Tuning Crystallization and Stability of Metastable Polymorph of DL-methionine by a Structurally Similar Additive
Crystal engineering is aiming at controlling crystal forms and desired properties from a view of molecular engineering, which is important to produce high-value-added products. DL-methionine (DL-met) usually crystallizes in a concomitant mixture with the known metastable α-form and stable β-form from pure aqueous solution. In this contribution, we firstly applied molecular dynamics (MD) simulation to study the effects of the tailor-made additive and found that DL-leucine (DL-leu) might enable us to stablize the metastable form. To validate our idea, we performed the experiments including cooling crystallization with isothermal and non-isothermal methods, nucleation induction time measurement, solvent mediated phase transformation and crystal growth rates measurement experiments. It can be found with increasing concentration of DL-leu, nucleation of β-form and the transformation of α-form are retarded. When the concentration of DL-leu reached above 0.02 mol/L, hexagonal crystals other than foliaceous crystals can be obtained. Hence, the metastable α-form of DL-met with fine crystal habit and high temporal stability were prepared.