Intermolecular Organization Research Articles

Molecular Organization, Structural Orientation, and Surface Topography of Monoacylated β-Cyclodextrins in Monolayers at the Air−Aqueous Interface

The surface behavior of monoacylated beta-cyclodextrins, with hydrocarbon chains of 16, 14, and 10 carbons, has been assessed by the measurement of the surface pressure, surface (dipole) potential, optical reflectivity, and surface topography in monolayers at the air-water interface. For all the derivatives studied, the intermolecular organization adopted along compression-decompression isotherms reveals a rich variety of packing states which imply profound reorganization of the hydrophobic and hydrophilic moieties of the beta-cyclodextrin derivatives in the film, depending on the lateral surface pressure. The intermolecular arrangements are consistent with the adoption of a different and defined orientation of the cyclic oligosaccharide unit, relative to the interfacial plane and the aqueous subphase. This is different from the behavior of the per-substituted derivatives, and none of the changes exhibited by the monosubstituted forms are consistent with the oligosaccharide ring remaining in a fixed orientation along the interface when the surface pressure is varied.

Aggregation Behavior and Dynamics of Synthetic Amphiphiles That Self‐Assemble to Anion Transporters

The amphiphilic heptapeptides-referred to as synthetic anion transporters (SATs)-mediate chloride transport in planar lipid bilayer membranes, synthetic liposomes, and mammalian cells. The SATs described have the general formula R1(2)NCOCH2OCH2CO-(Gly)3-Pro-(Gly)3-OR2. Substitution at R1 and R2 with various aliphatic or aromatic groups alters the ability of SATs to transport chloride through a phospholipid bilayer membrane. Despite extensive structure-activity relationship studies concerning Cl(-)-mediated transport by SATs, relatively little was known about the mechanism of insertion and pore-formation in the membrane. In the current study, the mechanistic behavior of SATs was investigated in aqueous solution and at the air-water interface. In the latter case, Langmuir trough studies and Brewster angle microscopy (BAM) revealed the extent of monolayer stability and organization for SATs. Dynamic light scattering and transmission electron microscopy (TEM) confirmed these results and defined the aggregation behavior of SATs in solution. SAT derivatives that showed low chloride transport activity organized into stable monolayers at the air-water interface, while more active SATs formed less stable monolayers. The relationship between intermolecular organization of SATs and pore-formation in the membrane is discussed along with its implications for chloride transport.

Poly(propylene oxide)−Poly(phenylene ethynylene) Block and Graft Copolymers

Block copolymers composed of a highly isotactic and nonracemic poly(propylene oxide) (PPO) segment joined to a poly(phenylene ethynylene) (PPE) segment, with either meta or para connectivity, were synthesized, and their aggregation and conformational behavior in solution properties was investigated by optical spectroscopy. Furthermore, graft copolymers based on a m-phenylene ethynylene backbone equipped with isotactic, nonracemic PPO side chains were synthesized and studied with regard to their backbone conformation in solution. The combination of flexible chiral polymeric segments (PPO) with rather rigid, extended aromatic segments (PPE) promises to be a fruitful concept for generating novel optoelectronic materials with defined levels of intra- and intermolecular organization, i.e., backbone conformation and self-assembly behavior.

Influence of Supramolecular Organization on Energy Transfer Properties in Chiral Oligo(p-phenylene vinylene) Porphyrin Assemblies

