Selective peptide binding using facially amphiphilic dendrimers

Amphiphilic dendrimers, which contain both hydrophobic and hydrophilic groups in every repeat unit, exhibit environment-dependent assemblies both in hydrophilic solvent, water, and in lipophilic solvent, toluene. Upon investigating the status of these assemblies in a mixture of immiscible solvents, these dendrimers were found to be kinetically trapped in the solvent in which they are initially assembled. This property has been exploited to selectively extract peptides from aqueous solution into an organic phase, where the peptides bind to the interior functionalities of the dendritic inverse micelles. While the corresponding small molecule surfactant does not exhibit any selective binding toward peptides, all dendrons (G1-G3) are capable of this selective binding. We show that the inverse micelle-type assembly itself is crucial for the binding event and that the assembly formed by the G1 dendron has a greater capability for binding compared to the G2 or G3 dendrons. We have also shown that the average apparent pKa of the carboxylic acid functionalities varies with generation, and this could be the reason for the observed differences in binding capacity.     

Hydroxyl-terminated dendritic oligomers from bile acids: synthesis and properties

[reaction: see text] The high reactivity of the chloroacetyl group has been exploited for the synthesis of bile acid based first and second generation dendrons with multiple hydroxyl groups. The synthesis involves only a few steps and avoids the use of protecting groups for the terminal hydroxyl groups. These dendritic structures with facially amphiphilic bile acid backbones on the periphery were able to solubilize cresol red, a hydrophilic dye, in a nonpolar solvent. HPLC analysis of the dendrons suggests that hydrophobicity increases with increase in oligomer size, but in each generation, the dendrons with a higher degree of branching are less hydrophobic.     

Self-immolative dendrimer biodegradability by multi-enzymatic triggering

New self-immolative dendritic molecules have been designed and synthesized. The dendrons are built with a multi-enzymatic triggering mechanism, which initiates their biodegradation through a self-immolative chain fragmentation to release a reporter group from the focal point. The dendritic backbone is constructed from polycarbamate linkages, which are stable to hydrolysis and enhance the dendrons' solubility in water. The degradation can readily take place under physiological conditions on enzymatic triggering.     

Synthesis, optical properties, and LFER analysis of solvent-dependent binding constants of Hamilton-receptor-connected merocyanine chromophores

A merocyanine dye equipped with a Hamilton-receptor unit has been synthesized that enables strong noncovalent binding of other merocyanine dyes bearing barbituric acid acceptor groups by six hydrogen bonds. NMR and UV/vis titration experiments in toluene, chloroform, dichloromethane, dioxane, and THF provide evidence for the formation of 1:1 complexes even in the dipolar solvents. An enhanced binding strength is observed for the more dipolar merocyanine dyes in the head-to-tail assembly structure with binding constants up to >10 (8) M (-1) in toluene. In the present bimolecular complexes two merocyanine chromophores are assembled in a head-to-tail fashion that affords increased dipole moments as demanded for efficient electric field induced poling processes in nonlinear optical and photorefractive polymeric hosts. The solvent dependency of the binding constants for various barbituric acid dye-Hamilton receptor complexes as well as a perylene imide-melamine complex reveals linear free energy relationships (LFER) that allow for an estimation of binding constants larger than 10 (12) M (-1) for Hamilton receptor organized head-to-tail merocyanine bimolecular complexes in aliphatic solvents. It is suggested that such LFER are valuable tools for the estimation of binding constants in solvents where experimental binding constants cannot be determined because of solubility or spectroscopic problems.     

Self-organization of amide dendrons and their dendronized macromolecules

A polymerizable methacryl unit was introduced at the focal moiety of the amide dendrons which have amide branches and alkyl periphery. Their dendronized polymers were also prepared by the radical polymerization of the methacryl units. The self-organization characteristics of dendrons and dendronized polymers were then investigated in both the organic and aqueous phases. The amide dendrons (1M and 2M) in which the focal carboxyl group was blocked with methacryl units did not form gel in organic media such as chloroform or THF, whereas amide dendrons with a free carboxyl group at the focal point form self-organized structures. In the aqueous phase, 1M and 2M formed spherical vesicular assemblies. The dendronized polymers with first and second generation dendrons, 1P and 2P, respectively, exhibited lamellar and columnar organization in toluene. In addition to hydrogen bonding between the dendritic amide branches and van der Waals interactions between the alkyl periphery, steric confinement of dendritic side groups along the polymer backbone played a key role in the packing process of the dendronized polymers. In aqueous phase, 1P and 2P showed spherical vesicular aggregates with persistent stability in the presence of Triton X-100.     

