Supramolecular Architecture through Self-Organization of Janus-Faced Homoazanucleosides.

Design of Janus-faced or double-headed homoazanucleosides with the possibility to undergo self-organization through base pairing has been conceptualized and accomplished. The synthetic strategy demonstrates the unique ability to introduce two similar or complementary nucleobases on opposite arms of a chiral polyhydroxypyrrolidine while also ensuring that their faces are anti to each other to allow only intermolecular interactions between the nucleobases, an essential requisite for self-assembly. Single-crystal X-ray structures were determined for all three types of homoazanucleosides, one possessing two adenine molecules, the other with two thymine moieties, and the third containing both adenine and thymine. The crystal structures of all three display noncovalent interactions, including Watson-Crick base pairing, Hoogsteen H-bonding, and π-π stacking, resulting in unusual supramolecular patterns. The most striking supramolecular motif among them, which emerged from the crystal structure of the homoazanucleoside containing both adenine and thymine, is a left-handed helix formed through Watson-Crick pairing between nucleobases. The present study thus forms a prelude to the design of Janus-faced building blocks to establish helical pillars as well as lateral branches that together define a three-dimensional (3D) lattice. The ready accessibility of these molecules is expected to spur the next generation of discoveries in the design of functional nanomaterials.