A comparative study on oligo(p-phenylene vinylene) (OPV)-appended porphyrins containing all trans-vinylene (either hydrophilic or lipophilic) or amide linkages (lipophilic) is presented. The type of supramolecular arrangement obtained in organic solvents proves to be strongly dependent on the nature of the covalent connection. In the case of all trans-vinylene linkages, a J-type intermolecular packing is obtained and the assemblies are only of moderate stability. Conversely, the supramolecular structures obtained from the amide-linked system display an H-type stacking arrangement of enhanced stability and chirality as a consequence of intermolecular hydrogen bonding along the stack direction, favorably interlocking the stacked building blocks. Interestingly, the observed differences in stability and organization are qualitatively illustrated by monitoring the sequential energy transfer process in both types of assemblies. Efficient intramolecular energy transfer from the OPVs (donors) to the respective porphyrin cores is followed by energy transfer from Zn-porphyrin (donor) to free-base porphyrin (acceptor) in both systems. However, the improved intermolecular organization for the amide-linked system increases the energy transfer efficiency along the stack direction. In addition, the water-soluble (OPV)-appended porphyrin system forms highly stable assemblies in an aqueous environment. Nevertheless, the poor energy transfer efficiency along the stack direction reveals a relative lack of organization in these assemblies.

Nanoscale Organization of the Pathogen Receptor DC‐SIGN Mapped by Single‐Molecule High‐Resolution Fluorescence Microscopy

DC-SIGN, a C-type lectin exclusively expressed on dendritic cells (DCs), plays an important role in pathogen recognition by binding with high affinity to a large variety of microorganisms. Recent experimental evidence points to a direct relation between the function of DC-SIGN as a viral receptor and its spatial arrangement on the plasma membrane. We have investigated the nanoscale organization of fluorescently labeled DC-SIGN on intact isolated DCs by means of near-field scanning optical microscopy (NSOM) combined with single-molecule detection. Fluorescence spots of different intensity and size have been directly visualized by optical means with a spatial resolution of less than 100 nm. Intensity- and size-distribution histograms of the DC-SIGN fluorescent spots confirm that approximately 80 % of the receptors are organized in nanosized domains randomly distributed on the cell membrane. Intensity-size correlation analysis revealed remarkable heterogeneity in the molecular packing density of the domains. Furthermore, we have mapped the intermolecular organization within a dense cluster by means of sequential NSOM imaging combined with discrete single-molecule photobleaching. In this way we have determined the spatial coordinates of 13 different individual dyes, with a localization accuracy of 6 nm. Our experimental observations are all consistent with an arrangement of DC-SIGN designed to maximize its chances of binding to a wide range of microorganisms. Our data also illustrate the potential of NSOM as an ultrasensitive, high-resolution technique to probe nanometer-scale organization of molecules on the cell membrane.

The local and intermediate range structures of the five amorphous ices at 80K and ambient pressure: A Faber-Ziman and Bhatia-Thornton analysis

Using isotope substitution neutron scattering data, we present a detailed structural analysis of the short and intermediate range structures of the five known forms of amorphous ice. Two of the lower density forms--amorphous solid water and hyperquenched glassy water--have a structure very similar to each other and to low density amorphous ice, a structure which closely resembles a disordered, tetrahedrally coordinated, fully hydrogen bonded network. High density and very high density amorphous ices retain this tetrahedral organization at short range, but show significant differences beyond about 3.1 A from a typical water oxygen. The first diffraction peak in all structures is seen to be solely a function of the intermolecular organization. The short range connectivity in the two higher density forms is more homogeneous, while the hydrogen site disorder in these forms is greater. The low Q behavior of the structure factors indicates no significant density or concentration fluctuations over the length scale probed. We conclude that these three latter forms of ice are structurally distinct. Finally, the x-ray structure factors for all five amorphous systems are calculated for comparison with other studies.

Localization of Functional Polypeptides in Bacterial Inclusion Bodies

Bacterial inclusion bodies, while showing intriguing amyloid-like features, such as a beta-sheet-based intermolecular organization, binding to amyloid-tropic dyes, and origin in a sequence-selective deposition process, hold an important amount of native-like secondary structure and significant amounts of functional polypeptides. The aggregation mechanics supporting the occurrence of both misfolded and properly folded protein is controversial. Single polypeptide chains might contain both misfolded stretches driving aggregation and properly folded protein domains that, if embracing the active site, would account for the biological activities displayed by inclusion bodies. Alternatively, soluble, functional polypeptides could be surface adsorbed by interactions weaker than those driving the formation of the intermolecular beta-sheet architecture. To explore whether the fraction of properly folded active protein is a natural component or rather a mere contaminant of these aggregates, we have explored their localization by image analysis of inclusion bodies formed by green fluorescent protein. Since the fluorescence distribution is not homogeneous and the core of inclusion bodies is particularly rich in active protein forms, such protein species cannot be passively trapped components and their occurrence might be linked to the reconstruction dynamics steadily endured in vivo by such bacterial aggregates. Intriguingly, even functional protein species in inclusion bodies are not excluded from the interface with the solvent, probably because of the porous structure of these particular protein aggregates.

