Bioavailability of Fullerene under Environmentally Relevant Conditions: Effects of Humic Acid and Fetal Bovine Serum on Accumulation in Lipid Bilayers and Cellular Uptake.

Carbon fullerene (C60) has emerged at the forefront of nanoscale research and application due to its unique properties. As the production of this nanoparticle rapidly increases, it can be released into natural aquatic environments and can accumulate in biological systems. This research examined the effects of humic acid and fetal bovine serum (FBS), which are ubiquitous in aquatic environments and representative of blood plasma in living organisms, respectively, on bioavailability of fullerene. Bioavailability was investigated using in vitro methods for lipid membrane accumulation and cellular uptake studies. Humic acid and FBS significantly changed the characteristics of fullerene including its particle size and surface charge. The effects of humic acid on lipid accumulation of fullerene depended on the lipid head charge. FBS also significantly decreased the lipid accumulation when positively charged and zwitterionic head groups were present on the lipids, possibly due to the higher steric repulsion of the protein coated nanoparticles. In addition, both humic acid and FBS protein effectively lowered the amounts of fullerene taken up by Caco-2 cells, which are derived from a human colorectal adenocarcinoma and have similar functions to the small intestinal epithelium. Results of this study suggest that surface modification of fullerene by environmentally relevant matrices can significantly affect the biological transport, as well as the possible toxicity of this nanomaterial.

Distribution of Fullerene Nanoparticles between Water and Solid Supported Lipid Membranes: Thermodynamics and Effects of Membrane Composition on Distribution.

The distribution coefficient (Klipw) of fullerene between solid supported lipid membranes (SSLMs) and water was examined using different lipid membrane compositions. Klipw of fullerene was significantly higher with a cationic lipid membrane compared to that with a zwitterionic or anionic lipid membrane, potentially due to the strong interactions between negative fullerene dispersions and positive lipid head groups. The higher Klipw for fullerene distribution to ternary lipid mixture membranes was attributed to an increase in the interfacial surface area of the lipid membrane resulting from phase separation. These results imply that lipid composition can be a critical factor that affects bioconcentration of fullerene. Distribution of fullerene into zwitterionic unsaturated lipid membranes was dominated by the entropy contribution (ΔS) and the process was endothermic (ΔH > 0). This result contrasts the partitioning thermodynamics of highly and moderately hydrophobic chemicals indicating that the lipid-water distribution mechanism of fullerene may be different from that of molecular level chemicals. Potential mechanisms for the distribution of fullerene that may explain these differences include adsorption on the lipid membrane surfaces and partitioning into the center of lipid membranes (i.e., absorption).

Effects of lipid composition on partitioning of fullerene between water and lipid membranes.

Fullerene partition coefficients (Klipw) between water and solid supported lipid membranes were determined for membranes of various lipid types and composition over a range of temperatures. The log Klipw (L/kg) values for fullerene, which range from 3.1 to 5.3, depend on the lipid type in the lipid membranes. Partition coefficients increased with increasing temperature, increasing acyl chain length of unsaturated lipids. The results indicate that lipid composition is a critical factor for bioconcentration of fullerene.

Evidence for multiple removal pathways in low-temperature (200-400 °C) thermal treatment of pentachlorophenol-laden soils.

Polychlorinated aromatic compounds (PCACs) pose significant remediation challenges, since their high soil affinities preclude mobile-phase partitioning and subsurface extraction. To enhance partitioning and desorption, subsurface temperatures are raised using a technique called thermal conduction heating-soil vapor extraction (TCH-SVE). While this technique improves PCAC partitioning, it can also promote several degradation reactions under temperatures typical of low-temperature TCH-SVE (200-400 °C). While these reactions are labile, the extent to which they occur in flow-through TCH-SVE is unclear. The current research used bench-scale, flow-through TCH-SVE to assess relative importance of three removal pathways: (1) target volatilization, (2) reductive dechlorination, and (3) oxidation via OH-addition. Pentachlorophenol was used as a representative PCAC, and pathway contributions, extents, and regioselectivity were examined as a function of temperature (225-375 °C) and gas-phase oxygen content (air vs. nitrogen). Across treatments, OH-addition and dechlorination occurred in parallel and accounted for significantly more removal than PCP volatilization. OH-addition byproducts had highest yields (regardless of oxygen content) and were consistent with surface-mediated OH production and ring addition. OH-addition increased with temperature while volatilization and dechlorination decreased. Notable exceptions occurred between 225 and 325 °C (where dechlorination dropped 10-fold) and 325 and 375 °C (where OH-addition fell 75%), signifying major mechanism shifts in these intervals.

