Dynamics of Dendrimeric Molecules Undergoing Simple Shear Flow: A Nonequilibrium Brownian Dynamics Study

Nonequilibrium Brownian dynamics (NEBD) simulations are used to model the dynamics of six generations of dendrimers undergoing shear flow. A coarse‐grained bead‐spring model is proposed, which incorporates springs with stretching and bending potentials. The bending constant is used as one of the primary independent variables to control the deformability of the molecules. Rheological and conformational properties, such as viscosity, normal stress differences, visco‐elastic moduli, flow birefringence, mean square radius of gyration, and degree of prolateness, are observed under both transient (startup and cessation of flow) and steady‐state conditions. Comparisons are made with the corresponding linear chain analogs of the same molecular weight. The model qualitatively describes many of the experimentally observed effects in these systems, most notably a Newtonian viscosity profile (no shear thinning) and a maximum in the dependence of the intrinsic viscosity on the molecular weight. The dendrimers are also characterized by negligible start‐up overshoots in the transient viscosity and birefringence.     

Effects of the Bead‐Solvent Interaction on the Dynamics of Macromolecules, 1

Summary: Hamiltonian dynamics and a chain model are used to study the dynamics of macromolecules immersed in a solution. From the Hamiltonian of the overall system, “macromolecule + solvent,” a master and a Fokker‐Planck equation are then derived for the phase‐space distribution of the macromolecule. In the Fokker‐Planck equation, all the information about the interaction among the beads of the macromolecule as well as the effects of the surrounding solvent is described by friction tensors, which are expressed in terms of the bead‐solvent interaction and the dynamic structure factor of the solvent. To explore the influence of the bead‐solvent potential on the dynamics of macromolecules, the friction tensors are calculated for a dumbbell molecule and for three choices of the interaction (Yukawa, Born‐Mayer, and Lennard‐Jones). Expressions are derived, in particular, for the friction tensor coefficients of the center‐of‐mass and the relative coordinates of the dumbbell. For the long‐term behaviour of the internal momentum autocorrelation function, moreover, an “algebraic decay” is found, in contrast to the (unphysical) exponential decay as known from phenomenological theory.

Yukawa, Born‐Mayer and Lennard‐Jones bead‐solvent interaction potentials.     

流体动力学作用对粒子聚集影响的布朗动力学模拟

采用布朗动力学模拟方法, 研究了流体动力学作用对稀溶液中悬浮粒子聚集过程的影响. 模拟中忽略了一个粒子同时与多个粒子碰撞聚集的可能, 引入了前人有关两粒子间流体动力学作用影响的研究成果. 模拟结果证实了流体动力学的作用在比较大的幅度上减缓了粒子的聚集过程, 是导致粒子聚集速率的实验值低于Smoluchowski理论值的重要原因之一. 另外, 在分别加入和排除重力作用, 以及考虑和忽略粒子间流体动力学作用在内的各种条件下模拟了粒子的聚集过程, 得到了两种因素相互耦合作用时各因素对粒子聚集过程影响的结果, 并从动力学的角度对这些因素的影响机制进行了相应的讨论.     

A Versatile Methodology for the Controlled Synthesis of Photoluminescent High‐Boron‐Content Dendrimers

Fluorescent star‐shaped molecules and dendrimers with a 1,3,5‐triphenylbenzene moiety as the core and 3 or 9 carborane derivatives at the periphery, have been prepared in very good yields by following different approaches. One procedure relies on the nucleophilic substitution of Br groups in 1,3,5‐tris(4‐(3‐bromopropoxy)phenyl)benzene with the monolithium salts of methyl and phenyl‐o‐carborane. The second method is the hydrosilylation reactions on the peripheral allyl ether functions of 1,3,5‐tris(4‐allyloxy‐phenyl)benzene and 1,3,5‐tris(4‐(3,4,5‐trisallyloxybenzyloxy)phenyl)benzene with suitable carboranyl‐silanes to produce different generations of dendrimers decorated with carboranyl fragments. This approach is very versatile and allows one to introduce long spacers between the fluorescent cores and the boron clusters, as well as to obtain a high loading of boron clusters. The removal of one boron atom from each cluster leads to high‐boron‐content water‐soluble macromolecules. Thermogravimetric analyses show a higher thermal stability for the three‐functionalized compounds than for those containing 9 clusters. All compounds exhibit photoluminescent properties at room temperature under ultraviolet irradiation with high quantum yields; these depend on the nature of the cluster and the substituent on the C_cluster. Cyclic voltammetry indicates that there is no electronic communication between the core and the peripheral carboranyl fragments. Due to the high boron content of these molecules, we currently focus our research on their biocompatibility, biodistribution in cells cultures, and potential applications for boron neutron capture therapy (BNCT).     

