An abiotic, tetrameric, eight-helix bundle†
Four helically folded aromatic oligoamide sequences containing either a chiral monomer based on 2-(2-aminophenoxy)-propionic acid, an N-terminal (1H)-camphanyl group, or both, were synthesized. Spectroscopic solution investigations using 1H NMR and circular dichroism (CD) demonstrated that the 2-(2-aminophenoxy)-propionic acid unit biases helix handedness quantitatively in chloroform and dichloromethane. It even quantitatively overcomes an opposing effect of the camphanyl group and thus ensures reliable helix handedness control. A series of nine sequences composed of two helically folded aromatic oligoamide segments separated by a flexible linker based on a di-, tri- or tetraethylene glycol unit were then synthesized. In these sequences, helix handedness was controlled by means of an N-terminal (1H)-camphanyl group or a 2-(2-aminophenoxy)-propionic acid units in either both helical segments, or only in the N-terminal segment, or in none of the segments. The helical segments all displayed hydroxy and carbonyl groups at their surfaces as hydrogen bond donors and acceptors so as to promote helix-to-helix hydrogen bonding. NMR and CD spectroscopic studies showed that, in some cases, well-defined, stable, discrete abiotic helix-turn-helix tertiary folds form in organic solvents. Molecular modelling suggests that these correspond to structures in which the two helix axes are at an angle. In one case, the absence of handedness control resulted in a complex and large aggregate. A solid state structure obtained by single crystal X-ray diffraction analysis revealed a tetrameric assembly composed of eight helices with both right and left handedness arranged in three subdomains consisting of two hydrogen-bonded three-helix bundles and one two-helix-bundle. Several helix-to-helix hydrogen bonds were mediated by bridging water molecules. This structure constitutes an important milestone in the construction of abiotic protein-like architectures.