Determination of the binding constants and ionic mobilities of diquat complexes with randomly sulfated cyclodextrins by affinity capillary electrophoresis.

The enantiomers of diquats (DQs), a new class of functional organic molecules, were recently separated by capillary electrophoresis (CE) with high resolution up to 11.4 within 5-7 min using randomly sulfated α-, ß-, and γ-cyclodextrins (CDs) as chiral selectors. These results indicated strong interactions between dicationic diquats and multiply negatively charged sulfated CDs (S-CDs). However, the binding strength of these interactions was not quantified. For that reason, in this study, affinity CE was applied for the determination of the binding constants and ionic mobilities of the complexes of DQ P- and M-enantiomers with CD chiral selectors in an aqueous medium. The non-covalent interactions of 10 pairs of DQ enantiomers with the above CDs were investigated in a background electrolyte (BGE) composed of 22 mM NaOH, 35 mM H3PO4, pH 2.5, and 0.0-1.0 mM concentrations of CDs. The average apparent binding constant and the average actual ionic mobility of the DQ-CD complexes were determined by nonlinear regression analysis of the dependence of the effective mobility of DQ enantiomers on the concentration of CDs in the BGE. The complexes were found to be relatively strong with the averaged apparent binding constants in the range 13 600-547 400 L/mol.

The separation of the enantiomers of diquats by capillary electrophoresis using randomly sulfated cyclodextrins as chiral selectors.

Diquats, derivatives of the widely used herbicide diquat, represent a new class of functional organic molecules. A combination of their special electrochemical properties and axial chirality could potentially result in their important applications in supramolecular chemistry, chiral catalysis, and chiral analysis. However, prior to their practical applications, the diquats have to be prepared in enantiomerically pure forms and the enantiomeric purity of their P- and M-isomers has to be checked. Hence, a chiral capillary electrophoresis (CE) method has been developed and applied for separation of P- and M-enantiomers of 11 new diquats. Fast and better than baseline CE separations of enantiomers of all 11 diquats within a short time 5-7 min were achieved using acidic buffer, 22 mM NaOH, 35 mM H3 PO4 , pH 2.5, as a background electrolyte, and 6 mM randomly sulfated α-, ß-, and γ-cyclodextrins as chiral selectors. The most successful selector was sulfated γ-cyclodextrin, which baseline separated the enantiomers of all 11 diquats, followed by sulfated ß-cyclodextrin and sulfated α-cyclodextrin, which baseline separated enantiomers of 10 and nine diquats, respectively. Using this method, a high enantiopurity degree of the isolated P- and M-enantiomers of three diquats with a defined absolute configuration was confirmed and their migration order was identified.

Helquat dyes targeting G-quadruplexes as a new class of anti-HIV-1 inhibitors.

The secondary structure of nucleic acids containing quartets of guanines, termed G-quadruplexes, is known to regulate the transcription of many genes. Several G-quadruplexes can be formed in the HIV-1 long terminal repeat promoter region and their stabilization results in the inhibition of HIV-1 replication. Here, we identified helquat-based compounds as a new class of anti-HIV-1 inhibitors that inhibit HIV-1 replication at the stage of reverse transcription and provirus expression. Using Taq polymerase stop and FRET melting assays, we have demonstrated their ability to stabilize G-quadruplexes in the HIV-1 long-terminal repeat sequence. Moreover, these compounds were not binding to the general G-rich region, but rather to G-quadruplex-forming regions. Finally, docking and molecular dynamics calculations indicate that the structure of the helquat core greatly affects the binding mode to the individual G-quadruplexes. Our findings can provide useful information for the further rational design of inhibitors targeting G-quadruplexes in HIV-1.

A Helquat-like Compound as a Potent Inhibitor of Flaviviral and Coronaviral Polymerases.

