Synthesis, characterization, and pH-triggered dethreading of alpha-cyclodextrin-poly(ethylene glycol) polyrotaxanes bearing cleavable endcaps

The synthesis, characterization, and degradation kinetics of three alpha-cyclodextrin (alpha-CD)-poly(ethylene glycol) (PEG) polyrotaxanes with endcaps that were installed using Cu(I)-catalyzed Huisgen cyclization is reported. PEG1500, azidated with azidoacetic acid, was threaded with alpha-CD to form a pseudopolyrotaxane that was then capped in up to 82% yield with three different substituents to provide polyrotaxanes that were either acid-, base-, or fluoride-sensitive. NMR, GPC, XRD, and AFM methods were used to characterize the polyrotaxanes. Dethreading rates upon exposure to mild deprotection conditions were monitored by turbidity analysis. The vinyl ether-endcapped polyrotaxane is stable at pH 7 for 16 h but is solubilized at approximately 0.0211 min(-1) at pH 4. The ester-endcapped polyrotaxane is solubilized at 0.0122 min(-1) at pH 12.1. Our results show that pH-triggerable polyrotaxanes can be readily and efficiently prepared from pseudopolyrotaxanes in high yield by Huisgen cyclization of azido- and alkynyl-modified precursors in the presence of Cu(I).     

Nuclear aggregates of polyamines

Nuclear aggregates of polyamines (NAPs) are cyclic supramolecular compounds made of polyamines and phosphate groups. Three different aggregates, s-NAP, m-NAP and l-NAP, with a molecular weight of 1035, 5175 and 9552 Da, respectively, are described. These molecules interact with genomic DNA. In consequence of this interaction, NAPs not only protect DNA from nucleases with extraordinarily greater efficiency than single polyamines (spermine, spermidine and putrescine), but also induce noticeable changes in DNA condensation status, as shown by temperature-dependent modifications of DNA electrophoretic properties. The biochemical characterization of these compounds has allowed the definition of a structural model for each NAP. According to this model, five s-NAPs assemble together to form a m-NAP unit. We hypothesize that the complexation of s-NAP into m-NAP favours the transition to Z-DNA through the progressive widening of DNA strands and the exposure of bases. We propose that NAPs, by wrapping the DNA helixes, form supramolecular tunnel-like structures that confer efficient protection without affecting DNA elasticity.     

Cyclodextrins and their uses: a review

Cyclodextrins are a family of cyclic oligosaccharides composed of α-(1,4) linked glucopyranose subunits. Cyclodextrins are useful molecular chelating agents. They possess a cage-like supramolecular structure, which is the same as the structures formed from cryptands, calixarenes, cyclophanes, spherands and crown ethers. These compounds having supramolecular structures carry out chemical reactions that involve intramolecular interactions where covalent bonds are not formed between interacting molecules, ions or radicals. The majority of all these reactions are of ‘host–guest’ type. Compared to all the supramolecular hosts mentioned above, cyclodextrins are most important. Because of their inclusion complex forming capability, the properties of the materials with which they complex can be modified significantly. As a result of molecular complexation phenomena CDs are widely used in many industrial products, technologies and analytical methods. The negligible cytotoxic effects of CDs are an important attribute in applications such as rug carrier, food and flavours, cosmetics, packing, textiles, separation processes, environment protection, fermentation and catalysis.     

X-ray scattering combined with coordinate-based analyses for applications in natural and artificial photosynthesis

Advances in X-ray light sources and detectors have created opportunities for advancing our understanding of structure and structural dynamics for supramolecular assemblies in solution by combining X-ray scattering measurement with coordinate-based modeling methods. In this review the foundations for X-ray scattering are discussed and illustrated with selected examples demonstrating the ability to correlate solution X-ray scattering measurements to molecular structure, conformation, and dynamics. These approaches are anticipated to have a broad range of applications in natural and artificial photosynthesis by offering possibilities for structure resolution for dynamic supramolecular assemblies in solution that can not be fully addressed with crystallographic techniques, and for resolving fundamental mechanisms for solar energy conversion by mapping out structure in light-excited reaction states.     