Modeling binding equilibrium in a competitive estrogen receptor binding assay.

Although the free concentration is more significant in the environmental chemistry and toxicology of receptor-mediated toxicants, few studies have been conducted to use it as a dose-metric. The relative binding affinity of three model endocrine disrupting compounds, diethylstilbestrol (DES), ethynylestradiol (EE2), and bisphenol A (BPA), were evaluated using a competitive ELISA with human estrogen receptor alpha. After measuring the available receptors and the dissociation constant for 17beta-estradiol, binding inhibition curves using the free concentration as the dose-metric were obtained by assuming species equilibrium in the ELISA system and compared with apparent inhibition curves generated using the nominal concentration as the dose-metric. Because ligand binding to estrogen receptors may reduce its free concentration in the assay system, the differences between the two curves for free and nominal concentrations are more significant for more strongly binding ligands. The ratio of a compound's nominal concentration causing 50% inhibition (IC50) to the IC50 of DES, the positive control, was strongly affected by specific assay conditions, while that estimated by modeling free concentration is independent of receptor concentration, indicating that the free concentration is a better dose-metric for a competitive binding assay.

Methyl bromide as a building disinfectant: interaction with indoor materials and resulting byproduct formation.

Several buildings were contaminated with Bacillus anthracis in the fall of 2001. These events required consideration of how to disinfect large indoor spaces for continued worker occupation. The interactions of gaseous disinfectants with indoor materials may inhibit the disinfection process, cause persistence of the disinfectant, and lead to possible byproduct formation and persistence. Methyl bromide (CH3Br) is a candidate for disinfection/deactivation of biological agents in buildings. In this study, 24 indoor materials were exposed to CH3Br for 16 hr at concentrations ranging from 100 to 2500 ppm in 48-L electropolished stainless steel chambers. CH3Br concentrations were measured during and after disinfection. Its interactions with materials were observed to be small, with nearly complete and rapid desorption. Between 3% and 8% of CH3Br adsorbed to four materials (office partition, ceiling tile, particle-board, and gypsum wallboard with satin paint), and the degree of adsorption decreased with increasing relative humidity. The percentage of adsorption to all other materials was <2%. This result suggests that when designing disinfection events with CH3Br, loss to indoor materials can be neglected in terms of disinfectant dose calculations. Possible reaction products were identified and/or quantified before and after exposure to CH3Br. Several monomethylated and dimethylated aliphatic compounds were observed in chamber air at low concentrations after the exposures of six materials to CH3Br. Concentration increases also occurred for chemicals that were observed to naturally off-gas from materials before exposure to CH3Br, suggesting that CH3Br may play a role in enhancing the natural off-gassing of chemicals, for example, by competitive displacement of compounds that already existed in the materials. The results described in this paper should facilitate the design of building disinfection systems involving CH3Br.

Partitioning of moderately hydrophobic endocrine disruptors between water and synthetic membrane vesicles.

The partition coefficient between water and lipid membrane vesicles (Klipw) has been used as an alternative to the 1-octanol-water partition coefficient (Kow) between water and organic solvent, because it more closely represents actual biological membranes. Despite theoretical differences, log Klipw correlates well with log Kow for conventional nonpolar organic pollutants. In the present study, Klipw values of 11 structurally diverse endocrine-disrupting chemicals (EDCs) were measured for three different types of lipid membrane vesicles from dipalmitoylphosphatidylcholine (DPPC), DPPC/cholesterol, and palmitoyloleoylphosphatidylcholine. Correlation analyses were conducted to evaluate the effects of hydrophobicity, molar liquid volume (MLV), and polar surface area (PSA) for 20 EDCs, including nine from a previous study. Correlations that include MLV and PSA reduce the predicted value of log Klipw, suggesting that lipid membranes are less favorable than 1-octanol for a hydrophobic solute because of the higher molar volume and higher hydrogen-bonding potential. These results suggested that Kow alone has limited potential for estimating Klipw and that additional descriptors are required. In addition, Klipw values vary by as much as two orders of magnitude because of the changes in membrane fluidity and the amount of cholesterol in the lipid bilayer. Therefore, lipid components should be chosen carefully to evaluate the bioconcentration of these compounds.

