| Abstract: | We have synthesized and characterized a series of cation-binding cyclic octapeptides which may function as potential ionophoric substances. The materials contain varying degrees of hydrophobic character, which was controlled systematically through the incorporation of N-alkylglycine residues where N-alkyl = methyl, n-hexyl, cyclohexyl, or n-decyl. The peptides reported include cyclo(Phe-Sar-Gly-Sar)2, cyclo(Glu(OBzl)-Sar-Gly-Sar-Glu(OBzl)-Sar-Gly-(N-decyl)Gly), cyclo(Glu(OBzl)-Sar-Gly-(N-decyl)Gly)2, cyclo(Glu(OBzl)-Sar-Gly-(N-hexyl)Gly)2, cyclo(Glu(OBzl)-Sar-Gly-(N-cyclohexyl)Gly)2, and the corresponding free diacid forms of the Glu-containing compounds. Using 13C- and 1H-nmr spectra, we demonstrated that the mixture of cis/trans peptide bond-isomer conformers, characteristic of the free-peptide benzyl esters in solution, was converted to unique C2-symmetric, presumably all-trans conformers on complexation with calcium ions. Cation-transport experiments, using the thick-liquid model of transport in a Pressman cell, established that these compounds transport a variety of cations and that one peptide examined in detail, cyclo(Glu(OBzl)-Sar-Gly-(N-decyl)Gly)2 (selectivity Ca2+ > Na+ > K+ > Mn2+ > Cu2+ > Mg2+ > Co2+ > Zn2+), transports calcium about an order of magnitude more efficiently than magnesium. |
