Physico-chemical properties of silica in relation to its toxicity

Surface chemistry architecture of silica nanoparticles determine the efficiency of in vivo fluorescence lymph node mapping

Near-infrared (NIR) imaging of the lymphatic system offers a sensitive, versatile, and accurate lymph node mapping to locate the first, potentially metastatic, draining nodes in the operating room. Many luminescent nanoprobes have received great attention in this field, and the design of nontoxic and bright nanosystems is of crucial importance. Fluorescent NIR-emitting dye doped silica nanoparticles represent valuable platforms to fulfill these scopes, providing sufficient brightness, resistance to photobleaching, and hydrophilic nontoxic materials. Here, we synthesized these highly stable core-shell nanoparticles with a programmable surface charge positioning and determined the effect of these physicochemical properties on their in vivo behavior. In addition, we characterized their fluorescence kinetic profile in the right axillary lymph node (RALN) mapping. We found that nanoparticles with negative charges hidden by a PEG shell are more appropriate than those with external negative charges in the mapping of lymph nodes. We also demonstrated the efficient excretion of these nanostructures by the hepatobiliary route and their nontoxicity in mice up to 3 months postinjection. These results indicate the potential future development of these fluorescent nanosystems for LN mapping.

In search of the chemical basis of the hemolytic potential of silicas

The membranolytic activity of silica particles toward red blood cells (RBCs) has been known for a long time and is sometimes associated with silica pathogenicity. However, the molecular mechanism and the reasons why hemolysis differs according to the silica form are still obscure. A panel of 15 crystalline (pure and commercial) and amorphous (pyrogenic, precipitated from aqueous solutions, vitreous) silica samples differing in size, origin, morphology, and surface chemical composition were selected and specifically prepared. Silica particles were grouped into six groups to compare their potential in disrupting RBC membranes so that one single property differed in each group, while other features were constant. Free radical production and crystallinity were not strict determinants of hemolytic activity. Particle curvature and morphology modulated the hemolytic effect, but silanols and siloxane bridges at the surface were the main actors. Hemolysis was unrelated to the overall concentration of silanols as fully rehydrated surfaces (such as those obtained from aqueous solution) were inert, and one pyrogenic silica also lost its membranolytic potential upon progressive dehydration. Overall results are consistent with a model whereby hemolysis is determined by a defined surface distribution of dissociated/undissociated silanols and siloxane groups strongly interacting with specific epitopes on the RBC membrane.

Mesoporous silica nanoparticle nanocarriers: biofunctionality and biocompatibility

The study of ordered mesoporous silica materials has exploded since their discovery by Mobil researchers 20 years ago. The ability to make uniformly sized, porous, and dispersible nanoparticles using colloidal chemistry and evaporation-induced self-assembly has led to many applications of mesoporous silica nanoparticles (MSNPs) as "nanocarriers" for delivery of drugs and other cargos to cells. The exceptionally high surface area of MSNPs, often exceeding 1000 m²/g, and the ability to independently modify pore size and surface chemistry, enables the loading of diverse cargos and cargo combinations at levels exceeding those of other common drug delivery carriers such as liposomes or polymer conjugates. This is because noncovalent electrostatic, hydrogen-bonding, and van der Waals interactions of the cargo with the MSNP internal surface cause preferential adsorption of cargo to the MSNP, allowing loading capacities to surpass the solubility limit of a solution or that achievable by osmotic gradient loading. The ability to independently modify the MSNP surface and interior makes possible engineered biofunctionality and biocompatibility. In this Account, we detail our recent efforts to develop MSNPs as biocompatible nanocarriers (Figure 1 ) that simultaneously display multiple functions including (1) high visibility/contrast in multiple imaging modalities, (2) dispersibility, (3) binding specificity to a particular target tissue or cell type, (4) ability to load and deliver large concentrations of diverse cargos, and (5) triggered or controlled release of cargo. Toward function 1, we chemically conjugated fluorescent dyes or incorporated magnetic nanoparticles to enable in vivo optical or magnetic resonance imaging. For function 2, we have made MSNPs with polymer coatings, charged groups, or supported lipid bilayers, which decrease aggregation and improve stability in saline solutions. For functions 3 and 4, we have enhanced passive bioaccumulation via the enhanced permeability and retention effect by modifying the MSNP surfaces with positively charged polymers. We have also chemically attached ligands to MSNPs that selectively bind to receptors overexpressed in cancer cells. We have used encapsulation of MSNPs within reconfigurable supported lipid bilayers to develop new classes of responsive nanocarriers that actively interact with the target cell. Toward function 4, we exploit the high surface area and tailorable surface chemistry of MSNPs to retain hydrophobic drugs. Finally, for function 5, we have engineered dynamic behaviors by incorporating molecular machines within or at the entrances of MSNP pores and by using ligands, polymers, or lipid bilayers. These provide a means to seal-in and retain cargo and to direct MSNP interactions with and internalization by target cells. Application of MSNPs as nanocarriers requires biocompatibility and low toxicity. Here the intrinsic porosity of the MSNP surface reduces the extent of hydrogen bonding or electrostatic interactions with cell membranes as does surface coating with polymers or lipid bilayers. Furthermore, the high surface area and low extent of condensation of the MSNP siloxane framework promote a high rate of dissolution into soluble silicic acid species, which are found to be nontoxic. Potential toxicity is further mitigated by the high drug capacity of MSNPs, which greatly reduces needed dosages compared with other nanocarriers. We anticipate that future generations of MSNPs incorporating molecular machines and encapsulated by membrane-like lipid bilayers will achieve a new level of controlled cellular interactions.

Evaluation of the antibacterial activity and biocompatibility for silver nanoparticles immobilized on nano silicate platelets

Silver nanoparticles (AgNPs) are known for their bactericidal abilities. The antibacterial potency is dependent on the particle size and dispersion status. In this study, we synthesized AgNP/NSP nanohybrids in two different weight ratios (1/99 and 8/92) using the fully exfoliated clay, i.e., nanosilicate platelets (NSP), as a dispersing agent and carrier for AgNPs. Due to the size of NSP, the immobilized AgNPs do not enter cells readily, which may lower the risk associated with the cellular uptake of AgNPs. The biocompatibility, immunological response, and antimicrobial activities of AgNP/NSP hybrids were evaluated. The results revealed that AgNP/NSP hybrids elicited merely mild inflammatory response and retained the outstanding antibacterial activity. The hybrids were further embedded in poly(ether)urethane (PEU) to increase the biocompatibility. At the same silver content (20 ppm), the PEU-AgNP/NSP nanocomposites were nontoxic to mouse skin fibroblasts, while simultaneously exhibiting nearly complete bacterial growth reduction (99.9%). PEU containing the same amount of free AgNPs did not display such an effect. Our results verify the better biosafety of the AgNPs/NSP hybrids and their polymer nanocomposites for further clinical use.