Positive-sense single-stranded RNA (+RNA) viruses have proven to be important pathogens that are able to threaten and deeply damage modern societies, as illustrated by the ongoing COVID-19 pandemic. Therefore, compounds active against most or many +RNA viruses are urgently needed. Here, we present PR673, a helquat-like compound that is able to inhibit the replication of SARS-CoV-2 and tick-borne encephalitis virus in cell culture. Using in vitro polymerase assays, we demonstrate that PR673 inhibits RNA synthesis by viral RNA-dependent RNA polymerases (RdRps). Our results illustrate that the development of broad-spectrum non-nucleoside inhibitors of RdRps is feasible.

Kinetic Target-Guided Synthesis of Small-Molecule G-Quadruplex Stabilizers.

The formation of a G-quadruplex motif in the promoter region of the c-MYC protooncogene prevents its expression. Accordingly, G-quadruplex stabilization by a suitable ligand may be a viable approach for anticancer therapy. In our study, we used the 4-(4-methylpiperazin-1-yl)aniline molecule, previously identified as a fragment library screen hit, as a template for the SAR-guided design of a new small library of clickable fragments and subjected them to click reactions, including kinetic target-guided synthesis in the presence of a G-quadruplex forming oligonucleotide Pu24. We tested the clickable fragments and products of click reactions for their G-quadruplex stabilizing activity and determined their mode of binding to the c-MYC G-quadruplex by NMR spectroscopy. The enhanced stabilizing potency of click products in biology assays (FRET, Polymerase extension assay) matched the increased yields of in situ click reactions. In conclusion, we identified the newly synthesized click products of bis-amino derivatives of 4-(4-methylpiperazin-1-yl)aniline as potent stabilizers of c-MYC G-quadruplex, and their further evolution may lead to the development of an efficient tool for cancer treatment.

Helquats as Promoters of the Povarov Reaction: Synthesis of 1,2,3,4-Tetrahydroquinoline Scaffolds Catalyzed by Helicene-Viologen Hybrids.

Helquats (HQs) are structurally linked to helicenes and viologens, and they represent an attractive field of research in chemistry and medicinal chemistry. In the present work they were used as catalysts for the synthesis of 1,2,3,4-tetrahydroquinolines in good yields by the Povarov reaction. The substrate scope and the capability of different helquats to promote Povarov reactions are demonstrated. Studies to elucidate mechanistic details revealed that helquats act as single-electron transfer oxidants through a cation-radical mechanism. The screening of the catalytic activity of HQs confirmed that an active HQ must have a LUMO energy below -8.67 eV and the standard redox potential higher (less negative) than -1.2 V vs. the ferrocene/ferrocenium redox couple.

Determination of binding constants of multiple charged cyclodextrin complexes by ACE using uncorrected and ionic strength corrected actual mobilities of the species involved.

In this study, the apparent binding constants and limiting mobilities of the multiply charged complexes of the Δ- and Λ-enantiomers of Ru(II)- and Fe(II)-polypyridyl associates ([Ru(2,2'-bipyridine)3 ]2+ , [Ru(1,10-phenanthroline)3 ]2+ , and [Fe(1,10-phenanthroline)3 ]2+ ) with single-isomer 2,3-diacetylated-6-sulfated-cyclodextrins (CDs) (12Ac-6S-α-CD, 14Ac-7S-ß-CD, and 16Ac-8S-γ-CD) were determined by ACE using uncorrected and ionic strength corrected actual mobilities of the species involved. Two limiting models were tested for the ionic strength correction of the actual mobilities based on an empirical relation for the ionic strength correction of multivalent ionic species. In model 1, the nominal values of the charge numbers (zS,nom ) and analytical concentrations (cS,nom ) of the above CD selectors in the BGEs were applied for calculation of the BGE ionic strength, as usual. In model 2, the CD selectors were considered as singly charged species (zS  = -1) with |zS,nom |-times higher concentrations in the BGE than their analytical concentrations (cS  = |zS,nom | × cS,nom ) in the calculation of the BGE ionic strength. In all three cases-with uncorrected actual mobilities as well as with actual mobilities corrected according to the two limiting models-the measured effective mobilities of the above enantiomers fit well the theoretical curves of their mobility dependences on the CD selectors concentrations in the BGE, with high average coefficients of determination (R2  = 0.9890-0.9995). Nevertheless, the best physico-chemically meaningful values of the apparent binding constants and the limiting mobilities of the enantiomer-CDs complexes with low RSDs were obtained using the actual mobilities of the species involved corrected according to model 2.

