Functional evaluation of an electrophilic focused library to identify a covalent inhibitor against intrinsically disordered circadian clock transcription factors.

In vitro screening of a focused library of compounds containing an electrophilic warhead identified N-chloroacetyl-bis(trifluoromethyl)aniline derivative 15 as a potent inhibitor of BMAL1-CLOCK heterodimer binding to an E-box DNA fragment. Kinetic analysis of thiol-reactivity demonstrated that iodoacetamide and structurally related 20 are significantly more reactive than or equally reactive as 15, respectively, whereas none inhibited BMAL1-CLOCK interaction with the E-box DNA fragment. These results suggest that 15 binds and reacts with a specific nucleophilic residue. This low-molecular-weight compound may serve as a useful lead for further development of BMAL1-CLOCK inhibitors.

Structure-activity-relationship study of semi-synthetically modified fusicoccins on their stabilization effect for 14-3-3-phospholigand interactions.

The diterpene glucoside fusicoccin-A (FC-A) is a fungal phytotoxin that stabilizes the interaction of plant 14-3-3 protein and plasma membrane H+-ATPase by forming a stable ternary complex. Previous studies demonstrated that structurally modified FC-A derivatives exhibit significant antitumor activities but their synthesis involves an explosive reagent, limiting their utility and opportunities for further structure-activity-relationship studies. In this study, we synthesized a series of FC derivatives by introducing various substituents on the fusicoccan scaffold and on the glucoside moiety, and evaluated their stabilization effects on the binding of 14-3-3 to fluorescently labeled mode-1 and mode-3 phosphopeptides. The results showed that introducing an amino group at the 6'-position of the glucoside moiety improves stabilization. Furthermore, cell-based evaluation demonstrated that 6'-amino benzyl 21b exhibits higher antiproliferative activity than previously developed FC agents.

Optimizing Photochirogenic Performance by Solvent-Driven Conformational Fixation in Enantiodifferentiating Photoisomerization of (Z)-Cyclooctene Mediated by Sensitizing ß-Cyclodextrin Hosts.

Catalytic enantiodifferentiating photoisomerization of cyclooctene (1Z) included and sensitized by regioisomeric 6-O-(o-, m-, and p-methoxybenzoyl)-ß-cyclodextrins (CDs) was performed under a variety of solvent conditions for higher enantioselectivities. The enantiomeric excess (ee) of chiral (E)-isomer (1E) produced was a critical function of all the internal and external factors examined, in particular, the sensitizer structure and the solvent conditions, to afford (R)-1E in record-high ee's of up to 67% upon sensitization with the meta-substituted ß-CD host in water and salt solutions but neither in 50% aqueous ethanol nor with the ortho- and para-substituted hosts. The mechanistic origin of the sudden ee enhancement achieved under the specific conditions is discussed on the basis of the circular dichroism spectral behaviors upon substrate inclusion and the compensatory enthalpy-entropy relationship of the activation parameters for the enantiodifferentiating photoisomerization.

Pressure-driven, solvation-directed planar chirality switching of cyclophano-pillar[5]arenes (molecular universal joints).

Planar chiral cyclophanopillar[5]arenes with a fused oligo(oxyethylene) or polymethylene subring (MUJs), existing as an equilibrium mixture of subring-included (in) and -excluded (out) conformers, respond to hydrostatic pressure to exhibit dynamic chiroptical property changes, leading to an unprecedented pressure-driven chirality inversion and the largest ever-reported leap of anisotropy (g) factor for the MUJ with a dodecamethylene subring. The pressure susceptivity of MUJs, assessed by the change in g per unit pressure, is a critical function of the size and nature of the subring incorporated and the solvent employed. Mechanistic elucidations reveal that the in-out equilibrium, as the origin of the MUJ's chiroptical property changes, is on a delicate balance of the competitive inclusion of subrings versus solvent molecules as well as the solvation of the excluded subring. The present results further encourage our use of pressure as a unique tool for dynamically manipulating various supramolecular devices/machines.

A Supramolecular Strategy for Enhancing Photochirogenic Performance through Host/Guest Modification: Dicationic γ-Cyclodextrin-Mediated Photocyclodimerization of 2,6-Anthracenedicarboxylate.

