articles in 2017

Nineteen ortho-substituted PCBs are chiral and found enantioselectively enriched in ecosystems. Their differential actions on biological targets are not understood. PCB 95 (2,2′,3,5′,6-pentachlorobiphenyl), a chiral PCB of current environmental relevance, is among the most potent toward modifying ryanodine receptors (RyR) function and Ca2+ signaling. PCB 95 enantiomers are separated and assigned aR- and aS-PCB 95 using three chiral-column HPLC and circular dichroism spectroscopy. Studies of RyR1-enriched microsomes show aR-PCB 95 with >4× greater potency (EC50 = 0.20 ± 0.05 μM), ∼ 1.3× higher efficacy (Bmax = 3.74 ± 0.07 μM) in [3H]Ryanodine-binding and >3× greater rates (R = 7.72 ± 0.31 nmol/sec/mg) of Ca2+ efflux compared with aS-PCB 95, whereas racemate has intermediate activity. aR-PCB 95 has modest selectivity for RyR2, and lower potency than racemate toward the RyR isoform mixture in brain membranes. Chronic exposure of hippocampal neuronal networks to nanomolar PCB 95 during a critical developmental period shows divergent influences on synchronous Ca2+ oscillation (SCO): rac-PCB 95 increasing and aR-PCB 95 decreasing SCO frequency at 50 nM, although the latter’s effects are nonmonotonic at higher concentration. aS-PCB95 shows the greatest influence on inhibiting responses to 20 Hz electrical pulse trains. Considering persistence of PCB 95 in the environment, stereoselectivity toward RyRs and developing neuronal networks may clarify health risks associated with enantioisomeric enrichment of PCBs.

The circular dichroisms (CDs) of dimethoxy[2.2]-, [3.2]-, and [3.3]pyridinophanes and their protonated forms were investigated experimentally and theoretically. Characteristic multisignate Cotton effects (CEs), typical for planar chiral cyclophane derivatives, were observed. The CD spectral pattern was quite comparable for the staggered forms of [2.2]-, [3.2]-, and [3.3]cyclophanes, but significantly differed for the eclipsed forms. More interestingly, the patterns resembled, but the CE signs were practically opposite between staggered and eclipsed [2.2]pyridinophanes. Upon protonation, the signs of most CEs were inverted in both forms of cyclophanes, due to the reversal of dipole moment in the pyridine against the pyridinium moiety. Such a change in CD spectrum upon protonation was not apparent in [3.2]pyridinophane, and the CD spectral behavior was more complex in [3.3]pyridinophanes. The variation of CD caused by the protonation/deprotonation process was temperature-dependent and hence utilized as a thermal sensor. The protonated forms of the homologous pyridinophanes with different tether lengths in staggered and eclipsed forms served as a model system for systematically studying the cation−π interaction and its effects on chiroptical properties. A steady increase of electronic interaction became apparent for the smaller-sized cyclophanes from the increased excitation energy and electronic coupling element of the charge-transfer (CT) band, while the observed CE at the CT band was a more complex function of the original transition dipole of donor/acceptor pair and linker atoms, as well as the strength of the electronic interaction.

Circular dichroisms (CDs) of neutral and protonated [3.3]anthracenopyridinophane (1 and 1-H+) were investigated experimentally and theoretically. Introducing an anthracene moiety with extended conjugation affected the cyclophane structure with the bent angles being appreciably　reduced from those of parent [3.3]pyridinophane. The Cotton effects (CEs) observed at the 1Bb band for both 1 and 1-H+ were fairly strong and apparently bisignate, which, however, turned out not to be a simple exciton couplet but to be composed of multiple transitions. In contrast, the CEs were much weaker in the 1La band region. The spectral changes upon protonation were less significant compared with the parent pyridinophane, being dominated by the local transitions of anthracene. Nevertheless, the CD spectra of 1 and 1-H+ were well reproduced by theoretical calculations to allow us an unambiguous absolute configuration determination of the first high-performance liquid chromatography (HPLC) elute (from Chiralcel IB column) as Sp. The transannular interactions between the anthracene and pyridine/pyridinium units were examined by UV-vis and fluorescence spectroscopy to reveal a charge-transfer (CT) band in the low-energy region, particularly for 1-H+. Despite the comparable CT interactions, the CE at the CT band was much stronger for the anthracenopyridinophane than for the parent pyridinophane, affording an anisotropy (g) factor as large as 4 × 10-3.

Oxidative fusion reactions of ortho-phenylene-bridged cyclic hexapyrroles and hexathiophenes furnished novel closed helicenes in a selective manner. X-Ray diffraction analysis unambiguously revealed the structures to be a closed pentaaza[9]helicene, the longest azahelicene reported so far, and an unexpected double-helical structure of hexathia[9]/[5]helicene, whose formation was assumed to result from multiple oxidative fusion along with a 1,2-aryl shift. The pentaaza[9]helicene exhibited well-defined emission with high fluorescence quantum yield (ΦF=0.31) among the known [9]helicenes. Chiral resolution of the racemic pentaaza[9]helicene and hexathia[9]/[5]helicene were achieved by chiral-phase HPLC and the enantiomers were characterized by circular dichroism spectra and DFT calculations.

