Purity drop in optical resolution of dl-methionine by the Diastereomer method

Abstract

We carried out optical resolution of dl-methionine-ethyl-ester ( dl) by the diastereomer method using acethyl- d-methionine ( d′) as a resolving agent in order to elucidate the mechanism of purity drop in the course of optical resolution. We obtained about 90–95% of less soluble salt ( d′ · d) in the agitated system. However, the purity dropped to 85–90% in the non-agitated system. This purity drop in the non-agitated system was explained as follows with the help of calculated composition profiles of d′ · d and d′ · l in the stagnant film that developed near the d′ · d crystal surface. d′ · l is transported with the bulk flow induced by molecular diffusion of d′ · d to the crystal–solution interface and accumulates on the crystal surface. This accumulation leads to nucleation of d′ · l (impurity). Nucleation of d′ · l causes the purity drop, especially in the non-agitated case. Although this type of nucleation can also occur in the agitated case, agitation enhances the deposition of required salt ( d′ · d), and hence the purity of agitated crystals is higher than that of non-agitated crystals.