Transport of chloride and carboxyfluorescein through phospholipid vesicle membranes by heptapeptide amphiphiles

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Abstract
Seven synthetic anion transporters (SAT) of the general form R2N-COCH2OCH2CO-(Gly)3-Pro-(Gly)3-OR′ were prepared. Three pairs of compounds each contained twin n-hexyl, n-decyl, and n-octadecyl (R) groups at the N-terminus and one contained twin n-tetradecyl groups. Three of the compounds were C-terminated by benzyl and three by heptyl (R′) residues. The ability of these compounds to mediate ion release from phospholipid vesicles was assessed. Chloride release was measured by ion selective electrode measurements and by chloride quenching of the fluorescent dye lucigenin. Transport of the anion carboxyfluorescein (CF) was measured by fluorescence dequenching. Differences in both the C- (R′) and N-terminal (R) residues within the ionophores affected anion transport. The chloride release data acquired by ion selective electrode and fluorescence methods were similar but not identical. A possible carrier mechanism for Cl− transport was discredited. Both Cl− and CF anions were released from vesicles by these compounds. The results of CF and Cl− transport showed good consistency when the ionophore's N-terminal chains were either decyl or octadecyl but not when they were hexyl. The transport of CF and Cl− appears to be fundamentally different when R is C6 compared to C10 or C18. Differences between the behavior of SATs with Cl− and CF were also reflected in negative ion mass spectrometric studies.

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