KINETIC EFFECT OF L-PHENYLALANINE ON GROWTH-PROCESS OF L-GLUTAMIC ACID POLYMORPH

The mechanism of additive effect of L-phenylalanine (L-Phe) on the gro wth kinetics of L-glutamic acid (L-Glu) polymorphs was investigated. W ith L-Phe concentration the growth rate of the (1 0 1) face of the bet a crystal decreased and finally stopped at a critical concentration C- p. As for alpha crystal the growth rate in the [1 1 1] direction was suppressed and the new (1 1 0) face appeared, while the growth rate of the (0 0 1) face was not influenced by L-Phe. The fluctuation of the growth rates was observed for both polymorphs at the higher concentrat ion of L-Phe. The critical concentration of L-Phe for the (1 1 0) face of alpha is about twice of that for the (1 0 1) face of beta. At the constant L-Phe concentration each polymorph grew at higher supersatura tions than the critical relative supersaturation sigma. The value of sigma for the beta crystal was a few times larger than that for the a lpha crystal. The relationship between the growth rates and the concen tration of the additive was expressed using the growth model of a curv ed step in connection with the adsorption density of the additive. The dependence of the growth rate of L-Glu on the L-Phe concentration and the relative supersaturation could be represented by Freundlich-type equations for both polymorphs. The calculated values of the adsorption density and the distance between the adsorbed neighboring L-Phe molec ules also showed the preferential adsorption of L-Phe on beta crystals . The method to determine the optional supersaturation at the certain concentration of additives was proposed. (C) 1998 Elsevier Science B.V . All rights reserved.