Processing pathway dependence of amorphous silica nanoparticle toxicity: colloidal vs pyrolytic

We have developed structure/toxicity relationships for amorphous silica nanoparticles (NPs) synthesized through low-temperature colloidal (e.g., Stöber silica) or high-temperature pyrolysis (e.g., fumed silica) routes. Through combined spectroscopic and physical analyses, we have determined the state of aggregation, hydroxyl concentration, relative proportion of strained and unstrained siloxane rings, and potential to generate hydroxyl radicals for Stöber and fumed silica NPs with comparable primary particle sizes (16 nm in diameter). On the basis of erythrocyte hemolytic assays and assessment of the viability and ATP levels in epithelial and macrophage cells, we discovered for fumed silica an important toxicity relationship to postsynthesis thermal annealing or environmental exposure, whereas colloidal silicas were essentially nontoxic under identical treatment conditions. Specifically, we find for fumed silica a positive correlation of toxicity with hydroxyl concentration and its potential to generate reactive oxygen species (ROS) and cause red blood cell hemolysis. We propose fumed silica toxicity stems from its intrinsic population of strained three-membered rings (3MRs) along with its chainlike aggregation and hydroxyl content. Hydrogen-bonding and electrostatic interactions of the silanol surfaces of fumed silica aggregates with the extracellular plasma membrane cause membrane perturbations sensed by the Nalp3 inflammasome, whose subsequent activation leads to secretion of the cytokine IL-1β. Hydroxyl radicals generated by the strained 3MRs in fumed silica, but largely absent in colloidal silicas, may contribute to the inflammasome activation. Formation of colloidal silica into aggregates mimicking those of fumed silica had no effect on cell viability or hemolysis. This study emphasizes that not all amorphous silicas are created equal and that the unusual toxicity of fumed silica compared to that of colloidal silica derives from its framework and surface chemistry along with its fused chainlike morphology established by high-temperature synthesis (>1300 °C) and rapid thermal quenching.

Hemolytic properties of synthetic nano- and porous silica particles: the effect of surface properties and the protection by the plasma corona

Novel silica materials incorporating nanotechnology are promising materials for biomedical applications, but their novel properties may also bring unforeseen behavior in biological systems. Micro-size silica is well documented to induce hemolysis, but little is known about the hemolytic activities of nanostructured silica materials. In this study, the hemolytic properties of synthetic amorphous silica nanoparticles with primary sizes of 7-14 nm (hydrophilic vs. hydrophobic), 5-15 nm, 20 nm and 50 nm, and model meso/macroporous silica particles with pore diameters of 40 nm and 170 nm are investigated. A crystalline silica sample (0.5-10 μm) is included for benchmarking purposes. Special emphasis is given to investigations of how the temperature and solution complexity (solvent, plasma), as well as the physicochemical properties (such as size, surface charge, hydrophobicity and other surface properties), link to the hemolytic activities of these particles. Results suggests the potential importance of small size and large external surface area, as well as surface charge/structure, in the hemolysis of silica particles. Furthermore, a significant correlation is observed between the hemolytic profile of red blood cells and the cytotoxicity profile of human promyelocytic leukemia cells (HL-60) induced by nano- and porous silica particles, suggesting a potential universal mechanism of action. Importantly, the results generated suggest that the protective effect of plasma towards silica nanoparticle-induced hemolysis as well as cytotoxicity is primarily due to the protein/lipid layer shielding the silica particle surface. These results will assist the rational design of hemocompatible silica particles for biomedical applications.

The toxicological mode of action and the safety of synthetic amorphous silica-a nanostructured material

Synthetic amorphous silica (SAS), in the form of pyrogenic (fumed), precipitated, gel or colloidal SAS, has been used in a wide variety of industrial and consumer applications including food, cosmetics and pharmaceutical products for many decades. Based on extensive physico-chemical, ecotoxicology, toxicology, safety and epidemiology data, no environmental or health risks have been associated with these materials if produced and used under current hygiene standards and use recommendations. With internal structures in the nanoscale size range, pyrogenic, precipitated and gel SAS are typical examples of nanostructured materials as recently defined by the International Organisation for Standardisation (ISO). The manufacturing process of these SAS materials leads to aggregates of strongly (covalently) bonded or fused primary particles. Weak interaction forces (van der Waals interactions, hydrogen bonding, physical adhesion) between aggregates lead to the formation of micrometre (μm)-sized agglomerates. Typically, isolated nanoparticles do not occur. In contrast, colloidal SAS dispersions may contain isolated primary particles in the nano-size range which can be considered nano-objects. The size of the primary particle resulted in the materials often being considered as "nanosilica" and in the inclusion of SAS in research programmes on nanomaterials. The biological activity of SAS can be related to the particle shape and surface characteristics interfacing with the biological milieu rather than to particle size. SAS adsorbs to cellular surfaces and can affect membrane structures and integrity. Toxicity is linked to mechanisms of interactions with outer and inner cell membranes, signalling responses, and vesicle trafficking pathways. Interaction with membranes may induce the release of endosomal substances, reactive oxygen species, cytokines and chemokines and thus induce inflammatory responses. None of the SAS forms, including colloidal nano-sized particles, were shown to bioaccumulate and all disappear within a short time from living organisms by physiological excretion mechanisms with some indications that the smaller the particle size, the faster the clearance is. Therefore, despite the new nomenclature designating SAS a nanomaterial, none of the recent available data gives any evidence for a novel, hitherto unknown mechanism of toxicity that may raise concerns with regard to human health or environmental risks. Taken together, commercial SAS forms (including colloidal silicon dioxide and surface-treated SAS) are not new nanomaterials with unknown properties, but are well-studied materials that have been in use for decades.

Sorption of aromatic ionizable organic compounds to montmorillonites modified by hexadecyltrimethyl ammonium and polydiallyldimethyl ammonium

Environmental residues of aromatic ionizable organic compounds (AIOCs) have received considerable attention due to their potential human health and ecological risks. The main objective of this study was to investigate the key factors and mechanisms controlling sorption of a series of anionic and zwitterionic AIOCs (two aromatic sulfonates, 4-methyl-2,6-dinitrophenol, tetracycline, sulfamethoxazole, and tannic acid) to montmorillonites modified with hexadecyltrimethyl ammonium (HDTMA) and polydiallyldimethyl ammonium (PDADMA). Compared with naphthalene (a nonpolar and nonionic solute), all AIOCs showed stronger sorption (the sorbent-to-solution distribution coefficient was in the order of 10-10 L kg) to the two organoclays in spite of the much lower hydrophobicity, indicating the predominance of electrostatic interaction in sorption. The proposed electrostatic mechanism of the tested AIOCs was supported by the pH dependency of sorption to the two organoclays. The two organoclays manifested weaker sorption affinity but faster sorption kinetics for bulky AIOCs than commercial activated carbon, resulting from the high accessibility of sorption sites in the open, ordered clay interlayer. The findings of this study highlight the potential of using HDTMA- and PDADMA-exchanged montmorillonites as effective sorbents for AIOCs in water and wastewater treatments.

