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 prod ucts. We have studied the separation of D,L-phenylalanine (Phe) by choleste ryl-L-glutamate:Cu-II (CLG:Cu-II) anchored in nonionic micelles (intrinsic enantioselectivity alpha (D/L,int) = 1.9). A cascaded system is needed to c omplete the separation, as a single stage is insufficient to obtain > 99% o ptically pure products. It is shown that the complexation and decomplexatio n processes are not instantaneous; hence, elucidation of the complexation k inetics is essential to the design of a multistage system. Linear driving f orce (LDF) models describe both the complexation and decomplexation rates o f 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), respectivel y, are not limited by enantiomer diffusion in the hydrophilic shell of the micelles. Consequently, the formation and rearrangement of the chelate comp lexes must be rate-limiting. In addition, decomplexation of both enantiomer s is even slower, on the order of 10(-6) s(-1). Fortunately, ultrafiltratio n 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 .