Molecular modeling of the carbohydrate corona formation on a polyvinyl chloride nanoparticle and its impact on the adhesion to lipid bilayers.

The pervasive presence of nanoplastics (NPs) in the environment has gained increasing attention due to their accumulation in living organisms. These emerging contaminants inevitably interact with extracellular polymeric substances along respiratory or gastrointestinal tracts, and diverse organic coating on the surface of NPs, known as bio- or eco-corona, is formed. Although its impact on altering the NP properties and potential cell internalization has been extensively examined, studies on its role in NP partitioning in the cell membrane are elusive yet. In this work, molecular dynamics is used to investigate the formation of chitosan (CT) corona centered on a polyvinyl chloride (PVC) nanoparticle and the uptake of the resulting complex onto lipid membranes. Coarse-grained models compatible with the newly developed Martini 3.0 force field are implemented for the two polymers employing the atomistic properties as targets in the parameterization. The reliability of the coarse-grained polymer models is demonstrated by reproducing the structural properties of the PVC melt and of solvated CT strands, as well as by determining the conformation adopted by the latter at the NP surface. Results show that the spontaneous binding of CT chains of high and intermediate protonation degrees led to the formation of soft and hard corona that modulates the interaction of PVC core with model membranes. The structural changes of the corona adsorbed at the lipid-water interface enable a subsequent transfer of the NP to the center of the saturated lipid membranes and a complete or partial transition to a snorkel conformation depending on the hydrophilic/hydrophobic balance in the CT-PVC complex. Overall, the computational investigation of the coarse-grained model system provides implications for understanding how the eco-corona development influences the uptake and implicit toxicology of NPs.

Coarse-Grained Model of Phytic Acid for Predicting the Supramolecular Architecture of Ionically Cross-Linked Chitosan Hydrogels.

Phytic acid is a polyphosphate whose ionized form is used as a cross-linking agent to formulate chitosan-based nanoparticles and hydrogels as carriers with remarkable adhesivity and biocompatibility. To predict the underlying cross-linking pattern responsible for the structural arrangement in the chitosan hydrogels, we put forth coarse-grained parametrization of the phytic acid compatible with the Martini 2.3P force field. The bonded parameters giving the distinctive representation of the phosphate substitutes to the myo-inositol ring of phytic acid are optimized by a structural comparison to the conformation sampled with the GROMOS 56ACARBO force field. The chitosan strand is coarse-grained following a similar approach, and the cross-interaction terms are optimized to reproduce the atomistic features of phytate-mediated cross-linking. The predicted binding motifs of the phytic acid-chitosan complexation enable us to rationalize the structural characteristics of the reticulated chitosan in a semi-dilute solution. The model describes a network topology affected by the phytic acid concentration and a nonmonotonous behavior of the mean pore size caused by a poor predilection for the parallel strand alignment near the charge neutralization of the phytic acid-chitosan complex.

Polymorphism of chitosan-based networks stabilized by phytate investigated by molecular dynamics simulations.

Chitosan can associate in the presence of polyphosphates into insoluble hydrogels capable of drug encapsulation and safe and efficient release. On the one hand, chitosan hydrogels were synthesized using the phytate anion as a crosslinking agent and were characterized by employing dynamic light scattering (DLS) and Fourier transform infrared spectroscopy (FTIR). On the other hand, an effective chitosan-phytate model with atomistic details was created to examine the underlying physical crosslinking pattern, and the structure and dynamics of the chitosan-phytate complex were systematically investigated by using molecular dynamics (MD) simulations. To harbor the crosslinker potential for obtaining chitosan-based hydrogels, the impact of the phytate concentration and the functional groups of the chitosan on the reticulation process was addressed. The phytate association was determined by the phosphates' capacity for H-bonding to the amine and hydroxyl groups belonging to two consecutive glucosidic units. The physical crosslinking pattern was determined by the number of chitosan chains bound by one phytate anion and the phytate orientation relative to the glucopyranose neighbors. Cross-linking of two up to six chitosan chains mediated by a phytate anion represented favorable states, and the number distribution of cross-linked chains depended on the phytate concentration. The circular distribution of the cross-linkable phosphates regulated the nearly isotropic orientation of the chitosan chains and phytate at the junction, and the variety of topological crosslinking demonstrated the phytate ion's potential for developing chitosan-based hydrogels with improved structural attributes.