Hofmeister effects in supramolecular and biological systems

Specific ion effects, representative of near-universal Hofmeister phenomena, are illustrated in three different systems. These are the formation of supramolecular assemblies from cyclodextrins, the optical rotation of L-serine, and the growth rate of two kinds of microorganisms (Staphylococcus aureus and Pseudomonas aeruginosa). The strong specific ion effects can be correlated with the anion polarizabilities and related physico-chemical parameters. The results show the relevance of dispersion (non-electrostatic) forces in these phenomena.     

Synthesis and characterization of polyrotaxane-based polymeric continuous beds for capillary electrochromatography

The continuous bed technique with its attractive features, such as fritless design, one-step in situ synthesis, low back pressure and no need for pressurising the electrode vessels to suppress bubble formation was applied to form polyrotaxane-based stationary phases for capillary electrochromatography (CEC). Rotaxanes are synthesized from two classes of substances, namely linear reactive monomers and inert cyclic compounds. Upon polymerisation, a gel forms with the cyclic molecules mechanically immobilized (see Fig. 1). We have employed this simple approach, using charged derivatives of cyclodextrins in order to introduce charged groups into continuous beds and thus render them appropriate for electrochromatography. The self-assembly of supramolecular structures to form rotaxanes during the synthesis of the continuous beds is treated. The electroosmotic and chromatographic properties of the various polyrotaxane-based stationary phases synthesized are discussed, as well as the synthesis of the continuous beds, including how to affect their porosity and its influence on the efficiency of the electrokinetic separation. The applicability of the rotaxane-based continuous bed is demonstrated by separation of model compounds by reversed- and normal-phase chromatography. A separation of enantiomers is also presented. This experiment is of particular interest because it indicates that the interaction with the cavity of beta-cyclodextrin (beta-CD) is not a fundamental requirement for enantioseparations.     

Inhibitory effect of supramolecular polyrotaxane-dipeptide conjugates on digested peptide uptake via intestinal human peptide transporter

The effect of polyrotaxane-dipeptide (Val-Lys) conjugates on the uptake of a model dipeptide (Gly-Sar) was examined via human peptide transporter (hPEPT1) on HeLa cells. Here, Val-Lys groups are introduced to alpha-CDs, which are threaded onto a poly(ethylene oxide) chain capped with bulky end-groups (polyrotaxane). The Gly-Sar uptake via hPEPT1 was significantly inhibited in the polyrotaxane conjugates, and this inhibitory effect was not explained by the sum of interaction between hPEPT1 and alpha-CD-Val-Lys conjugates. Further, the inhibition was significantly greater than those observed in dextran-Val-Lys conjugates. Therefore, our data clearly suggests that supramolecular structure in the polyrotaxane conjugates contributes considerably to the inhibitory effect via multivalent binding of Val-Lys groups with hPEPT1.     

Synthesis and characterization of amphiphilic per-(6-thio-2,3-trimethylsilyl)cyclodextrin: Application to Langmuir film formation

New amphiphilic cyclodextrins are synthesized by selective modification. Two synthetic strategies are investigated. Per-(6-thio-2,3-trimethylsilyl)cyclodextrins can only be obtain by synthesizing first the per-6-thio-cyclodextrin and then silylating the remaining hydroxy functions. Characterizations of the products were achieved by ESI-MS and intramolecular coupling between thiol functions was evidenced. These new compounds are shown to be amphiphilic, and form stable Langmuir–Blodgett films that can be transferred on the upstroke and the downstroke of a substrate with a transfer ratio close to unity.     