Diquats with Robust Chirality: Facile Resolution, Synthesis of Chiral Dyes, and Application as Selectors in Chiral Analysis.

Diquats with extremely high racemization barriers with ΔG≠theor of 233 kJ mol-1 at 180 °C are described. Reported configurational robustness is due to a combination of two structural features: the rigid o-xylylene tether connecting the nitrogen atoms and the presence of two substituents in the bay region of the bipyridinium scaffold. The straightforward synthesis of diquats, plus facile resolution and derivatization make them attractive for chiral application studies. This is demonstrated by: 1) synthesis of the first non-racemic diquat dyes with pronounced chiroptical properties, and 2) capability of diquats to interact stereospecifically with chiral molecules. This suggests potential for diquat derivatives to be used as chiral selectors in separation methods.

Dutch Resolution of a configurationally stable [5]helquat.

Synthesis and nontrivial optical resolution of a helicene-like dication, helquat 1, has been accomplished. Starting with gram scale of the racemic helquat 1 sample, Dutch Resolution using family of 3 tartrate anions was key to achieve successful separation of M and P helical enantiomers of 1. Hundreds of milligrams of each enantiomer of this configurationally stable C2 -symmetric helquat have been obtained. Racemization barrier of 1 has been determined. To our knowledge this is the first report on Dutch Resolution performed with a helicene-like compound. Moreover, there are no literature precedents for Dutch Resolution of chiral quaternary ammonium cations.

Intense redox-driven chiroptical switching with a 580 mV hysteresis actuated through reversible dimerization of an azoniahelicene.

Electrochemical reduction of an azoniahelicene affords a dimer, accompanied by a strong change in the electronic circular dichroism. The fast dimerisation event leads to a >500 mV shift of the oxidation potential, affording a large area of bistability, where the chiroptical signal only depends on the redox history.

Tunable Chiral Second-Order Nonlinear Optical Chromophores Based on Helquat Dications.

Fourteen new dipolar cations have been synthesized, containing methoxy or tertiary amino electron donor groups attached to helquat (Hq) acceptors. These Hq derivatives have been characterized as their TfO- salts by using various techniques including NMR and electronic absorption spectroscopies. UV-vis spectra show intense, relatively low energy absorptions with λmax ≈ 400-600 nm, attributable to intramolecular charge-transfer (ICT) excitations. Single-crystal X-ray structures have been solved for two of the chromophores, one as its PF6- salt, revealing centrosymmetric packing arrangements (space groups Pbca and P1̅). Molecular quadratic nonlinear optical (NLO) responses have been determined directly by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and indirectly via Stark (electroabsorption) spectroscopy for the low energy absorption bands. The obtained static first hyperpolarizabilities ß0 range from moderate to large: (9-140) × 10-30 esu from HRS in MeCN and (44-580) × 10-30 esu from the Stark data in PrCN. The magnitude of ß0 increases upon either extending the π-conjugation length or replacing a methoxy with a tertiary amino electron donor substituent. Density functional theory (DFT) and time-dependent DFT calculations on selected tertiary amino chromophores confirm that the low energy absorptions have ICT character. Relatively good agreement between the simulated and experimental UV-vis absorption spectra is achieved by using the CAM-B3LYP functional with the 6-311G(d) basis set. The ßtot values predicted by using DFT at the same level of theory are large ((472-1443) × 10-30 esu in MeCN). Both the theoretical and experimental results show that para-conjugation between Hq and electron donor fragments is optimal, and enlarging the Hq unit is inconsequential with respect to the molecular quadratic NLO response.

Chiral analysis of α-diimine Ru(II) and Fe(II) complexes by capillary electrophoresis using sulfated cyclodextrins as stereoselectors.

