Early warning sensors for monitoring mercury in water.

Poly-4-vinylpyridine grafted poly(vinylidene difluoride) (P4VP-g-PVDF) nanoporous polymer electrodes were found to be sensitive for Hg(II) analysis. The fabrication and characterization of functionalized nanoporous membrane-electrodes by FESEM and FTIR are presented. Functionalized nanopore charge state versus a large range of pH (1-10) was investigated by registering the streaming potential. This isoelectric point is achieved at the pKa of P4VP (pH = 5). Mercury adsorption at solid-liquid interface obeys a Langmuir law. A protocol for accurate Hg(II) analysis at ppb level was established. Calibration curves were performed and different real water samples (mineral water, ground water, surface water) were spiked and analyzed. The resulting sensor is intended to be integrated into existing systems or used standalone as portable devices. A first generation prototype exhibiting its own integrated potentiostat, its software and set of membrane-electrode pads is presented.

The concept of entropic rectifier facing experiments.

The transport of molecules in confined media is subject to entropic barriers. So theoretically, asymmetry of the confinement length may lead to molecular ratchets with entropy as the only driving force for the biased transport. We address experimentally this question by performing alternative ionic current measurements on electrolytes confined in neutral conical nanopores. In case anions and cations widely differ in size, we show that rectification of ionic current can be obtained that depends on ions size and cycle frequency, consistently with the entropic ratchet mechanism.

A method to Quantify the Affinity of Cabazitaxel for PLA-PEG Nanoparticles and Investigate the Influence of the Nano-Assembly Structure on the Drug/Particle Association.

PURPOSE: To study the impact of the size and the structure of the nano-assembly on the drug/particle association, determining the intrinsic partition coefficient, in order to better master the encapsulation and release properties of the carrier. METHODS: An experimental methodology is proposed to characterize the drug/nanoparticle association by mean of a partition coefficient between the PLA-PEG nanoparticles and the suspending aqueous medium, referred to as Kp. The determination was made from apparent values (referred to as Kp (ap)) measured in the presence of solubilizing agents (albumin and hydroxypropyl-ßcyclodextrin) and extrapolation to zero concentration. The structure of nanoparticles was investigated by Transmission Electron Microscopy and static light scattering. RESULTS: Depending on the manufacturing process and the PEG length of the copolymer, the nanoparticles structured either as aggregates of copolymer chains or micelles exhibiting significantly different Kp values. CONCLUSION: The methodological tool described here showed that the difference in cabazitaxel/nanoparticle association between aggregates and micelles could be attributed to the difference in PLA-PEG chains packing.

Peptide pores in lipid bilayers: voltage facilitation pleads for a revised model.

We address the problem of antimicrobial peptides that create pores in lipid bilayers, focusing on voltage-temperature dependence of pore opening. Two novel experiments (voltage clamp with alamethicin as an emblematic representative of these peptides and neutron reflectivity of lipid monolayer at solid-water interface under electric field) serve to revise the only current theoretical model. We introduce a general contribution of peptide adsorption and electric field as being responsible for an unbalanced tension of the two bilayer leaflets and we claim that the main entropy cost of one pore opening is due to the corresponding excluded area for lipid translation.

Crowding effect on helix-coil transition: beyond entropic stabilization.

We report circular dichroism measurements on the helix-coil transition of poly(L-glutamic acid) in solution with polyethylene glycol (PEG) as a crowding agent. The PEG solutions have been characterized by small angle neutron scattering and are well described by the picture of a network of mesh size ξ, usual for semi-dilute chains in good solvent. We show that the increase of PEG concentration stabilizes the helices and increases the transition temperature. But more unexpectedly, we also notice that the increase of concentration of crowding agent reduces the mean helix extent at the transition, or in other words reduces its cooperativity. This result cannot be taken into account for by an entropic stabilization mechanism. Comparing the mean length of helices at the transition and the mesh size of the PEG network, our results strongly suggest two regimes: helices shorter or longer than the mesh size.

Pink noise of ionic conductance through single artificial nanopores revisited.

We report voltage-clamp measurements through single conical nanopore obtained by chemical etching of a single ion track in polyimide film. Special attention is paid to the pink noise of the ionic current (i.e., 1/f noise) measured with different filling liquids. The relative pink-noise amplitude is almost independent of concentration and pH for KCl solutions, but varies strongly using ionic liquids. In particular, we show that depending on the ionic liquid, the transport of charge carriers is strongly facilitated (low noise and higher conductivity than in the bulk) or jammed. These results show that the origin of the pink noise can be ascribed neither to fluctuations of the pore geometry nor to the pore wall charges, but rather to a cooperative effect on ions motion in confined geometry.