Liquid‐Crystalline Thiol‐ and Disulfide‐Based Dendrimers for the Functionalization of Gold Nanoparticles. Preliminary Communication

Liquid‐crystalline dendrons carrying either a thiol or disulfide function which display nematic, smectic A, columnar, or chiral nematic phases have been synthesized. Their mesomorphic properties are in agreement with the nature of the mesogenic units and structure of the dendrons. The first‐generation poly(aryl ester) dendron containing two cyanobiphenyl mesogenic units was used to functionalize gold nanoparticles. For full coverage, a smectic‐like supramolecular organization on the nanometer scale is observed, when the gold nanoparticles are spread onto carbon‐coated copper grids. This result indicates that the dendritic ligands reported here act as self‐organization promoters.     

Hydrodynamic and Thermophoretic Effects on the Supramolecular Chirality of Pyrene‐Derived Nanosheets

Chiroptical properties of two‐dimensional (2D) supramolecular assemblies (nanosheets) of achiral, charged pyrene trimers ( Py₃ ) are rendered chiral by asymmetric physical perturbations. Chiral stimuli in a cuvette can originate either from controlled temperature gradients or by very gentle stirring. The chiroptical activity strongly depends on the degree of supramolecular order of the nanosheets, which is easily controlled by the method of preparation. The high degree of structural order ensures strong cooperative effects within the aggregates, rendering them more susceptible to external stimuli. The samples prepared by using slow thermal annealing protocols are both CD and LD active (in stagnant and stirred solutions), whereas for isothermally aged samples chiroptical activity was in all cases undetectable. In the case of temperature gradients, the optical activity of 2D assemblies could be recorded for a stagnant solution due to migration of the aggregates from the hottest to the coldest regions of the system. However, a considerably stronger exciton coupling, coinciding with the J‐band of the interacting pyrenes, is developed upon subtle vortexing (0.5 Hz, 30 rpm) of the aqueous solution of the nanosheets. The sign of the exciton coupling is inverted upon switching between clockwise and counter‐clockwise rotation. The supramolecular chirality is evidenced by the appearance of CD activity. To exclude artefacts from proper CD spectra, the contribution from LD to the observed CD was determined. The data suggest that the aggregates experience asymmetrical deformation and alignment effects because of the presence of chiral flows.     

Dark‐Field‐Based Observation of Single‐Nanoparticle Dynamics on a Supported Lipid Bilayer for In Situ Analysis of Interacting Molecules and Nanoparticles

Observation of single plasmonic nanoparticles in reconstituted biological systems allows us to obtain snapshots of dynamic processes between molecules and nanoparticles with unprecedented spatiotemporal resolution and single‐molecule/single‐particle‐level data acquisition. This Concept is intended to introduce nanoparticle‐tethered supported lipid bilayer platforms that allow for the dynamic confinement of nanoparticles on a two‐dimensional fluidic surface. The dark‐field‐based long‐term, stable, real‐time observation of freely diffusing plasmonic nanoparticles on a lipid bilayer enables one to extract a broad range of information about interparticle and molecular interactions throughout the entire reaction period. Herein, we highlight important developments in this context to provide ideas on how molecular interactions can be interpreted by monitoring dynamic behaviors and optical signals of laterally mobile nanoparticles.     

