Separation of racemic mixture by ultrafiltration of enantioselective micelles. 2 (De) complexation kinetics

The application of enantioselective micelles in ultrafiltration systems can be an alternative route to meet the increasing demand for enantiopure products. We have studied the separation of D,L-phenylalanine (Phe) by cholesteryl-L-glutamate:CuII (CLG:CuII) anchored in nonionic micelles (intrinsic enantioselectivity D/L,int = 1.9). A cascaded system is needed to complete the separation, as a single stage is insufficient to obtain >99␘ptically pure products. It is shown that the complexation and decomplexation processes are not instantaneous; hence, elucidation of the complexation kinetics is essential to the design of a multistage system. Linear driving force (LDF) models describe both the complexation and decomplexation rates of enantiomers. It can be concluded that the complexation rates of D- and L-Phe, (32 ± 11) x 10-5 s-1 and (28 ± 14) x 10-5 s-1, respectively, are not limited by enantiomer diffusion in the hydrophilic shell of the micelles. Consequently, the formation and rearrangement of the chelate complexes must be rate-limiting. In addition, decomplexation of both enantiomers is even slower, on the order of 10-6 s-1. Fortunately, ultrafiltration experiments indicate that a rapid exchange rate of bound L-Phe by unbound D-Phe improves the decomplexation of L-Phe to (360 ± 250) x 10-5 mM-1 s-1. This exchange process can be described by a second-order LDF model