Spectroscopic, molecular dynamics simulation and biological studies of Flavin MonoNucleotide and Flavin Adenine Dinucleotide in biomimetic systems.

The present study describes a comprehensive investigation of the spectroscopic characteristics, stability and in vitro antioxidant and cytotoxic properties of the Flavin MonoNucleotide (FMN) and Flavin Adenine Dinucleotide (FAD) in Dextran70 (Dx70) and Dx70/phospatidylcholine (PC) biomimetic systems by means of the UV-Vis absorption, fluorescence spectroscopy, chemiluminescence and Neutral Red assay. The affinity of FMN, FAD and the precursor riboflavin (RF) to an unsaturated phospholipid bilayer model as well as the location of the probes within the lipid bilayer were assessed from united-atom molecular dynamics simulations carried out on an unsaturated phospholipid bilayer model system, and the theoretical and experimental characterization of the two probes within biomembranes was complemented with the light microscopy survey of the cell morphology of L929 fibroblast cells cultivated in the presence of various dosage of FAD/FMN. In lipid bilayers, FMN/FAD resulted in a noticeable improvement of the antioxidant activity (the scavenging of reactive oxygen species up to 40%) and a significant effect on cellular viability in the L929 fibroblast cells. The results are important in the oxidative stress process concerning the redox reactions of flavins in humans as well as in further studies on different systems belonging to the category of flavoenzymes/flavoproteins, required for cellular respiration.

Chitosan-polyglycidol complexes to coating iron oxide particles for dye adsorption.

The study sheds light on the interaction between chitosan (Ch) and polyglycidol (PGL) and uses their interpolymer complex in hydrophilic coating of iron oxide particles (M). Preliminary investigations were performed by modeling chitosan and polyglycidol chains interactions using coarse grained beads approximation and molecular dynamics simulations. The results revealed that Ch and PGL chains associate together forming weak strength complexes, which was experimentally confirmed by surface tension, fluorescence and FTIR. The Ch-PGL mixture (C) and sodium dodecylsulfate (S) were used for layer-by-layer preparation of hydrophilic multilayer coatings of M. The successful covering, demonstrated by DLS, Zeta potential, FTIR, EDAX, preserved the particles super-paramagnetic properties. The most stable multilayer nanocomposite (MSCS) efficiently adsorbed methylene blue from water. The Freundlich model fitted well the equilibrium isotherm data, indicating a heterogeneous, multilayer adsorption. Benefiting from both nano-size and magnetic properties, this adsorbent could be an effectively, cheaply and eco-friendly wastewater treatment means.

Structural behavior of amphiphilic polyion complexes interacting with saturated lipid membranes investigated by coarse-grained molecular dynamic simulations.

Neutral polyelectrolyte complexes (PECs) made from an amphiphilic multiblock copolymer of type (A n B n ) m and an oppositely charged polyion and interacting with a dipalmitoylphosphatidylcholine (DPPC) lipid membrane have been examined employing a coarse-grained model with implicit solvent and molecular dynamics simulations. One systematically explored the influence of the size of the hydrophobic block B and of the number of these blocks per chain on the PEC tendency to adhere to the membrane surface and to intercalate into the membrane core. Simulation results showed that PECs bound irreversibly to the lipid bilayer without polyion unwinding from the complex and the adsorbed conformation was strongly affected by the size of the hydrophobic block B. The adsorption kinetics at low B size were characterized by a relaxation phase dominated by the spreading of PEC constituents along the outer leaflet of the membrane. Upon increasing the size of the hydrophobic block B to reach core-shell organization of the free PEC, the relaxation pathway of the complex corona in close contact with the headgroup lipids facilitated the transient exposure of the PEC hydrophobic core to the lipids and its subsequent cooperative internalization and solubilization in the membrane inner part associated with an internal reorganization of the lipid bilayer. In the generated snorkeling-type conformation, the charged blocks A and the oppositely charged polyion were confined to the headgroup region of the top leaflet, without spontaneous flipping to the headgroup region of the distal leaflet.

