Large Supramolecular Structures Research Articles

Aggregation effects in aqueous solutions of Star-polymers by spectroscopic investigations

A detailed spectroscopic analysis performed on aqueous solutions of uncharged Star-Polymers, having a porphyrin core and four poly(ethylene glycol) methoxy terminated arms, at different molecular weight values, by Elastic and Quasi-Elastic Light scattering and Fourier Transform Infrared absorption (FT-IR) has been reported. The aim of the present work is essentially to characterize the structural and dynamical properties of the studied samples by applying different spectroscopic techniques which permit to investigate different features of an unique problem. In particular from the light scattering measurements we showed that the species having higher molecular weight (3600 and 8500 Da), have a very low scattered intensity compatible with the presence of the molecules in the monomer form. At the lowest investigated molecular weight (2100 Da) molecules aggregate to form large supramolecular structures. This tendency to aggregate was confirmed by FT-IR measurements performed in the same water solutions at low frequency (200–700) cm −1. Also the high frequency region, in the OH stretching vibrational range, (2800–3800) cm −1, has been investigated versus the molecular mass of the star-polymer. From the comparison of the IR spectra in the case of star-polymers aqueous solutions with the absorbance spectrum of bulk water, we showed a destructuring effect for the ‘collective’ band centred at about 3200 cm −1, which is related to the OH stretching of tightly bonded water molecules (tetrahedral water).

A green chemistry approach to the synthesis of a crystalline organic inclusion compound

A large channel supramolecular structure composed entirely of organic molecules has been synthesised using a green chemistry approach, combining mechanochemistry and solid-state photodimerisation. Co-crystals of 1,2-bis(4-pyridyl)ethylene (bpe) and benzene-1,2,4,5-tetracarboxylic acid (bta) (stoichiometry bta·2bpe) have been obtained by solid-state grinding of a starting physical mixture. Within the resulting co-crystals, bpe molecules are crystallographically arranged such that, after UV irradiation, photodimerisation of bpe readily occurs to give a product consisting of bta and the photodimer tpcb. A large channel structure is then obtained by grinding this mixture of bta·tpcb in the presence of dimethylsulfoxide.

Solution NMR techniques for large molecular and supramolecular structures.

Transverse relaxation-optimized spectroscopy (TROSY) or generation of heteronuclear multiple quantum coherences during the frequency labeling period and TROSY during the acquisition period have been combined either with cross-correlated relaxation-induced polarization transfer (CRIPT) or cross-correlated relaxation-enhanced polarization transfer (CRINEPT) to obtain two-dimensional (2D) solution NMR correlation spectra of (15)N,(2)H-labeled homo-oligomeric macromolecules with molecular weights from 110 to 800 kDa. With the experimental conditions used, the line widths of the TROSY-components of the (1)H- and (15)N-signals were of the order of 60 Hz at 400 kDa, whereas, for structures of size 800 kDa, the line widths were about 75 Hz for (15)N and 110 Hz for (1)H. This paper describes the experimental schemes used and details of their setup for individual measurements. The performance of NMR experiments with large structures depends critically on the choice of the polarization transfer times, the relaxation delays between subsequent recordings, and the water-handling routines. Optimal transfer times for 2D [(15)N,(1)H]-CRIPT-TROSY experiments in H(2)O solutions were found to be 6 ms for a molecular weight of approximately 200 kDa, 2.8 ms for 400 kDa, and 1.4 ms for 800 kDa. These data validate theoretical predictions of inverse proportionality between optimal transfer time and size of the structure. The proton longitudinal relaxation times in H(2)O solution were found to be of the order of 0.8 s for structure sizes around 200 kDa, 0.4 s at 400 kDa, and 0.3 s at 800 kDa, which enabled the use of recycle times below 1 s. Since improper water handling results in severe signal loss, the water resonance was kept along the z-axis during the entire duration of the experiments by adjusting each water flip-back pulse individually.

Analysis of nuclear protein import and export in vitro using fluorescent cargoes.

AbstractNucleocytoplasmic transport of protein is central to the regulation of many cellular functions. Nuclear import and export takes place through nuclear pore complexes (NPCs; reviewed in refs. 1 and 2), large supramolecular structures that span the nuclear envelope. Cargo proteins contain specific signals for nuclear transport, which are often short stretches of amino acids. Nuclear localization signals (NLSs) direct the nuclear import of proteins, whereas nuclear export sequences (NESs) specify protein export from the nucleus. Both types of signals are recognized by nucleocytoplasmic shuttling receptor proteins belonging to the importin/karyopherin β family (reviewed in refs. 3 and 4). After binding to cargo, these receptors interact with proteins of the NPC to mediate cargo translocation into or out of the nucleus. The best-characterized nuclear-import pathway utilizes the major import-receptor importin β, which mediates the transport of proteins with basic amino-acid-rich NLSs. Importin (3 interacts with cargo molecules either directly (5) or via an adapter protein, importin α (3). Export of many proteins from the nucleus is mediated by the export receptor chromosome region maintenance 1 (CRM1), which recognizes leucine-rich NESs (6-10).KeywordsPermeabilized CellNuclear ExportNuclear ImportTransport ReactionNuclear TransportThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Tetraspanins and Intercellular Interactions

ABSTRACTThe superfamily of tetraspanins comprises a group of polypeptides with four transmembrane domains that form large supramolecular structures in the plasma membrane through their associations to multiple integral membrane proteins. They are involved in homo‐ and heterotypic intercellular interactions in different processes such as hematopoiesis, lymphocyte activation, cancer metastasis, and fertilization. Intercellularly located tetraspanins regulate the juxtacrine activity of growth factors, cell fusion, and myelin formation. On the other hand, in motile cells they relocalize from cell‐cell junctions to actin‐based structures such as filopodia or growth cones and regulate cell motility in wound healing and angiogenesis processes.

