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How halogenation affects protein or peptide folding and self-assembly hierarchically? This study tries to answer this question by using the halogen bonding mediated self-assemblies on cyclodipeptide scaffolds. Single-functionalized cyclodipeptides (Cyclo-GX) based on para-halogenated phenylalanine in the solid state form homochiral helical nanotubes via consecutive X···O bonds (X = Cl, Br, and I) independent of halogen kinds. In contrast, double-functionalized cyclodipeptides (Cyclo-XX) feature versatile self-assembly architectures depending on the para-substituents (X = H, F, Cl, Br, and I), affording nanotubular, lamellar, and triple helical nanotubular architectures. Cyclo-BrBr exclusively adopts intramolecular Type-IV X···X interaction that alters the molecular folding and packing, which also gives rise to opposite chirality at molecular folding (secondary structure), stacking (tertiary structure), and self-assembled nanohelices (quarternary structure) at macroscopic scale. It unveils how halogenation impacts on the self-assembly and chirality at hierarchical levels in specific peptides. Clusteroluminescence is found for the cyclodipeptides, achieving high quantum yield up to 71%, whereby circularly polarized luminescence is realized with tunable handedness by controlling halogen substituents.
Wang et al. (Wed,) studied this question.