Dendritic Architectures Research Articles

Polymers in Biomedicine and Electronics

The versatility of polymeric materials originates from the broad variety in their chemical structure and in their hierarchical organization as well as from the possibility to realize a wide spectrum of functions. Often such functions result from a combination ofamoleculararchitecture/morphology with a physical process and therefore require integration in a suitable system (e.g. medical device, electric circuit) or environment (e.g. physiological environment). Frequently, research in the area of functional polymers is motivated by applications. This special issue focuses on two application areas, in which polymer research as well as product development are progressing rapidly: Biomedicine and Electronics. Here (multi)functional polymers are an enabling technology and in this way create substantial and sustainable value. The significance of this topic is also highlighted at the conference ‘‘Polymers in Biomedicine and Electronics’’, which takes place from Oct 3 to 5, 2010 in Berlin combining the biannual meetings of the Gesellschaft Deutscher Chemiker (GDCh) Division ‘‘Macromolecular Chemistry’’ and of the Berlin-Brandenburgischer Verband fur Polymerforschung (BVP). Topics of the conference cover the design and the synthesis of functional polymers, physical principles underlying the function, the characterization of such functionswithin a system as well as product-oriented industrial research. Smart biomaterials, biocompatible coatings and their applications in controlled drug delivery systems or in regenerative therapies will be discussed in the biomedicine-oriented sessions of the conference. Photovoltaic, charge transport, light emission aswell as sensors andactuatorswill be subjects in the area of electronics.

One Rule to Grow Them All: A General Theory of Neuronal Branching and Its Practical Application

Understanding the principles governing axonal and dendritic branching is essential for unravelling the functionality of single neurons and the way in which they connect. Nevertheless, no formalism has yet been described which can capture the general features of neuronal branching. Here we propose such a formalism, which is derived from the expression of dendritic arborizations as locally optimized graphs. Inspired by Ramón y Cajal's laws of conservation of cytoplasm and conduction time in neural circuitry, we show that this graphical representation can be used to optimize these variables. This approach allows us to generate synthetic branching geometries which replicate morphological features of any tested neuron. The essential structure of a neuronal tree is thereby captured by the density profile of its spanning field and by a single parameter, a balancing factor weighing the costs for material and conduction time. This balancing factor determines a neuron's electrotonic compartmentalization. Additions to this rule, when required in the construction process, can be directly attributed to developmental processes or a neuron's computational role within its neural circuit. The simulations presented here are implemented in an open-source software package, the “TREES toolbox,” which provides a general set of tools for analyzing, manipulating, and generating dendritic structure, including a tool to create synthetic members of any particular cell group and an approach for a model-based supervised automatic morphological reconstruction from fluorescent image stacks. These approaches provide new insights into the constraints governing dendritic architectures. They also provide a novel framework for modelling and analyzing neuronal branching structures and for constructing realistic synthetic neural networks.

Optimization of neuronal morphologies for pattern recognition

Previous studies have shown that the morphology of a neuron can affect its firing pattern [1,2]. Specifically, some neuronal morphologies tend to favour bursting, where short sequences of spikes are interspersed with pauses in firing [1,2]. This type of bursting behaviour has been observed in cerebellar Purkinje cells (PCs), and previous work on associative memory in PCs has shown that the generation of burst-pause sequences can be important for information storage in the cerebellum [3]. These results have implications for the coding of information in the brain, but they are specific to one particular neuron with a highly specialised morphology. In this study we therefore use a general approach to optimise generic neuronal structures for pattern recognition, while analysing how their morphology influences their firing pattern. To study how the ability of a neuron to perform pattern recognition depends on morphology, we have built a genomic representation of neuronal models, focusing as a first objective on optimising dendritic architectures. The optimization process uses an evolutionary algorithm and involves four steps. Firstly, genotypes are generated, which specify binary tree structures [4]. Secondly, the genotype is expressed as a model neuron phenotype, in which the branching pattern is derived from the genotype, and which is then converted to a multi-compartmental model written in NEURON simulation code. Thirdly, the fitness values are assessed by evaluating the pattern recognition performance. Finally, genetic variation is introduced, using a process where the genes are modified by crossover and mutation operators. Unlike previous work that focussed on generating a subset of realistic neuronal morphologies for specific computational tasks [5], our representation ensures that the algorithm can generate the set of all possible morphologies for a specific number of terminal branches. The fitness function evaluates pattern recognition performance as described previously [3,6], by storing a number of input patterns based on changing synaptic weights and quantifying the ability of the model to distinguish the set of stored patterns from a set of novel patterns. The discrimination between stored and novel patterns is evaluated for different features of the spike response and quantified by calculating a signal-to-noise ratio. The evolved artificial neuronal morphologies are compared with reconstructed morphologies from real neurons. An extension of the work involves optimizing other neuronal features such as types and distributions of ion channels and the spatial structure of inputs in patterns.

