Complexation, liquid–liquid extraction, and transport through a liquid membrane of protonated peptides using crown ethers Original Research Article

By means of an ion-pairing mechanism, some protonated peptides (glycyl-glycine, glycyl-l-α-alanine, glycyl-l-valine, glycyl-l-leucine, glycyl-l-phenylalanine, glycyl-glycyl-glycine, glycyl-l-aspartic acid, and l-leucyl-glycyl-glycine) were extracted by crown ethers from an aqueous phase into a chloroform phase and transported though a chloroform liquid membrane in the presence of counterions. The peptides under study exhibited good extractability by crown ethers from the aqueous phase into the organic phase. The extraction constants of the peptides involved were determined. Active transport, assisted by pH gradient of peptides in protonated form as ion pairs in the presence of tropaeolin 00 using 18-crown-6 (18C6), benzo-18-crown-6 (B18C6) and dibenzo-18-crown-6 (DB18C6) as carriers through the chloroform liquid membrane was carried out. Determination of complex stabilities for the complexation between crown ethers and peptides was accomplished by calorimetric titration. It was also suggested that the extractability and the rate of the transport of peptides are controlled by factors like the structure of both the ligand and peptide, the nature of the anion used as the counterion and thermodynamic properties. A relevant correlation between the structural properties of the protonated peptides under study and their physicochemical characteristics was established on the basis of the experimental results. The carriers employed, namely 18C6, B18C6, and DB18C6, exhibited different transport selectivities relatively to the peptides under study. As a consequence, one main goal of our investigations was to determine the optimal conditions for separation the peptides through liquid membranes.