Co-crystallization And Crystal Growth Of L-amino Acids

Amino acids are the basic units for proteins,and as an indispensable material for life they are widely used in medicine,food additives and chemical industry etc.At present,the main production method of amino acids is microbialfermentation.By this mean,the target amino acid often generates accompanied with other impurity amino acids.The impurity amino acids in fermentation broth will affect the purity and crystal morphology of the target crystal then influence the quality of the product.In this paper,we investigate the interaction of L-valine,L-norvaline and L-alanine in co-crystallization process and research the effect and mechanism of impurity amino acids on the growth of L-alanine.According to wet residue method,the ternary phase diagrams of L-valine/L-norvaline and L-val/L-alanine in pure water were determined at 20°C,and the equilibrium solid phase was analyzed by PXRD.L-valine/L-norvaline is found to be a compelete solid solution system,while L-valine/L-alanine is a partial solid solution system.Based on this,the distributions of L-norvaline and L-alanine in the solid phase and liquid phase were analyzed,and the results show that the distribution coefficient of L-norvaline is larger than L-alanine.The solid-state samples of L-valine/L-norvaline and L-valine/L-alanine were prepared by evaporation metnod.The crystal structure properties of the two types of solid solutions were compared by a series of characterizations.Finally,the three kinds of amino acids were investigated from the perspectives of hydrophobicity,solubility parameters and crystal structure.The reasons for the formation of two types of solid solutions were explained from both macroscopeic properties and microscopic structures.Using L-norvaline and L-valine as additives,the effect of the two additives with different concentrations on the growth rates and crystal habits of L-alanine crystal were investgated by single crystal growth method.The results indicate that the influence of the two amino acids on the growth of L-alanine is similar.A very small amount of additives can inhibit the growth of the crystal along b-axis,but the growth rates of the crystal along c-axis show different trend with the variation of additive’s concentration.It is caused by the joint action of two effects of additive molecule.Occupying the active growth site reduces the growth rate of the crystal face;its desolvation action accelerates the diffusion rate of the solute molecules and promotes the growth of crystal face.The dissolvation action is proved to be related to hydrophobicity of the amino acid side chains.In addition,a certain concentration of additives can change the morphology of L-alanine,making it change from rod-shaped particle to needle-like particle.