Non-planar Conformation Research Articles

Slow Energy Transfer in Self‐Doped β‐Conformation Film of Steric Polydiarylfluorenes toward Stable Dual Deep‐Blue Amplified Spontaneous Emission

AbstractExciton behavior is crucial for improving the optoelectronic property of a light‐emitting conjugated polymer. Herein, the photoexcitation dynamics of exciton migration and energy transfer in a self‐doped β‐conformation film of the polydiarylfluorenes (poly[4‐(octyloxy)‐9,9‐diphenylfluoren‐2,7‐diyl]‐co‐[5‐(octyloxy)‐9,9‐ diphenylfluoren‐2,7‐diyl], PODPF) are demonstrated. Compared to the first generation of the β‐conformation polyfluorene, poly(9,9‐dioctylfluorene) (PFO), energy transfer occurs in PODPF β‐conformation films in a time period of ≈150 ps, much longer than those of the PFO ones (<5 ps), associated with the effective intrachain energy transfer (few hundred picoseconds), rather than interchain Förster energy transfer (a few picoseconds). Similar to PFO, the PODPF β‐conformation also displays well‐resolved vibronic emission peaks at 20 K, attributed to the planar and rigid conformation. Interestingly, a residual 0‐0 band emission of nonplanar conformation chain segments (435 nm, 2.85 eV) at 20 K also further confirms the exciton migration from the amorphous state to the β‐conformation domain in PODPF films. Therefore, the stable dual amplified spontaneous emission (ASE) behavior of the PODPF self‐doped films at 461 nm (2.69 eV) and 483 nm (2.57 eV), originates from the individual amorphous and β‐conformation domains.

Kinematic Reconstruction of Cyclic Peptides and Protein Backbones from Partial Data.

We present an algorithm, QBKR (Quaternary Backbone Kinematic Reconstruction), a fast analytical method for an all-atom backbone reconstruction of proteins and linear or cyclic peptide chains from Cα coordinate traces. Unlike previous analytical methods for deriving all-atom representations from coarse-grained models that rely on canonical geometry with planar peptides in the trans conformation, our de novo kinematic model incorporates noncanonical, cis-trans, geometry naturally. Perturbations to this geometry can be effected with ease in our formulation, for example, to account for a continuous change from cis to trans geometry. A simple optimization of a spring-based objective function is employed for Cα-Cα distance variations that extend beyond the cis-trans limit. The kinematic construction produces a linked chain of peptide units, Cα-C-N-Cα, hinged at the Cα atoms spanning all possible planar and nonplanar peptide conformations. We have combined our method with a ring closure algorithm for the case of ring peptides and missing loops in a protein structure. Here, the reconstruction proceeding from both the N and C termini of the protein backbone (or in both directions from a starting position for rings) requires freedom in the position of one Cα atom (a capstone) to achieve a successful loop or ring closure. A salient feature of our reconstruction method is the ability to enrich conformational ensembles to produce alternative feasible conformations in which H-bond forming C-O or N-H pairs in the backbone can reverse orientations, thus addressing a well-known shortcoming in Cα-based RMSD structure comparison, wherein very close structures may lead to significantly different overall H-bond behavior. We apply the fixed Cα-based design to the reverse reconstruction from noisy Cryo-EM data, a posteriori to the optimization. Our method can be applied to speed up the process of an all-atom description from voluminous experimental data or subpar electron density maps.

Vacancies in the molecular crystal of 2-(2'-hydroxyphenyl)benzothiazole

Structure of molecular units is calculated for the periodic model corresponding to the crystal lattice of 2-(2'-hydroxyphenyl)benzothiazole with vacancies. 2-(2' -hydroxyphenyl)benzothiazole is a luminescent organic substance undergoing excited state intramolecular proton transfer. The calculations are performed with density-functional based tight-binding methods usding Van der Waals interaction empirical correction. It is found that the dihedral angles formed by benzothiazole and phenol parts of the molecules deviate in the vicinity of the vacancy. The vacancy provides enough space for non-planar conformation of the molecules in the ground state. At the same time the increase in energy of the periodic structure with the vacancies caused by appearance of the non-planar conformation is larger than the corresponding increase in the isolated molecule.

