Daniel G. Angelescu

Viruses as supramolecular self-assemblies: modelling of capsid formation and genome packaging

Viruses are considered the simplest form of live. A virus is basically composed of genomic material surrounded by a protecting capsid. Today, full molecular details of many viruses are known, and several subclasses can be discerned. In this review, we present recent advances made over the past few years obtained from theoretical considerations and model simulations to improve our understanding on two vital aspects of the physics of viruses: viz. viral capsid self-assembly and viral genome packaging. Many processes, such as the self-assembly pathway, genome packing, and ultimately the infection mechanism, differ between viruses containing double-stranded polynucleotides on one hand and single-stranded polynucleotides on the other hand. We believe that these differences to a large degree originate from the different genome flexibilities.

Structural transitions of encapsidated polyelectrolytes

Abstract.Conformations and structural transitions of polyelectrolytes strictly confined onto a spherical 2D surface have been investigated by scaling descriptions based on physical arguments concerning polyelectrolyte adsorption onto planar surface and liquid crystals as well as by Monte Carlo simulations using a bead-spring model with short-range and electrostatic repulsions. In case of the electrostatic screened regime, a disordered-ordered (spiral) transition at increasing persistence length of the chain was found. It was predicted that the transition occurred when the persistence length is comparable with the mean spacing between adjacent strands of the ordered chain. The presence of a non-screened electrostatic repulsion led to a more complex behavior with i) a re-entrant order-disorder transition and ii) a tennis ball texture as an additional smectic/nematic structure. The various competing structures given by the theory were recovered by the Monte Carlo simulations, which also indicated that the tennis ball texture was favored over the spiral structure by the long-range interactions for semi-flexible chains.

Microemulsions as reaction media for the synthesis of Pt nanoparticles

For the synthesis of Pt nanoparticles we used water-in-oil droplet microemulsions as templates. The focus was on the correlation between the size of the microemulsion droplets and that of the resulting Pt particles. To study this correlation in a systematic way, all particles were synthesized at the water emulsification failure boundaries where the microemulsion droplets are spherical and where their size can easily be tuned by the amount of added water. The metallic particles were synthesized by mixing two microemulsions one of which contains the metal salt H(2)PtCl(6) and the other the reducing agent NaBH(4). The size and structure of the microemulsion droplets was studied via small-angle X-ray scattering, while the Pt particles were characterized by high-resolution transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy and selected area electron diffraction. The clear correlation between droplet and particle size was further supported by accompanying Monte Carlo simulations.

The antimicrobial reagent role on the degradation of model cellulose film.

The effect of the antimicrobial agent TMPAC (3-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride) on the cellulase activity on model cellulose substrate was investigated by in situ-null ellipsometry. The cellulases used were extracted from Trichoderma viride and Aspergillus niger, and the model cellulose film was prepared by spin-coating silicon oxide wafers with cellulose solubilized in N-methylmorpholine-N-oxide/dimethyl sulfoxide solution. Upon enzyme addition to the previously equilibrated cellulose film, the initial enzyme adsorption on the substrate was followed by an overall decrease in film mass owing to enzymatic digestion of the cellulose. The loss of cellulose film mass was associated with a non-monotonously behavior of the cellulose film thickness. The activities of the two enzymes were different, a much higher degradation rate being observed for the Trichoderma viride cellulase. The degradation rate with this cellulase decreased significantly when the cellulose film was treated with the antimicrobial agent. The antimicrobial agent did not affect the cellulose degradation catalyzed by the Aspergillus niger cellulase. It was, hence, demonstrated for the first time that, depending on the cellulase type, the antimicrobial agent can inhibit enzymatic activity at the solid-liquid interface.

Size control of PtPb intermetallic nanoparticles prepared via microemulsions

PtPb intermetallic nanoparticles (3-6 nm) with precise control (±0.8 nm) were prepared under mild conditions by using water-in-oil (w/o) microemulsions as reaction media. The amount of the aqueous phase has no significant effect on the composition of the NPs, while it indeed affects the size of the resulting NPs in a systematic way. A precise size control can be obtained by controlling the size of the w/o-microemulsions containing the metal salts and the reducing agent, respectively.

Influence of the shell thickness and charge distribution on the effective interaction between two like-charged hollow spheres.

The mean-force and the potential of the mean force between two like-charged spherical shells were investigated in the salt-free limit using the primitive model and Monte Carlo simulations. Apart from an angular homogeneous distribution, a discrete charge distribution where point charges localized on the shell outer surface followed an icosahedral arrangement was considered. The electrostatic coupling of the model system was altered by the presence of mono-, trivalent counterions or small dendrimers, each one bearing a net charge of 9 e. We analyzed in detail how the shell thickness and the radial and angular distribution of the shell charges influenced the effective interaction between the shells. We found a sequence of the potential of the mean force similar to the like-charged filled spheres, ranging from long-range purely repulsive to short-range purely attractive as the electrostatic coupling increased. Both types of potentials were attenuated and an attractive-to-repulsive transition occurred in the presence of trivalent counterions as a result of (i) thinning the shell or (ii) shifting the shell charge from the outer towards the inner surface. The potential of the mean force became more attractive with the icosahedrally symmetric charge model, and additionally, at least one shell tended to line up with 5-fold symmetry axis along the longest axis of the simulation box at the maximum attraction. The results provided a basic framework of understanding the non-specific electrostatic origin of the agglomeration and long-range assembly of the viral nanoparticles.