Possessing an extra anionic handle for chiral supramolecular interactions, 2,6-anthracenedicarboxylate exhibited greater photochirogenic performance than 2-anthracenecarboxylate to afford the anti-cyclodimer in up to 94% yield and -72% enantiomeric excess upon photoirradiation with dicationic γ-cyclodextrins.

Hydrostatic Pressure-Induced Spectral Variation of Reichardt's Dye: A Polarity/Pressure Dual Indicator.

The famous solvatochromic Reichardt's dye was applied to quantify hydrostatic pressure in media. The UV/vis spectra of the dye in various organic solvents are shifted bathochromically or hypsochromically at the shorter- or longer-wavelength band, respectively, upon hydrostatic pressurization. The E T value, determined by an absorption maximum, in ethyl acetate increases from 38.5 kcal mol-1 at 0.1 MPa to 39.2 kcal mol-1 at 300 MPa, which is mostly equal to the one in chloroform at 0.1 MPa. These spectroscopic origins were supported by the time-dependent density functional theory (TD-DFT) calculations. The concept and approach proposed in this paper, i.e., a dual indicator, should attract the attention of a broad spectrum in multidisciplinary science.

An Ultimate Stereocontrol in Supramolecular Photochirogenesis: Photocyclodimerization of 2-Anthracenecarboxylate Mediated by Sulfur-Linked ß-Cyclodextrin Dimers.

Stereoisomeric ß-cyclodextrin (CD) dimers linked with a sulfur atom or an arene spacer were designed to create a tethered dual CD capsule for precisely manipulating the regio- and enantioselectivities of the photocyclodimerization of 2-anthracenecarboxylate (AC) to four stereoisomeric classical 9,10:9',10'-cyclodimers and two nonclassical 5,8:9',10'-cyclodimers. Among the dimeric CD hosts prepared, exo-3-thia-ß-CD dimer formed 1:1 and 1:2 host-guest complexes with AC in aqueous solutions, the former of which hindered but the latter facilitated the AC photocyclodimerization with regio- and enantioselectivities much higher than those obtained with native ß-CD or the rest of the ß-CD dimers. The stereochemical outcomes turned out to be highly sensitive to and hence critically manipulable by the linking position and configuration of the connected saccharide units and the linker length, as well as the external variants, such as temperature, pH, and added salt. Eventually, the photocyclodimerization of AC mediated by the dimeric ß-CD host gave enantiopure syn-head-to-tail-9,10:9',10'-cyclodimer in 97-98% yield in a pH 5.1 buffer solution at 0.5 °C and also in an aqueous CsCl solution at -20 °C.

Combined Experimental and Theoretical Study on Circular Dichroism and Circularly Polarized Luminescence of Configurationally Robust D3-Symmetric Triple Pentahelicene.

Pentahelicene (PH) exhibits the largest absorption ( gabs) and luminescence ( glum) dissymmetry factors among the helicene family but is configurationally and (photo)chemically labile, encumbering its application to chiroptical materials. To bypass the pitfalls, three PH units are merged in a single molecule to build D3-symmetric triple pentahelicene, hexabenzotriphenylene (HBT), which attains indeed the configurational and (photo)chemical robustness through equilibrium with a C2-symmetric conformer that interrupts the racemization and photocyclization. UV-vis, circular dichroism (CD), and circularly polarized luminescence (CPL) spectral examinations reveal the significantly larger gabs and glum values for HBT than for any of configurationally robust single [ n]helicene ( n ≥ 6) and C2-symmetric triple pentahelicene, trinaphthotriphenylene (TNT). Theoretical calculations precisely reproduce the main features of the experimental CD and CPL spectra of PH, HBT, and TNT, and the relevant electric and magnetic transition moments and their mutual angles well rationalize the relative CD and CPL intensities of all the single and triple pentahelicenes.

X-ray observations of single bio-supramolecular photochirogenesis.

