Design, synthesis, and crystal structure of self-assembling norbornene (nbe)-supported two-helix bundles: a unique example of janus helicity in the solid-state structure of nbe(aib₅)₂

Abstract

Norbornene-supported bis-helical peptides with the general structure NBE(Aib_n)₂ (NBE: 2,3-trans-norbornene dicarbonyl unit; Aib: α,α'-dimethyl glycine unit; n = 4,5) have been synthesized and examined for self-assembly preferences in the solid state. An x-ray study has revealed a phenomenon of Janus helicity in the solid state structure of NBE(Aib₅)₂. The lower homologue NBE(Aib₄)₂, however, shows an identical screw sense for both the helical arms. The difference in the handedness of left and right arms is reflected in the self-assembly patterns. Thus, while the NBE(Aib₄)₂ molecule self-assembles to form an infinite hydrogen-bonded superhelical ladder, the Janus molecule NBE(Aib₅)₂ crystallizes as individual units surrounded by water molecules. The structures of Z–Aib₄–OMe and Z–Aib₅–OMe are also presented to compare their conformations with the helical arms of the title compound and also to the already known structures of other X–Aib_n –Y compounds. The helices in all the molecules are the 3₁₀-type.