Facile Synthesis, Spectral, and Electrochemical Redox Properties of π-Extended ‘Push-Pull’ Corroles, Porphyrins and Chlorins

Unsymmetrical meso-functionalized ‘push-pull’ porphyrinoid derivatives have been widely used in the areas of solar cells, photocatalysis, toxic ion sensing and nonlinear optics (NLO).1 They have been widely explored due to their ease of synthesis and facile functionalization whereas limited reports on β-substituted ‘push-pull’ π-extended porphyrinoids due to lack of synthetic methodologies. However, it is found that the latter ones exhibit unique physicochemical and electrochemical redox properties with interesting material and medicinal applications.Recently, our group has reported the facile synthesis of β-functionalized, β-meso-o-phenyl and β-β’ fused corroles, chlorins, and porphyrins with mixed substitutents pattern.2,3 The crystal structure analyses of highly substituted corroles, porphyrins and chlorins revealed quasiplanar to nonplanar saddle shape conformation. Notably, nonplanarity of the porphyrinoid core was controlled and modified by varying in size, shape, number and the electronic nature of β-substituents. These porphyrinoids exhibited highly red-shifted electronic spectra upto NIR region with dramatic decrement in HOMO-LUMO gap. In addition, the redox tunability was achieved by introducing both electron donating and withdrawing β-substituents into the tetrapyrrole skeleton which led to nonplanarity with enormous ‘cross polarization’ which has great potentiality to use in nonlinear optics (NLO). In this presentation, the facile synthesis, spectral and intriguing redox properties of these porphyrins and their potential application in materials chemistry will be discussed in detail. REFERENCES: (a) Urbani, M.; Grätzel, M. ; Nazeeruddin, M. K.; Torres, T. Chem. Rev., 2014, 114, 12330-12396. (b) Hiroto, S.; Miyake, Y.; Shinokubo, H. Chem. Rev., 2017, 117, 2910-3043.(a) Kumar, R.; Sankar, M. Inorg. Chem., 2014, 53, 12706-12719. (b) Yadav, P.; Sankar, M. Dalton Trans., 2014, 43, 14680-14688. (c) Kumar, R.; Chaudhri, N.; Sankar, M. Dalton Trans., 2015, 44, 9147-9157. (d) Kumar, R.; Chaudhary, N.; Sankar, M.; Maurya, M. R. Dalton Trans., 2015, 44, 17720-17729. (e) Grover, N.; Sankar, M.; Song Y.; Kadish, K. M. Inorg. Chem., 2016, 55, 584-597. (f) Chahal, M. K.; Sankar, M. Dalton Trans., 2016, 45, 16404-16412. (g) Sankar, M. et al. Inorg. Chem., 2017, 56, 8527-8537.(a) Chaudhri, N.; Grover, N.; Sankar, M. Inorg. Chem., 2017, 56, 424-437. (b) Chaudhri, N.; Grover, N.; Sankar, M. Inorg. Chem., 2017, 56, 11532-11545. (c) Sankar, M. et al, J. Mater. A, 201 7, 5, 6263-6276. (d) Sankar, M. et al, ACS Appl. Energy Mater., 2018, 1, 2793-2801. (e) Sankar, M. et al. Inorg. Chem., 2018, 57, 1490-1503. (f) Chaudhri, N.; Grover, N.; Sankar, M. Inorg. Chem., 2018, 57, 6658-6668. (g) Chaudhri, N.; Grover, N.; Sankar, M. Inorg. Chem., 2018, 57, 11349-11360. (h) Sankar, M. et al. Inorg. Chem., 2018, 57, 13213-13224. (i) Grover, N.; Chaudhri, N.; Sankar, M. Inorg. Chem., 2019, 58, 2514-2522. (j) Dar, T. A.; Uprety, B.; Sankar, M.; Maurya, M. R. Green Chem., 2019, 21, 1757-1768. (k) Rathi, P.; Butcher, R.; Sankar, M. Dalton Trans., 2019, 48, 15002-15011. (l) Chahal, M. K. et al. Inorg. Chem., 2019, 58, 14361-14376. (m) Chaudhri, N. et al. Inorg. Chem., 2020, 59, 1481-1495. (n) Grover, N and Sankar, M. Chem. Asian J. 2020, 15, 2192-2197.

