Chiral Lipophilic Ligands. 2. Cu(II)-Mediated Transport of α-Amino Acids Across a Bulk Chloroform Membrane

Article abstract of DOI:10.1016/0040-4020(94)00935-N

A number of achiral (1-4) and chiral (5-10) 1,2-diamino ethane derivatives bearing on one or both nitrogen atoms long paraffinic chains have been synthesized and successfully tested as carriers for the transport of Cu(II) and α-amino acids across a bulk chloroform membrane from a buffered (pH=5.5) source phase to a receiving solution of EDTA.This, being a highly hydrophilic ligand with an affinity for Cu(II) several order of magnitude larger than that of the carrier, allows an effective "up-hill" co-transport of ions and amino acids.The results indicate that the transport occurs via the formation of a ternary complex comprising Cu(II), the lipophilic diamine, and the amino acid.The lipophilicity of the amino acid is the main factor affecting the rate of transport using the same carrier, on the other hand, for the same amino acid the best carrier is that having affinity constant for Cu(II) close to that of the amino acid transported.The rate of transport is apparently governed by the rate of release of the metal ion and the amino acid from the source phase to the organic phase.The enantioselectivities observed in the co-transport of natural amino acids employing chiral ligands are generally modest; in the best cases, the enantioselectivity ratios (the initial rates of transport of the faster enantiomer relative to the slower) are slightly higher than 2.The enantioselectivity appears to depend more on thermodynamic rather than kinetic factors.