Crystallization of Amino Acids on a 21-well Circular PMMA Platform using Metal-Assisted and Microwave-Accelerated Evaporative Crystallization.

We describe the design and the use of a circular poly(methyl methacrylate) (PMMA) crystallization platform capable of processing 21 samples in Metal-Assisted and Microwave-Accelerated Evaporative Crystallization (MA-MAEC). The PMMA platforms were modified with silver nanoparticle films (SNFs) to generate a microwave-induced temperature gradient between the solvent and the SNFs due to the marked differences in their physical properties. Since amino acids only chemisorb on to silver on the PMMA platform, SNFs served as selective and heterogeneous nucleation sites for amino acids. Theoretical simulations for electric field and temperature distributions inside a microwave cavity equipped with a PMMA platform were carried out to determine the optimum experimental conditions, i.e., temperature variations and placement of the PMMA platform inside a microwave cavity. In addition, the actual temperature profiles of the amino acid solutions were monitored for the duration of the crystallization experiments carried out at room temperature and during microwave heating. The crystallization of five amino acids (L-threonine, L-histidine, L-leucine, L-serine and L-valine HCl) at room temperature (control experiment) and using MA-MAEC were followed by optical microscopy. The induction time and crystal growth rates for all amino acids were determined. Using MA-MAEC, for all amino acids the induction times were significantly reduced (up to ~8-fold) and the crystal growth rates were increased (up to ~50-fold) as compared to room temperature crystallization, respectively. All crystals were characterized by Raman spectroscopy and powder x-ray diffraction, which demonstrated that the crystal structures of all amino acids grown at room temperature and using MA-MAEC were similar.