Tracing the substrate translocation mechanism in P-glycoprotein.

P-glycoprotein (Pgp) is a prototypical ATP-binding cassette (ABC) transporter of great biological and clinical significance.Pgp confers cancer multidrug resistance and mediates the bioavailability and pharmacokinetics of many drugs (Juliano and Ling, 1976; Ueda et al., 1986; Sharom, 2011). Decades of structural and biochemical studies have provided insights into how Pgp binds diverse compounds (Loo and Clarke, 2000; Loo et al., 2009; Aller et al., 2009; Alam et al., 2019; Nosol et al., 2020; Chufan et al., 2015), but how they are translocated through the membrane has remained elusive. Here, we covalently attached a cyclic substrate to discrete sites of Pgp and determined multiple complex structures in inward- and outward-facing states by cryoEM. In conjunction with molecular dynamics simulations, our structures trace the substrate passage across the membrane and identify conformational changes in transmembrane helix 1 (TM1) as regulators of substrate transport. In mid-transport conformations, TM1 breaks at glycine 72. Mutation of this residue significantly impairs drug transport of Pgp in vivo, corroborating the importance of its regulatory role. Importantly, our data suggest that the cyclic substrate can exit Pgp without the requirement of a wide-open outward-facing conformation, diverting from the common efflux model for Pgp and other ABC exporters. The substrate transport mechanism of Pgp revealed here pinpoints critical targets for future drug discovery studies of this medically relevant system.

Halogenated building blocks for 2D crystal engineering on solid surfaces: lessons from hydrogen bonding.

Halogen bonding has emerged as a promising tool in two-dimensional (2D) crystal engineering. Since halogen bonds are similar to hydrogen bonds in a number of aspects, the existing knowledge of hydrogen bonded systems can be applied to halogenated systems. Here we evaluate the applicability of a retrosynthetic approach based on topological similarity between hydrogen and halogen bonds to obtain predictable halogen bonded networks. The self-assembly of 1,3-dibromo-5-alkoxybenzene derivatives was studied in analogy with well-explored alkoxy isophthalic acids using a combination of experimental and theoretical tools. Scanning tunneling microscopy (STM) characterization of the networks formed at the liquid-graphite interface revealed that while the retrosynthetic approach works at the level of small clusters of molecules within the 2D network, the overall structure of the network deviates from the anticipated structure. The monolayers consist of fractured rows of halogen-bonded modules instead of the expected continuous lamellar structure. Each module consists of a discrete number of halogen-bonded molecules. The interactions responsible for the stabilization of halogen bonded dimers are delineated through detailed density functional theory (DFT) calculations coupled with natural bonding orbitals (NBO) and perturbation analysis. A modified force field that includes an extra charged site to imitate the σ hole on the halogen atom was developed and applied to extract total potential energies of the anticipated and observed networks. Plausible reasons for the deviation from the anticipated structure are discussed. Finally, a modified molecular design that allows successful application of the hydrogen bond-halogen bond analogy was tested experimentally.

Tailoring atomic layer growth at the liquid-metal interface.

Engineering atomic structures at metal surfaces represents an important step in the development of novel nanomaterials and nanodevices, but relies predominantly on atomic/molecular beam epitaxy under ultrahigh vacuum conditions, where controlling the deposition processes remains challenging. By using solution-borne nanosized gold clusters as a precursor, here we develop a wet deposition protocol to the fabrication of atomically flat gold nanoislands, so as to utilize the dynamic exchange of surface-active molecules at the liquid-metal interface for manipulating the growth kinetics of ultrathin metallic nanostructures. While remarkable shape and size selection of gold nanoislands is observed, our experimental and theoretical investigations provide compelling evidences that organic adsorbates can impart a bias to the island orientation by preferred adsorption and alignment and intervene in the assembly and disassembly of adatom islands by complexing with Au adatoms. This approach offers a simple solution to regulate atomic layer growth of metals at ambient conditions.

Inhibit or Evade Multidrug Resistance P-Glycoprotein in Cancer Treatment.