Synthesis of new amphiphilic dendrons bearing aliphatic hydrocarbon surface sectors and a monocarboxylic or dicarboxylic acid focal point

[structure: see text]. A new series of amphiphilic G1-G3 dendrons containing purely aliphatic hydrocarbon dendritic surface sectors and one or two carboxylic acid group(s) at the focal point was synthesized in good yields. The key branching steps involve diallylation reactions of diethyl malonate or Meldrum's acid. These dendrons are highly soluble in hexane despite having highly polar carboxylic acid groups at the focal point.     

Poly(aryl ether) Dendrons with Monopyrrolotetrathiafulvalene Unit‐Based Organogels exhibiting Gel‐Induced Enhanced Emission (GIEE)

A series of poly(aryl ether) dendrons with a monopyrrolo‐tetrathiafulvalene unit linked through an acyl hydrazone linkage were designed and synthesized as low molecular mass organogelators (LMOGs). Two of the dendrons could gelate the aromatic solvents and some solvent mixtures, but the others could not gel all solvents tested except for n‐pentanol. A subtle change on the molecular structure produces a great influence on the gelation behavior. Note that the dendrons could form the stable gel in the DMSO/water mixture without thermal treatment and could also form the binary gel with fullerene (C₆₀) in toluene. The formed gels undergo a reversible gel–sol phase transition upon exposure to external stimuli, such as temperature and chemical oxidation/reduction. A number of experiments (SEM, FTIR spectroscopy, ¹H NMR spectroscopy, and UV/Vis absorption spectroscopy, and XRD) revealed that these dendritic molecules self‐assembled into elastically interpenetrating one‐dimensional fibrillar aggregates and maintain rectangular molecular‐packing mode in organogels. The hydrogen bonding, π–π, and donor–acceptor interactions were found to be the main driving forces for formation of the gels. Moreover, the gel system exhibited gel‐induced enhanced emission (GIEE) property in the visible region in spite of the absence of a conventional fluorophore unit and the fluorescence was effectively quenched by introduction of C₆₀.     

Controlling solubility and modulating peripheral function in dendrimer encapsulated dyes

The synthesis of large dendrons and dendrimers with site-isolated dyes at their core has been explored. The dyes selected for this work were coumarin 343 and pentathiophene, as energy transfer processes prevail when the two dyes are intimately mixed but each should behave independently of the other if site-isolation is achieved. Because the two dyes have very different functional characteristics, a protocol involving orthogonal protecting groups and allowing the use of a single family of electroactive dendrons for their encapsulation had to be developed. The synthetic protocol must balance the need to incorporate electroactive groups at the periphery of the dendrons with the requirement for high solubility and a size sufficient to fully encapsulate the central dye. Because of their poor solubility and tendency to crystallize, dendrons with uniform triarylamine substitution proved unsatisfactory leading to the development of new unsymmetrical dendrons with alternating branched alkyl groups and triarylamine moieties at their periphery. These dendrons, which show excellent solubility and no tendency to crystallize, were assembled into large dendrimers using a modular protocol with the light emitting dye at their core. It is expected that the large size of the dendritic shell will provide effective site-isolation for the encapsulated central dyes enabling them to exhibit their intrinsic emission properties with minimal energy transfer between neighboring core fluorophores when processed in bulk thin films.     