Cryo-EM structure of amyloid fibrils formed by the entire low complexity domain of TDP-43

Amyotrophic lateral sclerosis and several other neurodegenerative diseases are associated with brain deposits of amyloid-like aggregates formed by the C-terminal fragments of TDP-43 that contain the low complexity domain of the protein. Here, we report the cryo-EM structure of amyloid formed from the entire TDP-43 low complexity domain in vitro at pH 4. This structure reveals single protofilament fibrils containing a large (139-residue), tightly packed core. While the C-terminal part of this core region is largely planar and characterized by a small proportion of hydrophobic amino acids, the N-terminal region contains numerous hydrophobic residues and has a non-planar backbone conformation, resulting in rugged surfaces of fibril ends. The structural features found in these fibrils differ from those previously found for fibrils generated from short protein fragments. The present atomic model for TDP-43 LCD fibrils provides insight into potential structural perturbations caused by phosphorylation and disease-related mutations.

Solid-State Reversible Dual Fluorescent Switches for Multimodality Optical Memory.

Fluorescent photoswitches are highly attractive, because they hold great promises for photonic devices and imaging. However, a limited number of reversible switches with a response to light have been achieved in the solid state. Here, we report reversible dual fluorescent photoswitching characteristics in the solid state of spiropyran (SP)-functionalized tetraphenylethene (TPE) derivatives. These photoswitches exhibit two distinct and selectively addressable states, a cyan fluorescence and a red fluorescence, which can be conveyed into each other in a reversible feature upon irradiation with alternating UV and visible light. Detailed spectroscopic and theoretical studies suggest that the nonplanar molecular conformation of TPE moieties leads to large free volumes, which facilitates the reversible photoisomerization of SP. The excellent reversibility and high-contrast fluorescence of solid-state photoswitches enable great applications in multimodality anticounterfeiting and optical writing and erasing fluorescent devices.

Passivating Nucleobases Bring Charge Transfer Character to Optically Active Transitions in Small Silver Nanoclusters.

DNA-wrapped silver nanoclusters (DNA-AgNCs) are known for their efficient luminescence. However, their emission is highly sensitive to the DNA sequence, the cluster size, and its charge state. To get better insights into photophysics of these hybrid systems, simulations based on density functional theory (DFT) are performed. Our calculations elucidate the effect of the structural conformations, charges, solvent polarity, and passivating bases on optical spectra of DNA-AgNCs containing five and six Ag atoms. It is found that inclusion of water in calculations as a polar solvent media results in stabilization of nonplanar conformations of base-passivated clusters, while their planar conformations are more stable in vacuum, similar to the bare Ag5 and Ag6 clusters. Cytosines and guanines interact with the cluster twice stronger than thymines, due to their larger dipole moments. In addition to the base-cluster interactions, hydrogen bonds between bases notably contribute to the structure stabilization. While the relative intensity, line width, and the energy of absorption peaks are slightly changing depending on the cluster charge, conformations, and base types, the overall spectral shape with five well-resolved bands at 2.5-5.5 eV is consistent for all structures. Independent of the passivating bases and the cluster size and charge, the low energy optical transitions at 2.5-3.5 eV exhibit a metal to ligand charge transfer (MLCT) character with the main contribution emerging from Ag-core to the bases. Cytosines facilitate the MLCT character to a larger degree comparing to the other bases. However, the doublet transitions in clusters with the open shell electronic structure (Ag5 and Ag6+) result in appearance of additional red-shifted (<2.5 eV) and optically weak band with negligible MLCT character. The passivated clusters with the closed shell electronic structure (Ag5+ and Ag6) exhibit higher optical intensity of their lowest transitions with much higher MLCT contribution, thus having better potential for emission, than their open shell counterparts.