Interaction of piroxicam with bovine serum albumin investigated by spectroscopic, calorimetric and computational molecular methods.

The binding of drugs to serum proteins is governed by weak non-covalent forces. In this study, the nature and magnitude of the interactions between piroxicam (PRX) and bovine serum albumin (BSA) was assessed using spectroscopic, calorimetric and computational molecular methods. The fluorescence data revealed an atypical behavior during PRX and BSA interaction. The quenching process of tryptophan (Trp) by PRX is a dual one (approximately equal static and dynamic quenched components). The FRET results indicate that a non-radiative transfer of energy occurred. The association constant and the number of binding sites indicate moderate PRX and BSA binding. The competitive binding study indicates that PRX is bound to site I from the hydrophobic pocket of subdomain IIA of BSA. The synchronous spectra showed that the microenvironment around the BSA fluorophores and protein conformation do not change considerably. The Trp lifetimes revealed that PRX mainly quenches the fluorescence of Trp-213 situated in the hydrophobic domain. The CD and DSC investigation show that addition of PRX stabilizes the protein structure. ITC results revealed that BSA-PRX binding involves a combination of electrostatic, hydrophobic and hydrogen interactions. The analysis of the computational data is consistent with the experimental results. This thorough investigation of the PRX-BSA binding may provide support for other studies concerning moderate affinity drugs with serum protein.Communicated by Ramaswamy H. Sarma.

Coarse-grained simulation studies on the adsorption of polyelectrolyte complexes upon lipid membranes.

Coarse-grained molecular dynamics simulations have been applied to explore the adsorption of oppositely charged polyelectrolyte complexes (PECs) on an electronically neutral dipalmitoylphosphatidylcholine (DPPC) lipid bilayer. The membrane model implied an implicit solvent description, and the DPPC parametrization is capable of reproducing relatively well the main physical properties of the bilayer such as the area per lipid, bending modulus, bilayer thickness, orientation order parameter and internal pressure distribution. It has been furthermore shown that the lipid model can be applied to investigate the dynamics and adsorption structures of PECs with a varying polyanion-to-polycation charge ratio. Irreversible adsorption has been observed for the overcharged PECs, the polyanion beads being in close contact with the choline group and without internalization into the membrane core. The bound PEC undergoes a relaxation phase characterized by steady spreading on the outer leaflet of the membrane. The effect of the charge excess on this phase has been expressed by variable shapes of PECs ranging from oblate discs to prolate spheroids, net charge gathering at the bilayer surface and partial unwinding of the polyanion. The response membrane was characterized by slightly tighter lipid packing upon PEC attachment and an increase of the order parameter in the membrane core following the PEC relaxation phase. Analysis of truncating electrostatic interactions indicated that the cutoff distance did not influence the dynamics of the adsorption process and induced an artificial ordering of the lipid tails.

Tubular and Spherical SiO2 Obtained by Sol Gel Method for Lipase Immobilization and Enzymatic Activity.