CE using randomly highly sulfated α-, ß-, and γ-CDs (S-α-CD, S-ß-CD, S-γ-CD), sulfobutylether-ß-CD (SBE-ß-CD), single isomer (6-O-sulfo) α-, ß-, and γ-CDs, and their derivatives as stereoselectors was applied to chiral analysis of polypyridyl complexes of [Ru(bpy)3 ]2+ , [Ru(phen)3 ]2+ , and [Fe(phen)3 ]2+ (bpy = 2,2'-bipyridine; phen = 1,10 phenanthroline). The best separations of Δ- and Λ-enantiomers of the these complexes with high resolution (up to R1,2  = 7.0) and short analysis times (10-20 min) were achieved in the BGE composed of 22 mM NaOH/35 mM H3 PO4 , pH 2.4, containing 1.5-6.0 mM S-α-CD or S-ß-CD, or SBE-ß-CD as chiral selectors. The developed method was applied to the assessment of enantiomeric purity of several samples of [Ru(bpy)3 ]2+ catalyst. CE experiments were performed in a homemade analyzer equipped with bare or hydroxypropylcellulose-coated fused-silica capillaries (total/effective length 40/29 cm, id/od 50/375 µm) and an UV absorption detector operating at 206 nm. In addition to chiral analysis, apparent binding constants of the complexes of [Ru(bpy)3 ]2+ , [Ru(phen)3 ]2+ , and [Fe(phen)3 ]2+ enantiomers with five sulfated CDs (S-α-CD, S-ß-CD, S-γ-CD, SBE-ß-CD, and 16Me-8S-γ-CD) were determined from the dependence of their effective electrophoretic mobilities on the concentration of the CDs in the BGE by nonlinear regression analysis. Calculated apparent binding constants of these complexes were found to be in the (1.10-4.66) × 103  L/mol range. Moreover, it was shown that at selected concentrations of some S-CDs and suppressed or very low electroosmotic flow, the exceptional enantioseparations with infinite resolution could be achieved.

Ferrocenyl helquats: unusual chiral organometallic nonlinear optical chromophores.

Three new dipolar cations have been synthesised, containing ferrocenyl (Fc) electron donor groups attached to helquat (Hq) acceptors. These organometallic Hq derivatives have been characterised as their TfO- salts by using various techniques including NMR and electronic absorption spectroscopies and electrochemical measurements. UV-vis spectra show multiple intense low energy absorptions attributable to intramolecular charge-transfer (ICT) excitations. Each compound displays a reversible Fc+/0 redox process, together with two reversible one-electron reductions of the Hq fragment. Molecular quadratic nonlinear optical (NLO) responses have been determined by using hyper-Rayleigh scattering at 1064 nm, and Stark (electroabsorption) spectroscopic studies on the visible absorption bands. The obtained first hyperpolarizabilities ß are moderate, consistent with the relatively short π-conjugation lengths between the Fc and attached pyridinium group. A single-crystal X-ray structure has been solved for one of the complexes as its PF6- salt, revealing a centrosymmetric packing in the triclinic space group P1[combining macron]. Density functional theory (DFT) and time-dependent DFT calculations indicate that the lowest energy absorption bands have mainly metal-to-ligand charge-transfer character. The donor orbitals involved in the electronic transitions forming the next lowest energy ICT band also have substantial contributions from the Fe atom. Good agreement between the simulated and experimental UV-vis absorption spectra is achieved by using the PBE0 functional with the 6-311++G(d)/LANL2DZ mixed basis set, and the theoretical ß values are reasonably large. Oxidation of the Fc unit is predicted to cause the ßtot value to decrease by more than 80% in one of the complexes.

Interactions of helquats with chiral acidic aromatic analytes investigated by partial-filling affinity capillary electrophoresis.