Supramolecular Aggregates of Water Soluble Dendritic Polyglycerol Architectures for the Solubilization of Hydrophobic Compounds

Dendritic core-shell architectures which are based on hyperbranched polyglycerol for the solubilization of hydrophobic drugs have been synthesized and characterized. The core of hyperbranched polyglycerol has been modified with hydrophobic biphenyl groups or perfluorinated chains to increase the core hydrophobicity of the macromolecules. These amphiphilic core-shell type architectures were then used to solubilize pyrene, nile red, and a perfluoro tagged diazo dye, as well as the drug nimodipine in water. Specific host-guest interactions such as fluorous-fluorous interactions could be tailored by this flexible core design and determined by UV spectroscopy. The transport capacity increased 450-fold for nile red, 47-fold for nimodipine, and 37-fold for pyrene at a polymer concentration of only 0.1 wt.-%. Surface tension measurements and scanning force microscopy (SFM) were used to reveal the aggregation properties of these complexes. The formation of supramolecular aggregates with diameters of ≈20 nm and critical aggregate concentrations of 2 × 10(-6) mol · L(-1) have been observed. This indicates the controlled self-assembly of the presented amphiphilic dendritic core-shell type architectures.

A Facile Synthesis of Novel Pyrrolidine Dendrimers by Terminal Group Modification through 1,3-Dipolar Cycloaddition Reaction

Abstract: A series of novel dendrimers with pyrrolidine as surfacegroup has been achieved in high yields by terminal-group modifica-tion on chalcone dendrimers through the application of 1,3-dipolarcycloaddition reaction. In 1,3-dipolar cycloaddition, azomethineylides (AMY) with alkenes afforded five-membered N -methyl pyr-rolidine. The formation of N -methyl pyrrolidine in the dendriticstructure would be an important candidate in the field of biologyand pharmacology. Key words: dendrimer, pyrrolidine, chalcone, 1,3-dipolar cycload-dition Design and synthesis of dendritic architectures have beenactive areas of research because of their countless applica-tions in the field of biology and material sciences. 1–4 Den-drimers are highly branched, regularly orderedmonodisperse three-dimensional architecture. The combi-nation of discrete numbers of functionalities and high lo-cal densities of active groups in dendrimers has attracteda lot of attention from those active in medicinal chemis-try.

Chirally Modified Platinum Nanoparticles Stabilized by Dendritic Core-Multishell Architectures for the Asymmetric Hydrogenation of Ethyl Pyruvate

In this paper we present the asymmetric hydrogenation of α-keto esters with platinum nanoparticles homogeneously stabilized in dendritic core-multishell architectures. The main focus lies on recycling and metal leaching, because little is reported so far about these aspects. It is shown that the stabilizing polymer allows for the efficient modification of the Pt surface with the chiral alkaloid cinchonidine, thereby inducing enantioselectivity and enhancing the reaction rate in the asymmetric hydrogenation of ethyl pyruvate. After optimization of the reaction conditions 63% ee for (R)-ethyl lactate was obtained. During recycling it was found that this value could even be increased upon ultrafiltration of the catalyst prior to use. Recycling was accomplished for 10 cycles with stable activity and enantioselectivity (∼73% ee) in the first eight runs. Aggregation of the initially well dispersed nanoparticles was observed by transmission electron microscope (TEM) analysis, leading to reduced conversion after the 8 th cycle, but metal leaching into the product has been observed only in the very first run.

