Facilitated transport of organics of biological interest: I. A new alternative for the separation of amino acids by fixed-site crown-ether polysiloxane membranes

Fixed-site heteropolysiloxane membranes containing grafted macrocyclic receptors can separate a mixture of amino acids. These membranes have an intermediate configuration between liquid membranes (selective complexation by a specific carrier) and solid membranes (charge interactions). The solution–diffusion model, which was used to analyze experimental transport results for these membranes, provided evidence for a new dual transport mechanisms. With an acidic pH of the feed phase, the selectivity of the transport (symport of protons) relative to l-α-alanine (Ala) was high and dependent on the relative hydrophobicity of the amino acids (l-phenylaniline (Phe), leucine (Leu)) (S=7–10), whereas no selectivity was obtained when the pH of the feed phase was higher. The proton-driven transport increased the flux and caused the transport rates of amino acids to be widely spread out due to different molecular recognition principles effecting transport. An active transport mechanism of amino acids is possibly present in the solid dense polymeric matrix.