Stefan Matile

Experimental evidence for the functional relevance of anion– π interactions

Attractive in theory and confirmed to exist, anion-pi interactions have never really been seen at work. To catch them in action, we prepared a collection of monomeric, cyclic and rod-shaped naphthalenediimide transporters. Their ability to exert anion-pi interactions was demonstrated by electrospray tandem mass spectrometry in combination with theoretical calculations. To relate this structural evidence to transport activity in bilayer membranes, affinity and selectivity sequences were recorded. pi-acidification and active-site decrowding increased binding, transport and chloride > bromide > iodide selectivity, and supramolecular organization inverted acetate > nitrate to nitrate > acetate selectivity. We conclude that anion-pi interactions on monomeric surfaces are ideal for chloride recognition, whereas their supramolecular enhancement by pi,pi-interactions appears perfect to target nitrate. Chloride transporters are relevant to treat channelopathies, and nitrate sensors to monitor cellular signaling and cardiovascular diseases. A big impact on organocatalysis can be expected from the stabilization of anionic transition states on chiral pi-acidic surfaces.

Photoproduction of Proton Gradients with π-Stacked Fluorophore Scaffolds in Lipid Bilayers

Rigid p-octiphenyl rods were used to create helical tetrameric π-stacks of blue, red-fluorescent naphthalene diimides that can span lipid bilayer membranes. In lipid vesicles containing quinone as electron acceptors and surrounded by ethylenediaminetetraacetic acid as hole acceptors, transmembrane proton gradients arose through quinone reduction upon excitation with visible light. Quantitative ultrafast and relatively long-lived charge separation was confirmed as the origin of photosynthetic activity by femtosecond fluorescence and transient absorption spectroscopy. Supramolecular self-organization was essential in that photoactivity was lost upon rod shortening (from p-octiphenyl to biphenyl) and chromophore expansion (from naphthalene diimide to perylene diimide). Ligand intercalation transformed the photoactive scaffolds into ion channels.

Synthetic ion channels and pores (2004–2005)

This critical review covers synthetic ion channels and pores created between January 2004 and December 2005 comprehensively. The discussion of a rich collection of structural motifs may particularly appeal to organic, biological, supramolecular and polymer chemists. Functions addressed include ion selectivity and molecular recognition, as well as responsiveness to light, heat, voltage and membrane composition. The practical applications involved concern certain topics in medicinal chemistry (antibiotics, drug delivery), catalysis and sensing. An introduction to principles and methods is provided for the non-specialist; some new sources of inspiration from fields beyond chemistry are highlighted.

Anion–π Slides for Transmembrane Transport

The recognition and transport of anions is usually accomplished by hydrogen bonding, ion pairing, metal coordination, and anion-dipole interactions. Here, we elaborate on the concept to use anion-pi interactions for this purpose. Different to the popular cation-pi interactions, applications of the complementary pi-acidic surfaces do not exist. This is understandable because the inversion of the aromatic quadrupole moment to produce pi-acidity is a rare phenomenon. Here, we suggest that pi-acidic aromatics can be linked together to produce an unbendable scaffold with multiple binding sites for anions to move along across a lipid bilayer membrane. The alignment of multiple anion-pi sites is needed to introduce a cooperative multi-ion hopping mechanism. Experimental support for the validity of the concept comes from preliminary results with oligonaphthalenediimide (O-NDI) rods. Predicted by strongly positive facial quadrupole moments, the cooperativity and chloride selectivity found for anion transport by O-NDI rods were consistent with the existence of anion-pi slides. The proposed mechanism for anion transport is supported by DFT results for model systems, as well as MD simulations of rigid O-NDI rods. Applicability of anion-pi slides to achieve electroneutral photosynthesis is elaborated with the readily colorizable oligoperylenediimide (O-PDI) rods. To clarify validity, scope and limitations of these concepts, a collaborative research effort will be needed to address by computer modeling and experimental observations the basic questions in simple model systems and to design advanced multifunctional anion-pi architectures.

Ditopic Ion Transport Systems: Anion–π Interactions and Halogen Bonds at Work

Single-atom exchange series are introduced to extract the individual contributions of halogen bonds and anion–π interactions to the transport of anions across lipid bilayer membranes (see picture). Known cation binding sites are used for counterion activation of the neutral calix[4]arene transporters. The experimental evidence for anion transport with halogen bonds is unprecedented.

Rigid-rod anion–π slides for multiion hopping across lipid bilayers

Shape-persistent oligo-p-phenylene-N,N-naphthalenediimide (O-NDI) rods are introduced as anion-pi slides for chloride-selective multiion hopping across lipid bilayers. Results from end-group engineering and covalent capture as O-NDI hairpins suggested that self-assembly into transmembrane O-NDI bundles is essential for activity. A halide topology VI (Cl > F > Br approximately I, Cl/Br approximately Cl/I > 7) implied strong anion binding along the anion-pi slides with relatively weak contributions from size exclusion (F >or= OAc). Anomalous mole fraction effects (AMFE) supported the occurrence of multiion hopping along the pi-acidic O-NDI rods. The existence of anion-pi interactions was corroborated by high-level ab initio and DFT calculations. The latter revealed positive NDI quadrupole moments far beyond the hexafluorobenzene standard. Computational studies further suggested that anion binding occurs at the confined, pi-acidic edges of the sticky NDI surface and is influenced by the nature of the phenyl spacer between two NDIs. With regard to methods development, a detailed analysis of the detection of ion selectivity with the HPTS assay including AMFE in vesicles is provided.