Investigation of the interface in silica-encapsulated liposomes by combining solid state NMR and first principles calculations

In the context of nanomedicine, liposils (liposomes and silica) have a strong potential for drug storage and release schemes: such materials combine the intrinsic properties of liposome (encapsulation) and silica (increased rigidity, protective coating, pH degradability). In this work, an original approach combining solid state NMR, molecular dynamics, first principles geometry optimization, and NMR parameters calculation allows the building of a precise representation of the organic/inorganic interface in liposils. {(1)H-(29)Si}(1)H and {(1)H-(31)P}(1)H Double Cross-Polarization (CP) MAS NMR experiments were implemented in order to explore the proton chemical environments around the silica and the phospholipids, respectively. Using VASP (Vienna Ab Initio Simulation Package), DFT calculations including molecular dynamics, and geometry optimization lead to the determination of energetically favorable configurations of a DPPC (dipalmitoylphosphatidylcholine) headgroup adsorbed onto a hydroxylated silica surface that corresponds to a realistic model of an amorphous silica slab. These data combined with first principles NMR parameters calculations by GIPAW (Gauge Included Projected Augmented Wave) show that the phosphate moieties are not directly interacting with silanols. The stabilization of the interface is achieved through the presence of water molecules located in-between the head groups of the phospholipids and the silica surface forming an interfacial H-bonded water layer. A detailed study of the (31)P chemical shift anisotropy (CSA) parameters allows us to interpret the local dynamics of DPPC in liposils. Finally, the VASP/solid state NMR/GIPAW combined approach can be extended to a large variety of organic-inorganic hybrid interfaces.

Aqueous colloidal mesoporous nanoparticles with ethenylene-bridged silsesquioxane frameworks

Aqueous colloidal mesoporous nanoparticles with ethenylene-bridged silsesquioxane frameworks with a uniform diameter of ∼20 nm were prepared from bis(triethoxysilyl)ethenylene in a basic aqueous solution containing cationic surfactants. The nanoparticles, which had higher hydrolysis resistance under aqueous conditions, showed lower hemolytic activity toward bovine red blood cells than colloidal mesoporous silica nanoparticles.

Influence of size, surface area and microporosity on the in vitro cytotoxic activity of amorphous silica nanoparticles in different cell types

Identifying the physico-chemical characteristics of nanoparticles (NPs) that drive their toxic activity is the key to conducting hazard assessment and guiding the design of safer nanomaterials. Here we used a set of 17 stable suspensions of monodisperse amorphous silica nanoparticles (SNPs) with selected variations in size (diameter, 2-335 nm), surface area (BET, 16-422 m(2)/g) and microporosity (micropore volume, 0-71 microl/g) to assess with multiple regression analysis the physico-chemical determinants of the cytotoxic activity in four different cell types (J774 macrophages, EAHY926 endothelial cells, 3T3 fibroblasts and human erythrocytes). We found that the response to these SNPs is governed by different physico-chemical parameters which vary with cell type: In J774 macrophages, the cytotoxic activity (WST1 assay) increased with external surface area (alphas method) and decreased with micropore volume (r(2) of the model, 0.797); in EAHY926 and 3T3 cells, the cytotoxic activity of the SNPs (MTT and WST1 assay, respectively) increased with surface roughness and small diameter (r(2), 0.740 and 0.872, respectively); in erythrocytes, the hemolytic activity increased with the diameter of the SNP (r(2), 0.860). We conclude that it is possible to predict with good accuracy the in vitro cytotoxic potential of SNPs on the basis of their physico-chemical characteristics. These determinants are, however, complex and vary with cell type, reflecting the pleiotropic interactions of nanoparticles with biological systems.

Interactions between giant unilamellar vesicles and charged core-shell magnetic nanoparticles

This work combined two tools, giant unilamellar vesicles (GUVs) and core-shell magnetic nanoparticles (CSMNs), to develop a simplified model for studying interactions between the cell membrane and nanoparticles. We focused on charged functionalized CSMNs that can be either cationic or anionic. Using optical, electron, and confocal microscopy, we found that giant vesicle-nanoparticle interactions did not result from a simple electrostatic phenomenon because cationic CSMNs tended to bind to positively charged bilayers, whereas anionic CSMNs remained inert.

The use of magnetite nanoparticles for implant-assisted magnetic drug targeting in thrombolytic therapy

Implant-assisted targeting of magnetic particles under the influence of an external magnetic field has previously been verified through mathematical modeling, in vitro studies, and in vivo studies on rat carotid arteries as a feasible method for localized drug delivery. The present study focuses on the development of nanoparticles for the treatment of in-stent thrombosis. Magnetic nanoparticles in the size-range 10-30 nm were synthesized in a one-pot procedure by precipitation of ferrous hydroxide followed by oxidation to magnetite. The nanoparticles were silanized with tetraethyl orthosilicate in the presence of triethylene glycol and/or polyethylene glycol. The surface coated magnetite nanoparticles were activated with either N-hydroxysulfosuccinimide or tresyl chloride for covalent immobilization of tissue plasminogen activator (tPA). Hysteresis loops showed saturation magnetizations of 55.8, 44.1, and 43.0 emu/g for the naked nanoparticles, the surface coated nanoparticles, and the tPA-nanoparticle conjugates, respectively. The hemolytic activity of the nanoparticles in blood was negligible. An initial in vivo biocompatibility test in pig, carried out by intravascular injection of the nanoparticles in a stented brachial artery, showed no short-term adverse effects. In vitro evaluation in a flow-through model proved that the nanoparticles were captured efficiently to the surface of a ferromagnetic coiled wire at the fluid velocities typical for human arteries. A preliminary test of the tPA-nanoparticle conjugates in a pig model suggested that the conjugates may be used for treatment of in-stent thrombosis in coronary arteries.

