Inclusion of amino acids and the effect on growth kinetics of l-glutamic acid

The effect of hydrophobic amino acids as additives on the crystallization kinetics of α l-glutamic acid (l-Glu) crystals and the inclusion behaviors of these additives in l-Glu crystals were investigated. Four amino acids l-valine (l-Val), l-leucine (l-Leu), l-isoleucine (l-Ile) and l-norleucine (l-Nle) were used as additives. The crystallization rate of l-glutamic acid (l-Glu) was retarded by these additives and it was found that the effect of l-Val is larger than l-Leu isomers, and in l-Leu isomers, the retardation effect is the largest for l-Ile and the smallest for l-Nle. In every case, additive molecules are included in l-Glu crystal and the maximum molar ratio of additive to l-Glu is about 2.5×10−2. l-Val with the smallest molecule size is most included in l-Glu crystals and the l-Leu isomers take similar R value. It was suggested that the effect of l-Val on the crystallization rate is due to the high adsorption density and the readiness of the inclusion of l-Val molecule in l-Glu crystals rather than the steric hindrance of the substituted group. In l-Leu isomers, the methyl group at the β carbon seems to be effective in inhibiting the growth of l-Glu. The amount of additive included in crystals could be expressed well by isotherms of either Langmuir or Freundlich type.