A Systematic Study of Peripherally Multiple Aromatic Ester‐Functionalized Poly(benzyl ether) Dendrons for the Fabrication of Organogels: Structure–Property Relationships and Thixotropic Property

A new class of peripherally multiple aromatic ester‐functionalized poly(benzyl ether) dendrons and/or dendrimers with different focal point substituents, surface groups, interior structures, as well as different generations have been synthesized and their structure–property relationships with respect to their gelation ability have been investigated systematically. Most of these dendrons are able to gel organic solvents over a wide polarity range. Evident dendritic effects were observed not only in gelation capability but also in thermotropic, morphological, and rheological characterizations. It was disclosed that subtle changes in peripheral ester functionalities and interior dendritic structures affected the gelation behavior of the dendrons significantly. Among all the dendrons studied, the second‐ and third‐generation dendrons G₀G₂‐Me and G₀G₃‐Me with dimethyl isophthalates (DMIP) as peripheral groups exhibited the best capability in gelation, and stable gels were formed in more than 22 aromatic and polar organic solvents. The lowest critical gelation concentration (CGC) reached 2.0 mg mL^?1, indicating that approximately 1.35×10⁴ solvent molecules could be entrapped by one dendritic molecule. Further study on driving forces in gel formation was carried out by using a combination of single‐crystal/powder X‐ray diffraction (XRD) analysis and concentration‐dependent (CD)/temperature‐dependent (TD) ¹H NMR spectroscopy. The results obtained from these experiments revealed that the multiple π–π stacking of extended π‐systems due to the peripheral DMIP rings, cooperatively assisted by non‐conventional hydrogen‐bonding, is the key contributor in the formation of the highly ordered supramolecular and fibrillar network. In addition, these dendritic organogels exhibited unexpected thixotropic‐responsive properties, which make them promising candidates with potential applications in the field of intelligent soft materials.     

Dendron-protected Au nanoparticles--effect of dendritic structure on chemical stability

A series of gold nanoparticles stabilised by 'Newkome-type' dendritic branching has been synthesised and fully characterised. In particular, the properties and behaviour of these hybrid materials are compared with those of a previously reported set of nanoparticles stabilised by dendrons constructed using l-lysine building blocks. The rates of cyanide-induced nanoparticle decomposition were determined, and it was found that the rate of decomposition increased on the introduction of dendritic branching. Furthermore, 'Newkome-type' dendrons were significantly more effective at protecting the encapsulated gold nanoparticle than the l-lysine based dendrons. It is proposed that this observation can be explained on the basis of more effective packing and surface coverage by the 'Newkome-type' dendrons. Importantly, this study therefore demonstrates that the organic chemical structure of dendritic ligands plays a crucial role in controlling the reactivity of self-assembled hybrid nanostructures.     

FeCp]+-induced hexafunctionalization of hexamethylbenzene with dendrons for the direct synthesis of redox-active iron-centered metallodendrimers

Phenol tri- and nonaallyl dendrons (3 and 7, respectively) were functionalized at the focal position to give the new triallyl dendrons 4 and 6 and the nonaallyl dendrons 11 and 13 that contain a iodoalkyl or a bromobenzyl termini. All these dendrons were used for the [FeCp]+-induced hexafunctionalization of hexamethylbenzene in [FeCp(eta6-C6Me6)][PF6] (1) under mild conditions in the presence of KOH. These reactions directly yielded the 18-allyl and 54-allyl dendrimers 9, 10, and 14 with a [FeCp(eta6-arene)]+ unit located at the dendrimer core. Cyclic voltammetry studies were recorded in THF and DMF with these metallodendrimers and compared with those of analogous dendrimers or complexes of smaller size that contain a [FeCp(eta6-arene)]+ unit at the core. The decreased rate of heterogeneous electron transfer when the dendritic size increases first disclosed by Diederich and Gross is confirmed. The variation of the redox potential of the Fe(II/I) redox system with increasing dendritic size is negligible even in a solvent of high dielectric constant such as DMF. This trend is attributed to fact that the involved "redox" orbital is buried on the metal center, well protected by the shell of alkyl chains (electron-reservoir nature), unlike in ferrocene. The chemical irreversibility increases in THF as the dendrimer size increases, due to more facile ligand substitution with THF at the 19-electron level when the chain bulk increases.     

