l -Glutamic Acid Polymorph Control Using Amino Acid Additives

Additive is an effective factor in controlling crystallization of polymorph. l -Tryptophan, l -leucine, l -threonine, l -histidine, l -alanine, and l -valine, as additives during l -glutamic acid crystallization, were investigated for probing their effects on the polymorphic transformation. l -Tryptophan was then chosen as a representative case to evaluate the effect of addition concentration on the morphology, nucleation, and polymorphic transformation rate of l -glutamic acid. It is shown that all these additives have a certain influence on the polymorphic transformation and the presence of a bulky side chain (phenyl) in l -tryptophan and l -histidine is identified as an essential feature in achieving stabilization of the α-polymorph. When the concentration of l -tryptophan reaches above 0.008 M, the elongation of crystals occur. Increasing l -tryptophan concentration, the transformation time and nucleation rate of l -glutamic acid are retarded. The inclusion behavior of additive molecules in l -Glu crystals was investigated, and the results show that the effect of additive molecule on the polymorphic transformation may be through incorporating into crystal structures by adsorption.