Crystal Growth of L-Alanine with Glycine-Based Oligopeptides: the Revelation for the Competitive Mechanism

The morphology of L-alanine can be controlled with additives to extend its application in medicine design and nonlinear material development. This study indicates the competitive mechanism of glycine-based oligopeptides, new types of green additives, on regulating the growth of an L-alanine crystal at room temperature and a supersaturation of 1.005. It is found that the morphology of L-alanine changes from rod-like to needle-like in the presence of glycine, which is quite different from a previous report. Meanwhile, this special case of the effect of weak hydrophilic tailor-made additives on the growth of L-alanine demonstrates the one-sidedness of analysis based on the single mechanism. However, in the presence of glycyl-glycine and glycyl-glycyl-glycine, which have a similar structure and the same weak hydrophilia as glycine, the L-alanine crystal grows as block-like morphology. The simulations and experiments reveal the competition between face adsorption, which reduces the kink sites, and face roughening, which increases kink sites on the (1 2 0) face in the presence of glycine, glycyl-glycine, and glycyl-glycyl-glycine. Differently, the controlling mechanism of glycine and these two oligopeptides on the growth of the (0 1 1) face of L-alanine can be changed from the competitive mechanism to single mechanism as the peptide chain length increases. Besides, relying on the fluorescence of oligopeptide, laser confocal technology is used creatively for the intuitive observation of the action sites of oligopeptide molecules in the L-alanine crystals, which verifies the incorporation of glycyl-glycine and the availability of the prediction based on the simulation.