Mapping Cell Membrane Organization and Dynamics Using Soft Nanoimprint Lithography.

Membrane shape is a key feature of many cellular processes, including cell differentiation, division, migration, and trafficking. The development of nanostructured surfaces allowing for the in situ manipulation of membranes in living cells is crucial to understand these processes, but this requires complicated and limited-access technologies. Here, we investigate the self-organization of cellular membranes by using a customizable and benchtop method allowing one to engineer 1D SiO2 nanopillar arrays of defined sizes and shapes on high-performance glass compatible with advanced microscopies. As a result of this original combination, we provide a mapping of the morphology-induced modulation of the cell membrane mechanics, dynamics and steady-state organization of key protein complexes implicated in cellular trafficking and signal transduction.

Electrostatics of cell membrane recognition: structure and activity of neutral and cationic rigid push-pull rods in isoelectric, anionic, and polarized lipid bilayer membranes.

Design, synthesis, and structural and functional studies of rigid-rod ionophores of different axial electrostatic asymmetry are reported. The employed design strategy emphasized presence of (a) a rigid scaffold to minimize the conformational complexity, (b) a unimolecular ion-conducting pathway to minimize the suprastructural complexity and monitor the function, (c) an extended fluorophore to monitor structure, (d) variable axial rod dipole, and (e) variable terminal charges to create axial asymmetry. Studies in isoelectric, anionic, and polarized bilayer membranes confirmed a general increase in activity of uncharged rigid push-pull rods in polarized bilayers. The similarly increased activity of cationic rigid push-pull rods with an electrostatic asymmetry comparable to that of alpha-helical bee toxin melittin (positive charge near negative axial dipole terminus) is shown by fluorescence-depth quenching experiments to originate from the stabilization of transmembrane rod orientation by the membrane potential. The reduced activity of rigid push-pull rods having an electrostatic asymmetry comparable to that in alpha-helical natural antibiotics (a positive charge near the positive axial dipole terminus) is shown by structural studies to originate from rod "ejection" by membrane potentials comparable to that found in mammalian plasma membranes. This structural evidence for cell membrane recognition by asymmetric rods is unprecedented and of possible practical importance with regard to antibiotic resistance.

Topological diversity of artificial beta-barrels in water.

Rigid-rod beta-barrels are composed of interdigitating, short, amphiphilic peptide strands flanked by stabilizing rigid-rod "staves". We here report studies on the topological diversity of these recently devised artificial beta-barrels with regard to their length. For this purpose, homologous p-octiphenyl, p-sexiphenyl, and p-quarterphenyl rods were equipped with complementary tripeptide strands based on the sequences Lys-Leu-Lys and Glu-Leu-Glu. The stability of rigid-rod beta-barrels of different length was determined by denaturation with guanidinium chloride. Free energies of delta GH2O = -5.2 kcalmol-1, delta GH2O = -2.9 kcalmol-1, and delta GH2O < -0.3 kcalmol-1 found for homologous p-octiphenyl, p-sexiphenyl, and p-quarterphenyl beta-barrels demonstrated strong dependence of beta-barrel stability on beta-barrel length. These results revealed a very qualitative minimal (approximately 23 A) and an "ideal" beta-barrel length (approximately 34 A), synergistic formation (alpha = 1.4) and remarkable stability for "ideal" p-octiphenyl beta-barrels exceeding that of several proteins and most synthetic models. Rigid-rod beta-barrels with p-oligophenyl "staves" longer than approximately 34 A will be very difficult to make and study because of rapidly decreasing rod solubilities. However, a strategy to bypass this apparent upper limitation of beta-barrel length is introduced: supramolecular matching of mismatched rods yielded elongated beta-barrels (61 A) of acceptable stability (delta GH2O = 2.2 - 3.1 kcalmol-1).

p-octiphenyl beta-barrels with ion channel and esterase activity.

Design, synthesis, and esterase and ion channel activity of a novel barrel-stave supramolecule with hydrophobic exterior and histidine-rich interior are reported. Voltage-dependent binding of pyrenyl-8-oxy-1,3,6-trisulfonates by histidines within p-octiphenyl beta-barrels (and not monomers) via ionic (and not hydrophobic) interactions (K(D), K(I), K(M) < 1 microM) is the basis for superb esterolytic proficiency up to (k(cat)/K(M))/k(uncat) = 9.6 x 10(5) in water and bilayer membranes. The conductance of labile ion channels formed in planar bilayer membranes is shown to be reduced by 8-hydroxypyrene-1,3,6-trisulfonate on the single- and multichannel level. [reaction: see text]

Bioorganic chemistry à la baguette: studies on molecular recognition in biological systems using rigid-rod molecules.