Multidrug resistance (MDR) is a major cause of failure in cancer chemotherapy. P-glycoprotein (P-gp), a promiscuous drug efflux pump, has been extensively studied for its association with MDR due to overexpression in cancer cells. Several P-gp inhibitors or modulators have been investigated in clinical trials in hope of circumventing MDR, with only limited success. Alternative strategies are actively pursued, such as the modification of existing drugs, development of new drugs, or combination of novel drug delivery agents to evade P-gp-dependent efflux. Despite the importance and numerous studies, these efforts have mostly been undertaken without a priori knowledge of how drugs interact with P-gp at the molecular level. This review highlights and discusses progress toward and challenges impeding drug development for inhibiting or evading P-gp in the context of our improved understanding of the structural basis and mechanism of P-gp-mediated MDR.

Hierarchical self-assembly of enantiopure and racemic helicenes at the liquid/solid interface: from 2D to 3D.

The performance of organic nanostructures is closely related to the organization of the functional molecules. Frequently, molecular chirality plays a central role in the way molecules assemble at the supramolecular level. Herein we report the hierarchical self-assembly of benzo-fused tetrathia[7]helicenes on solid surfaces, from a single surface-bound molecule to well-defined microstructures, using a combination of various characterization techniques assisted by molecular modeling simulations. Similarities as well as discrepancies are revealed between homochiral and heterochiral aggregations by monitoring the hierarchical nucleation of helicenes on surfaces, where the impact of enantiopurity, concentration and adsorbate-substrate interaction on molecular organization are disclosed.

Tunable doping of graphene by using physisorbed self-assembled networks.

One current key challenge in graphene research is to tune its charge carrier concentration, i.e., p- and n-type doping of graphene. An attractive approach in this respect is offered by controlled doping via well-ordered self-assembled networks physisorbed on the graphene surface. We report on tunable n-type doping of graphene using self-assembled networks of alkyl-amines that have varying chain lengths. The doping magnitude is modulated by controlling the density of the strong n-type doping amine groups on the surface. As revealed by scanning tunneling and atomic force microscopy, this density is governed by the length of the alkyl chain which acts as a spacer within the self-assembled network. The modulation of the doping magnitude depending on the chain length was demonstrated using Raman spectroscopy and electrical measurements on graphene field effect devices. This supramolecular functionalization approach offers new possibilities for controlling the properties of graphene and other two-dimensional materials at the nanoscale.

Diastereoselective Strategies towards Thia[n]helicenes.

In the present study, we have investigated different strategies for diastereoselective synthesis of thia[n]helicenes. We describe the introduction of different chiral auxiliaries at various positions and investigated their effect in the photocyclization reaction. Different chiral groups were placed at the sterically hindered position of the helical core and their interactions with various solvents and metals like copper were investigated. The use of Cu(I) salts has led to high diastereoselectivity in the photocyclization process and we were successful in obtaining the thia[5]helicene in enantiomerically pure form in good yield. The single diastereomer obtained was characterized by X-ray crystallography. From the study of the barrier of racemization of these thia[5]helicenes, the stability was found to be comparable to unsubstituted tetrathia[7]helicenes and substituted diazadithia[7]helicenes. This approach provides an easy access to enantiopure helicenes.

Insights into dynamic covalent chemistry at surfaces.

The potential of surface confined self-assembly to influence the chemical equilibrium of Schiff base formation and bias the yield and distribution of reaction products is explored.

Synthesis, functionalization, and optical properties of chiral carbazole-based diaza[6]helicenes.

In the present study, carbazole-based diaza[6]helicenes were synthesized utilizing versatile quinoline and 9-(2-ethylhexyl)-2,7-dimethoxycarbazole-3-carbaldehyde building blocks via the Wittig reaction-photocyclization strategy. The presence of bifunctional units comprising electrophilic chloroquinoline and electron-rich carbazole has opened up new opportunities. The chloro group was substituted with a chiral amine, allowing diastereomeric separation, and the chiral forms were monofunctionalized via electrophilic substitution on the carbazole unit. Postcyclization functionalization via substituting the carbazole unit provides a platform for the synthesis of chiral functionalized materials with potential application in fields such as asymmetric synthesis and organic electronics. The configuration of the diaza[6]helicene diastereomers was demonstrated by time-dependent density functional theory (TD-DFT) calculations. Furthermore, on the basis of the DFT calculations of the HOMO-LUMO energy levels of the chiral forms, these compounds can be potentially of interest as hole-transporting compounds.