Influence of Intermolecular Orientation on the Photoinduced Charge Transfer Kinetics in Self-Assembled Aggregates of Donor−Acceptor Arrays

The kinetics of photoinduced charge transfer reactions in covalently linked donor-acceptor molecules often undergoes dramatic changes when these molecules self-assemble from a molecular dissolved state into a nanoaggregate. Frequently, the origin of these changes is only partially understood. In this paper, we describe the intermolecular spatial organization of three homologous arrays, consisting of a central perylene bisimide (PERY) acceptor moiety and two oligo(p-phenylene vinylene) (OPV) donor units, in nanoaggregates and identify both face-to-face (H-type) and slipped (J-type) stacking of the OPV and PERY chromophores. For the J-type aggregates, short intermolecular OPV-PERY distances are created that give rise to a charge-transfer absorption band. The proximity of the donor and acceptor groups in the J-type aggregates enables a highly efficient photoinduced charge separation with a rate (k(cs) > 10(12) s(-1)) that significantly exceeds the rate of the intramolecular charge transfer of the same compounds when molecularly dissolved, even in the most polar media. In the H-type aggregates, on the other hand, the intermolecular OPV-PERY distance is not reduced compared to the intramolecular separation, and hence, the rates of the electron transfer reactions are not significantly affected compared to the molecular dissolved state. Similar to the forward electron transfer, the kinetics of the charge recombination in the aggregated state can be understood by considering the different interchromophoric distances that occur in the H- and J-type aggregates. These results provide the first consistent rationalization of the remarkable differences that are observed for photoinduced charge-transfer reactions of donor-acceptor compounds in molecularly dissolved versus aggregated states.

Calcium-Induced Changes to the Molecular Conformation and Aggregate Structure of β-Casein at the Air−Water Interface

The influence of calcium on interactions of beta-casein at the air-water interface has been studied by several techniques, including interfacial rheology, atomic force microscopy (AFM), infrared reflectance-absorbance spectroscopy (IRRAS), and zeta potential measurements. In the absence of calcium, a weak interfacial gel forms after about 2.5 h. Also in the absence of calcium, the adsorbed beta-casein film exhibits some degree of both intra- and intermolecular structural organization. For example, IRRAS spectra show a measurable amount of alpha-helix content, and AFM images indicate the presence of interfacial aggregates with a characteristic lateral length scale of 20-30 nm, which we interpret as hemimicelles. Upon the addition of calcium, particularly at Ca:beta-casein molar ratios above approximately 5:1, a stronger interfacial gel forms more quickly; for example, the interfacial shear moduli increase twice as rapidly. Also under these conditions (5:1 Ca:beta-casein ratio) there is little evidence of structural organization; i.e., the alpha-helix peaks are very weak, and AFM images show a disordered, but continuous film, without distinct hemimicelles. On the basis of these findings, we hypothesize that calcium binding destabilizes the coupled intra- and intermolecular structural organization, and that the loss of organization permits more rapid interfacial gelation. These phenomena are characteristic of the air-water interface; they are not accompanied by analogous structural changes in bulk solution.

Synthesis of hexazincporphyrin and hexaamine substituted Ru(bipy) 3 complexes for alternate self-assembly by tritopic coordinations

Syntheses of Ru(4,4′-zincporphynyl-bipyridine) 3 ( 1) and Ru(4,4′-aminomethyl-bipyridine) 3 ( 2) complexes are reported. The two octahedral complexes interact strongly each other by tritopic coordination of three amino groups to three zincporphyrin moieties. The remaining three amino groups and three zincporphyrin moieties can be used for further intermolecular organization to give linear 2 : 1, 1 : 1 and 1 : 2 complexes of 1 and 2 , depending on their relative molar ratios.

