Molecular Modeling on the Role of Local Concentration in the Crystallization of L-Methionine from Aqueous Solution

摘要

In the present work, the importance of local concentration of the growth of L-methionine (L-Met) was explored by the spiral growth model and interfacial structure (IS) analysis. The spiral growth model shows that the decrease in the local concentration of growth units at crystal faces plays a key role in an appearance of thin hexagonal plate-like morphology of L-Met crystals from water. The (001)_2 layer with hydrogen bonds on its under surface was found to require a relatively large amount of energy for detachment of growth units from kink sites. This situation results in lower local concentration and leads to the decrease in net flux of solute molecules at kink sites compared with the (001)_1 layer interacting with its under surface by van der Waals force. Furthermore, the interfacial structure analysis provides important information on the growth of L-Met. Hydrophilic NH3+ and COO- groups of L-Met on the (001)_1 surface were found to have a tendency to form hydrogen bonding with water, and thus L-Met molecules should overcome the energy barrier required to detach water molecules adsorbed on the crystal surface for continuous growth of the (001) face. From those results, it can be concluded that the limitation of approaching growth units for the formation of the (001)_2 layer onto the (001)_1 surface is responsible for very slow growth of the [001] direction of L-Met crystals.