Asymmetric induction under confinement

Confinement may efficiently condition the stereochemical outcome of a reaction through space constriction and molecular close contact. This article briefly reviews recent approaches of supramolecular chemistry to achieve chiral confinement. Crystallization is not always possible and the use of chiral crystals or clathrates lacks generality. The construction of solid supramolecular assemblies circumvents some of the problems of the crystal chemistry. In this regard, molecular imprinting of polymeric matrices with orifices mimicking the transition state of an enantioselective process is a very young, promising technique. Zeolites provide porous, rigid environments to host molecules without the need of lucky crystallizations, yet zeolites are not chiral per se and must be chirally modified. Besides, the limited dimension of their pores restricts the size of the guest molecules. Despite these problems, useful asymmetric photochemical reactions have been performed on zeolites. Finally, the formation of pillared lamellar structures, from inorganic salts of tetravalent transition metals covalently grafted with organic chains, is considered. The adequate selection of functionality and chirality of the organic pillars would afford custom-made, highly porous, 3D hybrid organo-inorganic scaffolds. However, the production of asymmetric processes within these layered materials still remains to be seen.     

The formation of a novel supramolecular structure by amyloid of poly-L-glutamic acid

Polyglutamic acid (PE) has been shown to form amyloid fibrils in vitro under pH value of 4.0. However, under the pH of 2.0, a further self-association process resulting in a novel supramolecular structure was observed. These supramolecular assemblies had diameters ranging from 1 to 20 μm and lengths up to several hundred microns, which were significantly larger than those of typical “amyloid fibrils”. The existence of amyloid-like structure within these assemblies was confirmed with Fourier transform infrared spectroscopy and Thioflavin T fluorescence assay. The aggregation process of PE was studied by direct observation of electronic microscopy. The supramolecular assemblies appeared to be formed in a hierarchical process in which the preformed amyloid-like subunits self-assembled into higher-order assemblies in a well-organized pattern.     

Differential effects of alpha-, beta- and gamma-cyclodextrins on human erythrocytes

Alpha-, beta- and gamma-cyclodextrins are cyclic hexamers, heptamers, and octamers of glucose, respectively, and thus are hydrophilic; nevertheless, they have the ability to solubilize lipids through the formation of molecular inclusion complexes. The volume of lipophilic space involved in the solubilization process increases with the number of glucose units in the cyclodextrin molecule and, consequently, cyclodextrins were found to have different effects on human erythrocytes: (a) in the induction of shape change from discocyte to spherocyte the potency was observed to be alpha greater than gamma, but with beta-cyclodextrin hemolysis occurred before the change was complete; (b) in the increase of fluorescence intensity of 1-anilinonaphthalene-8-sulfonate in cyclodextrin-pretreated membranes, the observed potency was beta much greater than gamma greater than alpha; (c) in the release of potassium and hemoglobin, the potency was beta greater than alpha greater than gamma. The potencies of cyclodextrin for solubilizing various components of erythrocytes were alpha greater than beta much greater than gamma for phospholipids, beta much greater than gamma greater than alpha for cholesterol and beta much greater than gamma greater than alpha for proteins. The solubilization potencies were derived from concentration/final-effect curves. The above processes occurred without entry of solubilizer into the membrane, since (a) beta-[14C]cyclodextrin did not bind to erythrocytes and (b) cyclodextrins did not enter the cholesterol monolayer. A study of the [3H]cholesterol in erythrocytes indicated that beta-cyclodextrin extracted this lipid from membrane into a new compartment located in the aqueous phase which could equilibrate rapidly with additional erythrocytes. Therefore, the effects of cyclodextrins differ from those of detergents which first incorporate themselves into membranes then extract membrane components into supramolecular micelles.     

Cyclic assemblies formed by metal ions, pyrimidines and isogeometrical heterocycles: DNA binding properties and antitumour activity

In this contribution, we discuss the use of simple pyrimidine derivatives and geometrically related heterocycles in combination with protected metal ions to build cyclic polynuclear coordination assemblies. The resulting assemblies range from conformationally rigid triangles to corrugated octagons with positively charged cavities and surfaces that can interact with anionic species in a supramolecular fashion. Special attention has been focussed to the supramolecular interaction of these systems towards mononucleotides and DNA as a possible novel interaction of DNA binding metallo-drugs. The results show that these systems efficiently interact with both mononucleotides and ds-DNA. The molecular recognition of ds-DNA leads to significant conformational changes, however, this interaction gives only rise to moderate cytotoxic effects towards tumour cell lines.     