Characterization of anticholinesterase-active 3-alkylpyridinium polymers from the marine sponge Reniera sarai in aqueous solutions.

From the marine sponge Reniera sarai 3-alkylpyridinium oligomers and polymers have been isolated. 3-Alkylpyridinium polymers are potent anticholinesterase agents; in addition, they show hemolytic and cytotoxic activities. Oligomers with a molecular weight lower than 3000 Da do not possess any significant activity. We report structural characterization of 3-alkylpyridinium polymers and their behavior in aqueous solutions. We found that biologically active polymers are composed of head-to-tail 3-alkylpyridinium units. According to MALDI-TOF spectrometry two species of polymers exist, the smaller with a molecular weight of 5520 Da and the larger with a molecular weight of 18,900 Da. Both polymers are soluble only in water, while low molecular oligomers are readily soluble in organic solvents. Polymers form large water-dissolved supramolecular structures with an average hydrodynamic radius of 23 +/- 2 nm and, therefore, cannot be separated with size-exclusion chromatography.

Porphyrin aggregation in aqueous solutions: small angle and quasielastic light scattering results

The porphyrin trans-bis( N-methylpyridinium-4-yl)diphenylporphyine ( t-H 2P agg) is known to form in solution large assemblies and supramolecular structures in the presence of polyanions, such as nucleic acids, as a template support. We have studied the aggregation of the unbound porphyrin in 0.1 M NaCl aqueous solutions. The aggregation process was monitored by a time resolved small angle light scattering apparatus. The intensity profile of the scattered light as a function of the exchanged wavevector q, conforms to a scattered law for fractal aggregates in the range 0.2–31.4 μm −1. An analysis of the intensity data reveals a value of the fractal dimension d f equal to 2.5. Additional dynamic light scattering measurements confirm the presence of almost monodispersed aggregates in the mesoscopic range.

On the Definition of Chirality

Should Lord Kelvin's definition of chirality be modified to accommodate concepts associated with the chirality of large random supramolecular structures? Stimulated by a recent article by D. Avnir et al. (Chem. Eur. J. 1996, 2, 174), L. D. Barron discusses this question. In their reply D. Avnir et al. further clarify and discuss their original thesis.

Microtubular protein in its polymerized or nonpolymerized states differentially modulates in vitro and intracellular fluxes catalyzed by enzymes of carbon metabolism

The fluxes through HK/G6PDH and PK/LDH coupled-enzymatic reactions were quantified in the presence of physiological concentrations (1-15 microM) of polymerized or non-polymerized microtubular protein (MTP) from rat brain and in a permeabilized yeast cell system. In vitro enzymatic fluxes were increased by either polymerized or nonpolymerized brain MTP mainly in the lower range of MTP concentration. At fixed MTP concentrations in the flux stimulatory range of HK/G6PDH (1 mg/ml MTP) or PK/LDH (0.4 mg/ml MTP), a hyperbolic and sigmoidal response to NADP and PEP, respectively, was detected. That dependence varied according to the polymeric status of MTP. The specificity of the phenomenon observed in vitro, was tested for the PK/LDH and HK/G6PDH enzymatic couples in the presence of neutral polymers such as glycogen (< or = 10 mg/ml), poly(ethylene glycol) (up to 10% w/w) or G-actin (< or = 1 mg/ml). In permeabilized Saccharomyces cerevisiae cells, the PK-catalyzed flux was sensitive to microtubule disruption by nocodazole (15 micrograms/ml). The HK/G6PDH system was not affected by nocodazole showing values of kinetic parameters close to those obtained in vitro in the presence of polymerized brain MTP. Indirect immunofluorescence with specific antibodies against tubulin allowed to confirm the microtubules disruption in the presence of nocodazole in permeabilized yeast cells under the same conditions in which enzymes were assayed intracellularly. The experimental evidence is in agreement with the observed phenomenon of increase in fluxes in the enzymatic reactions assayed to be specifically induced by MTP either in vitro or in situ. The results presented are discussed in terms of the assembly of large supramolecular structures as a supraregulatory mechanism of synchronization of systemic cellular processes such as metabolic fluxes.

Large supramolecular structures in water-alcohol mixtures evidenced by elastic light scattering

We report the results of different light scattering experiments, both elastic and quasi-elastic, performed as a function of temperature at different concentrations on a water-butoxyethanol (C4E1) mixture. The comparison of the obtained data with the results of SANS gives evidence of the amphiphilic character of the alcohol molecules. In particular, we observe such a property gives rise to a well-defined micellarlike structure. At low temperatures (T≤5°C) we find that the mixture exhibits a surprising behaviour, namely very large structures (of the order of 2000 A) built by an intermicellar aggregation process. In addition when the light scattering data show the presence of such large aggregates, neutron data reveal the persisting presence of micelles of about 20 A. Such a structural picture agrees with the behaviour of the viscosity data.

On the observation of micellar structures by dielectric spectroscopy

Many cationic surfactants form, in the presence of certain organic acids, large supramolecular micellar structures in water. The dielectric response of one such system (cetyl trimethyl ammonium bromide-salicylic acid, CTMAS) has been studied as a function of frequency, concentration and temperature. The results are compared with dynamic mechanical data on the same system, which has been published in the literature.