Diels-Alder Additions as Mechanistic Probes-Interception of Silyl-Isoindenes: Organometallic Derivatives of Polyphenylated Cycloheptatrienes and Related Seven-Membered Rings.

The intermediacy of short-lived isoindenes, generated in the course of metallotropic or silatropic shifts over the indene skeleton, can be shown by Diels-Alder trapping with tetracyanoethylene, leading to the complete elucidation of the dynamic behaviour of a series of polyindenylsilanes. Cyclopentadienones, bearing ferrocenyl and multiple phenyl or naphthyl substituents undergo [4 + 2] cycloadditions with diaryl acetylenes or triphenylcyclopropene to form the corresponding polyarylbenzenes or cycloheptatrienes. The heptaphenyltropylium cation, [C7Ph7+], was shown to adopt a nonplanar shallow boat conformation. In contrast, the attempted Diels-Alder reaction of tetracyclone and phenethynylfluorene yielded electroluminescent tetracenes. Finally, benzyne addition to 9-(2-indenyl)anthracene, and subsequent incorporation of a range of organometallic fragments, led to development of an organometallic molecular brake.

Mesophase in melt-spun poly(ϵ-caprolactone) filaments: Structure–mechanical property relationship

Fine poly(ϵ-caprolactone) (PCL) filaments (diameter: 59–92 μm) were successfully melt-spun with modified drawing setups to prevent draw instabilities. Depending on the production parameters, different mechanical properties were obtained (tensile strength: 302–456 MPa, elongation at break: 69–88%). These variations are related to subtle structural differences, which we have analyzed with wide-angle x-ray diffraction (WAXD) and small-angle x-ray scattering (SAXS). SAXS was used to determine crystal widths and the spacing between crystals along the filament axis. Additionally, 2D WAXD patterns were simulated and WAXD profiles were fitted. The detailed 2D WAXD analysis revealed that a highly-oriented non-crystalline mesophase is present in drawn PCL filaments, which is most-likely situated in-between PCL crystals. A large amount of this mesophase (>16%), combined with high crystalline orientation and perfect crystals, led to higher tensile strength values. We also confirmed that PCL chains pack with non-planar chain conformations in the unit cell.

Hydrolysis of Twisted Amides inside a Self-Assembled Coordination Cage

Mechanical twisting of amides is anticipated to enhance their reactivity considerably. Recent work published in Nature Chemistry by Fujita and co-workers demonstrates that reactive non-planar amide conformers can be bound inside a self-assembled coordination cage. The twisted amides are stabilized by the rigid cavity of the cage and are thus up to 14 times more reactive toward hydrolysis under basic conditions.

Achieving High-Performance Photothermal and Photodynamic Effects upon Combining D-A Structure and Nonplanar Conformation.

