The crystal morphology of amino acids can be altered in a controlled manner through inclusion of tailor-made additives in their structure, in order to widen their scope for applications in drug design and targeted delivery. In this study, the effect of multiadditive combinations of hydrophobic and hydrophilic amino acids on the growth and morphology of l-alanine was investigated. Theoretical calculations were performed using two crystal growth models in Materials Studio software: (1) build-in model; (2) surface docking model. Crystallization experiments were carried out using the metal-assisted and microwave accelerated evaporative crystallization (MA-MAEC) technique with multiple hydrophobic and hydrophilic amino acids added in stoichiometric amounts to l-alanine solution. The crystal morphology was established and compared with predicted crystal morphology. The use of hydrophilic and hydrophobic additives was predicted to have significant changes in the morphology of l-alanine crystals. Multiadditive combinations with hydrophobic amino acids resulted in elongation of l-alanine crystals through the (120) face. Experimental data corroborates with the theoretical predictions in relation to the morphological changes due to additives, indicating the accuracy of theoretical models in predicting the impact of additives in crystal growth.

In this work, we demonstrated that the change in the morphology of l-alanine crystals can be controlled with the addition of l-leucine using the metal-assisted and microwave accelerated evaporative crystallization (MA-MAEC) technique. Crystallization experiments, where an increasing stoichiometric amount of l-leucine is added to initial l-alanine solutions, were carried out on circular poly(methyl methacrylate) (PMMA) disks modified with a 21-well capacity silicon isolator and silver nanoparticle films using microwave heating (MA-MAEC) and at room temperature (control experiments). The use of the MA-MAEC technique afforded for the growth of l-alanine crystals with different morphologies up to ∼10-fold faster than those grown at room temperature. In addition, the length of l-alanine crystals was systematically increased from ∼380 to ∼2000 μm using the MA-MAEC technique. Optical microscope images revealed that the shape of l-alanine crystals was changed from tetragonal shape (without l-leucine additive) to more elongated and wire-like structures with the addition of the l-leucine additive. Further characterization of l-alanine crystals was undertaken by Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy and powder X-ray diffraction (PXRD) measurements. In order to elucidate the growth mechanism of l-alanine crystals, theoretical simulations of l-alanine’s morphology with and without l-leucine additive were carried out using Materials Studio software in conjunction with our experimental data. Theoretical simulations revealed that the growth of l-alanine’s {011} and {120} crystal faces were inhibited due to the incorporation of l-leucine into these crystal faces in selected positions.

Rapid crystallization of a model protein, i.e., lysozyme, on blank and silvered circular crystallization platforms with 21 sample capacity, using the metal-assisted and microwave-accelerated evaporative crystallization (MA-MAEC) technique is described. The effectiveness of the MA-MAEC technique for the crystallization of lysozyme was compared to the conventional crystallization technique (i.e., at room temperature without microwave heating) based on the following parameters: crystallization time, crystal size, crystal number, and crystal quality. Using silvered platforms, the growth of lysozyme crystals was concluded within 857 ± 31 min and 565 ± 64 min, at room temperature and using the MA-MAEC technique (microwave power level 1 or duty cycle of 3 s in a kitchen microwave oven), respectively. On blank platforms (silver is omitted), the growth of lysozyme crystals was concluded within 1190 ± 14 min and 955 ± 50 min, at room temperature and using microwave heating. The largest sizes of lysozyme crystals (200 ± 50 μm) were grown on silvered platforms and microwave heating at power level 1. In addition, superior intraplatform and interplatform repeatability of growth of lysozyme crystals was observed with silvered platforms and microwave heating at power level 1. Although the same well-defined tetragonal shapes of lysozyme crystals were observed at both room temperature and using the MA-MAEC technique, the quality of lysozyme crystals was found to slightly deteriorate with the use of microwave heating.