Molecular Size and Electronic Structure Combined Effects on the Electrogenerated Chemiluminescence of Sulfurated Pyrene‐Cored Dendrimers

The electrochemistry, photophysics, and electrochemically generated chemiluminescence (ECL) of a family of polysulfurated dendrimers with a pyrene core have been thoroughly investigated and complemented by theoretical calculations. The redox and luminescence properties of dendrimers are dependent on the generation number. From low to higher generation it is both easier to reduce and oxidize them and the emission efficiency increases along the family, with respect to the polysulfurated pyrene core. The analysis of such data evidences that the formation of the singlet excited state by cation–anion annihilation is an energy‐deficient process and, thus, the ECL has been justified through the triplet–triplet annihilation pathway. The study of the dynamics of the ECL emission was achieved both experimentally and theoretically by molecular mechanics and quantum chemical calculations. It has allowed rationalization of a possible mechanism and the experimental dependence of the transient ECL on the dendrimer generation. The theoretically calculated Marcus electron‐transfer rate constant compares very well with that obtained by the finite element simulation of the whole ECL mechanism. This highlights the role played by the thioether dendrons in modulating the redox and photophysical properties, responsible for the occurrence and dynamics of the electron transfer involved in the ECL. Thus, the combination of experimental and computational results allows understanding of the dendrimer size dependence of the ECL transient signal as a result of factors affecting the annihilation electron transfer.     

Contributions of Various Noncovalent Bonds to the Interaction between an Amide and S‐Containing Molecules

N‐Methylacetamide, a model of the peptide unit in proteins, is allowed to interact with CH₃SH, CH₃SCH₃, and CH₃SSCH₃ as models of S‐containing amino acid residues. All of the minima are located on the ab initio potential energy surface of each heterodimer. Analysis of the forces holding each complex together identifies a variety of different attractive forces, including SH???O, NH???S, CH???O, CH???S, SH???π, and CH???π H‐bonds. Other contributing noncovalent bonds involve charge transfer into σ* and π* antibonds. Whereas some of the H‐bonds are strong enough that they represent the sole attractive force in several dimers, albeit not usually in the global minimum, charge‐transfer‐type noncovalent bonds play only a supporting role. The majority of dimers are bound by a collection of several of these attractive interactions. The SH???O and NH???S H‐bonds are of comparable strength, followed by CH???O and CH???S.     

The Development of G‐Quadruplex‐Based Assays for the Detection of Small Molecules and Toxic Substances

G‐Quadruplexes can be induced to form guanine‐rich DNA sequences by certain small molecules or metal ions. In concert with an appropriate signal transducer, such as a fluorescent dye or a phosphorescent metal complex, the ligand‐recognition event can be transduced into a luminescent response. This focus review aims to highlight recent examples of aptamer‐based and metal‐mediated G‐quadruplex assays for the detection of small molecules and toxic substances in the last three years. We discuss the mechanisms and features of the different assays and present an outlook and a perspective for the future of this field.     

A Molecular‐Dynamics Study of Size and Chirality Effects on Glass‐Transition Temperature and Ordering in Carbon Nanotube‐Polymer Composites

Molecular‐dynamics simulations of single‐walled carbon nanotubes (CNTs) embedded in a coarse‐grained amorphous monodisperse polyethylene‐like model system have been carried out. The roles of nanotube diameter and chirality on the physical and structural properties of the composite are thoroughly discussed for several CNTs with different diameter and chirality. It is shown that the glass‐transition temperature of the polymer matrix increases with the diameter of the CNT while chirality effects are negligible. A denser and ordered layered region of polymer matrix is found in the vicinity of the nanotube surface. The density and ordering of this layer increases with the CNT diameter. All simulations indicate that chirality does not affect the atomic structure of the highly ordered layer surrounding the CNTs. Despite the simplicity of the polymer model, results of this study are qualitatively comparable with those obtained from experiments and numerical simulations that consider a chemically specific polymer matrix.

     

A Continuous‐Flow Process for the Synthesis of Artemisinin

Isolation of the most effective antimalarial drug, artemisinin, from the plant sweet wormwood, does not yield sufficient quantities to provide the more than 300 million treatments needed each year. The high prices for the drug are a consequence of the unreliable and often insufficient supply of artemisinin. Large quantities of ineffective fake drugs find a market in Africa. Semisynthesis of artemisinin from inactive biological precursors, either dihydroartemisinic acid (DHAA) or artemisinic acid, offers a potentially attractive route to increase artemisinin production. Conversion of the plant waste product, DHAA, into artemisinin requires use of photochemically generated singlet oxygen at large scale. We met this challenge by developing a one‐pot photochemical continuous‐flow process for the semisynthesis of artemisinin from DHAA that yields 65 % product. Careful optimization resulted in a process characterized by short residence times. A method to extract DHAA from the mother liquor accumulated during commercial artemisinin extractions, a material that is currently discarded as waste, is also reported. The synthetic continuous‐flow process described here is an effective means to supplement the limited availability of artemisinin and ensure increased supplies of the drug for those in need.     

