Redox and optically active carbohelicene layers prepared by potentiodynamic polymerization

Hexahelicene DNA-binding: Minor groove selectivity, semi-intercalation and chiral recognition

In this contribution, we focused on a fundamental study targeting the interaction of water-soluble [6]helicene derivative 1 (1-butyl-3-(2-methyl[6]helicenyl)-imidazolium bromide) with double-stranded (ds) DNA. A synthetic 30-base pair duplex, plasmid, chromosomal calf thymus and salmon DNA were investigated using electrochemistry, electrophoresis and spectroscopic tools supported by molecular dynamics (MD) and quantum mechanical approaches. Both experimental and theoretical work revealed the minor groove binding of 1 to the dsDNA. Both the positively charged imidazole ring and hydrophobic part of the side chain contributed to the accommodation of 1 into the dsDNA structure. Neither intercalation into the duplex DNA nor the stable binding of 1 to single-stranded DNA were found in topoisomerase relaxation experiments with structural components of 1, i.e. [6]helicene (2) and 1-butyl-3-methylimidazolium bromide (3), nor by theoretical calculations. Finally, the binding of optically pure enantiomers (P)-1 and (M)-1 was studied using circular dichroism spectroscopy, isothermal titration calorimetry and UV Resonance Raman (UVRR) methods. Using MD and quantum mechanical methods, minor groove and semi-intercalation were proposed for compound 1 as the predominant binding modes. From the UVRR findings, we also can conclude that 1 tends to preferentially interact with adenine and guanine residues in the structure of dsDNA.

The synthesis and crystal structure of pH-sensitive fluorescent pyrene-based double aza- and diaza[4]helicenes

Novel pyrene-based double aza- and diaza[4]helicenes have been prepared through a five-step synthetic sequence in overall good yields. Commercially available 2,3-dihaloazines (2,3-dibromopyridine, 2,3-dichloropyrazine and 2,3-dichloroquinoxaline) were used as starting materials. The synthesis employs electrophile-induced cyclizations of ortho-alkynyl bihetaryls as the key steps, leading to the formation of a helical skeleton. To discern the effect of merging azine and pyrene moieties within a helical skeleton, the X-ray structures, UV-vis absorption and fluorescence spectra of the helicenes were investigated and compared with those of the parent [4]helicene, aza- and diaza[4]helicenes. It was found that the emission properties of the synthesized helicenes can be modulated as a function of pH. The basicity of pyrene-based double aza[4]helicenes was estimated by the direct fluorimetric titration method; the pK_a value was found to be equal to 1.4.

Flavin-Helicene Amphiphilic Hybrids: Synthesis, Characterization, and Preparation of Surface-Supported Films

This work reports on the preparation and structural characterization of flavo[7]helicene 1 (flavin-[7]helicene conjugate), which was subsequently characterized at the molecular level in either an aqueous environment or an organic phase, at the supramolecular level in the form of polymeric layers, and also embedded in a lipidic mesophase environment to study the resulting properties of such a hybrid relative to its parent molecules. The flavin benzo[g]pteridin-2,4-dione (isoalloxazine) was selected for conjugation because of its photoactivity and reversible redox behavior. Compound 1 was prepared from 2-nitroso[6]helicene and 6-methylamino-3-methyluracil, and characterized using common structural and spectroscopic tools: circular dichroism (CD), circularly polarized luminescence (CPL) spectroscopy, cyclic voltammetry (CV), and DFT quantum calculations. In addition, a methodology that allows the loading of 1 enantiomers into an internally nanostructured lipid (1-monoolein) matrix was developed.

Helicity: A Non-Conventional Stereogenic Element for Designing Inherently Chiral Ionic Liquids for Electrochemical Enantiodifferentiation

Configurationally stable 5-aza[6]helicene (1) was envisaged as a promising scaffold for non-conventional ionic liquids (IL)s. It was prepared, purified, and separated into enantiomers by preparative HPLC on a chiral stationary phase. Enantiomerically pure quaternary salts of 1 with appropriate counterions were prepared and fully characterized. N-octyl-5-aza[6]helicenium bis triflimidate (2) was tested in very small quantities as a selector in achiral IL media to perform preliminary electrochemical enantiodifferentiation experiments on the antipodes of two different chiral probes. The new organic salt exhibited outstanding enantioselection performance with respect to these probes, thus opening the way to applications in the enantioselective electroanalysis of relevant bioactive molecules.