Length-Dependent Formation of Transmembrane Pores by 3₁₀-Helical α-Aminoisobutyric Acid Foldamers

Abstract

The synthetic biology toolbox lacks extendable and conformationally controllable yet easy-to-synthesize building blocks that are long enough to span membranes. To meet this need, an iterative synthesis of α-aminoisobutyric acid (Aib) oligomers was used to create a library of homologous rigid-rod 310-helical foldamers, which have incrementally increasing lengths and functionalizable N- and C-termini. This library was used to probe the inter-relationship of foldamer length, self-association strength, and ionophoric ability, which is poorly understood. Although foldamer self-association in nonpolar chloroform increased with length, with a ∼14-fold increase in dimerization constant from Aib6 to Aib11, ionophoric activity in bilayers showed a stronger length dependence, with the observed rate constant for Aib11 ∼70-fold greater than that of Aib6. The strongest ionophoric activity was observed for foldamers with >10 Aib residues, which have end-to-end distances greater than the hydrophobic width of the bilayers used (∼2.8 nm); X-ray crystallography showed that Aib11 is 2.93 nm long. These studies suggest that being long enough to span the membrane is more important for good ionophoric activity than strong self-association in the bilayer. Planar bilayer conductance measurements showed that Aib11 and Aib13, but not Aib7, could form pores. This pore-forming behavior is strong evidence that Aibm (m ≥ 10) building blocks can span bilayers.

Ligand-modulated conformational switching in a fully synthetic membrane-bound receptor

“…Comparing the ratio of monomer emission intensities from the I and III emission bands from the pyrene fluorophore 35 of Cu-1 in organic solvents to Cu-1 in a bilayer indicates that the pyrenes are in a polar environment to which water has access. This suggests that the pyrenes lie within the membrane but close to the interface with water, possibly on the other side of the bilayer (foldamers of similar length and structure are capable of spanning the bilayer 19 ).…”

“…Synthetic oligomers built solely from Aib fold into 310 helices (Figure 1a, "Relay"), 18 with decamers being sufficiently long to span a bilayer. 19 In solution, these "foldamers" 20 populate almost exclusively 21 two rapidly exchanging conformational states of left or right handed helical screw sense, with the relative population of these two states being remarkably sensitive to chiral influences. [22][23][24][25] A chiral ligand covalently 15 or non-covalently 16 bound to one terminus of an Aib foldamer will propagate its conformational influence through the entire foldamer length.…”

An α‐Helix‐Mimicking 12,13‐Helix: Designed α/β/γ‐Foldamers as Selective Inhibitors of Protein–Protein Interactions

“…Foldamers are unnatural oligomers that adopt well-defined secondary and tertiary conformations. [1][2][3][4] As bioinspired structures,s ome of them have been validated as useful reagents to modulate (therapeutically important) biological processes and systems, [4][5][6][7][8][9] and others as building blocks for use in synthetic biology [10][11][12][13][14] or the construction of functional materials. [15,16] Ap articularly fertile area is centred on the search for foldamers that mimic natural secondary structures (specifically a-helices) and thereby act as inhibitors of protein-protein interactions (PPIs).…”

Artificial Signal Transduction across Membranes

“…[81] Subsequently,t he same group expanded this approach to successful coupling of Kir6.2 to the b 2 -adrenergicr eceptor ( Figure 4A), [82] an important drug target because of its involvement in smooth muscle relaxation, [83] and to opsin,t he GPCR that is part of the light-sensitive rhodopsin, [84] to produce al ight-sensitive ion channel. [65] C) Rings of cyclic peptidess elf-assemble into peptidenanotubes in al ipid bilayer,c reating apassagef or ions. Peptide-based channels.…”