Studies of solvation of ketocyanine dyes in homogeneous and heterogeneous media by UV/Vis spectroscopic method

Solvation characteristics of ketocyanine dyes (I-VI) have been investigated in pure solvents and heterogeneous media by absorption and fluorescence studies. The dyes are good reporters of solvent polarity. In protic solvents they exist as equilibrium mixtures of bare and hydrogen-bonded form in the ground state (S0), the latter being the emitting species. In aprotic solvents of low polarity association of the S1 state of the dye takes place. In aqueous micellar media the dye resides at the micelle water interface. The binding constant for dye-micelle interaction has been determined. Fluorescence data in beta-cyclodextrine solution resemble that for that neutral micellar solution indicating that the interaction between the -OH group of the heterogeneous part (micelle/cyclodextrine cavity) and the carbonyl oxygen of the dye is important in both the cases.     

Synthesis of Polyisoprene Terpenoid Dendrons and their Applications in Oligo(phenylene ethynylene)s as “Shells”

The novel double‐stage convergent synthesis of a new class of polyisoprene terpenoid (PIPTP) dendrons is described. PIPTP dendrons bear a highly branched aliphatic hydrocarbon skeleton and a hydrophilic hydroxy focal point functionality. These dendrons have the specific formula C_(5×2^G+1_‐5)H_(5×2^G+2_‐8)O, and each dendritic layer is constructed from an isoprene unit. The key branching steps involve a double alkyl‐metal addition to an ester functionality, followed by deoxygenation of the resulting tertiary alcohol by triethylsilane and trifluoroacetic acid, then hydrogenation or hydrogenolysis. The dendrons were also attached to oligo(phenylene ethynylene)s (OPEs) so as to function as protective shells to allow fine tuning of the nanoscopic environment around the OPE moiety, and to exert precise control of the packing density and intermolecular interaction between the OPE cores. Fluorescence quantum yield data reveal that the OPE core is better encapsulated by the PIPTP dendrons than by Fréchet dendrons.     

The Effect of Incorporating Fréchet Dendrons into Rotaxanes and Molecular Shuttles Containing the 1,2‐Bis(pyridinium)ethane⊂[24]Crown‐8 Templating Motif

Fréchet‐type dendrons (G0–G3) were added as both axle stoppering units and cyclic wheel appendages in a series of [2]rotaxanes, [3]rotaxanes, and molecular shuttles that employ 1,2‐bis(pyridinium)ethane axles and 24‐membered crown ethers wheels. The addition of dendrimer wedges as stoppering units dramatically increased the solubility of simple [2]rotaxanes in nonpolar solvents. The X‐ray structure of a G1‐stoppered [2]rotaxane shows how the dendritic units affect the structure of the interlocked components. Increased solubility allows observation of how the interaction of dendritic units on separate components in interlocked molecules influences switching properties and molecular size. In a series of [2]rotaxane molecular shuttles incorporating two recognition sites, it was demonstrated that an increase in generation on either the stoppering unit or cyclic wheel could influence both the rate of shuttling and the site preference of the wheel on the axle.     

Convergent synthesis of noncovalent metallodendrimers containing hydrophobic dendrons at the periphery.

The noncovalent synthesis of "layer-block" metallodendrimers containing hydrophobic shells of covalent dendritic wedges at the periphery is described. Starting from first- and second-generation Fréchet wedges having phosphines at their focal point, convergent dendritic growth yields third- and fourth-generation metallodendrimers in which the coordination of nitriles, pyridines, and phosphines to SCS Pd(II) pincers is used as the assembly motif. In this convergent growth, the number of terminal hydrophobic phosphine wedges increases with generation. The solubility of the dendritic structures in apolar organic solvents such as chloroform and dichloromethane increases accordingly, in contrast to previously reported metallodendrimers. All dendritic structures were characterized by (1)H and (31)P NMR spectroscopy, elemental analysis, and MALDI-TOF mass spectrometry.     