A wide range of hybrid biomaterials has been designed in order to sustain bioremediation processes by associating sol-gel SiO2 matrices with various biologically active compounds (enzymes, antibodies). SiO2 is a widespread, chemically stable and non-toxic material; thus, the immobilization of enzymes on silica may lead to improving the efficiency of biocatalysts in terms of endurance and economic costs. Our present work explores the potential of different hybrid morphologies, based on hollow tubes and solid spheres of amorphous SiO2, for enzyme immobilization and the development of competitive biocatalysts. The synthesis protocol and structural characterization of spherical and tubular SiO2 obtained by the sol gel method were fully investigated in connection with the subsequent immobilization of lipase from Rhizopus orizae. The immobilization is conducted at pH 6, lower than the isoelectric point of lipase and higher than the isoelectric point of silica, which is meant to sustain the physical interactions of the enzyme with the SiO2 matrix. The morphological, textural and surface properties of spherical and tubular SiO2 were investigated by SEM, nitrogen sorption, and electrokinetic potential measurements, while the formation and characterization of hybrid organic-inorganic complexes were studied by UV-VIS, FTIR-ATR and fluorescence spectroscopy. The highest degree of enzyme immobilization (as depicted from total organic carbon) was achieved for tubular morphology and the hydrolysis of p-nitrophenyl acetate was used as an enzymatic model reaction conducted in the presence of hybrid lipase⁻SiO2 complex.

Assembled viral-like nanoparticles from elastic capsomers and polyion.

Molecular dynamics simulations are carried out on a coarse-grained model to describe the polyion driven co-assembly of elastic capsomers as viral-like aggregates. The kinetics and structural properties of the complexes are examined using cationic capsomers, an anionic polyion, both modelled using beads connected by springs, and counterions neutralizing separately the two charged species. Polyion overcharging the capsid is encapsulated owing to combined effects of the capsomer-capsomer short-range interactions, the polyion ability to follow a Hamiltonian path, and Donnan equilibrium. Conditions leading to a high yield of viral-like nanoparticles are found, and the simulations demonstrate that the capsomer elasticity provides mechanisms that improve the reliability toward correctly formed capsids. These mechanisms are related to a highly irregular capsomer cluster growth followed by the appearance of two stable capsomer clusters with the polyion acting as a tether between them. Elevated capsomeric flexibility provides an additional pathway to anneal the kinetically trapped structures by the ejection of a capsomeric monomer from a malformed complex followed by a rebinding step to form a correct capsid.

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.

Branched-linear polyion complexes at variable charge densities.

Structural behavior of complexes formed by a charged and branched copolymer and an oppositely charged and linear polyion was examined by Monte Carlo simulations employing a coarse-grained bead-spring model. The fractional bead charge and the branching density were systematically varied; the former between 0e and 1e and the latter such that both the comb-polymer and the bottle-brush limits were included. The number of beads of the main chain of the branched copolymer and of the linear polyion was always kept constant and equal, and a single side-chain length was used. Our analysis involved characterization of the complex as well as investigation of size, shape, and flexibility of the charged moieties. An interplay between Coulomb interaction and side-chain repulsion governed the structure of the polyion complex. At strong Coulomb interaction, the complexes underwent a gradual transition from a globular structure at low branching density to an extended one at high branching density. As the electrostatic coupling was decreased, the transition was smoothened and shifted to lower branching density, and, eventually, a behavior similar to that found for neutral branched polymer was observed. Structural analogies and dissimilarities with uncharged branched polymers in poor solutions are discussed.

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.

Adsorption of branched-linear polyethyleneimine-ethylene oxide conjugate on hydrophilic silica investigated by ellipsometry and Monte Carlo simulations.

The adsorption of and conformation adopted by a branched-linear polymer conjugate to the hydrophilic silica-aqueous solution interface have been studied by in situ null ellipsometry and Monte Carlo simulations. The conjugate is a highly branched polyethyleneimine structure with ethyleneoxide chains grafted to its primary and secondary amino groups. In situ null ellipsometry demonstrated that the polymer conjugate adsorbs to the silica surface from water and aqueous solution of 1 mM asymmetric divalent salt (calcium and magnesium chloride to emulate hard water) over a large pH range. The adsorbed amount is hardly affected by pH and large charge reversal on the negatively charged silica surface occurred at pH = 4.0, due to the adsorption of the cationic polyelectrolyte. The Monte Carlo simulations using an appropriate coarse-grained model of the polymer in solution predicted a core-shell structure with no sharp boundary between the ethyleneimine and ethyleneoxide moieties. The structure at the interface is similar to that in solution when the polymer degree of protonation is low or moderate while at high degree of protonation the strong electrostatic attraction between the ethyleneimine core and oppositely charged silica surface distorts the ethyleneoxide shell so that an "anemone"-like configuration is adopted. The adsorption of alkyl benzene sulfonic acid (LAS) to a preadsorbed polymer layer was also investigated by null ellipsometry. The adsorption data brought additional support for the existence of a strong polymer adsorption and showed the presence of a binding which was further enhanced by the decreased solvency of the surfactant in the salt solution and confirmed the surface charge reversal by the polymer adsorption at pH = 4.0.