The binding and photochirogenic behaviour of 2-anthracenecarboxylate (AC) with human serum albumin (HSA) have hitherto been investigated and comprehended as time-averaged statistical events by spectroscopic examinations and product analyses. In this study, we employed a diffracted X-ray tracking (DXT) technique to visualize the single-molecular dynamics of free and AC-loaded HSA (AC:HSA = 0, 1, 5 and 10), as well as the AC-HSA complex under photoirradiation, all of which were tethered to gold nanocrystals and hence traceable in real time by DXT. This enabled us to draw a more dynamic picture of the bio-supramolecular photochirogenesis at a single-molecule resolution, detailing the softening and flexibility enhancement of HSA upon binding of ACs to its inter-subdomain IIA-IIB site and the dynamic extrusion of AC dimers produced upon photoirradiation.

Solvent and Temperature Effects on Dynamics and Chiroptical Properties of Propeller Chirality and Toroidal Interaction of Hexaarylbenzenes.

Because of the unique interaction of radial aromatic blades, propeller-shaped hexaarylbenzenes (HABs) attract much research interest and find various practical applications. By introducing a small point-chiral group at the tip of aromatic blade(s), HAB becomes propeller-chiral to exhibit strong Cotton effects. Because of the dynamic nature of propeller chirality, the chiroptical properties of HAB critically responded to minute changes in the environment. Using a series of chiral HABs with one to six ( R)-1-methylpropyloxy substituent(s) introduced at the blade tip, we elucidated how the smallest chiral auxiliary at the HAB periphery progressively and cooperatively boosts the overall chiroptical properties and also how subtle changes in temperature and solvent structure affect the propeller dynamics and thus the chiroptical responses. The unique features of propeller-chiral HABs further enabled us to switch on/off their circularly polarized luminescence.

Supramolecular Photochirogenesis Driven by Higher-Order Complexation: Enantiodifferentiating Photocyclodimerization of 2-Anthracenecarboxylate to Slipped Cyclodimers via a 2:2 Complex with ß-Cyclodextrin.

Chiral slipped 5,8:9',10'-cyclodimers were preferentially produced over classical 9,10:9',10'-cyclodimers upon supramolecular photocyclodimerization of 2-anthracenecarboxylate (AC) mediated by ß-cyclodextrin (ß-CD). This photochirogenic route to the slipped cyclodimers, exclusively head-to-tail (HT) and highly enantioselective, has long been overlooked in foregoing studies but is dominant in reality and is absolutely supramolecularly activated by 2:2 complexation of AC with ß-CD. The intricate structural and photophysical aspects of this higher-order complexation-triggered process have been comprehensively elucidated, while the absolute configurations of the slipped cyclodimers have been unambiguously assigned by comparing the experimental and theoretical circular dichroism spectra. In the 2:2 complex, two ACs packed in a dual ß-CD capsule are not fully overlapped with each other but are only partially stacked in a slipped anti- or syn-HT manner. Hence, they do not spontaneously cyclodimerize upon photoexcitation but instead emit long-lived excimer fluorescence at wavelengths slightly longer than the monomer fluorescence, indicating that the slipped excimer is neither extremely reactive nor completely relaxed in conformation and energy. Because of the slipped conformation of the AC pair in the soft capsule, the subsequent photocyclodimerization becomes manipulable by various internal or external factors, such as temperature, pressure, added salt, and host modification, enabling us to exclusively obtain the slipped cyclodimers with high regio- and enantioselectivities. In this supramolecularly driven photochirogenesis, the dual ß-CD capsule functions as a chiral organophotocatalyst to trigger and accelerate the nonclassical photochirogenic route to slipped cyclodimers by preorganizing the conformation of the encapsulated AC pair, formally mimicking a catalytic antibody.

Entropy-Driven Diastereoselectivity Improvement in the Paternò-Büchi Reaction of 1-Naphthyl Aryl Ethenes with a Chiral Cyanobenzoate through Remote Alkylation.

The precise stereocontrol of photocycloaddition reactions is still a significant challenge owing to their mechanistic complexity and the involvement of highly reactive and short-lived intermediates. Attempts have hitherto been made through structural modifications, mostly by introducing steric conflicts, to increase the difference between the enthalpic barriers. Herein, we show that entropy plays a crucial role in influencing the diastereoselectivity of a Paternò-Büchi reaction. Remote meta alkylation of the donor caused nominal changes in its photophysical properties as well as those of the exciplexes derived thereof. Nevertheless, the diastereomeric excess of the oxetane product was greatly improved by about 40 %. This enhancement, which is not accompanied by any significant changes in the photophysical properties, is difficult to rationalize by conventional enthalpic control concepts based on repulsive steric and/or attractive intermolecular interactions as well as electronic perturbations. Differential activation parameters and compensatory enthalpy-entropy relationships revealed that the diastereoselectivity enhancement is not simply enthalpic but also entropic in origin.