Lysobisphosphatidic acid controls endosomal cholesterol levels

Most cell types acquire cholesterol by endocytosis of circulating low density lipoprotein, but little is known about the mechanisms of intra-endosomal cholesterol transport and about the primary cause of its aberrant accumulation in the cholesterol storage disorder Niemann-Pick type C (NPC). Here we report that lysobisphosphatidic acid (LBPA), an unconventional phospholipid that is only detected in late endosomes, regulates endosomal cholesterol levels under the control of Alix/AlP1, which is an LBPA-interacting protein involved in sorting into multivesicular endosomes. We find that Alix down-expression decreases both LBPA levels and the lumenal vesicle content of late endosomes. Cellular cholesterol levels are also decreased, presumably because the storage capacity of endosomes is affected and thus cholesterol clearance accelerated. Both lumenal membranes and cholesterol can be restored in Alix knockdown cells by exogenously added LBPA. Conversely, we also find that LBPA becomes limiting upon pathological cholesterol accumulation in NPC cells, because the addition of exogenous LBPA, but not of LBPA isoforms or analogues, partially reverts the NPC phenotype. We conclude that LBPA controls the cholesterol capacity of endosomes.

Rigid Oligoperylenediimide Rods: Anion–π Slides with Photosynthetic Activity†

To combine ion-channel and photosynthetic activity productively is one of the great challenges of synthetic multifunctional nanoarchitecture. One expectation is the ability to compensate the transmembrane electronic imbalance caused by photoinduced active electron transport with passive anion transport in the other direction. Such electroneutral electron–anion antiport should prevent saturation of the system by membrane polarization. The problem is the high, but so far elusive, anion selectivity required to avoid collapse of the entire multifunctional system. The recent introduction of rigid-rod oligo(p-phenylene)N,N-naphthalenediimide (O-NDI) anion–p slides has provided a new approach to transmembrane anion transport that focuses on anion–p interactions for selectivity and multi-ion hopping. Photosynthetic activity remains, however, inaccessible with O-NDI anion–p slides for synthetic reasons. Herein, we introduce the differently colored oligo(p-phenylene)-N,N-perylenediimide (O-PDI) rods 1 and 2 as anion–p slides with photosynthetic activity (Figure 1). PDIs are p-acidic n-semiconductors. These properties are compatible with electron transport in one direction and anion transport by anion–p interactions in the other. The ideal topology of the PDI chromophore to align with the lipid tails in one leaflet of a bilayer membrane has been noted recently. In rigid O-PDI rod 1 (Figure 1), two PDIs are unbendably placed in line to reach the matching length to span a lipid bilayer membrane. Anionic triglutamate tails were added at one end to obtain the water solubility needed to assure delivery to membrane vesicles, intervesicular transfer, oriented partitioning, and parallel self-assembly. Pyrrolidinyl substituents were added to the PDI core because the 1,7bis(pyrrolidin-1-yl)-3,4:9,10-perylene-bis(dicarboximide) chromophore was shown by Wasielewski and co-workers to have optoelectrical characteristics similar to those of chlorophyll, the green pigment of plants (Figure 1). For instance, face-to-face dimers of these green PDIs were found to undergo symmetry-breaking, relatively long-lived photoinduced charge separation (lifetime t 4 ns, depending on solvent; photoinduced charge separation in linear dimers was less favorable). Rigid O-PDI rod 1 was synthesized in 12 steps from commercially available starting materials, such as perylene dianhydride and glutamate (Scheme S1 and Figures S1–S3, Supporting Information). Synthetic procedures and analytical and spectroscopic data for all new compounds can be found in the Supporting Information. Egg yolk phosphatidylcholine large unilamellar vesicles (EYPC LUVs) loaded with the pH-sensitive fluorescent probe 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) were used to evaluate the activity of rigid O-PDI rod 1 as an anion–p slide. 10] Vesicles with internal NaBr were diluted with isoosmolar buffer containing different salts MX, and rod 1 was added (Figure 2). This version of the HPTS assay fails to detect the ability of rod 1 to dissipate the applied transmembrane NaX and MBr gradients by transmembrane

Catalysis with Anion–π Interactions

The conclusion is inevitable: Increasing stabilization of an anionic transition state with increasing π-acidity of the catalyst is observed; thus, anion-π interactions can contribute to catalysis.

Direct Observation of Anion-Mediated Translocation of Fluorescent Oligoarginine Carriers into and across Bulk Liquid and Anionic Bilayer Membranes

The recent hypothesis that counteranion‐mediated dynamic inversion of charge and solubility might contribute to diverse functions of oligoarginines in biomembranes was tested with two fluorescently labelled oligomers, FL‐R8, one of the most active cell‐penetrating peptides, and its longer version, FL‐R16. We report evidence for counteranion‐mediated phase transfer from water into bulk chloroform and anionic lipid‐bilayer membranes as well as reverse‐phase transfer from bulk chloroform and across intact lipid‐bilayer membranes into water. The differences found between FL‐R8 and FL‐R16 with regard to location in the bilayer and reverse‐phase transfer from bulk and lipid‐bilayer membranes into water implied that the reported results may be relevant for biological function.