Disassembly via an environmentally friendly and efficient fluorous phase constructed with dendritic architectures

AbstractTraditionally the fluorous phase is generated with perfluorinated alkyl groups that are usually perfluorooctyl or longer and are bioaccummulative and biopersistent and therefore, are considered environmentally unfriendly. Here we report a new concept for the construction of the fluorous phase. This concept is based on the amplification of the fluorous effect with the help of dendritic architectures containing very short semifluorinated groups on their periphery. This new concept was demonstrated by the convergent synthesis of the first and second generation AB3 and AB2 benzyl ether dendrons functionalized on their periphery via catalytic nucleophilic addition of their phenolates to perfluoropropyl vinyl ether. The resulting dendrons are liquids. Their fluorous phase affinity was analyzed and demonstrated that the dendritic architecture amplifies the fluorous phase at a specific generation by the number of functional groups on the dendron periphery, and at different generations by increasing their generation number. Therefore, this concept is very efficient for the design and synthesis of new fluorous materials. In addition, by contrast with dendrons containing perfluoroalkyl groups on their periphery, the current dendrons mediate the disassembly of their parent building blocks but do not mediate the self‐assembly in a supramolecular architecture. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2498–2508, 2010

Carbon-Nanotube-Guiding Oriented Growth of Gold Shrubs on TiO2 Nanotube Arrays

Shape-controlled synthesis of gold nanoparticles is common, while the crystal growth of dendritic gold architectures is rare, especially the perpendicular growth of shrub-like gold crystals on substrate. Herein, a simple potential-static electrodeposition method was developed to fabricate shrub-like gold nanostructures on TiO2 nanotube (NT) arrays assisted by acidified carbon nanotubes (CNTs). Au crystals were electro-stacked along the electric field lines that are perpendicular to the TiO2 substrate, constructing beautiful shrubs on TiO2 NT arrays. In this process, the acidified CNTs with rich electrons attract Au3+ through electric attraction and work as primers guiding the Au growth along the direction of the electric field. The Au shrubs are single crystalline with intertwined symmetrical nanostructures. The as-prepared Au shrub/TiO2 NTs exhibited a more sensitive response in sensing highly toxic As3+ than Au films due to the higher surface area and unique three-dimensional structures.

Computational Approach for Understanding the Interactions of UV-Degradable Dendrons with DNA and siRNA

In this work, we present a molecular dynamics study to gain an insight into the binding of nucleic acids with spermine functionalized dendrons. We compare UV-degradable dendrons with nondegradable dendrons studied in our previous works. These two dendritic architectures have the same functional surface; however, the branching scaffold exhibits different flexibilities. Here, we explore how the different branching scaffolds influence the ability to interact with DNA and siRNA. The free energies of binding, calculated with the well-known molecular mechanics Poisson-Boltzmann surface area method, are in good accordance with the experimentally observed binding behavior, demonstrating that the theoretical models are reliable and deliver an accurate description of the systems. In general, the interaction of dendrons with the more flexible siRNA is higher than with rigid double-stranded DNA. Importantly, while binding enthalpy best describes the attraction in general--being in direct relationship with the number of opposite charges interacting in the system--binding entropy is correlated with the distribution of these interactions along the binding interface. Higher uniformity in the binding allows it to maintain a strong enthalpic attraction toward the nucleic acid at lower entropic cost. This entropic cost is due to a loss of degrees of freedom in binding, which is related to the nonuniformity in the energetic contribution of the individual ligands. These findings suggest that the estimation of the pure attraction is not enough to fully understand the binding, but also, information about how this attraction is distributed is needed. This proposes new criteria in the design of DNA and siRNA binding agents.

Functional dendritic polymer architectures as stimuli-responsive nanocarriers

Stimuli-responsive polymer architectures are molecular systems which evolve with an external signal. The observed changes are mainly decomposition, isomerization, polymerization, activation, supramolecular aggregation, and structural modifications of these molecules. The external stimuli, which can be combined in order to provoke these molecular changes, are numerous. In this review, we have chosen to present an overview on different mechanisms to impart responsiveness to dendritic polymers, with the particular aim of delivery and release of bioactive molecules.

Dendritic carbon architectures formed by nanotube core-directed diffusion-limited aggregation of nanoparticles

A regular array of fractal patterns with macroscopic dendritic carbon architecture was prepared by catalytic chemical vapor deposition (CVD). The dendritic carbon architectures have micrometre-sized stems and hyperbranches evolved by lateral growth, and they are formed by diffusion-limited aggregation of carbon-encapsulated iron nanoparticle building blocks generated from catalytic pyrolysis of toluene, which is directed by carbon nanotube cores, and followed by subsequent restructuring from surface to bulk. Incorporation of such proposed processes in Monte Carlo simulations generates dendritic architectures similar to the morphologies observed from the experiments. The findings provide direct information to the time resolved evolution of the morphology and microstructure of the dendritic carbon architecture, which mimic the nature behavior as snowflake attaching on the tree branches. Those will be important to understand the growth of vapor grown carbon fibers and carbon filamentous structures, and further possibility to control branching out of vapor grown carbon fibers.