Various organic nanoagents have been developed for photothermal therapy (PTT) and photodynamic therapy (PDT) under near-infrared (NIR) irradiation. Among them, small molecule-based nanoagents are very attractive due to their advantages of well-defined chemical structures, high purity, good reproducibility, and easy processability. However, only a few small molecule-based nanoagents have been developed for PDT under NIR irradiation. Moreover, the mechanism of PDT under NIR is still elusive. Herein, a semiconducting small molecule (BTA) with donor-acceptor-donor structure and twisted conformation is developed for PDT/PTT under NIR irradiation. A large π-conjugated electron-deficient unit is used as the core to couple with two electron-donating units, ensuring the strong absorption under 808 nm. Moreover, the donor-acceptor structures and twisted conformation can reduce the energy gap between the singlet and triplet states (∆EST ) to afford effective intersystem crossing, beneficial for reactive oxygen species generation. The mechanism is probed by experimental and theoretical evidence. Moreover, the BTA nanoparticles exhibit excellent biocompatibility and PTT/PDT in vitro performance under NIR irradiation. This provides a strategy for designing highly efficient PDT/PTT molecular materials.

Azomethine phthalimides fluorescent E→Z photoswitches

Herein, we report the synthesis and E→Z photoswitching behavior of two 4-substituted azomethine phthalimides containing anthracenyl and 4-(dimethylamino)phenyl moieties (EAMP1 and EAMP2). These compounds represent newly synthesized and unstudied photoswitches with dual fluorescence properties as E-isomers and at photostationary state (PSS) depending on the solvent polarity. Steady-state fluorescence measurements were performed in various solvents and the results show strong sensitivity on the environmental polarity. The kinetics of E→Z photoswitching to PSS was studied in AcCN by visible light activation at 410 nm (EAMP1) and long wavelength UV-light activation at 350 nm (EAMP2). The quantitative and qualitative performance of the switching behavior was evaluated by the degree of photoisomerization (R) and the rate constant (k). It was found for EAMP1 R = 6.95 %, k = 8.87 × 10−4 s−1 and EAMP2 R = 88.72 %, k = 4.00 × 10−4 s−1, respectively. The reason for the lower photoconversion of EAMP1 compared to the EAMP2 was analyzed through optimization of the molecular geometry of E- and Z-isomers in the ground state (S0) and first excited state (S1) by DFT/TD-DFT calculations with B3LYP/6-31+G(d,p) level of theory using IEFPCM in AcCN. It was found that E-isomers in the S0 have nonplanar conformation, while the Z-isomer of EAMP1 prefers twisted conformation and the Z-isomer of EAMP2 T-shaped conformation is energetically advantageous compared to the twisted one. The reason is the weak H…..π noncovalent interaction (NCI) between 4-(dimethylamino)phenyl moiety and phthalimide ring. Moreover, the Z-isomer of EAMP2 is unusual stable up to 600 min at room temperature in dark compared to the EAMP1, which undergoes full Z→E relaxation for less than 60 min at the same conditions. The Z→E relaxation of EAMP2 is achieved for 90 min at 60 °C. The fluorescence E→Z switching behavior was studied by emission measurements in AcCN and 1,4-DOX as E-isomers and at PSS in room and liquid nitrogen (77 K) temperatures. In the polar and nonpolar solvents, red-shifted emissions with increased fluorescence quantum (Φfl) yields have been observed at PSS compared to the E-isomers. The molecular rotor behavior was studied in the binary mixture of glycerol:ethanol and the results show a sensitivity of the emission bands depending on the environmental viscosity. Time-resolved fluorescence decay measurements were performed in AcCN and 1,4-DOX as E-isomers and at PSS to estimate the mechanism of fundamental fluorescence bands. We found that dyes at PSS have longer lifetime (τ) compared to the E-isomers, especially in less polar 1,4-DOX.

A Hidden Side of the Conformational Mobility of the Quercetin Molecule Caused by the Rotations of the O3H, O5H and O7H Hydroxyl Groups: In Silico Scrupulous Study