Supramolecular self-assembly of porphyrinic materials by design. Non-centrosymmetric architectures of the 5-(3′-pyridyl)-10,15,20-tris(4′-carboxyphenyl) and 5-(2′-quinolyl)-10,15,20-tris(4′-hydroxyphenyl) porphyrins

This study reports the design of polarized porphyrin network motifs sustained by mixed hydrogen bonding algorithms, aiming at the synthesis of non-centrosymmetric supramolecular architectures. It is based on suitably functionalized porphyrin scaffolds with mixed proton–donor and proton–acceptor groups that exhibit reduced C2/C2v point symmetry, in relation to the square-planar tetra-substituted 4′-carboxyphenyl, 4′-hydroxyphenyl or 4′-pyridyl porphyrin derivatives (D4h) widely utilized in the construction of porphyrin framework solids thus far. Representative examples are provided by supramolecular self-assembly of the free-base 5-(3′-pyridyl)-10,15,20-tris(4′-carboxyphenyl)-porphyrin (1) and 5-(2′-quinolyl)-10,15,20-tris(4′-hydroxyphenyl) porphyrin (2) moieties. In both cases, each porphyrin building block is connected to neighboring species in a cooperative but asymmetric manner by a combination of O–H⋯O and O–H⋯N inter-porphyrin hydrogen bonds, where optimization of the O–H⋯N interactions involves molecules related by twofold screw symmetry. This induces the formation of two-dimensionally networked arrays characterized by polarized intermolecular organization and by corrugated surfaces. Stacking of these layers along the normal direction is characterized by either translation or twofold screw symmetry, affording non-centrosymmetric crystal lattices with solvent species incorporated within the interporphyrin voids.

Using an electronic microbalance technique to study the stick–slip behavior of lubricated polypropylene fibers

AbstractBy use of an electronic microbalance, fiber/fiber friction measurements were made on cleaned and lubricated polypropylene (PP) filaments. The filaments were coated with a 0.14‐μm‐thick layer of one of the five lubricants: two different hydroxylated oils (hydroxylated oleate and hydroxylated stearate) and three different ethoxylated surfactants [(i) ethoxylated 10 OE dioleate; (ii) ethoxylated 15 OE dioleate; and (iii) ethoxylated 20 OE trioleate]. Despite the thick layer of lubricant applied, stick–slip persisted. Theoretical considerations of experimental conditions (load, speed, and viscosity) show that the pressure is very high at the contact point, and this would induce film thinning and thus stick–slips arising from phase transitions. Statistical evaluations and atomic force microscopy images show that contacts between PP surfaces take place in presence of lubricants because surface asperity heights are larger than the lubricant film thickness. Oils or surfactants having similar surface tensions give different interfiber cohesion. Some explanations correlating the lubricant chemical structure and its spatial conformation, as well as its capacity to form intermolecular bonds and associative organization, to interfiber friction are given. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 645–654, 2003

Neutron and x-ray scattering study of fully deuterated poly(3-n.decylpyrrole)

The structure of a fully deuterated sample of the conducting polymer poly(3-n.decylpyrrole) was investigated by neutron and x-ray diffraction. The different sensitivity of the two probes to deuterium atoms allowed us to obtain novel insights about the intermolecular organization of the polymer, with special focus on the alkyl substituent. The polymer turns out to be arranged in parallel layers. Within each layer, neighboring pyrrole rings maintain a coplanar disposition, whereas the alkyl decane tails are kinked out of the pyrrole plane. Monomers belonging to different adjacent layers display a high degree of superposition, due to a considerable overlap of both pyrrole rings and decane tails. Thus, despite its folded conformation, the alkyl substituent does not affect the short-range order of the compound, but rather contributes to determine its layered structure.