An integrated approach to the study of the interaction between proteins and nanoparticles

The rapid development of nanotechnology has raised some concerns about the effects of engineered nanoparticles (NPs) on human health and the environment. At the same time, NPs have attracted intense interest because of their potential applications in biomedicine. Hence, the requirement of detailed knowledge of what takes place at the molecular level when NPs get inside living organisms is a necessary step in assessing and likely predicting the behavior of an NP. The elicited effects strongly depend on the early events occurring when NPs reach biological fluids, where the interaction with proteins is the primary process. Whereas the adsorption of proteins on biomaterials has been thoroughly investigated, the mechanisms underlying the interaction of proteins with NPs are still largely unexplored. Here we report a study of the behavior of four model proteins differing in their resistance to conformational changes, net charge, and surface charge distributions, adsorbed on two nanometric silica powders with distinct hydrophilicity. An integrated picture of the adsorption process has been obtained by applying a whole set of techniques: the extent of coverage of the silica surface and the reversibility of the process were evaluated by combining the adsorption isotherms with the changes in the zeta potential and the point of zero charge for NPs at different protein coverages; the occurrence of protein deformation was evaluated by Raman spectroscopy, and EPR spectroscopy of spin-labeled proteins provided insight into their orientation on the silica surface. We have found that the extent of coverage of the nanoparticle surface is strongly influenced by the protein structural stability as well as by the distribution of charges at the protein surface.

Biomimetic nanoparticles: preparation, characterization and biomedical applications

Mimicking nature is a powerful approach for developing novel lipid-based devices for drug and vaccine delivery. In this review, biomimetic assemblies based on natural or synthetic lipids by themselves or associated to silica, latex or drug particles will be discussed. In water, self-assembly of lipid molecules into supramolecular structures is fairly well understood. However, their self-assembly on a solid surface or at an interface remains poorly understood. In certain cases, hydrophobic drug granules can be dispersed in aqueous solution via lipid adsorption surrounding the drug particles as nanocapsules. In other instances, hydrophobic drug molecules attach as monomers to borders of lipid bilayer fragments providing drug formulations that are effective in vivo at low drug-to-lipid-molar ratio. Cationic biomimetic particles offer suitable interfacial environment for adsorption, presentation and targeting of biomolecules in vivo. Thereby antigens can effectively be presented by tailored biomimetic particles for development of vaccines over a range of defined and controllable particle sizes. Biomolecular recognition between receptor and ligand can be reconstituted by means of receptor immobilization into supported lipidic bilayers allowing isolation and characterization of signal transduction steps.

Cell viability in a wet silica gel

A modified two-step sol-gel route using silicon ethoxide (TEOS) has been used to synthesize amorphous sol-gel-derived silica, which has been successfully used as a cell encapsulation matrix for 3T3 mouse fibroblasts and CRL-2595 epithelial cells due to its non-toxicity. The sol-gel procedure comprised a first, low pH hydrolysis step, followed by a neutral condensation-gelation step. A high water-to-TEOS ratio and the addition of d-glucose as a porogen and source of nutrients were chosen to minimize silica dissolution and improve the biocompatibility of the process. Indeed, the cell integrity in the encapsulation process was preserved by alcohol removal from the starting solution. Cells were then added in a buffered medium, causing rapid gelation and entrapment of the cells within a randomly structured siloxane matrix in the shape of a monolith, which was maintained in the wet state. MTT and alamarBlue assays were used to check the cytotoxicity of the silica gels and the viability of entrapped cells at initial times in contact with silica. To improve cell attachment, cell clumping experiments - where groups of cells were formed - were designed, rendering improved viability. The obtained materials are therefore excellent candidates for designing tissue-culture scaffolds and implantable bioreactors for biomedical applications.

Surface-Dependent Quartz Uptake by Macrophages: Potential Role in Pulmonary Inflammation and Lung Clearance

Inhalation of quartz particles is associated with a variety of adverse lung effects. Since particle surface is considered to be crucial for particle pathogenicity, we investigated the influence of quartz surface properties on lung burden, inflammation (bronchoalveolar lavage cells), and cytotoxicity (protein, lactate dehydrogenase, beta-glucuronidase) 90 days after a single intratracheal instillation of 2 mg DQ12 into rats. The role of particle surface characteristics was investigated by comparative investigation of native versus surface-modified quartz, using polyvinylpyridine N-oxide (PVNO) or aluminum lactate (AL) coating. Uptake and subcellular localization of quartz samples as well as tumor necrosis factor (TNF)-alpha release were determined using NR8383 rat alveolar macrophages. Surface modification of quartz particles resulted in marked in vivo and in vitro changes. Compared to native quartz, modified quartz samples showed lower lung burden at 90 days, as well as decreased inflammatory and cytotoxic responses. Coating with polyvinylpyridine N-oxide (PVNO) appeared to be more effective than aluminium lactate (AL). PVNO-coating of quartz also resulted in an enhanced particle uptake by macrophages up to 24 h, whereas AL coating caused a transient reduction of quartz uptake at 2 h. At 24 h differences with the native quartz were absent. Subcellular localization of quartz particles was not affected by surface modifications. However, surface modification resulted in a reduced release of TNF-alpha. In conclusion, surface properties of quartz particles appear to be crucial for rate and extent of in vitro particle uptake in macrophages. Our in vivo findings also indicate that quartz surface properties may affect clearance kinetics. Particle surface-specific interactions between quartz and macrophages may therefore play a major role in the pulmonary pathogenicity of quartz.

Biomembrane phospholipid-oxide surface interactions: crystal chemical and thermodynamic basis

Quartz has the least favored surface among many oxides for bacterial attachment and for lipid bilayer or micelle interactions. Tetrahedrally coordinated crystalline silica polymorphs are membranolytic toward liposomes, lysosomes, erythrocytes, and macrophages. Amorphous silica, the octahedral silica polymorph, (stishovite), and oxides such as Al2O3, Fe2 O3, and TiO2 are less cytotoxic. Existing theories for membrane rupture that invoke interactions between oxide surfaces and cell membrane phospholipids (PLs) do not adequately explain these differences in membranolytic potential of the oxides. The author presents a crystal chemical, thermodynamic model for the initial interaction of oxide surfaces with the quaternary ammonium component of the PL's polar head group. The model includes solvation energy changes and electrostatic forces during adsorption, represented by the dielectric constant of the solid and the charge-to-radius ratio of the adsorbing solute. The nature of oxide-solute interactions compared with oxide-water, solute-water, and water-water interactions determines the membranolytic activity of the oxide, where the solute is TMA+, the quaternary ammonium moeity. Significant membrane rupture, as on quartz, requires unfavorable adsorption entropy (DeltaS(ads,TMA+)<0) to maximize disruption of normal membrane structure and requires favorable Gibbs free energy of exchange between TMA+ and the ambient Na+ ions (DeltaG(exc,TMA+/Na+) = DeltaG(ads,TMA+)-DeltaG(ads,Na+)<0) to maximize the extent of membrane affected. For amorphous silica, DeltaS(ads,TMA+) >0, so disruption of structure is limited, even though G(exc,TMA+/Na+) is <0. Stishovite and other oxides have DeltaS(ads,TMA+) <0, but now DeltaG(exc,TMA+/Na+) is>0 at the acidic to circumneutral pHs of cellular and subcellular organelle fluids. The model predicts the correct sequence of membranolytic ability: quartz > or = amorphous SiO2 >Al2O3 >Fe2O3 >TiO2. The model thus explains the relatively poor adhesion of bacterial cells to quartz and the lack of quartz as a biomineral. It is proposed that one function of extracellular polymeric substances exuded by bacteria is to render mineral surfaces more hydrophilic.