Circular Dichroisms of Mono- and Dibromo[2.2]paracyclophanes: A Combined Experimental and Theoretical Study.

Circular dichroisms (CDs) of planar chiral 4-bromo[2.2]paracyclophane (1) and three isomeric dibromo[2.2]paracyclophanes (p-2, m'-2, and o'-2) were investigated experimentally and theoretically. They all exhibited strong multisignate Cotton effects (CEs) at the 1Lb, 1La, and 1B transitions of the component (bromo)benzene chromophore and were comparable to each other. For all of the cyclophanes examined, the enantiomer that eluted earlier from a chiral high-performance liquid chromatography column (Chiralcel IA or IB) exhibited negative and positive CEs at the 1Lb and 1La bands, respectively, which were followed by a more complicated pattern of CDs at the higher-energy bands. These CD features were well reproduced by quantum chemical calculations, allowing us to unambiguously assign the absolute configurations of the first-eluted enantiomers as R p in all of the cases examined. Interestingly, the CDs of 1 and 2, although largely comparable in shape, were still sensitive to the number and pattern of bromine substitution, showing closer resemblance between m'-2 and o'-2 and between p-2 and 1. The theoretical calculations also reproduced successfully these spectral resemblance between them. The anisotropy (g) factors for the 1Lb bands of these cyclophanes were considerably large (∼10-2), whereas those for the 1La band were conventional in the order of 10-3. In addition, a weak CE was observed in the low-energy region at around 320 nm, which turned out to originate from the interplanar interaction and is hence assigned to the "cyclophane band". The experimental g factors of this band were fairly large in the order of 10-2, but the computation turned out to be quite challenging and were less well reproduced theoretically, ascribable to the forbidden nature of the transition.

Enantioselectivity of 2,2',3,5',6-Pentachlorobiphenyl (PCB 95) Atropisomers toward Ryanodine Receptors (RyRs) and Their Influences on Hippocampal Neuronal Networks.

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.

A Combined Experimental and Theoretical Study on the Circular Dichroism of Staggered and Eclipsed Forms of Dimethoxy[2.2]-, [3.2]-, and [3.3]Pyridinophanes and Their Protonated Forms.

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.

Chiroptical properties of dithia[3.3]cyclophanes composed of anthracene and pyridine/pyridinium moieties: A combined experimental and theoretical study.

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 1 Bb 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 1 La 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 .

Oligosaccharide Sensing in Aqueous Media by Porphyrin-Curdlan Conjugates: A Prêt-á-Porter Rather Than Haute-Couture Approach.

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.

Temperature-Driven Planar Chirality Switching of a Pillar[5]arene-Based Molecular Universal Joint.

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.

Regioselective Molecularly Imprinted Reaction Field for [4 + 4] Photocyclodimerization of 2-Anthracenecarboxylic Acid.

Molecularly imprinted cavities have functioned as a regioselective reaction field for the [4 + 4] photocyclodimerization of 2-anthracenecarboxylic acid (2-AC). Molecularly imprinted polymers were prepared by precipitation polymerization of N-methacryloyl-4-aminobenzamidine as a functional monomer to form a complex with template 2-AC and ethylene glycol dimethacrylate as a crosslinking monomer. The 2-AC-imprinted cavities thus constructed preferentially bound 2-AC with an affinity greater than that toward structurally related 9-anthracenecarboxylic acid, 2-aminoanthracene, and unsubstituted anthracene. Moreover, from the four possible regioisomeric cyclodimers, they mediated the [4 + 4] photocyclodimerization of 2-AC specifically to the anti-head-to-tail (anti-HT) isomer. This indicates that the imprinted cavities accommodate two 2-AC molecules in an anti-HT manner, thereby facilitating the subsequent regioselective photocyclodimerization.