Synthesis of triazole dendrimers with a dimethyl isophthalate surface group and their application to dye-sensitized solar cells

Dendritic architectures with 1,2,3-triazole as the building unit and dimethyl isophthalate as the surface group are synthesized through a convergent approach employing click chemistry and tested for their application in dye-sensitized solar cells (DSSCs). The studies revealed that the presence of triazole and isophthalate groups in dendritic structures significantly altered the absorption, electrochemical and DSSC behaviors. In DSSCs, the dendritic structures exhibited higher Voc values and higher power conversion efficiency (η) in the I−/I3− redox couple under simulated conditions at 40 mW cm−2.

One rule to grow them all: A general theory of neuronal branching and its practical application

Event Abstract Back to Event One rule to grow them all: A general theory of neuronal branching and its practical application Hermann Cuntz1, 2*, Friedrich Forstner2, Alexander Borst2 and Michael Hausser1 1 University College London, United Kingdom 2 Max Planck Institute of Neurobiology , Germany Understanding the principles governing axonal and dendritic branching is essential for unravelling the functionality of single neurons and the way in which they connect. Nevertheless, no formalism has yet been described which can capture the general features of neuronal branching. Here we propose such a formalism, which is derived from the expression of dendritic arborizations as locally optimized graphs [1][2]. Inspired by Ramón y Cajal’s laws of conservation of cytoplasm and conduction time in neural circuitry [3], we show that this graphical representation can be used to optimize these variables. This approach allows us to generate synthetic branching geometries which replicate morphological features of any tested neuron. The structure of a neuronal tree is captured by its spatial extent and by a single parameter, a balancing factor weighing the costs of conservation of cytoplasm and conduction time. This balancing factor allows a neuron to adjust its preferred electrotonic compartmentalization. In the context of the network, competitive growth and spatial tiling are governed by this same rule. Realistic large-scale synthetic networks of neurons can therefore be generated by simply defining their input and output organization. We have developed an open-source software package to implement these simulations, the "TREES toolbox", which provides a general set of tools for analyzing, manipulating and generating dendritic structure. The package includes a tool to generate artificial members of any particular cell group and a method for model-based supervised automatic morphological reconstruction of multiple cells from fluorescent image stacks. These approaches provide new insights into the constraints governing dendritic architectures. They also provide a novel framework for modelling and analyzing neuronal branching structures and for constructing realistic artificial neural networks. Acknowledgements This work was supported by the Gatsby Charitable Foundation, the Wellcome Trust, the Alexander von Humboldt Foundation, and the Max Planck Society.

Highly sensitive detection of DNA by electrogenerated chemiluminescence amplification using dendritic Ru(bpy)32+-doped silica nanoparticles

This study describes the development and characterization of a novel dendritic signal amplification strategy. It relies on the use of two different Ru(bpy)(3)(2+)-doped silica nanoparticles (Probe(1,2)RSNP and Probe(2c)RSNP) coated with complementary DNAs, which can be simply and conveniently self-assembled to build sandwich-type dendritic architectures on a gold grid. The performance of this dendritic amplification route was demonstrated in conjunction with the electrogenerated chemiluminescent (ECL) detection of the target DNA. Compared to normal amplification, dendritic amplification allowed a 5-fold enhancement of the ECL signals. The higher sensitivity allowed by the dendritic amplification route was attributed to the hybridization between the DNA (Probe(2)DNA) on Probe(1,2)RSNP (normal amplification) and the complementary DNA (Probe(2c) DNA) on the additional Probe(2c)RSNP. As low as 1 fM of 22-bp-long target DNA was clearly detected. The experimental results demonstrated that the ECL intensity achieved through dendritic amplification showed a good linear relationship with the concentration of the target DNA over a wide linear range (10 fM-10 pM).