Microstructure of polyelectrolyte nanoaggregates studied by fluorescence probe method.

The microstructure of water soluble nanoaggregates based on polyelectrolyte complex formed by the cationic comb-type copolymer poly(acrylamide -co-[3- (methacryloyl-amino)propyl] trimethylammonium chloride)-graft- polyacrylamide [P(AM-co-MAPTAC)-g-PAM] and the anionic linear polyelectrolyte sodium polyacrylate (NaPA) was investigated using the fluorescence probe technique. The fluorescence probe were 1-anilinonaphthalene-8-sulfonic acid (ANS), pyrene (Py) and 1,10-bis(1-pyrene) decane (PD). The fluorescence properties in polyelectrolyte complex solutions, which are sensitive to either micropolarity (ANS, Py) or microviscosity (PD), were related to the quantities obtained in different pure or mixed solvents. Micropolarities were quantified utilizing the polarity common index (Reichardt) ET(30). ANS and Py showed a variation of the micropolarity with the charge ratio of the two polymers, with the lowest polarity reached at the complex neutralization. The PD probe, by its excimer-to-monomer fluorescence intensities ratio, enabled us to evidence the effect of the composition and the comb-type copolymer grafting density on the microviscosity of the interpolyelectrolytes aggregates. It has been found that the microviscosity increased with the density of the grafting PAM chains.

Physicochemical Characterization and In Vitro Cytotoxic Effect of 3-Hydroxyflavone in a Silver Nanoparticles Complex.

The aim of this work was to characterize the physico-chemical properties of 3-hydroxyflavone (3-HF) in a silver nanoparticles complex (SNPs) using UV–vis and Fluorescence spectroscopy, Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) analysis. One also evaluated its effect on the cell viability and morphology of L929 mouse fibroblast cells in vitro. The contribution of the carrier protein, Bovine Serum Albumin (BSA) to 3-HF properties has also been investigated. 3-HF in BSA/SNPs systems presented no cytotoxic effect in L929 mouse fibroblast cells at any of the tested concentrations. The results are discussed with relevance to the oxidative stress process.

Branched-linear polyion complexes investigated by Monte Carlo simulations

Complexes formed by one charged and branched copolymer with an oppositely charged and linear polyion have been investigated by Monte Carlo simulations. A coarse-grained description has been used, in which the main chain of the branched polyion and the linear polyion possess the same absolute charge and charge density. The spatial extension and other structural properties, such as bond-angle orientational correlation function, asphericity, and scaling analysis of formed complexes, at varying branching density and side-chain length of the branched polyion, have been explored. In particular, the balance between cohesive Coulomb attraction and side-chain repulsions resulted in two main structures of a polyion complex. These structures are (i) a globular polyion core surrounded by side chains appearing at low branching density and (ii) an extended polyion core with side chains still being expelled at high branching density. The globule-to-extended transition occurred at a crossover branching density being practically independent of the side chain length.

The influence of hydroxy propyl β-cyclodextrin on the micellar to gel transition in F127 solutions investigated at macro and nanoscale levels

Micellar to gel phase transition in the aqueous triblock copolymer F127 solutions occurs at higher temperatures in the presence of cyclodextrin and the temperature range corresponding to the gel phase is contracted. These features were observed by dynamic rheometry and the tube inversion method. We investigated further the behaviours of various molecular probes in Pluronic F127 solutions in the absence and in the presence of hydroxypropyl-β-cyclodextrin (HPB) as a function of temperature by continuous wave electron paramagnetic resonance (cw-EPR), Electron Spin Echo Envelope Modulation (ESEEM) and fluorescence spectroscopy, to correlate the observed macroscopic and microscopic changes. The use of molecular probes with different structural particularities allowed the targeting of different regions of F127 micelles which are inhomogeneous at the nanoscale level and the elucidation of the mode in which HPB influences the phase transition in such systems. Dansyl-TEMPO (DT), with a dual character (paramagnetic and fluorescent), spin labelled cyclodextrin (MTCYC), spin labelled Pluronics – L62NO and F127NO, dansyl β-CD (D-β-CD) and pyrene were employed as molecular probes. Both cw-EPR and fluorescence measurements revealed that molecular probes with a more hydrophobic character are more sensitive to phase transition occurring in the studied systems.