In this study at the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p) level of quantum-mechanical theory it was explored conformational variety of the isolated quercetin molecule due to the mirror-symmetrical hindered turnings of the O3H, O5H and O7H hydroxyl groups, belonging to the A and C rings, around the exocyclic C–O bonds. These dipole active conformational transformations proceed through the 72 transition states (TSs; C1 point symmetry) with non-orthogonal orientation of the hydroxyl groups relatively the plane of the A or C rings of the molecule (HO7C7C8/HO7C7C6 = ±(89.9–93.3), HO5C5C10 = ±(108.9–114.4) and HO3C3C4 = ±(113.6–118.8 degrees) (here and below signs ‘±’ corresponds to the enantiomers)) with Gibbs free energy barrier of activation ΔΔGTS in the range 3.51–16.17 kcal·mol−1 under the standard conditions (T = 298.1 K and pressure 1 atm): ΔΔGTSO7H (3.51–4.27) &lt; ΔΔGTSO3H (9.04–11.26) &lt; ΔΔGTSO5H (12.34–16.17 kcal mol−1). Conformational dynamics of the O3H and O5H groups is partially controlled by the intramolecular specific interactions O3H…O4, C2′/C6′H…O3, O3H…C2′/C6′, O5H…O4 and O4…O5, which are flexible and cooperative. Dipole-active interconversions of the enantiomers of the non-planar conformers of the quercetin molecule (C1 point symmetry) is realized via the 24 TSs with C1 point symmetry (HO3C3C2C1 = ±(11.0–19.1), HC2′/C6′C1′C2 = ±(0.6–2.9) and C3C2C1′C2′/C3C2C1′C6′ = ±(1.7–9.1) degree; ΔΔGTS = 1.65–5.59 kcal·mol−1), which are stabilized by the participation of the intramolecular C2′/C6′H…O1 and O3H…HC2′/C6′ H-bonds. Investigated conformational rearrangements are rather quick processes, since the time, which is necessary to acquire thermal equilibrium does not exceed 6.5 ns.

Complementary Fluorescence Emission and Second Harmonic Spectra Improve Bilayer Characterization.

Complementary investigation of Laurdan fluorescence emission and second harmonic (SH) spectra in nonpolar, protic and aprotic polar solvents and phospholipid bilayers was carried out. SH of spectra computed using methods familiar in electro spin resonance spectroscopy yielded better resolution. Spectra were fit to log-normal distributions. SH spectra showed presence of two emissions in protic polar and nonpolar solvents and in both bilayer gel and liquid phases and a single line in aprotic polar solvents. Each of the half maximal positions of each line in both homogenous solvents and bilayers, expresses similar linearity with peak position. This shared feature suggests planar and nonplanar Laurdan conformation respectively in the longer (red) and shorter (blue) wavelength emitting states. The weaker 432nm blue line, not detected before in the gel phase, is distinguishable in the SH. Temperature trajectories of areas and peak positions of the individual lines bring new insight into the nature of lipid packing and evolution of domains, indicating inhomogeneous lipid packing even in the gel phase. The blue line identifies as emission from Laurdan in tighter packed regions and the dominant 445-448nm red line in the gel phase shifting to 484nm in the liquid phase as emission from Laurdan-water coupled states that are in varying stages of relaxation according to temperature and phase. Unexpected increase in the area of the blue line with temperature through the gel-liquid transition is consistent with coexisting low and high curvature domains and Laurdan's preference for less polar low curvature domains.

355 nm Photodissociation of N2O3 Revealed by Velocity-Mapped Ion Imaging.

Dinitrogen trioxide is proposed as a precursor to forming nitrous acid, an important source of hydroxyl radicals in the atmosphere. The spectroscopy and properties of N2O3 have been studied at high pressures or low temperatures, but there are no reports of its gas-phase photodissociation. This study investigates the 355 nm photodissociation of N2O3 in a DC-sliced velocity-mapped ion imaging apparatus using linearly polarized nanosecond pump and probe lasers. The N2O3 sample was generated by expanding NO and NO2 seeded in a He carrier gas. After photodissociation, a high fraction of the available energy ends up in translation of the products. Time-dependent density functional theory calculations confirm the parallel transition dipole assignment if the dissociation occurs from a nonplanar N2O3 conformation. The vector correlations are nearly at the physical limits for a system where μ||v⊥J. The DC-sliced velocity-mapped ion imaging technique is well-suited to investigate N2O3 photodissociation since it resolves product speeds and differentiates among the sources of NO+ in an expansion containing NO, NO2, HONO, and N2O3.