Nuclear polyglutamine-containing protein aggregates as active proteolytic centers

Protein aggregates and nuclear inclusions (NIs) containing components of the ubiquitin-proteasome system (UPS), expanded polyglutamine (polyQ) proteins, and transcriptional coactivators characterize cellular responses to stress and are hallmarks of neurodegenerative diseases. The biological function of polyQ-containing aggregates is unknown. To analyze proteasomal activity within such aggregates, we present a nanoparticle (NP)-based method that enables controlled induction of sodium dodecyl sulfate-resistant inclusions of endogenous nuclear proteins while normal regulatory mechanisms remain in place. Consistent with the idea that the UPS maintains quality control, inhibition of proteasomal proteolysis promotes extra large protein aggregates (1.4-2 mum), whereas formation of NP-induced NIs is found to be inversely correlated to proteasome activation. We show that global proteasomal proteolysis increases in NP-treated nuclei and, on the local level, a subpopulation of NIs overlaps with focal domains of proteasome-dependent protein degradation. These results suggest that inclusions in the nucleus constitute active proteolysis modules that may serve to concentrate and decompose damaged, malfolded, or misplaced proteins.

Development of an empirical force field for silica. Application to the quartz-water interface

Interactions of pulverized crystalline silica with biological systems, including the lungs, cause cell damage, inflammation, and apoptosis. To allow computational atomistic modeling of these pathogenic processes, including interactions between silica surfaces and biological molecules, new parameters for quartz, compatible with the CHARMM empirical force field were developed. Parameters were optimized to reproduce the experimental geometry of alpha-quartz, ab initio vibrational spectra, and interactions between model compounds and water. The newly developed force field was used to study interactions of water with two singular surfaces of alpha-quartz, (011) and (100). Properties monitored and analyzed include the variation of the density of water molecules in the plane perpendicular to the surface, disruption of the water H-bond network upon adsorption, and space-time correlations of water oxygen atoms in terms of Van Hove self-correlation functions. The vibrational density of states spectra of water in confined compartments were also computed and compared with experimental neutron-scattering results. Both the attenuation and shifting to higher frequencies of the hindered translational peaks upon confinement are clearly reproduced by the model. However, an upshift of librational peaks under the conditions of model confinement still remains underrepresented at the current empirical level.

A comparative study of unrelaxed surfaces on quartz and kaolinite, using the periodic density functional theory

To investigate surface properties of fractured silica particles, which are commonly connected to the etiology of silica toxicity, models of low-index unrelaxed surfaces of quartz and kaolinite were constructed and analyzed using the periodic density functional theory calculations. The models were used to investigate surface sites that emerge in the processes of heterolytic and homolytic cleavage of quartz. It is found that the quartz surface is stabilized by two types of interactions. One, due to a more even charge distribution of sites, was characterized by surface energies of up to 0.025 eV x A(-2) and the other, due to a more even oxygen distribution between complementary surfaces, was up to 0.036 eV x A(-2). The total specific surface energies of unrelaxed surfaces ranged from 0.161 to 0.200 eV x A(-2) for quartz and from 0.017 to 0.158 eV x A(-2) for kaolinite. For the conchoidal fracture of quartz an average specific surface energy of 0.187 eV x A(-2) was obtained. These results provide a foundation for further characterization of the surface properties of mechanically comminuted respirable silica particulate and for reduction of occupational health hazards due to pulverized silica.

Structure and stability of the (001) α-quartz surface

The structure and surface energies of the cleaved, reconstructed, and fully hydroxylated (001) alpha-quartz surface of various thicknesses are investigated with periodic density functional theory (DFT). The properties of the cleaved and hydroxylated surface are reproduced with a slab thickness of 18 atomic layers, while a thicker 27-layer slab is necessary for the reconstructed surface. The performance of the hybrid DFT functional B3LYP, using an atomic basis set, is compared with the generalised gradient approximation, PBE, employing plane waves. Both methodologies give similar structures and surface energies for the cleaved and reconstructed surfaces, which validates studying these surfaces with hybrid DFT. However, there is a slight difference between the PBE and B3LYP approach for the geometry of the hydrogen bonded network on the hydroxylated surface. The PBE adsorption energy of CO on a surface silanol site is in good agreement with experimental values, suggesting that this method is more accurate for hydrogen bonded structures than B3LYP. New hybrid functionals, however, yield improved weak interactions. Since these functionals also give superior activation energies, we recommend applying the new functionals to contemporary issues involving the silica surface and adsorbates on this surface.

Impact of silanol surface density on the toxicity of silica aerosols measured by erythrocyte haemolysis

Exposures to silica-containing dusts are associated with a risk of developing life-threatening lung diseases. However, the mechanism of silica toxicity is poorly understood. In this work the atomic structure of the surfaces of different silica polymorphs was determined, and a relationship with in vitro silica toxicity was examined. The density of geminal and single silanol groups was quantitatively estimated for different silica polymorphs using a novel molecular modeling method. An association was found between the reported haemolytic activity and modeled densities of surface geminal (but not single) silanol groups on several silica polymorphs. These findings suggest a new view of aerosol toxicity based on the estimation of surface site densities. The results can be used in the development of new toxicological assays for respirable particulates, including nanomaterials.

The genotoxic risk of underground coal miners from Turkey

A cytogenetic monitoring study was carried out on a group of workers from a bituminous coal mine in Zonguldak province of Turkey, to investigate the genotoxic risk of occupational exposure to coal mine dust. Cytogenetic analysis, namely sister chromatid exchanges (SCEs), chromosomal aberrations (CAs) and micronucleus (MN) tests were performed on a strictly selected group of 39 workers and compared to 34 controls matched for gender, age, and habit. Smoking and age were considered as modulating factors. Both SCE and CA frequencies in coal miners appeared significantly higher than in controls. Similarly, there was a significant increase in the frequency of total micronuclei in exposed group as compared to control group. The effect of smoking on the level of SCE and MN was significant in the control group. A positive correlation between the age and the level of SCE was also found in controls. The frequencies of both SCE and CA were significantly enhanced with the years of exposure. The results of this study demonstrated that occupational exposure to coal mine dust leads to a significant induction of cytogenetic damage in peripheral lymphocytes of workers engaged in underground coal mining.

