Insights Into The Nucleation And Polymorphic Transformation Mechanism Of L-glutamic Acid:a Combined Experimental And Simulation Study

Crystallization has been widely used in chemical and pharmaceutical industry because of its low energy cost, high efficiency, high product quality, and mild operating conditions. In addition, crystallization is also frequently employed in research field such as physics, chemistry, materials and life science. Solution crystallization, which has a long history and is in common use, involves complex nucleation and crystal growth behind the seemingly simple process, and can be affected by many inner and outer factors, such as temperature, supersaturation, solvents and additives. So far, the process of crystallization has not been completely understood, and its design and optimization strongly depend on practical experience. In order to effectively controll the process and thus meet varied product demands, a fundamental understanding of nucleation and crystal growth mechanism and the dependence of the crystallization behaviour on the operational factors is desirable. Among the factors, supersaturation and solvents are considered to be basically important as the controlling factors. Although great efforts have been made and a number of models have been proposed to investigate the effects of solvent and supersaturation on crystal nucleation and polymorphic transformation, the research is still far from being systematic or clear,In the present work, L-glutamic acid (L-Glu) is chosen as the model system to investigate the clustering behaviors of solute molecules in solution before nucleation, the supersaturation effect on nucleation and polymorphic transformation, and the role of surface and solvent during the secondary nucleation of β form on the surfaces of a form by combining experiments and molecular simulations. The details are presented as follows:1. UV spectroscopy was used to study self-association of solute molecules in different solvents as a function of concentration. In addition, on the basis of the structural analysis, the self-association behaviors of L-Glu in solution were also investigated by molecular dynamics simulation. The results indicate that in highly concentrated aqueous solutions, the L-Glu molecules tend to form one hydrogen bond dimers or bigger clusters which further explained the experimental results.2. A direct relationship between the species self-association in L-glutamic acid solution and the polymorphic outcome is established based on the solution behavior of different supersaturated solution detected by Fourier transform infrared (FTIR) spectroscopy. Supersaturation can determine the polymorph of the products by means of affecting the major conformers which can precipitate to polymorph. For L-Glu, the stable β form nucleates directly in a lower supersaturated solution, while the metastable a form first occurs in a higher supersaturated solution.3. Powder X-ray diffraction was used in the quantitative analysis of L-Glu polymorphs, and standard curve was calculated to determine the ratios of each polymorph during the solution-mediated transformation process. Furthermore, it has been found that the transformation time will decreases with the increase of supersaturation, because the supersaturation is found to determine the morphology of the metastable a form which influences the secondary nucleation rate of the stable β form.4. Experimental results show that the β form can nucleate on the three main surfaces of the a form during solution-mediated transformation process, and the corresponding probability follows the order of{011}>{111}>{001}. Molecular simulation of the adsorption of molecules initially with a or β conformation on the three surfaces indicates that the adsorption energies decrease in the order of {011}>{111}>{001} regardless of the initial conformation of the solute molecule, which is consistent with the experimental results. The adsorption energies of the βclusters on the three surfaces of a form follow the same order as that of single β molecules. Moreover, it reveals that the three surfaces have different abilities to induce the conformational change of a to β. Especially, for a molecule adsorbed on the{011} surface initially with a conformation, it will evolve into the β conformation. In addition, the molecules with β conformation is liable to self-associate on the {011} surface.5. Solvent is involved in the diffusion, adsorption and association behaviors of solute molecules. Therefore, a crystal-solution model was built to demonstrate the role of solvents in the secondary nucleation of the β form on the three solvated surfaces of a form by using molecular dynamics simulation. By comparing the simulation results on the solvated surfaces and those on the un-solvated surfaces, it is revealed that water molecules have not made any change in the order of the adsorption energies for the solute molecules on the three surfaces, but reduced the absolute values of adsorption energies with the extent increasing in the order of {011}<{111}<{001}, and retarded the clustering behaviors of solute molecules with the same order. In addition, water molecules also weaken the inductive effects of the surfaces on the conformational change of a to β. In general, solvent molecules exert a retarding effect on the polymorphic transformation of L-Glu.