Chiral Separation of Racemic Mixtures with Chiral Polyamide Membranes Containing Aspartyl Residues in their Main Chains
Novel chiral polyamides with chiral environment in their main chains were obtained from aromatic diamines, such as 1,3- phenylenediamine (1,3-PDA) or 1,4-phenhylenediamine (1,4-PDA), and the D-isomer or L-isomer of N-α-protected aspartic acid, such as N-α-benzyloxycarbonyl-D-aspartic acid (Z-D-Asp-OH) or N-α-benzyloxycarbonyl-L-aspartic acid (Z-L-Asp-OH), in the presence of triphenyl phosphite (TPP). Four types of newly prepared polyamide showed optical rotation, implying that there were asymmetric carbons in their main chains. CD studies demonstrated that resulting chiral polyamides took a helical structure. Surface plasmon resonance (SPR) spectroscopy studies revealed that the polyamide from Z-D-Asp and 1,4-PDA (1,4-PDA-Z-D-Asp) incorporated the L-isomer of glutamic acid (Glu) in preference to D-Glu and the adsorption selectivity toward L-Glu was determined to be 1.30 and vice versa; in other words, D-Glu was selectively adsorbed in 1,4-PDA-Z-L-Asp with the adsorption selectivity toward the D-isomer of 1.22. Contrary to the adsorption phenomena of the chiral polyamides consisting of 1,4-PDA, two types of 1,3-PDA-Z-Asp polyamide hardly showed adsorption selectivity toward racemic Glu. Chiral separation ability of those four types of polyamide was investigated by adopting concentration gradient or applied potential difference as a driving force for membrane transport. Membranes from chiral polyamide with 1,4-PDA as a diamine component showed permselectivity toward racemic mixture of Glu. The permselcetivity was dependent not only on the absolute configuration of diacid component but also on the nature of medium containing racemic mixture. In the enantioselective electrodialysis in an aqueous ethanol system, the permselectivity toward L-Glu for 1,4-PDA-Z-D-Asp membrane was determined to be 1.52 and that toward D-Glu for 1,4-PDA-Z-L-Asp membrane to be 1.77.
Keywords: Chiral polyamide, Chiral separation, Membrane, Optical resolution, Permselectivity, Polyamide, Racemic Mixtures, Aspartyl Residues
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