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(Aib(5))(2)

Norbornene-supported bis-helical peptides with the general structure NBE(Ai bn)(2) (NBE: 2,3-trans-norbornene dicarbonyl unit; Aib: alpha,alpha'-dimeth yl glycine unit; n = 4,5) have been synthesized and examined for self-assem bly preferences in the solid state. An x-ray study has revealed a phenomeno n of Janus helicity in the solid state structure of NBE(Aib(5))(2). The low er homologue NBE(Aib(4))(2), however, shows an identical screw sense for bo th the helical arms. The difference in the handedness of left and right arm s is reflected in the self-assembly patterns. Thus, while the NBE(Aib(4))(2 ) molecule self-assemblies to form an infinite hydrogen-bonded superhelical ladder, the Janus molecular NBE(Aib(5))(2) crystallizes as individual unit s surrounded by water molecules. The structures of Z-Aib(4)-OMe and Z-Aib(5 )-OMe are also presented to compare their conformations with the helical ar ms 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 3(10)-type. (C) 2000 John Wiley & Sons, Inc.dagger