Novel Pulsed Electron Paramagnetic Resonance Techniques for the Studies of Structure and Dynamics of Photo‐excited Triplet State of Organic Molecules: A Professional Journey

We present a few novel pulsed electron paramagnetic resonance techniques developed in our laboratory for the studies of structure and dynamics of the photo‐excited triplet state of organic molecules. We discuss many aspects of these new techniques and the significances of these measurements: (1) enhancing NMR signal intensity by dynamic nuclear polarization ‐ integrated solid effect, (2) performing magnetic resonance in zero‐field and low‐field by pulsed microwave, (3) mapping molecular motion of organic crystals by pulsed zero‐field and low‐field experiments, (4) probing spin dynamics at level anti‐crossing by fast field switching, (5) measuring hyperfine interaction by electron spin echo envelop modulation and spin‐echo electron nuclear double resonance and (6) detecting spin dynamics, nuclear quantum oscillation, entanglements and new avenues for quantum computer. We have employed the highly electron spin polarized pentacene triplet state as the model system in all of our pulsed EPR experiments. We performed most of our experiments at room temperature. The goals of our studies are aiming to improve spin detectability, to probe molecular dynamics, to determine electronic structures, to measure molecular interaction and motion, and to examine quantum coherence and oscillation which may yield new avenues in the applications of pulsed EPR techniques to quantum computer.     

Self‐Assembly and Dispersion of Chromogenic Molecules: A Versatile and General Approach for Self‐Assessing Polymers

Summary: Self‐assessing polymer blends based on poly(ethylene terephthalate glycol) or linear low‐density polyethylene and small amounts (0.5–2% w/w) of chromogenic sensor dyes are prepared and investigated. The cyano‐substituted oligo(p‐phenylene vinylene) dyes employed in the study exhibit pronounced optical absorption changes upon self‐assembly, because of charge‐transfer interactions or conformation changes. The extent of dye aggregation (and therewith the optical absorption characteristics) in these blends is significantly influenced by exposure to external stimuli. Subjecting appropriately processed samples to either temperatures above their glass transition or mechanical deformation can significantly change the extent of aggregation, which in turn leads to a color change.

Mechano‐optical response of a 1.0% w/w LLDPE/C18‐RG blend film. Pristine films are orange due to aggregated dye molecules. Deformation leads to dispersion of the dye and irreversibly changes the color to yellow.     

Site‐Specific Investigation of the Steady‐State Kinetics and Dynamics of the Multistep Binding of Bile Acid Molecules to a Lipid Carrier Protein

The investigation of multi‐site ligand–protein binding and multi‐step mechanisms is highly demanding. In this work, advanced NMR methodologies such as 2D ¹H–¹⁵N line‐shape analysis, which allows a reliable investigation of ligand binding occurring on micro‐ to millisecond timescales, have been extended to model a two‐step binding mechanism. The molecular recognition and complex uptake mechanism of two bile salt molecules by lipid carriers is an interesting example that shows that protein dynamics has the potential to modulate the macromolecule–ligand encounter. Kinetic analysis supports a conformational selection model as the initial recognition process in which the dynamics observed in the apo form is essential for ligand uptake, leading to conformations with improved access to the binding cavity. Subsequent multi‐step events could be modelled, for several residues, with a two‐step binding mechanism. The protein in the ligand‐bound state still exhibits a conformational rearrangement that occurs on a very slow timescale, as observed for other proteins of the family. A global mechanism suggesting how bile acids access the macromolecular cavity is thus proposed.     

C_（60）分子间相互作用势及其晶体压缩系数的计算

根据Ｃ（６０）晶体升华焓的实验值求得了Ｃ（６０）分子间相互作用势的解析表达式。用该表达式计算了Ｃ（６０）分别为面心立方和六方晶格时晶体的结合能，讨论了不同晶格的稳定性问题，解释了Ｃ（６０）晶体一般是面心立方的原因。本文还根据Ｃ（６０）分子间相互作用势计算了Ｃ（６０）晶体的压缩系数，并与文献的实验值进行了比较，所得结果与实验值相符合。