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.

For the concept of aromaticity, energetic quantification is crucial. However, this has been elusive for excited-state (Baird) aromaticity. Here we report our serendipitous discovery of two nonplanar thiophene-fused chiral [4n]annulenes Th4 COT Saddle and Th6 CDH Screw , which by computational analysis turned out to be a pair of molecules suitable for energetic quantification of Baird aromaticity. Their enantiomers were separable chromatographically but racemized thermally, enabling investigation of the ring inversion kinetics. In contrast to Th6 CDH Screw , which inverts through a nonplanar transition state, the inversion of Th4 COT Saddle , progressing through a planar transition state, was remarkably accelerated upon photoexcitation. As predicted by Baird’s theory, the planar conformation of Th4 COT Saddle is stabilized in the photoexcited state, thereby enabling lower activation enthalpy than that in the ground state. The lowering of the activation enthalpy, i.e., the energetic impact of excited-state aromaticity, was quantified experimentally to be as high as 21–22 kcal mol–1.

Saccharide sensing in aqueous media is an intriguing but challenging goal in current chemistry. Herein we report the oligosaccharide-sensing behavior of newly synthesized porphyrin–curdlan conjugates, which are random coils in DMSO but become globules in aqueous solutions to induce circular dichroism (ICD) in the biologically accessible spectral region due to the conformational fixation of porphyrin reporters. The magnitude of ICD was significantly varied specifically in the presence of acarbose, a drug for type-2 diabetes, enabling us to detect the aminosaccharide at concentrations down to 200 μm. This result demonstrates that the prêt-á-porter approach, using less-defined reporter-curdlan conjugates, is more advantageous than the traditional haute-couture approach with highly sophisticated hosts in particular in oligosaccharide sensing.

The study of an enantiopure bicyclic pillar[5]arene-based molecular universal joint (MUJ) by single-crystal X-ray diffraction allowed for the first time the unequivocal assignment of the absolute configuration of a planar chiral pillar[5]arene by circular dichroism spectroscopy. Crucially, the absolute configuration of the MUJ was switched reversibly by temperature, with an accompanying sign inversion of the anisotropy factor that varied by as much as 0.03, which is the largest value ever reported. Mechanistically, the reversible chirality switching of the MUJ is driven by the threading/dethreading motion of the fused ring and hence is dependent on both the size and nature of the ring and the solvent employed, reflecting the critical balance between the self-complexation of the ring by pillar[5]arene, the solvation to the excluded ring, and the inclusion of solvent molecules in the cavity.

Circular dichroisms (CDs) of the o,p′-isomers of 1,1,1-trichloro- and 1,1-dichloro-2,2-bis(chlorophenyl)ethanes (DDT and DDD) were investigated experimentally and theoretically. A series of strong Cotton effect peaks in a characteristic negative–negative–positive–negative, or its mirror-imaged, pattern were observed in the CD spectra of these persistent organic pollutants. The theoretical CD spectra at the SAC-CI/B95(d) and RI-CC2/def2-TZVPP levels well reproduced the experimental ones, enabling us to unambiguously assign the absolute configuration of (+)-DDT and (−)-DDD as S.

[28]Hexaphyrin GeIV and SnIV complexes were synthesized in high yields by reactions of [28]hexaphyrin with GeCl4 or SnCl4 in the presence of triethylamine. Both complexes display distinct 28π Möbius aromatic character and possess a trigonal bipyramidal geometry at the central GeIV or SnIV atom. The equatorial hydroxy group of the GeIV complex was smoothly exchanged with neutral nucleophiles, such as phenol derivatives and thiophenol, with retention of configuration. In the SnIV complex, intersystem crossing to the T1 state is remarkably enhanced owing to the effective heavy-atom effect, thus allowing the formation of the T1 tate in high yield. The T1 states of the GeIV and SnVI complexes were found to be antiaromatic on the basis of the transient absorption features in line with the Baird rule.

A novel dioxa[6]helicene-based supramolecular chirogenic system (1) as a specific chiral recognition host for enantiopure trans-1,2-cyclohexanediamine (2) is reported. Host 1 with an inherent free phenolic group and a (1S)-camphanate chiral handle on the opposite terminal rings of the helicene chromophore acted as an efficient turn on fluorescent sensor for S,S-2 with an excellent enantioselective factor, α = KSS/KRR = 6.3 in benzene. This specific host–guest interaction phenomenon is found to be solvent-dependent, which leads to an enantioselective chiral (camphanate) group transfer to the diamine guest molecule. In the case of R,R-2, the de value is up to 68% even at room temperature. Intriguingly, the induced helicity in dioxa[6]helicene diol 6, upon supramolecular hydrogen-bonding interactions, is of opposite sense with positive helicity for S,S-2 and negative helicity for R,R-2, as shown by circular dichroism spectroscopy and in combination with theoretical calculations. This chiral supramolecular system is found to be an excellent host–guest pair for enantiomeric recognition of 2, based on their electronic and steric factors.