Polyamine-stimulated phosphorylation of prostatic spermine-binding protein is mediated only by cyclic AMP-independent protein kinases.

Spermine-binding protein (a rat ventral prostatic protein with high affinity for spermine) was phosphorylated in situ through the action of intrinsic cellular protein kinase(s), suggesting it to be a phosphoprotein in vivo. The purified protein served as a substrate in a number of cyclic AMP-independent protein kinase reactions in vitro, but not for cyclic AMP-dependent, Ca2+ + calmodulin-dependent or Ca2+ + phospholipid-dependent protein kinases. Available data indicate that at least one of the cyclic AMP-independent protein kinases (cytosolic protein kinase C2) may be physiologically relevant in mediating the phosphorylation of this protein. The phosphorylation reaction was stimulated several-fold in the presence of spermine. Spermidine was somewhat less effective, whereas putrescine, cadaverine and 1,6-hexanediamine were minimally active. Phospho amino acid analysis of 32P-labelled spermine-binding protein indicated that phosphoserine was the only labelled phospho amino acid. Spermine-binding protein did not undergo autophosphorylation, or modify the stimulative effect of spermine on the phosphorylation of other substrates such as non-histone proteins. In situ the phosphorylation of spermine-binding protein in tissue from castrated rats was markedly diminished as compared with the normal. Since the phosphorylation of spermine-binding protein appears to be mediated by cyclic AMP-independent protein kinase(s) whose activity in the prostate is under androgenic control, it is suggested that androgen-dependent modulation of the protein kinase(s) exerts a regulatory control (via phosphorylation-dephosphorylation) on the spermine-binding activity and stability of this protein in vivo. Further, since this protein is a substrate for only the cyclic AMP-independent protein kinases, it could serve as a tool for the investigation of such kinases.     

Nuclear pore complex assembly through the cell cycle: regulation and membrane organization

In eukaryotes, all macromolecules traffic between the nucleus and the cytoplasm through nuclear pore complexes (NPCs), which are among the largest supramolecular assemblies in cells. Although their composition in yeast and metazoa is well characterized, understanding how NPCs are assembled and form the pore through the double membrane of the nuclear envelope and how both processes are controlled still remains a challenge. Here, we summarize what is known about the biogenesis of NPCs throughout the cell cycle with special focus on the membrane reorganization and the regulation that go along with NPC assembly.     

Vascular prostheses with controlled release of antibiotics Part 1: Surface modification with cyclodextrins of PET prostheses

Vascular prostheses were functionalised with the aim to obtain a slow release of antibiotics in order to reduce postoperative infections. The original process that we present in this paper is based on the use of a family of cage molecules named cyclodextrins (CD). These compounds have the ability to form reversible inclusion complexes with drugs such as antibiotics. The aim of this work was to graft CD onto the prosthesis, so that an antibiotic can be bound on it by this inclusion phenomenon, and then be progressively released over a prolonged period by a complex dissociation mechanism. This paper presents the first part of this research program and concerns mainly the study of the functionalization parameters. It presents surface characterization results of the modified prostheses. The PET prostheses were immersed into a solution containing a cross linking agent, cyclodextrins (beta-CD, gamma-CD, HP-beta-CD and HP-gamma-CD) and a catalyst and were padded. Grafting occurred by the mean of a thermofixation step at a temperature comprised between 140 and 180 degrees C. It was observed that the support was permanently modified when the CD polymer that coated the fibres resisted to the final washing process. Grafting rates of 12 wt% in CD polymer could be reached. It was also observed that the fibre coating reaction induced an increase of the permeability of the grafts.     

