Molecular Design and Synthesis of Artificial Ion Channels Based on Cyclic Peptides Containing Unnatural Amino Acids

Abstract

A series of novel cyclic peptides composed of 3 to 5 dipeptide units with alternating natural-unnatural amino acid units, have been designed and synthesized, employing 5-(N-alkanoylamino)-3-aminobenzoic acid with a long alkanoyl chain as the unnatural amino acid. All cyclic peptides with systematically varying pore size, shape, and lipophilicity are found to form ion channels with a conductance of ca. 9 pS in aqueous KCl (500 mM) upon examination by the voltage clamp method. These peptide channels are cation selective with the permeability ratio P(Cl(-))/P(K(+)) of around 0.17. The ion channels formed by the neutral, cationic, and anionic cyclic peptides containing L-alanine, L-lysine, and L-aspartate, respectively, show the monovalent cation selectivity with the permeability ratio P(Na(+))/P(K(+)) of ca. 0.39. On the basis of structural information provided by voltage-dependent blockade of the single channel current of all the tested peptides by Ca(2+), we inferred that each channel is formed from a dimer of the peptide with its peptide ring constructing the channel entrance and its alkanoyl chains lining across the membrane to build up the channel pore. The experimental results are consistent with an idea that the rate of ion conduction is determined by the nature of the hydrophobic alkanoyl chain region, which is common to all the channels.

Anion recognition by cyclic peptides

“…[33][34][35] UV-vis titration experiments in DMSO indicated that the disodium salt of 4-nitrophenyl phosphate forms 1 : 1 complexes with the receptors [K a = 1.2 Â 10 6 M À1 for cyclo-(Ala-Aba) 3 (4)]. Additional evidence for the binding mode was obtained by 1 H NMR spectroscopy, which indicates that 4 has C 3 -symmetry that is maintained on binding the phosphoester.…”

Expanding the Genetic Code

“…[60] Intramolecular native chemical ligation has also been used to create cyclic peptides and proteins. [61][62][63] Chemical synthesis allows a large variety of unnatural amino acids to be incorporated into peptides and proteins. It is particularly useful for the incorporation of amino acid analogues that are toxic to cells or incompatible with the translational machinery (for example, the synthesis of an all d-amino acid protein [64,65] ).…”

The determination of the ion selectivity of synthetic ion channels and pores in vesicles

“…For an organic chemist entering the field without BLMs, [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] LUVs provide a user friendly and, according to the comparative studies discussed below, valid alternative. Among several possible methods to determine anion/cation selectivity of synthetic ion channels in vesicles, the HPTS assay worked best in our hands (Fig.…”