Photoactive Nanowires in Fullerene−Ferrocene Dyad Polyelectrolyte Multilayers

Layer-by-layer (LBL) films of a fulleropyrrolidinium−androstane−ferrocene dyad revealed a surprisingly high level of intermolecular organization. In particular, in every adsorption layer, the films self-assemble into linear nanowire superstructures. Given the inherent intramolecular charge transfer performance of the dyads, such a film organization facilitates the charge diffusion from the photoactive centers to the electrodes, due to extensive interpenetration of the layers, which leads to multiple contacts between the fullerene−ferrocene nanowires.

Intermolecular organisation of triphenylene­based discotic mesogens by interdigitation of alkyl chains

Two series of hexakis(alkyloxy)triphenylenes have been synthesised and characterised. All materials contain two different lengths of n-alkyloxy chains. Series I (C5Cn) materials contain three –OC5H11 chains and three –OCnH2n + 1 chains. Series II (CmCn) materials contain three –OCmH2m + 1 chains and three –OCnH2n + 1 chains, such that m + n = 10. Hexagonal columnar mesophases are observed in Series I for n ≥ 3 and, in Series II, for C5C5 and C6C4. Investigation by XRD shows that, within Series I, increasing the Cn chain length increases the intercolumnar spacing. Furthermore, the intercolumnar spacings for constitutional isomers are identical. The difference in the lengths of the two types of alkyl chain at the periphery of the molecules is defined as the Interdigitation Length (IL) and is a measure of the maximum extent of interdigitation that is possible between neighbouring molecules. Series I and II materials have been studied to probe the effect of IL on the change in enthalpy (ΔHCol–I) and entropy (ΔSCol–I) for the mesophase to isotropic liquid phase transition. In contrast to CxCx materials, which show a decrease in ΔHCol–I and ΔSCol–I with increasing chain length, Series I and II mesogens all exhibit ΔHCol–I and ΔSCol–I values similar to those of long chain CxCx materials, irrespective of chain length.

Kinetic steps for the hydrolysis of sphingomyelin by Bacillus cereus sphingomyelinase in lipid monolayers

The sphingomyelinase (Sphmase) activity degrading sphingomyelin (Sphm) monolayers shows a slow-reaction latency period before exhibiting constant rate catalysis. These two kinetic regions are regulated independently by the lateral surface pressure and by lipids that are biomodulators of cell function such as ceramide, glycosphingolipids, fatty acids, and lysophospholipids. Knowledge of the interfacial adsorption of Sphmase, precatalytic activation, initiation of effective catalysis, and the corresponding kinetic parameters is necessary for studying the level at which different lipids modulate the activity. We dissected some kinetic steps and determined the rate constants for degradation of Sphm, under controlled intermolecular organization, by Sphmase. Six models, adapted to two dimensions, were used to elucidate possible mechanisms for the interfacial activation of Sphmase during the lag time. The models consider enzyme binding to the substrate monolayer and a subsequent, essentially irreversible interfacial activation; this is supported experimentally by monolayer transfer experiments. Some mechanisms involve enzyme-substrate binding and associated states of the enzyme in the bulk subphase or at the interface, prior to complete activation. The activity of Sphmase is consistent with kinetics involving enzyme partitioning into the interface followed by substrate association, and by a process that proceeds with bimolecular kinetic dependence on the interfacial Sphmase concentration, and a subsequent slow step of activation. A possible equilibrium between the apparent monomolecular and bimolecular activated states of the interfacial enzyme, coupled to a slow activation, constitute rate-limiting steps that can explain the existence of lag time and the achievement of a maximum constant rate of degradation of Sphm monolayers by Sphmase.—Fanani, M. L., and B. Maggio. Kinetic steps for the hydrolysis of sphingomyelin by Bacillus cereus sphingomyelinase in lipid monolayers. J. Lipid Res. 2000. 41: 1832–1840.