Synthesis of novel dendritic 2,2'-bipyridine ligands and their application to Lewis acid-catalyzed diels-alder and three-component condensation reactions

A series of dendritic ligands with a 2,2'-bipyridine core was synthesized through the coupling of 4,4'-dihydroxy-2,2'-bipyridine with poly(aryl ether) dendrons. The corresponding dendritic Cu(OTf)2 catalysts were used for Diels-Alder and three-component condensation reactions. The dendritic Cu(OTf)2-catalyzed Diels-Alder reaction proceeded smoothly, and these dendritic catalysts could be recycled without deactivation by reprecipitation. Three-component condensation reactions such as Mannich-type reactions also proceeded not only in dichloromethane but also in water. Furthermore, a positive dendritic effect on chemical yields was observed in both Diels-Alder reactions and aqueous-media three-component condensation reactions.     

对甲苯磺酰氯对聚酰胺胺树枝形聚合物的端基修饰与表征

用对甲苯磺酰氯对聚酰胺胺(PAMAM)树枝形聚合物进行了端基修饰,以第5代为例采用元素分析,IR和^1H NMR等方法对产物进行了表征,结果表明：以83％的较高产率得到了预期的产品,产品的官能化程度达到99％。新的壳层使其有不同于PAMAM的溶解性,显示出在含芳基有机溶剂感应方面的潜在应用。     

Studies of solvation in homogeneous and heterogeneous media by electronic spectroscopic method

Solvation characteristics in homogenous (pure and mixed binary solvents) and heterogeneous media (aqueous micelles, beta- and gamma-cyclodextrine solutions) have been studied by monitoring the emission characteristics of a newly synthesised dye. The longest wavelength absorption and emission band of the dye arise due to transition between S(0) and S(1) state. The maximum energy of electronic transition involving intramolecular charge transfer is found to be dependent on both the hydrogen-bond donating ability and the polarity-polarisability in pure solvent. The dipole moment in the S(1) state, as determined by solvatochromic procedure, agrees well with the value obtained by theoretical calculation at the AM1 level. Preferential solvation of the dye by alcohols has been found to occur in ethanol+water, propan-1-ol+water, propan-2-ol+water binary mixtures. In aqueous micellar media the dye molecule is located at the water-micelle interface. The binding constant for the dye-micelle interaction has also been determined. The results have been compared with those for a structurally related symmetrical ketocyanine dye.     

The metathetic degradation of polyisoprene and polybutadiene in block copolymers using Grubbs second generation catalyst

A degradation study of polystyrene-polybutadiene-polystyrene and polyisoprene-polystyrene-polyisoprene in both dichloromethane and hexane solvents is presented. Alternative solvents for metathetic degradation provide the potential for greener chemistry, better selectivity, and control over the products. The catalyst concentration and solvent selection both determine the products formed. The degradation of polyisoprene and polybutadiene in a particular solvent was controlled by the solubility of polyisoprene/polybutadiene, and by its solubility relative to polystyrene. A large difference in solubility between the polymers in the selected solvent provides an additional driving force for block separation, encouraging reaction close to the interface between different blocks. Furthermore, solubility of the block copolymer speeds the degradation reaction. This tailoring of the reaction mechanism yields a new control over the products of polymer degradation.     

Polarity effects on the photophysics of dendrimers with an oligophenylenevinylene core and peripheral fullerene units

Highly soluble dendritic branches with fullerene subunits at the periphery and a carboxylic acid function at the focal point have been prepared by a convergent approach. They have been attached to an oligophenylenevinylene (OPV) core bearing two alcohol functions to yield dendrimers with two, four or eight peripheral C60 groups. Their photophysical properties have been systematically investigated in solvents of increasing polarity; that is, toluene, dichloromethane, and benzonitrile. Ultrafast OPV-->C60 singlet energy transfer takes place for the whole series of dendrimers, whatever the solvent. Electron transfer from the fullerene singlet is thermodynamically allowed in CH2Cl2 and benzonitrile, but not in apolar toluene. For a given solvent, the extent of electron transfer, signaled by the quenching of the fullerene fluorescence, is not the same along the series, despite the fact that identical electron transfer partners are present. By increasing the dendrimer size, electron transfer is progressively more difficult due to isolation of the central OPV core by the dendritic branches, which hampers solvent induced stabilization of charge separated couples. Compact structures of the hydrophobic dendrimers are favored in solvents of higher polarity. These structural effects are also able to rationalize the unexpected trends in singlet oxygen sensitization yields.