Initial studies using rigid-rod molecules or "baguettes" to address bioorganic topics of current scientific concern are reported. It is illustrated how transmembrane oligo(p-phenylene)s as representative model rods can be tuned to recognize lipid bilayer membranes either by their thickness or polarization. The construction of otherwise problematic hydrogen-bonded chains along transmembrane rods yields "proton wires," which act by a mechanism that is central in bioenergetics but poorly explored by means of synthetic models. Another example focuses on multivalent ligands assembling rigid-rod cell-surface receptors into transmembrane dynamic arene arrays. The potassium transport mediated by these ligand-receptor complexes provides experimental support for the potential biological importances of the controversial cation-pi mechanism. More complex supramolecular architecture is portrayed in the first artificial beta-barrels. It is shown how programmed assembly of toroidal rigid-rod supramolecules in detergent-free water permits control of diameter of the chemical nature of their interior. Reversed rigid-rod beta-barrels are assembled to function as self-assembled ionophores, ion channel models, and transmembrane nanopores. The potential of future intratoroidal chemistry is exemplified by encapsulation and planarization of beta-carotene in water and the construction of transmembrane B-DNA at the center of a second-sphere host-guest complex à al baguette.

Rapid access to synthetic lysobisphosphatidic acids using P(III) chemistry.

An expeditious route to synthetic lysobisphosphatidic acid S,S-1, its enantiomer, and regioisomers is reported. Synthetic difficulties concerning lipid stability and stereochemistry are bypassed using a phosphite triester approach in combination with multiple silyl protection. Spectroscopic studies evidence that acyl group migration in S,S-1 is accelerated by nonpolar solvents and inhibited by pyridine.

On the electrostatics of cell-membrane recognition: from natural antibiotics to rigid push-pull rods.

The question why pore-forming, alpha-helical natural antibiotics are not toxic is discussed within the general context of the interaction of electrostatically asymmetric "nanorods" with neutral, anionic, and polarized bilayer membranes. We suggest that simplification of the structural complexity of natural systems will be necessary to ultimately define the involved subtle balance between constructive and destructive electrostatic interactions.

Rigid-rod beta-barrels as lipocalin models: probing confined space by carotenoid encapsulation.

Herein, we describe the design, synthesis, structure, and function of synthetic, supramolecular beta-barrel models. Assembly of octi(p-phenylene)s with complementary -Lys-Leu-Lys-NH2 and Glu-Leu-Glu-NH2 side chains yielded water-soluble rigid-rod beta-barrels of precise length and with flexible diameter. A hydrophobic interior was evidenced by guest encapsulation. Host-guest complexes with planarized, monomeric beta-carotene within tetrameric rigid-rod beta-barrels, and disc micellar astaxanthin J-aggregates surrounded by about dodecameric rigid-rod "bicycle tires" were prepared from mixed micelles by dialytic detergent removal. The significance of these findings for future bioorganic chemistry in confined, intratoroidal space is discussed in comparison with pertinent biological examples.

Synthesis of multiply substituted, ion channel forming octi(p-phenylene)s: theme and variations.

[reaction: see text] To facilitate the access to unique models for biological processes, we examined six different synthetic routes to octi(p-phenylene) rods with lateral and terminal substituents R(L) and R(T). This systematic study allowed us to increase to overall yield for the synthesis of a new class of oligo(p-phenylene) ionophores about 20 times and to provide general insights into the practicability of synthetic routes to multiply substituted molecular rods.

Chiroptical rhythmicity, Part 1: Description of a novel phenomenon.

A novel chiroptical phenomenon named "chiroptical rhythmicity" is described. Chiroptical rhythmicity consists of multiple reversible circular dichroism Cotton effect sign inversions that follow a rhythmical pattern with respect to the continuous addition of a ligand (L-His) to the sample (a septi(p-phenylene)-based receptor bound to phosphatidylcholine bilayers). Decrease and blue-shift of the 1La absorption of the oligophenylene chromophore indicated that partial "H" aggregation occurs during chiroptical rhythmicity. Based on stereochemical considerations for monomeric and tetrameric oligophenylenes, a hypothetical mechanism is discussed.

Chiroptical rhythmicity, Part 2: Evidence for signal transduction by stereoselective ion pair formation at the membrane/water interface.

The initiation mechanism of chiroptical rhythmicity, a novel chiroptical phenomenon (Ghebremariam and Matile preceding contribution), was investigated by exploring the functionality of five structural analogs of the inducing L-His ligand. Disappearnace of chiroptical rhythmicity with L-His methyl esters as well as with D-His implied that stereoselective ion pair formation between carboxylate anion of L-His bound at the membrane/water interface and ammonium cations of phosphatidylcholine is essential for signal transduction to the hydrophobic core of the membrane. It was further shown that "H" aggregation of the asymmetric septi(p-phenylene) chromophore induced by multivalent L-His (i.e., poly-L-His) exceeds the extent required for chiroptical rhythmicity by far and ultimately results in the formation of achiral herringbone lattices.