An update on the detoxification processes for silica particles and asbestos fibers: successess and limitations

Inhalation of asbestos fibers and crystalline silica produces a number of diseases including fibrosis and cancer. Investigations into the mechanisms involved in mineral particle-induced toxicity indicated the importance of their surfaces in the pathological consequences. Masking of the surface sites has therefore featured prominently in a number of detoxification processes that have been investigated. The majority of the detoxification processes were, however, conducted to elucidate the involvement of a particular surface site in the toxicity of a specific mineral. Others were investigated with the aim of large industrial applications to be applied during mining, handling, processing, transporting, and disposing of minerals. It can be concluded that, to date, there is no single detoxification process that could be applied universally to all different types of mineral particles. Those that have shown some success could not completely abolish all adverse effects. Further elucidation of mechanisms of particle-induced toxicity may open new possibilities for detoxification processes.

Roles of plasma proteins in the formation of silicotic nodules in rats

The contribution of plasma protein(s) to the stabilization of fibroids formed in rat lungs exposed to acute silica dust inhalation was examined. Antibodies against component proteins of the nodules remaining insoluble in 2% SDS, 10M urea and 40 mM sulfhydryl reagents under prolonged boiling conditions were raised in rabbits and used to capture plasma proteins, which were identified by 2D-gel electrophoresis and MALDI-TOF analysis. The silica particles were encapsulated with extracellular protein composites whose amino acid compositions showed high levels of alanine, i.e., above those of glycine and proline, a building block of collagen. Antibody-captured plasma proteins showed the dominant presences of fibrinogen, albumin, and prealbumin (transthyretin), and other minor proteins, which included alpha-1-protease inhibitor, contraspin-like protease inhibitor, cathepsin B, etc. The presence of the N(epsilon)-(gamma-glutamyl) lysine isopeptide bond in the nodules was evidenced by direct chemical methods and by immunoreactivity for anti-isopeptide bonds. Immunostaining of affected lung tissue and of the fibroid regions showed elevated levels of transglutaminase (TGase) E and plasma factor XIII (F-XIII), but showed no reactivity towards other TGases. These findings suggest that the silica encapsulated nodules are a mixture of extracellular proteins that include collagen type I, fibrin and transthyretin, which is stabilized by TGase catalyzed crosslinking between plasma and extracellular proteins during fibrosis to eventually form insoluble nodules.

Reconstruction of Pristine and Hydrolyzed Quartz Surfaces

Reconstruction of the most common pristine and hydrolyzed surfaces of quartz was investigated with periodic density functional theory calculations. Surface energies of reconstructed pristine faces, pertinent to quartz growth morphologies in melts, are found to range from 0.071 eV/A2 for the (101) surface to 0.139 eV/A2 for the (001) surface, and they increase as (101) < (102) < (112) < [(100), (111)] < (110) < (001). Four types of reconstruction reactions are observed: (1) formation of two-membered rings from vicinal silyl and siloxy sites, (2) formation of a pair of tricoordinated/unicoordinated oxygen atoms, (3) formation of three-membered rings, and (4) transformation of silanone sites into siloxane sites. The main features of reconstructed pristine quartz surfaces are two-membered rings formed from bridged siloxy and silyl sites on all investigated surfaces, a stable site complex with geminal positively charged tricoordinated and negatively charged unicoordinated oxygen atoms revealed on the (112) surface, and charged nonbridged siloxy/silyl sites, which are more stable than radical siloxy/silyl sites. Hydrolyzed surface energies range from -0.010 eV/A2 for the (001) surface to 0.002 eV/A2 for the (101) surface and increase as (001) < (110) < (102) < (111) < (100) < (112) < (101). The hydrolyzed surface stability is found to depend strongly on inter-site silanol hydrogen bonding. Observed networks of hydrogen bonds are important for interactions between silica surfaces and biomolecules in an aqueous environment.

Signaling pathways controlling the production of inflammatory mediators in response to crystalline silica exposure: role of reactive oxygen/nitrogen species

Occupational exposure to crystalline silica has been linked to pulmonary fibrosis and lung cancer. Surface properties of crystalline silica are critical to the production of oxidant species, chemokines, inflammatory cytokines, and proliferative factors involved in the initiation and progression of silica-induced damage, inflammation, alveolar type II cell hyperplasia, fibroblast activation, and disease. The transcription factors nuclear factor kappaB (NF-kappaB) and activator protein 1 (AP-1) have been shown to play key roles in gene promotion for inflammatory mediators, oncogenes, and growth factors. This review summarizes evidence that in vitro and in vivo exposure to crystalline silica results in activation of NF-kappaB and AP-1. Signaling pathways for activation of these transcription factors are described. In addition, the role of silica-induced reactive oxygen species and nitric oxide in the activation of these signaling events is presented. Last, the generalizability of mechanisms regulating silica-induced pulmonary responses to pulmonary reactions to other occupational particles is discussed.

Montserrat volcanic ash induces lymph node granuloma and delayed lung inflammation

OBJECTIVES

A substantial amount of Montserrat volcanic ash, containing up to 24% of cristobalite (w/w), a fibrogenic crystalline silica, has been generated since the first documented eruption in 1995. The bioreactivity of the ash and its two major components: cristobalite and anorthite have been studied in vivo for a year following intratracheal instillation into rats.

METHODS

The rats (n=5) were instilled with a sterile vehicle solution (0.15 M NaCl) and/or three doses (1.0, 2.5 or 5.0 mg) of each of the dust, and were sacrificed at 13, 25 or 49 weeks post-instillation for quantitative biochemical and histopathological analyses in the lung and lymph nodes.

RESULTS

Cristobalite caused inflammation in the lung and granuloma in the hilar lymph nodes associated with significant size augmentation at 13 weeks post-instillation (P<0.05) and cristobalite (5.0 mg) induced fibrosis in the lung at 49 weeks post-exposure. However, the Montserrat volcanic ash caused inflammation in the lung at 49 weeks post-treatment without any fibrogenic response although the ash (5.0 mg) triggered significant lymph node enlargement without significant changes in the lung at 13 weeks post-treatment (P<0.05). Dose and time independent responses in the anorthite-exposed lung and lymph nodes suggest that a single instillation of 5.0 mg of poorly soluble mineral dust does not induce any change in the lung or lymph nodes.

CONCLUSION

The ash produces inflammatory reactions in lymph nodes at 13 weeks post-instillation in rats. These effects are seen much earlier than any inflammatory reaction in the lung. The onset of the lung inflammation is delayed until 49 weeks post-exposure. Despite the high cristobalite content of the ash, there is no evidence of any lung fibrogenic responses.

