing crystals by SAM. This unusual result may largely due to therepulsion between the positive charges of the LMH and L-cysteine.SAM still can induce LHM crystals into nuclear growth in the long time of inductiondeposition. And the crystal growth orientation could be controlled to some extent. At the sametime, we used the cooling method and ethanol solvent method to regulate it. The XRD resultindicated that the cooling rate and ethanol solvent effects can affect the orientation of thecrystal growth.The LMH-boric acid (mole ratio, LMH: H3BO3=1:1) system was introduced to study topromote the crystallization of LHM on SAM, and control the shape of the crystal. The XRDresults showed that boric acid was preferred growth on the SAM of L-cysteine, then the LMHcrystallization was improved remarkably after the crystallized of boric acid. After24h，wegot the diffraction peaks corresponded to the LMH lattice plane of (101) and (112). And theSEM results showed that the appearance of the LMH crystal also changed greatly, frommassive on the SAM to needle form on boric acid crystals. The results demonstrated thatboric acid could be used to accelerate the growth speed of LMH and influence its crystalmorphology significantly.Glutamic acid crystals have two different crystal forms, the-glutamic acid and-glutamic acid, which was typical in polymorphs of the amino acids. The glutamic acid waspreferred growth on the SAM of L-cysteine. In this paper, we reported some facile methods toobtain different polymorphs of L-glutamic acid on L-cysteine SAM. These ways included thecooling method, desiccant method, ethanol solvent method, dimethyl formamide solvent method and so on. They were used to change the process of reaching saturation. The key ofthese methods was to control the rate of nucleation and growth. The faster the rate is, thegreater chance it would be to gain the-glutamic acid.