Aggregation-induced emission compounds based on 9,10-dithienylanthracene and their applications in cell imaging

Three new symmetrical 9,10-dithienylanthracene (DTA) derivatives, including 9,10-bis-(4-(4-methoxyphenyl)-5-methylthiophene-2-yl)anthracene (BMPTA), 9,10-bis-(5-methyl-4-phenylthiophene-2-yl)anthracene (BPTA), and 9,10-bis-(4-(4-cyanophenyl)-5-methylthiophene-2-yl)anthracene (BCPTA), with excellent aggregation-induced emission (AIE) behaviors had been designed and synthesized successfully. Crystal BMPTA and BPTA had nonplanar conformations because of the multiple intermolecular hydrogen bonding interaction between the adjacent molecules, which restricted their intramolecular rotation and enabled them to emit intensely. Additionally, both BMPTA and BPTA exhibited reversible mechanofluorochromic (MFC) performance by grinding-fuming, which was systematically investigated with powder X-ray diffraction (XRD), differential scanning calorimetry (DSC) and photoluminescent (PL) lifetime. The results showed that substituent groups had significant influence on their molecular packing, MFC and AIE properties. Furthermore, these DTA luminogens could be applied in cell imaging as fluorochrome.

Inserting Nitrogen: An Effective Concept To Create Nonplanar and Stimuli-Responsive Perylene Bisimide Analogues.

Establishing design principles to create nonplanar π-conjugated molecules is crucial for the development of novel functional materials. Herein, we describe the synthesis and properties of dinaphtho[1,8-bc:1',8'-ef]azepine bisimides (DNABIs). Their molecular design is conceptually based on the insertion of a nitrogen atom into a perylene bisimide core. We have synthesized several DNABI derivatives with a hydrogen atom, a primary alkyl group, or an aryl group on the central nitrogen atom. These DNABIs exhibit nonplanar conformations, flexible structural changes, and ambipolar redox activity. The steric effect around the central nitrogen atom substantially affects the overall structures and results in two different conformations: a nonsymmetric bent conformation and a symmetric twisted conformation, accompanied by a drastic change in electronic properties. Notably, the nonsymmetric DNABI undergoes unique structural changes in response to the application of an external electric field, which is due to molecular motions that are accompanied by an orientational fluctuation of the dipole moment. Furthermore, the addition of a chiral Brønsted base to N-unsubstituted DNABI affords control over the helical chirality via hydrogen-bonding interactions.

Chiral Molecular Ruby [Cr(dqp)2]3+ with Long-Lived Circularly Polarized Luminescence.

The chiral resolution of a kinetically inert molecular ruby [Cr(dqp)2]3+ (1, dqp = 2,6-di(quinolin-8-yl)pyridine) displaying strong dual light emission at room temperature has been achieved. The wrapped arrangement of the six-membered dqp chelating ligands around the Cr(III) provided nonplanar helical conformations leading to the diastereoselective assembly of chiral bis-tridentate monometallic Cr(III)-helix. The PP-(+)-[Cr(dqp)2]3+ and MM-(-)-[Cr(dqp)2]3+ enantiomers could be separated and isolated by using cation-exchange chromatography and subsequent salt-metathesis with KPF6. X-ray crystallographic analysis based on Flack parameters assigned the absolute configurations of the two enantiomers. Circularly polarized luminescence (CPL) spectra showed two polarized emission bands within the NIR region corresponding to the characteristic metal-centered spin-flip Cr(2E → 4A2) and Cr(2T1 → 4A2) transitions with exceptionally high dissymmetry factors, |glum|, of 0.2 and 0.1, respectively, which are comparable to those reported for rare-earth chiral complexes. Photophysical properties also revealed an extremely long excited-state lifetime of 1.2 ms and a high quantum yield of 5.2% at room temperature in water. These properties make [Cr(dqp)2]3+ an ideal sensitizer for the preparation of enantiopure luminescent supramolecular energy-converting devices and also open up the possibility of using chiral Cr(III) chromophores for the construction of NIR-CPL materials and polarized photonic devices based on earth-abundant metals.