Diseases caused by silica: mechanisms of injury and disease development

While silica particles are considered to be fibrogenic and carcinogenic agents, the mechanisms responsible are not well understood. This article summarizes literature on silica-induced accelerated silicosis, chronic silicosis, silico-tuberculosis, bronchogenic carcinoma, and immune-mediated diseases. This article also discusses the generation of reactive oxygen species (ROS) that occurs directly from the interaction of silica with aqueous medium and from silica-stimulated cells, the molecular mechanisms of silica-induced lung injuries with focus on silica-induced NF-kappaB activation, including its mechanisms, possible attenuation and relationship to silica-induced generation of cyclooxygenase II and TNF-alpha. Silica-induced AP-1 activation, protooncogene expression, and the role of ROS in these processes are also briefly discussed.

Ultrastructural effects of silicic acid on primary lung fibroblasts in tissue culture

Transmission and scanning electron microscopic examination of primary lung fibroblasts exposed in tissue culture to polymeric silicic acid (PSA) revealed profound cellular changes in the cell surface membranes, resulting in rapid endocytosis of affected membranes and formation of multivesicular bodies. Exposure to monomeric silicic acid did not appear to exhibit any immediate adverse effects. Appearance of numerous cytoplasmic vacuoles within 1 h of PSA exposure was easily visible by light microscopy. Electron microscopy revealed that PSA exposure caused formation of an 'osmiophilic' cell surface membrane. Numerous osmiophilic cytoplasmic blebs on the surface and subsequent endocytotic vesicles appeared to collapse and aggregate into multivesicular bodies. This study provides ultrastructural evidence of the direct interaction between lung fibroblasts and polymeric silicic acid, which has a dramatic effect the surface membrane, its subsequent internalization and cytoplasmic processing. This interaction could be one of the key steps in the damaging effects of silica containing dust.

Aluminium lactate treatment of DQ12 quartz inhibits its ability to cause inflammation, chemokine expression, and nuclear factor-kappaB activation

In 1997, an IARC Working Group classified quartz (crystalline silica) as a Group 1 lung carcinogen, but only in some industries, i.e., the quartz hazard is a variable entity. The reactivity of the quartz surface may underlie its ability to cause inflammation, and treatments that ameliorate this reactivity will reduce the quartz hazard. In this study we treated quartz (Q) with aluminium lactate (AL), a procedure that is reported to decrease the quartz hazard, and explored the effect this had on the highly reactive quartz surface and on proinflammatory events in rat lungs. Aluminium lactate-treated quartz showed a reduced surface reactivity as measured by electron spin resonance and the hemolysis assay. Eighteen hours after instillation of Q into the rat lung, there was massive inflammation as indicated by the number of neutrophils in the bronchoalveolar lavage (BAL). In addition, Q induced an increase in BAL macrophage inflammatory protein-2 (MIP-2) while ALQ had no significant effect compared to control. Epithelial damage, as indicated by BAL protein and gamma glutamyl transpeptidase, also increased with Q but not with ALQ. Furthermore, Q induced an increase in MIP-2 mRNA by BAL cells while ALQ had no effect compared to controls. There was an increase in nuclear binding of the transcription nuclear factor kappaB (NF-kappaB) in the Q-exposed BAL cells and again no effect on nuclear NF-kappaB binding in BAL cells from ALQ-exposed rats. In conclusion, treatment of the quartz surface with aluminium lactate reduced the reactivity of the particles both in terms of hydroxyl radical generation and in terms of the induction of molecular signaling events leading to inflammation.

Silicosis and coal workers' pneumoconiosis

Exposure to coal mine dust and/or crystalline silica results in pneumoconiosis with initiation and progression of pulmonary fibrosis. This review presents characteristics of simple and complicated coal workers' pneumoconiosis (CWP) as well as pathologic indices of acute and chronic silicosis by summarizing results of in vitro, animal, and human investigations. These results support four basic mechanisms in the etiology of CWP and silicosis: a) direct cytotoxicity of coal dust or silica, resulting in lung cell damage, release of lipases and proteases, and eventual lung scarring; b) activation of oxidant production by pulmonary phagocytes, which overwhelms the antioxidant defenses and leads to lipid peroxidation, protein nitrosation, cell injury, and lung scarring; c) activation of mediator release from alveolar macrophages and epithelial cells, which leads to recruitment of polymorphonuclear leukocytes and macrophages, resulting in the production of proinflammatory cytokines and reactive species and in further lung injury and scarring; d) secretion of growth factors from alveolar macrophages and epithelial cells, stimulating fibroblast proliferation and eventual scarring. Results of in vitro and animal studies provide a basis for proposing these mechanisms for the initiation and progression of pneumoconiosis. Data obtained from exposed workers lend support to these mechanisms.

Vitronectin protects alveolar macrophages from silica toxicity

Silicosis is an interstitial lung disease caused by the inhalation of crystalline silicon dioxide. Current concepts suggest that a crucial step in the development of silicosis is silica-induced injury of alveolar macrophages (AM). The adhesive protein vitronectin is a natural constituent of the lung, in which its function is largely unexplored. This study investigated a possible role for vitronectin in protecting AM from silica exposure. In this study, the concentration of vitronectin was shown to be increased in the bronchoalveolar lavage fluid of silica-treated rats. Vitronectin affinity for silica was shown both in vitro and in vivo by immunostaining. Vitronectin reduced silica-induced injury to cultured AM as determined with the (51)Cr release assay. Vitronectin reduced silica-induced free radical production as determined with a cell-free thiobarbituric acid assay. Additionally, vitronectin reduced the silica-induced respiratory burst in AM as determined with chemiluminescence. This study suggests that vitronectin may protect AM during the initial exposure to silica.

Synthesis, structure, and thermal properties of 1,2-dipalmitoylgalloylglycerol (DPGG), a novel self-adhering lipid

A novel diacyl glycerol-based lipid with a polyphenolic head group has been synthesized and characterized. X-ray diffraction experiments show that this lipid, 1,2-dipalmitoylgalloylglycerol (DPGG), hydrates to form gel phase bilayers at 20 degrees C with extremely narrow interbilayer fluid separations, indicating that apposing DPGG bilayers strongly adhere to each other. Differential scanning calorimetry shows that fully hydrated DPGG exhibits a pretransition exotherm (3.7 kcal/mol) at 52 degrees C and a high enthalpy (11.3 kcal/mol) main endothermic transition at 69 degrees C. These thermal properties are similar to those of galactosylceramides with similar hydrocarbon chain compositions. The adhesive and thermal properties of DPGG are likely due to both intermolecular hydrogen-bonding and hydrophobic interactions between the aromatic rings on the gallic acids.