Noncovalent molecular interactions between helquats, a new class of dicationic helical extended diquats, and several chiral acidic aromatic drugs and catalysts have been investigated using partial-filling affinity capillary electrophoresis (PF-ACE). Helquats dissolved at 1mM concentration in the aqueous background electrolyte (40mM Tris, 20mM acetic acid, pH 8.1) were introduced as ligand zones of variable length (0-130mm) into the hydroxypropylcellulose coated fused silica capillary whereas 0.1mM solutions of negatively charged chiral drugs or catalysts (warfarin, ibuprofen, mandelic acid, etodolac, binaphthyl phosphate and 11 other acidic aromatic compounds) were applied as a short analyte zone at the injection capillary end. After application of electric field, analyte and ligand migrated against each other and in case of their interactions, migration time of the analyte was increasing with increasing length of the ligand zone. From the tested compounds, only isomers of those exhibiting helical chirality and/or possessing conjugated aromatic systems were enantioselectively separated through their differential interactions with helquats. Some compounds with conjugated aromatic groups interacted with helquats moderately strongly but non-enantiospecifically. Small compounds with single benzene ring exhibited no or very weak non-enantiospecific interactions. PF-ACE method allowed to determine binding constants of the analyte-helquat complexes from the changes of migration times of the analytes. Binding constants of the weakest complexes of the analytes with helquats were less than 50L/mol, whereas binding constants of the strongest complexes were in the range 1 000-1 400L/mol.

Linquats: Synthesis, Characterization, and Properties of Linear Extended Diquats.

We report an innovative synthetic route to linear extended diquats (linquats). Our approach is short and efficient and features a highly modular reaction sequence based on two-fold quaternization followed by the key intramolecular [2+2+2] alkyne cycloaddition. The physico-chemical properties of four new linquats were characterized by spectroscopic methods, X-ray crystallography, and electrochemistry complemented by information obtained from DFT calculations. Electron deficient N-heteroaromatic cations with linear extended diquat motif with high electron affinities have been recently recognized as attractive n-type semiconductors for chemical and biological sensing. Their advantageous redox properties such as very fast reversible electron transfers make the title compounds interesting for applications.

Intense chirality induction in nitrile solvents by a helquat dye monitored by near resonance Raman scattering.

Chirality induction phenomena attract attention because of their relevance to intermolecular interactions encountered in living matter. Usually, such effects are weak. However, enantiomers of a [6]helquat dye were found to induce exceptionally strong chirality in several achiral solvents containing nitrile groups. This effect was observable as an intense Raman optical activity (ROA) induced in acetonitrile, acetonitrile-d3, and liquid hydrogen cyanide solvents. The observation was verified by measurement of both helquat enantiomers which provided mirror image ROA spectra. Theoretical analysis indicated that the 532 nm laser excitation light was in a near resonance with electronic transitions of the dye, which made the effect observable in very dilute solutions (1 : 200 000 helquat to nitrile ratio) and thus the phenomenon can be generally useful in analytical chemistry.

Structural revisions of small molecules reported to cross-link G-quadruplex DNA in vivo reveal a repetitive assignment error in the literature.

Two molecules of mistaken identity are addressed. Uncovering these assignment errors led us to formulate more general guidelines about additional misassignments in cases of published bis-imines derived from 1,2-phenylenediamine and hydroxybenzaldehydes having no substituent in ortho-positions. The main purpose of this article is to highlight this repetitive assignment error in the literature and thus increase the likelihood of correct assignments in future papers.

Helquat Dyes: Helicene-like Push-Pull Systems with Large Second-Order Nonlinear Optical Responses.

Helquat dyes combine a cationic hemicyanine with a helicene-like motif to form a new blueprint for chiral systems with large and tunable nonlinear optical (NLO) properties. We report a series of such species with characterization, including determination of static first hyperpolarizabilities ß0 via hyper-Rayleigh scattering and Stark spectroscopy. The measured ß0 values are similar to or substantially larger than that of the commercial chromophore E-4'-(dimethylamino)-N-methyl-4-stilbazolium. Density functional theory (DFT) and time-dependent DFT calculations on two of the new cations are used to probe their molecular electronic structures and optical properties. Related molecules are expected to show bulk second-order NLO effects in even nonpolar media, overcoming a key challenge in developing useful materials.