Review: cyclodextrins and their interaction with amylolytic enzymes

This review is concerned with inhibition of amylases by cyclodextrins (cyclic maltooligosaccharides), the interaction that occurs between amylases and cyclodextrins and the application of cyclodextrin affinity chromatography in the purification of amylases. In many cases, amylases that are competitively inhibited by cyclodextrins can be purified by cyclodextrin affinity chromatography with the cyclodextrins interacting with the active site on such enzymes. Interestingly amylases that are not competitively inhibited by cyclodextrins may also be purified by cyclodextrin affinity chromatography. Therefore, cyclodextrin affinity chromatography can function in the purification of such amylolytic enzymes with the interaction occurring at a site removed from the active site. In such cases it appears that the cyclodextrin is interacting with an affinity site or binding site that is present on some amylolytic enzymes. It seems that certain similarities occur among the binding sites of such enzymes. Literature concerning amylases, and their subsequent purification using cyclodextrin affinity chromatography is reviewed and the fundamental basis of the interaction of the cyclodextrin with amylolytic enzymes is discussed here.     

Sequestration of cellular native factors by biomolecular assemblies: Physiological or pathological?

In addition to native-state structures, biomolecules often form condensed supramolecular assemblies or cellular membraneless organelles that are critical for cell life. These biomolecular assemblies, generally including liquid-like droplets (condensates) and amyloid-like aggregates, can sequester or recruit their interacting partners, so as to either modulate various cellular behaviors or even cause disorders. This review article summarizes recent advances in the sequestration of native factors by biomolecular assemblies and discusses their potential consequences on cellular function, homeostasis, and disease pathology.     

The structure and protein binding of amyloid-specific dye reagents

The self-assembling tendency and protein complexation capability of dyes related to Congo red and also some dyes of different structure were compared to explain the mechanism of Congo red binding and the reason for its specific affinity for beta-structure. Complexation with proteins was measured directly and expressed as the number of dye molecules bound to heat-aggregated IgG and to two light chains with different structural stability. Binding of dyes to rabbit antibodies was measured indirectly as the enhancement effect of the dye on immune complex formation. Self-assembling was tested using dynamic light scattering to measure the size of the supramolecular assemblies. In general the results show that the supramolecular form of a dye is the main factor determining its complexation capability. Dyes that in their compact supramolecular organization are ribbon-shaped may adhere to polypeptides of beta-conformation due to the architectural compatibility in this unique structural form. The optimal fit in complexation seems to depend on two contradictory factors involving, on the one hand, the compactness of the non-covalently stabilized supramolecular ligand, and the dynamic character producing its plasticity on the other. As a result, the highest protein binding capability is shown by dyes with a moderate self-assembling tendency, while those arranging into either very rigid or very unstable supramolecular entities are less able to bind.     

The thermal structural transition of α-crystallin inhibits the heat induced self-aggregation

α-crystallin, the major constituent of human lens, is a member of the heat-shock proteins family and it is known to have a quaternary structural transition at . The presence of calcium ions and/or temperature changes induce supramolecular self-aggregation, a process of relevance in the cataractogenesis. Here we investigate the potential effect of the bovine α-crystallin's structural transition on the self-aggregation process. Along all the temperatures investigated, aggregation proceeds by forming intermediate molecular assemblies that successively aggregate in clusters. The final morphology of the aggregates, above and below Tc, is similar, but the aggregation kinetics are completely different. The size of the intermediate molecular assemblies, and their repulsive energy barrier show a marked increase while crossing . Our results highlight the key role of heat modified form of α-crystallin in protecting from aggregation and preserving the transparency of the lens under hyperthermic conditions.     

Current viewpoint on structure and on evolution of collagens. II. Fibril-associated collagens

Fibril-associated collagens (FACITs) form one of subfamilies included in family of collagens. Being minor components of connective tissue of multicellular animals, FACITs play an important role in structurization of extracellular matrix whose peculiarities determine essential intertissue differences. FACITs participate in regulation of sizes of banded collagen fibrils as well as are connecting links between various components extracellular matrix and cells in different tissues. Functional characteristics of FACIT molecules are determined by peculiarities of structural organization of their α-chains (breakdowns in collagenous domains and module structure of N-terminal noncollagenous sites), trimeric molecules (domains of trimerization) and supramolecular assemblies (mainly association with banded collagen fibrils and the inability to form homopolymeric supramolecular aggregates). The problem of evolution of this group of collagen molecules is also discussed. A hypothetical model of structural changes leading to formation of the FACIT subfamily is proposed.