Dependence of NF-kappaB activation and free radical generation on silica-induced TNF-alpha production in macrophages

Tumor necrosis factor alpha (TNFalpha) plays an important role in the pathogenesis of silicosis and other chronic inflammatory lung diseases. The present study investigates the role nuclear transcription factor kappaB (NF-kappaB) and oxygen free radicals in silica-induced TNFalpha production in primary alveolar macrophages and RAW 264.7 cells. Using electrophoretic mobility shift assay (EMSA) and enzyme-linked immunoadsorbent assay (ELISA), we have demonstrated that silica can induce NF-kappaB activation and TNFalpha expression in a dose-dependent manner. Transient transfection assays with a plasmid construct containing NF-kappaB binding sites linked to a reporter gene further show that silica is able to induce the transcriptional activation of NF-kappaB-dependent gene. Inhibition of NF-kappaB activation by SN50, a specific NF-kappaB blocker, abolishes silica-induced TNFalpha production. Pretreatment of the cells with catalase (H2O2 scavenger) or deferoxamine (*OH scavenger) effectively inhibits NF-kappaB and TNFalpha activation, whereas superoxide dismutase (O2 scavenger) has an opposite effect. These results indicate that silica-mediated free radical generation and NF-kappaB activation play important roles in silica-induced TNFalpha gene expression.

An overview of the role of mineral solubility in silicosis and asbestosis

Asbestosis and silicosis are fibrotic diseases initiated by the inhalation of silica-containing dusts, asbestos and quartz. There are various approaches for explaining the causes of these diseases. At present, our knowledge on the matter indicates that silicic acid dissolved from these minerals, contact between macrophages and minerals, highly reactive and oxidative species formed on the mineral surface, and lysosomal enzymes released upon engulfment of particulate mineral of appropriate size all contribute to various extents to the initiation of fibrosis. Among these mineral solubility seems to have a substantial contribution as a causative factor.

Physical Adsorption of Bilayer Membranes on Silica

Isotherms for adsorption of bilayer-forming synthetic amphiphiles or phospholipids from vesicles onto hydrophilic silica particles (Aerosil OX-50) are obtained over a range of experimental conditions. Phosphatidylcholine (PC), dipalmitoylphosphatidylcholine (DPPC), dihexadecylphosphate (DHP), and dioctadecyldimethylammonium bromide (DODAB) dispersed in 10 mM Tris, pH 7.4, as small unilamellar vesicles present affinities for silica following the sequence DODAB > DPPC > PC > DHP. Among these, only DHP adsorption is below that expected for one bilayer deposition. Interaction at 65degreesC for 1 h between DPPC (or, at 25degreesC, for PC) vesicles and silica efficiently leads to bilayer deposition at maximal adsorption, if Tris is the buffer used. Preliminary centrifugation of the PC or DPPC vesicle dispersion is necessary to prevent overestimated adsorption. PC affinity for silica and its deposition as a bilayer depend on the nature of buffer used being much higher for Tris than for Hepes at pH 7.4. Formation of ion pairs between protonated amino groups in Tris and silanol groups on silica may lead to Tris adsorption and an increase in density of -OH groups on the solid surface. Therefore, formation of cooperative hydrogen bridges between -P=O in the phosphatidyl of the phospholipid bilayer and the -OH groups of Tris adsorbed on silica increases PC affinity for silica in the presence of Tris. For Hepes as buffer, PC affinity for silica is much lower and no plateau indicative of bilayer deposition is observed in the adsorption isotherm. Stabilization of supported phospholipid bilayers on solid surfaces requires several cooperative hydrogen bridges between the phospholipid and the solid surface. DODAB adsorption was unaffected by vesicle age and physical state of the bilayer vesicle. Adsorption isotherms for DODAB are of the high-affinity type with a maximum indicative of competition between intervesicle interactions and DODAB deposition on silica. Stabilization of DODAB bilayer deposition requires surface charge densities on silica higher than -1 μC/cm2. Copyright 1997Academic Press

The interaction of polyphenols with bilayers: conditions for increasing bilayer adhesion

Because proteins and other molecules with a high polyphenol content are commonly involved in adhesion processes, we are investigating the interactions between polyphenols and biological materials. A naturally occurring polyphenol that binds a variety of proteins and lipids is tannic acid (TA), which contains five digallic acid residues covalently linked to a central D-glucose. A previous study has shown that TA increases the adhesion between apposing phosphatidylcholine (PC) bilayers and over a very narrow concentration range collapses the interbilayer fluid space from about 15 A to 5 A. To determine the chemical requirements a polyphenolic molecule must possess to increase bilayer adhesion, we have synthesized several simpler TA analogs that vary in their size, shape, and number of gallic acid and hydroxyl groups. X-ray diffraction, absorbance, binding, and differential scanning calorimetry measurements were used to investigate the interaction of these polyphenolic molecules with egg PC (EPC) and dipalmitoyl PC (DPPC) bilayers. Of these synthetic polyphenols, only penta-O-galloyl-alpha-D-glucose (PGG) was able to completely mimic the effects of TA by collapsing the interbilayer fluid space from 15 A to 5 A, decreasing the dipole potential by about 300 mV, increasing the transition enthalpy of DPPC liposomes, and inducing an interdigitated phase in DPPC. Binding studies indicated that the fluid space was reduced to 5 A at an EPC:PGG mole ratio of 5:1. We conclude that these polyphenols collapse the fluid space of PC bilayers because they 1) are amphipathic and partition into the bilayers interfacial region, 2) are long enough to span the interbilayer space, 3) contain several gallic acids distributed so that they can partition simultaneously into apposing bilayers, and 4) have sufficient gallic acid residues to interact with all lipid headgroups and cover the bilayer surface. Under these conditions we conclude that the polyphenols from interbilayer bridges. We argue that these bridges are stabilized by increased adhesion arising from an increased van der Waals interaction between apposing bilayers, electrostatic interactions between the pi electrons in the phenol ring and the -(N+CH3)3 groups on the PC headgroups, decreased hydration repulsion between bilayers, and hydrogen bonds between the H-bond-donating moieties on the polyphenols and H-bond-accepting groups in the bilayer.

Mechanism of hemolysis of human erythrocytes exposed to monosodium urate monohydrate crystals. Preliminary characterization of membrane pores

Microcrystals of monosodium urate monohydrate (MSUM) have the ability to cause rapid hemolysis of erythrocytes. The nature of the initial MSUM crystal-erythrocyte membrane binding interaction was investigated over a range of different ionic strength media. There was negligible binding of MSUM to erythrocyte ghost membranes in low ionic strength media such as isotonic mannitol but binding was dramatically increased in isotonic NaCl/mannitol solutions or isotonic mannitol containing 1 mM Ca2+. Hemolysis induced by MSUM crystals was preceded by the leakage of K+ from the cells suggesting a colloid-osmotic mechanism of hemolysis. The inclusion of large (oligosaccharide) molecules in the extracellular media or the modulation of the extracellular solution tonicity inhibited both the rate and extent of hemolysis supporting the concept of MSUM-induced pores followed by colloid osmotic hemolysis.