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) E(T)(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.

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.

Monte Carlo simulations of flexible polyelectrolytes inside viral capsids with dodecahedral charge distribution.

Structural properties of encapsidated flexible polyelectrolytes in viral capsids with dodecahedral charge distribution have been investigated by Monte Carlo simulations using a coarse-grained model. Several capsid charge distributions ranging from a homogeneous surface charge distribution (lambda=0) to a complete dodecahedral distribution (lambda=1) at constant total capsid charge and fixed radial location of the capsid charges have been considered. The radial and lateral organizations of the polyelectrolyte have been examined as a function of the polyelectrolyte length and capsid charge distribution. With short polyelectrolytes a single polyelectrolyte layer was formed at the inner capsid surface, whereas at increasing polyelectrolyte length also a uniform polyelectrolyte density inside the surface layer was established. At low lambda , the polyelectrolyte layer was laterally isotropic, but at lambda> or =0.05 a dodecahedral structure started to appear. At lambda=1 , the polyelectrolyte followed essentially a path along the edges of a dodecahedron. With sufficiently long chains, all edges were decorated with polyelectrolyte, facilitated by loop formation. For an undercharged capsid, the capsid counterions inside the capsid also adopted a dodecahedral distribution.

Packaging of a flexible polyelectrolyte inside a viral capsid: effect of salt concentration and salt valence.

The effect of salt on the location and structure of a flexible polyelectrolyte confined inside a viral capsid and the Donnan equilibrium of the salt across the capsid have been examined using a coarse-grained model solved by Monte Carlo simulations. The polyelectrolyte was represented by a linear jointed chain of charged beads, and the capsid was represented by a spherical shell with embedded charges. At low salt concentration, the polyelectrolyte was strongly adsorbed onto the inner capsid surface, whereas at high salt concentration it was located preferentially in the central part of the capsid. Under the condition of equal Debye screening length, the electrostatic screening increased as the valence of the polyelectrolyte counterion was increased. The distribution of the small cations and anions was unequal across the capsid. An excess of polyelectrolyte counterions occurred inside the capsid, and the excess increased with the salt concentration. A simplified representation of the small ions through the use of the screened Coulomb potential provided only a qualitatively correct picture; the electrostatic screening originating from the small ions was exaggerated.

Monte Carlo simulations of polyelectrolytes inside viral capsids.

Structural features of polyelectrolytes as single-stranded RNA or double-stranded DNA confined inside viral capsids and the thermodynamics of the encapsidation of the polyelectrolyte into the viral capsid have been examined for various polyelectrolyte lengths by using a coarse-grained model solved by Monte Carlo simulations. The capsid was modeled as a spherical shell with embedded charges and the genome as a linear jointed chain of oppositely charged beads, and their sizes corresponded to those of a scaled-down T=3 virus. Counterions were explicitly included, but no salt was added. The encapisdated chain was found to be predominantly located at the inner capsid surface, in a disordered manner for flexible chains and in a spool-like structure for stiff chains. The distribution of the small ions was strongly dependent on the polyelectrolyte-capsid charge ratio. The encapsidation enthalpy was negative and its magnitude decreased with increasing polyelectrolyte length, whereas the encapsidation entropy displayed a maximum when the capsid and polyelectrolyte had equal absolute charge. The encapsidation process remained thermodynamically favorable for genome charges ca. 3.5 times the capsid charge. The chain stiffness had only a relatively weak effect on the thermodynamics of the encapsidation.