Crystallization ofdl-methionine polymorphs in the presence of structurally compatible additives

In general, amino acid additive is a key factor in controlling the crystallization of polymorphs. The effect of dl-leucine (dl-leu) on the growth mechanism of dl-methionine (dl-met) crystals was investigated under aqueous medium by varying the additive concentrations at ambient temperature. However, when the additive concentration was gradually increased, the disappearance of the prominent face of β-form (200) was observed with the emergence of (100) facets of α-form, which obviously indicates the change in the type of nucleation induced by the additive dl-leu. On the other hand, concomitant polymorphism (α and β-form) was identified at low (x = 0.001 mol/lit) additive concentration. Instead, for all other concentrations, α-form morphology was observed. Therefore, increasing the quantity of dl-leu (0.001 < x < 0.1 mol/lit) strongly influences the stability of the metastable state corresponding to α-form. Moreover, the lattice parameter values and the distinguished peaks ((600) and (113) planes) obtained from XRD analysis confirms the occurrence of α and β-form morphologies, respectively. Differential scanning calorimetry analysis indicates that the crystal undergoes a solid-state phase transformation from β form to α form. Hence, the overall premise is that addition of dl-leu to dl-met in aqueous medium is a promising technique to acquire pure α-form at ambient temperature.