Spectroscopic detection of endovesiculation by large unilamellar phosphatidylcholine vesicles: effects of chlorpromazine, dibucaine, and safingol.

Endovesiculation by large unilamellar vesicles (LUVs) induced by cationic amphiphiles is described in this work. A recent procedure to monitor phagocytosis of vesicles by macrophages by determining the amount of the simultaneously internalized water_soluble fluorescent dye HPTS with external quencher was adapted to LUVs (Daleke, D. L.; Hong, K.; Papahadjopoulos, D. Biochim. Biophys. Acta 1990, 1024, 352). Compared to dibucaine and safingol, the local anesthetic chlorpromazine (CPZ) was found to be the most efficient inducer of HPTS-internalization by LUVs. Control experiments using LUVs with entrapped HPTS indicated that the observed dye-internalization does not originate from transient lysis. A strong increase in activity above the critical micelle concentration of CPZ implies the importance of CPZ-micelles for endovesiculation. The significantly less efficient CPZ-induced HPTS-internalization by LUVs with 68 nm compared to 176 nm diameter further diminishes the likelihood of a micelle/bilayer fusion mechanism and supports the presence of 'zipper-type' endovesiculation by LUVs with diameters as small as 68 nm.

Spectroscopic studies of PhTX facilitated cation movement across membranes.

Philanthotoxins, noncompetitive inhibitors of the nicotinic acetylcholine receptor and various glutamate receptors, were found to be capable of mediating cation transport across lipid bilayers. With respect to the relatively weak binding constants of these amphiphilic polyamines to neuronal receptor proteins, this finding implies that their interaction with cell membranes might have to be considered in addition to that with protein receptors to fully understand the molecular mechanism of these neurotoxins.

Voltage-dependent ion channel formation by rigid rod-shaped polyols in planar lipid bilayers.

In this Letter, we describe the appearance of large, voltage-dependent currents in BLM induced by rigid rod-shaped polyols that function without charge and permanent dipole moment. The capacity of these symmetrical, nonpeptide models to form either short-living nanopores or small ion channels is shown to depend critically on the length of rigid-rod scaffold as well as the nature of the lateral side chains.

Exciton coupled circular dichroic studies of self-assembled brevetoxin-porphyrin conjugates in lipid bilayers and polar solvents.

BACKGROUND: Brevetoxins, involved in the 'red tide' as well as shellfish poisoning, are known to bind to cell membranes and membrane proteins. Brevetoxin B (BTX-B) interacts specifically with neuronal sodium channels. We recently found that BTX also induces selective ion movements across lipid bilayers through transmembrane BTX self-assemblies. RESULTS: We examined the self-assembly of several BTX derivatives in the presence and absence of cations and lipid bilayers using the powerful porphyrin chromophores as circular dichroism labels. BTX derivatives self-assemble into tubes, which can bind to metals both when soluble and when inserted into the bilayer to form transmembrane pores. Depending on the tendency of the BTX derivative to self-aggregate (the critical 'micelle' concentration, cmc), it may aggregate in solution before membrane insertion, or may insert itself into the membrane as a monomer before assembling the pore. CONCLUSIONS: The active BTX-B complex in lipid bilayers is a cyclic, transmembrane self-assembly consisting of antiparallel aligned BTX molecules that can mediate selective ion movement through membranes. The differences in pore formation mechanisms between BTX derivatives may be reflected in differences in pore formation by natural BTX variants, perhaps explaining their varying levels of toxicity.

Intramolecular porphyrin pi,pi-stacking: absolute configurational assignment of acyclic compounds with single chiral centers by exciton coupled circular dichroism.

We report a new concept based on exciton coupled circular dichroism (CD) for assigning absolute configurations to a single chiral center *CXYSL, where X is -OH or -NH2, Y is an acyclic chain with terminal OH or -NH2, and S (small) and L (large) represent sterically distinct groups. It consists of a one step attachment of porphyrins to X and Y followed by CD measurement. The key event is intramolecular porphyrin pi,pi-stacking, which converts the flexible, acyclic substrate into a rigid stacked conformation characterized by bisignate exciton split CD curves. Since the stacked conformer is sterically controlled by the two groups, S and L, the sign of the exciton split CD is directly governed by the spatial arrangement of these groups. This approach is applicable to various acyclic compounds with different C/C distances (1,3 approximately 1,15) between the functional groups, but not to 1,2-C/C.