Fluctuations of ionic current through lipid bilayers at the onset of peptide attacks and pore formation.

Voltage-clamp measurements on lipid bilayers at the onset of peptide attacks before pore formation are reported. With four different peptides [alamethicin, melittin, and two synthetic peptides of the leucine (L)-lysine(K) copolymers (LK series)], correlations of conductivity fluctuations slowly decay over four decades in time. This slow dynamics is interpreted as being due to fluctuations of peptide concentration at the crowded surface of the bilayer and found to be compatible with the t(-1/2) relaxation of the RSA model.

Scaling and continuum percolation model for enzyme-catalyzed gel degradation.

Enzyme-catalyzed gel degradation is inherently controlled by diffusion of enzymes in the gel. We report kinetics measurements on the gelatin-thermolysin system, varying solvent viscosity as well as gel and enzyme concentrations. Scaling relations and reduced variables are proposed which are shown to account for the experimental results. Finally, we argue that the nontrivial experimental dependence on enzyme concentration for the degradation time demonstrates that enzyme random walk is self-attracting, leading to a continuum percolation model for gel degradation.

Unfolding of proteins and long transient conformations detected by single nanopore recording.

We study the electrophoretic blockades due to entries of partially unfolded proteins into a nanopore as a function of the concentration of the denaturing agent. Short and long pore blockades are observed by electrical detection. Short blockades are due to the passage of completely unfolded proteins, their frequency increases as the concentration of the denaturing agent increases, following a sigmoidal denaturation curve. Long blockades reveal partially folded conformations. Their duration increases as the proteins are more folded. The observation of a Vogel-Fulcher law suggests a glassy behavior.

Aggregation during coniferyl alcohol polymerization in pectin solution: a biomimetic approach of the first steps of lignification.

Coniferyl alcohol was polymerized in pectin solution in order to mimic the lignification that is the final step of biosynthesis of plant cell wall. Dehydrogenated polymers (DHP = coniferyl alcohol polymers = synthetic lignin) interact with pectin to form hydrophobic clusters as monitored by pyrene fluorescence spectroscopy. The structure of these clusters was studied during the polymerization of synthetic lignin by static and quasielastic light scattering and small angle neutron scattering experiments. We show that synthetic lignin and pectin contribute to the same clusters, but the inner structure of these clusters is very heterogeneous and displays three phases. One observes a segregation between well separated pectin and lignin rich phases at length scales below approximately 30 nm. As a corollary of this segregation, clusters embody a large amount of solvent. On average, the density of the polymer rich phase (lignin plus pectin) inside clusters increases while its specific surface area decreases throughout the polymerization process. These results are discussed with respect to in vivo lignification of the plant cell wall.

Percolation model for enzyme gel degradation.

We study a model for the gel degradation by an enzyme, where the gel is schematized as a cubic lattice, and the enzyme as a random walker, that cuts the bonds over which it passes. The model undergoes a (reverse) percolation transition, which for low density of enzymes falls in a universality class different from random percolation. In particular, we have measured a gel fraction critical exponent beta=1.0+/-0.1, in excellent agreement with experiments made on the real system.

Enzyme-catalyzed gel proteolysis: an anomalous diffusion-controlled mechanism.

Enzyme-catalyzed proteolysis of gelatin gels has been studied. We report a gel degradation rate varying as the square of the enzyme concentration. The diffusion motion of enzymes in the gel has been measured by two-photon fluorescence correlation spectroscopy and identified as being anomalously slow. These experimental results are discussed from a theoretical point of view and interpreted in terms of a diffusion-controlled mechanism for the gel degradation. These results make a step toward the understanding of enzyme-catalyzed gel degradation and give new insight on biological processes such as the action of metalloproteinases in the extracellular matrix involved in cellular invasion.

Refolding of a high molecular weight protein: salt effect on collapse.