Dendritic Polyglycerols for Biomedical Applications

The application of nanotechnology in medicine and pharmaceuticals is a rapidly advancing field that is quickly gaining acceptance and recognition as an independent area of research called "nanomedicine". Urgent needs in this field, however, are biocompatible and bioactive materials for antifouling surfaces and nanoparticles for drug delivery. Therefore, extensive attention has been given to the design and development of new macromolecular structures. Among the various polymeric architectures, dendritic ("treelike") polymers have experienced an exponential development due to their highly branched, multifunctional, and well-defined structures. This Review describes the diverse syntheses and biomedical applications of dendritic polyglycerols (PGs). These polymers exhibit good chemical stability and inertness under biological conditions and are highly biocompatible. Oligoglycerols and their fatty acid esters are FDA-approved and are already being used in a variety of consumer applications, e.g., cosmetics and toiletries, food industries, cleaning and softening agents, pharmaceuticals, polymers and polymer additives, printing photographing materials, and electronics. Herein, we present the current status of dendritic PGs as functional dendritic architectures with particular focus on their application in nanomedicine, in drug, dye, and gene delivery, as well as in regenerative medicine in the form of non-fouling surfaces and matrix materials.

Efficient synthesis of fluorescent squaraine rotaxane dendrimers.

A squaraine rotaxane scaffold with four alkyne groups is readily converted into a range of dendritic architectures using high-yielding copper-catalyzed alkyne azide cycloaddition (CuAAC) chemistry. A convergent synthesis approach is more efficient than a divergent pathway. Dendritic squaraine rotaxanes with peripheral amine groups can be further functionalized to produce multivalent deep-red fluorescent derivatives that exhibit high brightness and outstanding chemical stability in biological solution. The surface groups on these functionalized fluorescent dendrimers include guanidinium, mannose, and phosphatidylcholine.

Phenols-useful templates for the synthesis of bi-functional orthogonally protected dendron building blocks via solid phase Mitsunobu reaction

Bi-functional dendritic building blocks for convergent dendrimer growth were successfully synthesized from phenolic templates in the solid phase via a Mitsunobu reaction. Each arm of the dendron building block carries an orthogonally protected secondary amine along the arm, and a peripheral primary amine or phenol group (building block type 1) or a tertiary amine junction with orthogonally protected peripheral primary amine or carboxyl groups (building block type 2). The synthetic routes reported in this work are general and applicable for the preparation of diverse building blocks, controlling protection, arm length, and peripheral moieties. These novel dendron units can form unusual dendritic architectures by solid-phase chemistry, which may be incorporated into specific complex structures expanding the scope of dendrimer science.

Synthesis and Characterization of Some Novel Dendritic Architectures Bearing Chalcone at the Periphery Through Click Approach

Synthesis of 1,2,3-triazole-based dendritic architectures bearing a chalcone unit at the surface group has been achieved through a click reaction by the convergent synthetic strategy, in which tri- and tetra-azide were used as core units.

Synthetic Aspects of Porphyrin Dendrimers

AbstractThis Microreview presents a concise overview of the different synthetic approaches that have been applied to porphyrin dendrimers over the years, with the porphyrin macrocycle(s) being embedded variously as the dendritic cores, at the peripheries, within the branching units, or spread throughout entire dendritic architectures. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Shape-Controlled Preparation of PbS with Various Dendritic Hierarchical Structures with the Assistance of l-Methionine

PbS fishbone-like architectures were synthesized by using a biomolecule (l-methionine)-assisted approach in a mixture solvent made of ethanolamine (EA) and distilled water. Additionally, other PbS homogeneous morphologies (e.g., hexapod-like, coralloid, and dendritic) were obtained through altering the experimental parameters, such as the molar ratio of the reactants and the volume ratio of the mixture solvent. It is noteworthy that our experiments were envionmentally friendly, because no disgusting scent (H2S) appeared in our experiments, which could be hardly avoided in other previous reports. The as-prepared PbS products were examined by using XRD, FESEM, TEM, HRTEM, SAED, and PL. Furthermore, a possible growth mechanism (an initial nucleating stage and a subsequent growth stage) was proposed to explain the formation of dendritic architectures on the basis of TEM observations, XRD, and FTIR analyses. Up to now, this is the first case of the direct growth of novel PbS with various hierarchical nanostructures with l-methionine’s assistance. The synthesized PbS fishbone-like structure was found to indicate strong photoluminescence in the blue-light range.