Oxidative transformation to naphthodithiophene and thia[7]helicenes by intramolecular Scholl reaction of substituted 1,2-bis(2-thienyl)benzene precursors.

We present here a strategy to synthesize a variety of substituted naphthodithiophene building blocks through DDQ/acid-mediated oxidative cyclizations. The versatility of the Scholl reaction using the DDQ/acid system was demonstrated by the preparation of a novel substituted tetrathia[7]helicene where three new C-C bonds were formed in a one-pot procedure. The new DDQ/acid method was compared to the known strategies such as FeCl3 oxidation and oxidative photocyclization. By protecting the 1,2-bis(2-thienyl)benzene precursors, it is possible to direct the intermediates to controlled cyclization and effectively suppressing the polymerization. The highly reactive α-position of the terminal thiophenes can allow for further functionalization. The efficient preparation of a variety of naphthodithiophene building blocks, the extension to a nonphotochemical synthesis of [n]helicenes, and the ease of isolation of the products are arguments for the use of DDQ/acid system for this Scholl reaction.

Diazadithia[7]helicenes: Synthetic Exploration, Solid-State Structure, and Properties.

Diazadithia[7]helicenes were synthesized from the readily available building block ethyl 7-chloro-8-formylthieno[3,2-f]quinoline-2-carboxylate by a Wittig reaction-photocyclization strategy. The helicene core was functionalized by nucleophilic aromatic substitution with a variety of nucleophiles (e.g., O-, N-, and C-centered) and palladium-catalyzed reactions such as Suzuki coupling and Buchwald-Hartwig amination. Racemization studies confirmed that the enantiopure forms of these [7]helicenes are conformationally stable compared to their lower analogues. The solid-state structures of the novel diazadithia[7]helicenes were determined by single-crystal X-ray diffraction. The crystal structures of these azathia[7]helicenes show columnar stacking in antiparallel fashion. The HOMO-LUMO gaps of the new compounds were determined on the basis of electrochemical and optical measurements.

A fragment based approach toward thia[n]helicenes.

The synthesis of [9]- and [11]thiahelicenes, as well as the preparation of lower homologues such as [5]-, [6]-, and [7]thiahelicenes is reported. Efficient palladium catalyzed coupling reactions were employed. Triorganoindium derivatives were selectively mono-cross-coupled with N-methyl-3,4-dibromomaleimide followed by Stille coupling with the readily available building block naphthodithiophene. Oxidative photocyclization of the conjugated precursors using visible light was employed to synthesize a series of thia[n]helicenes. This modular synthetic methodology is independent of the length of the helical backbone.

Synthesis and structural elucidation of diversely functionalized 5,10-diaza[5]helicenes.

Diversely functionalized diaza[5]helicenes have been synthesized starting from 6,9-dichloro-5,10-diaza[5]helicene, which was prepared from a readily available quinoline building block via Wittig reaction followed by photochemical electrocyclization. The helicene skeleton was substituted by a variety of O-, S-, N-, and C-centered nucleophiles using nucleophilic aromatic substitution reactions and palladium-catalyzed reactions like Suzuki coupling and Buchwald-Hartwig aminations. We have determined, using X-ray single-crystal diffraction, the crystal structures of the chloro- and methoxy-substituted diaza[5]helicenes. A resolution strategy based on diastereomeric separation by substitution of the dichloro derivative with a chiral amine has been shown.

Efficient synthesis of benzo fused tetrathia[7]helicenes.

An efficient route toward the synthesis of symmetrical and unsymmetrical benzo fused tetrathia[7]helicenes substituted with electron donor (ED) and electron acceptor (EA) groups is reported. A common, readily available precursor 1,2- bis-(2-thienyl)benzene was used to synthesize different helicenes through a Wittig reaction, Stille coupling, and modified oxidative photocyclization.