Small-angle neutron scattering experiments were performed on dilute solutions of a high molecular weight protein (fibronectin, M = 580 kg/mol) in four cases: native conditions; unfolded state obtained by a denaturing agent (urea); and two badly refolded (or collapsed) states obtained by progressive elimination of the denaturing agent in salt-containing or salt-free solutions. Our main result is concerned by the conformation of the protein as the attempt for refolding is driven with or without salt. In salt-containing solution, we observe unambiguously that the protein chain collapses at large length scales but still obeys to a Gaussian statistics at short length scales. In other words, the globule embodies a large quantity of solvent compared to the compact situation. In salt-free solutions, the badly refolded protein is not globular but displays both a coil-like and an open conformation at large length scales and a local high density area. This behavior is discussed with respect to the scaling theories for polymers and polyampholytes.

Temperature-induced beta-aggregation of fibronectin in aqueous solution.

Fibronectin structural reorganization induced by temperature has been investigated by Fourier-transform infrared (FT-IR) spectroscopy and light-scattering experiments. At 20 degrees C, from resolution enhanced by FT-IR spectra, 43% of beta sheet, 31% of turn and 26% of unordered structures were estimated. Static and quasi-elastic light-scattering results do not change significantly between 20 and 34 degrees C. Just below 50 degrees C, a decrease of 1/3 of beta sheet structures contents is observed, concomitantly with a corresponding increase of turn. The contribution of disordered structures is found to be temperature-independent. Above 50 degrees C, our data reveals the formation of intermolecular hydrogen bonding leading to the formation of intermolecular beta sheet structures. The IR band absorption at 1618 cm(-1) increases strongly as a function of temperature. The scattered intensity increases and becomes strongly q(2)-dependent. The dynamic structure factor is not a single exponential decay and becomes strongly dependent on the scattering angle. These results demonstrate that aggregation occurs in fibronectin solution. When temperature decreases, this aggregation is found irreversible. Fibronectin aggregation is driven by the formation of intermolecular hydrogen bonds responsible for intermolecular beta sheet structures.

Critical behavior of gelation probed by the dynamics of latex spheres.

We report a quasielastic light scattering study of the dynamics of large latex probe particles (R=225 nm) in gelatin solution undergoing gelation. We show that by focusing on the short-time and long-time behavior of the autocorrelation function, it is possible to simply interpret out data in terms of the divergence of the viscosity and emergence of the shear elastic modulus near the gel point. Our crude analysis allows us to grasp the critical behavior of gelation and to obtain the two critical exponents of the transport properties.

Gel-sol transition can describe the proteolysis of extracellular matrix gels.

We monitored the cell-free solubilization of extracellular matrix and fibronectin gels, catalyzed by exogenous proteinases. The corresponding measurements could not be interpreted according to usual proteinase kinetics. The observation that this experimental system did not consist in surface but in bulk degradation and appeared specific to gel substrates, incited us to use gelation-related approaches to describe these kinetics. We show that the gel-sol transition theory adequately describes the enzyme reactions. This allowed formulation and experimental confirmation of a power law relating macroscopic changes of the gel to enzyme kinetics. This approach could also be used for other power laws predicted by the gel-sol transition theory, allowing a better understanding of macroscopic modification of the extracellular matrix during proteolysis, which is implied in many biological situations, especially tumor dissemination.

Statistical conformation of human plasma fibronectin.

Fibronectin is a multifunctional glycoprotein (molecular mass, M = 530 kg/mol) of the extra cellular matrix (ECM) having a major role in cell adhesion. In physiological conditions, the conformation of this protein still remains debated and controversial. Here, we present a set of results obtained by scattering experiments. In "native" conditions, the radius of gyration (R(g) = 15.3 +/- 0.3 nm) was determined by static light scattering as well as small-angle neutron scattering. The hydrodynamic radius (R(H) = 11.5 +/- 0.1 nm) was deduced from quasi-elastic light scattering measurements. These results imply a low internal concentration compared to that of usual globular proteins. This is also confirmed by the ratio R(H)/R(g) = 0. 75 +/- 0.02 consistent with a Gaussian chain, whereas R(H)/R(g) = 1. 3 for spherical shaped molecules. However, adding a denaturing agent (urea 8 M) increases R(g) by a factor 2. This means that fibronectin "native" chain is not either completely unfolded. The average shape of fibronectin conformation was also probed by small-angle neutron scattering performed for reverse scattering vector q(-)(1) smaller than R(g) (0.2 < q(-)(1) < 15 nm). The measured form factor is in complete agreement with the form factor of a random string of 56 beads of 5 nm diameter. It rules out the possibility of unfolded chain as well as globular structures. These results have structural and biological implications as far as ECM organization is concerned.