Selective ion transport and ion extraction by synthetic cyclic octapeptide

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Abstract
Cyclic octapeptide, cyclo[Gly‐L‐Lys(Z)‐Sar‐L‐Pro]2 (CGLSP2) was synthesized as an ionophore model. Its ion‐transport ability through a liquid membrane was investigated in connection with ion extractability and conformational properties. CGLSP2 transported the picrate salts of Ba2+ and Ca2+ efficiently through chloroform membrane in a U‐tube (Ba2+> Ca2+ > K+ > Rb+ > Mg2+> Na+). Cyclo[Gly‐L‐Lys(Z)‐Sar‐L‐Pro] or Boc‐[Gly‐L‐Lys(Z)‐Sar‐L‐Pro]n‐OR (n = 1 or 2; R = H or CH3) showed a significant decrease in ion‐transport ability. Extraction equilibrium constants (Kex) of CGLSP2 with metal picrates were determined in chloroform‐water system. In this case, CGLSP2/cation complexes were found to have a stoichiometry of 1:1. The Kex sequences were Ba2+ > Ca2+ > Mg2+ and K+ > Rb+ > Na+, showing good agreement with the selectivity in ion transport. Conformation of CGLSP2 in chloroform (CDC13) was investigated by 1H and 13C‐n.m.r. spectroscopy. Predominant conformation takes a C2 ‐symmetric structure containing two cis Lys‐Sar peptide bonds. Upon complexation with Ca2+ in CDC13/CD3CN mixture (5/3, v/v), the conformation of CGLSP2 converged into a single C2 ‐symmetric one that was predominant in a free state in CDC13.