EFFICIENT AND HIGHLY SELECTIVE COPPER(II) TRANSPORT ACROSS A BULK LIQUID CHLOROFORM MEMBRANE MEDIATED BY LIPOPHILIC DIPEPTIDES

Several structurally simple N-monoalkylated and -dialkylated dipeptide s made of alpha-amino acids Gly, Phe, and Leu, 1-11, were synthesized and investigated as carriers for the transport of Cu(II), Zn(II), and Ni(II) from an aqueous pH = 5.6 buffer source to a 0.1 M HCl receiving phase across a bulk chloroform membrane. The proton-driven translocat ion was followed during the process by analyzing the metal ion concent rations in the three phases. The transport efficiency depends on the e ase of formation of a neutral complex with Cu(II) (the peptide group a nd carboxylic acid being deprotonated) at the source-chloroform interf ace and on that of its disruption by protonation at the receiving phas e: the carrier's Lipophilicity favors the metal ion uptake and not the release. By modulating the length of the N-alkyl chains and the hydro phobicity of the dipeptide moiety, a quite remarkable transport effici ency was observed for Cu(II), in most cases superior to that of the in dustrial extractant Kelex 100. Moreover, using L,L- and L,D-N-octyl-Ph eLeu as carriers, remarkable diastereomeric effects were observed in t he rate of uptake and release of Cu(II) ion although the differences m utually compensate in the overall transport rate. Under the conditions used the carriers are much less effective in the translocation of Zn( II) and Ni(II) and their transport efficiency drops dramatically in th e presence of Cu(II), the latter being favored by factors of 1.2 x 10( 3) and > 10(4), respectively. Such very high selectivities depend on t he fact that only Cu(II) among other transition metal ions Can form ne utral complexes at the pH value of the source phase.