Abstract 2752: Recognition of the hybrid-1 human telomeric G-quadruplex by a platinum(II)-based compound

Abstract The human telomeric DNA, which consists of tandem repeat sequences of d(TTAGGG)n, plays an important role in cancers and cell aging. Previous studies in human cancer cells have demonstrated DNA G-quadruplex (G4) structure formation in telomeres. G-quadruplexes are four-stranded structures formed in guanine-rich sequences that are held together by guanine-guanine Hoogsteen hydrogen bonding, and G-quadruplex stabilization by small molecules induces tumor cell senescence and apoptosis by repressing telomerase activity and the DNA damage response pathway. Additionally, telomeric-overhang DNA can form biologically relevant higher-order DNA structures containing consecutive G-quadruplexes which provides additional binding sites for small molecules. Thus, telomeric DNA G-quadruplexes have attracted increasing interest as potential drug targets for cancer therapy. Here, we found that a novel platinum (II)-based tripod (Pt-tripod) specifically recognizes the hybrid-1 human telomeric G4, which is formed in the physiologically relevant K+ solution. Pt-tripod strongly represses telomerase activity and exhibits DNA-targeted photodynamic therapy anticancer activity. Using NMR, we show that Pt-tripod binds the hybrid-1 human telomeric G4 over other G-quadruplex structures and dsDNA. We determined solution structures of the 1:1 and the dimeric 4:2 Pt-tripod-hybrid-1 telomeric G4 complexes by NMR. Our 1:1 complex structure shows preferential binding of the Pt-tripod to the 5′-end of hybrid-1 telomeric G4. At higher ligand ratio, Pt-tripod binds to the 3′-end of the hybid-1 G4 and induces an unprecedented dimeric 4:2 structure interlocked by a non-canonical A:A pair at the 3′-end. The specific binding of the Pt-tripod with the hybrid-1 G4 is achieved by unique interaction modes including the π-π stacking, hydrogen bonding, and electrostatic interactions with the loop and flanking segments. The non-planar tertiary amine conformation and three properly sized cationic platinum arms are critical for the specific and strong binding. Our structures provide significant insights into understanding the dynamic binding of small molecules with G-quadruplexes, and a structural basis for future rational anticancer drug design of non-planar small molecules targeting the human telomeric G4. Our studies also provide a potential handle to study the specific protein interactions and biological functions of hybrid-1 human telomeric G4. Citation Format: Wenting Liu, Clement Lin, Zong-wan Mao, Danzhou Yang. Recognition of the hybrid-1 human telomeric G-quadruplex by a platinum(II)-based compound [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2752.

The Development of Synthetic Routes to 1,1,n,n‐Tetramethyl[n](2,11)teropyrenophanes

A concise synthetic approach to 1,1,n,n‐tetramethyl[n](2,11)teropyrenophanes has been developed. It involves the construction of triply‐bridged pyrenophanes, during which the three bridges are installed successively using Friedel‐Crafts alkylation, Wurtz coupling and McMurry reactions. At the same time, the innate regiochemical preferences of pyrene toward electrophilic aromatic substitution are relied upon to control the substitution pattern. A cyclodehydrogenation reaction is then employed to generate the teropyrene system directly in a nonplanar conformation. The crystal structure of 1,1,7,7‐tetramethyl[7](2,11)teropyrenophane was determined and the teropyrene system was found to have an end‐to‐end bend angle of 177.9°.