Intermolecular organization of triphenylene-based discotic mesogens by interdigitation of alkyl chains

Two homologous series of 2,3,6,7,10,11-hexa-alkyloxy-substituted triphenylenes have been synthesized, such that alkyl chains of two different lengths alternate around the rigid triphenylene core in order to allow interdigitation of the chains between neighbouring columns in a hexagonal columnar mesophase. We report here the dependence of structural and thermal properties on the extent of interdigitation, as investigated by X-ray diffraction, polarized optical microscopy and differential scanning calorimetry.

Salivary Film Expresses a Complex, Macromolecular Binding Site for Streptococcus sanguis

Teeth in the oral cavity are coated with a salivary film or pellicle, which lacks apparent intermolecular organization. This heterogeneous film facilitates binding of early commensal colonizing bacteria, including Streptococcus sanguis. To test the hypothesis that sufficient intermolecular organization exists in salivary films to form binding sites for S. sanguis, an in vitro model of saliva-coated teeth was probed with murine anti-idiotypical monoclonal antibodies (mAb2, anti-ids). The anti-ids were harvested from hybridomas that were developed in response to first generation murine hybridomas that produced anti-S. sanguis adhesin monoclonal antibodies (mAb1). The anti-ids (i) reacted with experimental salivary films and inhibited S. sanguis adhesion in a dose-dependent fashion. In Western blots, the anti-ids (ii) recognized a high molecular weight salivary antigen and (iii) secretory IgA (sIgA) light chain and alpha-amylase. After isolation by gel filtration from whole saliva or mixed secretory IgA and alpha-amylase, the high molecular weight component, containing amylase activity and sIgA, bound to hydroxyapatite to promote adhesion of S. sanguis. Therefore, a complex enriched in secretory immunoglobulin A and alpha-amylase forms a S. sanguis-binding site.

Calix

Chiral calix[4]arene derivatives with four O-(N-acetyl-PhgOMe), (1), (Phg denotes R-phenylglycine), or O-(N-acetyl-LeuOMe) (2) strands have been synthesised. Both compounds exist in chloroform in stable cone conformations with a noncovalently organised cavity at the lower rim that is formed by circular interstrand amidic hydrogen bonds. Such organisation affects both the selectivity and extraction/transport properties of 1 and 2 toward metal cations. Calix[4]arene derivatives with one OCH2COPhgOMe strand (3), two OCH2COPhgOMe strands (5) and with 1,3-OMe-2,4-(O-CH2COPhgOMe) substituents (4) at the lower rim have also been prepared. For 3, a conformation stabilised by a circular hydrogen-bond arrangement is found in chloroform, while 4 exists as a time-averaged C2 conformation with two intramolecular NH ...OCH3 hydrogen bonds. Compound 5 has a unique hydrogen-bonding motif in solution and in the solid state with two three-centred NH-.. O and two OH...O hydrogen bonds at the lower rim. This motif keeps 5 in the flattened cone conformation in chloroform. The X-ray structure analysis of 1 revealed a molecular structure with C2 symmetry; this structure is organised in infinite chains by intra- and intermolecular H bonds. The solid-state and solution structures of the [1-Na]ClO4 complex are identical, C4 symmetric cone conformations.

Ceramide enhances susceptibility of membrane phospholipids to phospholipase A2 through modification of lipid organization in platelet membranes.

The effects of ceramide on agonist-stimulated phospholipase A2 (PLA2) activity were studied in platelets. Cell-permeable C6-ceramide (N-hexanoylsphingosine) exogenously added to platelet suspension enhanced U46619-stimulated arachidonic acid release and lysophosphatidylcholine production. Treatment of platelets with sphingomyelinase also led to an enhancement of the release. The enhanced arachidonic acid release by exogenous ceramide was completely inhibited by methyl arachidonyl fluorophosphonate, a cytosolic PLA2 inhibitor. However, U46619-stimulated PLA2 activity was not significantly potentiated by ceramide. These results suggest that enrichment of ceramide in membranes causes modification of intermolecular organization, leading to increased susceptibility of substrate phospholipids to PLA2.