AntiFreeze Proteins (=AFPs) bind to a surface of ice crystals and inhibit their growth. Some living organisms; fishes, insects and fungus, at low-temperature environment have several different types of AFPs. AFPs protect their body from freezing damages. To clarify the antifreeze effect of lpAFP (isolated from longsnout poacher, a fish living in the sea of Okhotsk) at single molecular level, we performed the single molecular observation using diffracted X-ray tracking (=DXT). DXT is the method to observe single molecular motions using X-rays and gold nanocrystals as probe. Gold nanocrystals are labeled on AFPs, and when irradiated with X-rays, diffracted spots from gold nanocrystals can be observed. We can observe the motion of the AFPs by tracking these spots. We performed the DXT experiments at SPring-8 BL40XU, and the spatial and time resolutions are the order of milli-radian and micro-second. In this study, we used lpAFP isolated from longsnout poacher, and observed the temperature dependency of the lpAFP’s single molecular brownian motion. As a result, lpAFP’s motion increased at 5 °C. Next, to ensure the relationship between the adsorption property and the maximum brownian motion, we observed the temperature dependency of the adsorption affinity to AgI thin film as ice surface. As a result, the adsorption amount to AgI of lpAFP was the largest at 2°C. These two experiments suggest that there is a strong correlation between lpAFP’s molecular motion and adsorption property. We acquired the temperature dependency of lpAFP using other experiments; circular dichroism (CD), atomic force microscopy (AFM), dynamic light scattering (DLS), and small angle X-ray scattering (SAXS, performed at SPring-8 BL40B2). In poster session, we discuss the detail experimental procedure and results.

The circular dichroisms (CDs) of planar chiral [2.2]- and [3.3]-pyridinophanes were investigated experimentally and theoretically. Strong multisignate Cotton effects, typical for cyclophane derivatives, were observed. The CD spectra of [2.2]- and [3.3]-paracyclophanes closely resembled in pattern each other, despite the much greater conformational variations in the latter. Upon protonation, both of the cyclophanes suffered dramatic CD spectral changes with accompanying complete sign inversion, which was attributed to the reversal of diploe moment of pyridinium versus pyridine moiety. This chiroptical property switching driven by protonation/deprotonation was temperature-dependent and hence applicable to thermal sensing. The protonated forms of pyridinophanes served as ideal model systems for studying the cation−π interactions and their effects on chiroptical properties. Thus, the molar CD (Δε) of the charge-transfer band of protonated [2.2]pyridinophane was 10-fold larger than that of protonated [3.3]pyridinophane, which exceeds the increased interplane electronic interactions assessed from the electronic coupling element values.

Chiral boron dipyrromethenes (BPs) enjoy high fluorescence efficiency at visible to near-IR wavelength regions with a reasonable range of dissymmetry factors. Here, we demonstrate that the (quasi)propeller chirality, similarly to hexagonal propeller in hexaarylbenzene, can be effectively induced in heptaarylated BP. In addition, supramolecular dimer was formed at low temperatures in nonpolar solvent, which exhibits strong bisignate Cotton effects at BP transitions (the couplet amplitude A = 193 M–1 cm–1) in the circular dichroism (CD). Due to the bulky substituents on the propeller blades, but with void space around boron atoms, BP chromophores in the dimer are aligned in a head-to-tail manner with a small torsion (φ ≈ 15°), to avoid fluorescence quenching usually observed in H-type dimer of BPs, exhibiting strong circularly polarized luminescence (CPL) signals (glum = 2.0 × 10–3, Φlum = 0.45). Such supramolecular dimer formation would be viewed as an alternative approach for designing and developing novel chiroptical materials.

The use of photoreactions such as photoredox catalysts in asymmetric synthesis has been expansively recognized as expedient procedure over the past decade. In this paper, a potential application of photoreaction of Charge-Transfer (CT) complex for asymmetric synthesis is described. In chiral donor-acceptor systems, excitation wavelength, along with other environmental factors such as solvent polarity and temperature, plays more substantial role in determining the stereoselectivity of the photochemical outcomes. Accordingly, the excited CT complex produced upon the selective CT-band excitation behaves completely different from the conventional exciplex, which is formed by the direct excitation of donor or acceptor. Such unique wavelength effect, together with other entropic controls, may possibly come to be valuable means in asymmetric photochemical synthesis for the future.