Use of a parallel artificial membrane system to evaluate passive absorption and elimination in small fish.

A parallel artificial lipid membrane system was developed to mimic passive mass transfer of hydrophobic organic chemicals in fish. In this physical model system, a membrane filter-supported lipid bilayer separates two aqueous phases that represent the external and internal aqueous environments of fish. To predict bioconcentration kinetics in small fish with this system, literature absorption and elimination rates were analyzed with an allometric diffusion model to quantify the mass transfer resistances in the aqueous and lipid phases of fish. The effect of the aqueous phase mass transfer resistance was controlled by adjusting stirring intensity to mimic bioconcentration rates in small fish. Twenty-three simple aromatic hydrocarbons were chosen as model compounds for purposes of evaluation. For most of the selected chemicals, literature absorption/elimination rates fall into the range predicted from measured membrane permeabilities and elimination rates of the selected chemicals determined by the diffusion model system.

Importance of doping, dopant distribution, and defects on electronic band structure alteration of metal oxide nanoparticles: Implications for reactive oxygen species.

Metal oxide nanoparticles (MONPs) are considered to have the potency to generate reactive oxygen species (ROS), one of the key mechanisms underlying nanotoxicity. However, the nanotoxicology literature demonstrates a lack of consensus on the dominant toxicity mechanism(s) for a particular MONP. Moreover, recent literature has studied the correlation between band structure of pristine MONPs to their ability to introduce ROS and thus has downplayed the ROS-mediated toxicological relevance of a number of such materials. On the other hand, material science can control the band structure of these materials to engineer their electronic and optical properties and thereby is constantly modulating the pristine electronic structure. Since band structure is the fundamental material property that controls ROS-producing ability, band tuning via introduction of dopants and defects needs careful consideration in toxicity assessments. This commentary critically evaluates the existing material science and nanotoxicity literature and identifies the gap in our understanding of the role of important crystal structure features (i.e., dopants and defects) on MONPs' electronic structure alteration as well as their ROS-generation capability. Furthermore, this commentary provides suggestions on characterization techniques to evaluate dopants and defects on the crystal structure and identifies research needs for advanced theoretical predictions of their electronic band structures and ROS-generation abilities. Correlation of electronic band structure and ROS will not only aid in better mechanistic assessment of nanotoxicity but will be impactful in designing and developing ROS-based applications ranging from water disinfection to next-generation antibiotics and even cancer therapeutics.

Chemical mass balance model with fractionation for apportioning PM2.5: a test case for Los Angeles traffic sources.

Chemical mass balance receptor models (CMBs) use measured pollutant concentrations, along with source information, to apportion the contributions of primary sources to the measured concentrations. CMBs can be used to evaluate the accuracy of the emission inventories that underlie State Implementation Plan (SIP) modeling, by providing an allocation of emissions to individual source categories. CMBs, however, traditionally have not accounted for the chemical reaction and differential deposition or fractionation that occur between the source and receptor. This means that they have historically had severe limitations in apportioning secondary particulate matter (PM), which is an especially important component of fine PM (PM2.5). Stafford and Liljestrand developed a method to account for fractionation in CMBs using depletion factors based on a solution of the steady-state advection-dispersion equation, including gravitational settling, dry deposition, and first-order chemical reaction.

Mercury speciation during in situ thermal desorption in soil.

Metallic mercury (Hg(0)) and its compounds are highly mobile and toxic environmental pollutants at trace level. In situ thermal desorption (ISTD) is one of the soil remediation processes applying heat and vacuum simultaneously. Knowledge of thermodynamic mercury speciation is imperative to understand the fate and transport of mercury during thermal remediation and operate the treatment processes in a cost-effective manner. Hence, speciation model for inorganic mercury was developed over a range of environmental conditions to identify distribution of dissolved mercury species and potential transformations of mercury at near source environment. Simulation of phase transitions for metallic mercury, mercury(II) chloride and mercury sulfide with temperature increase showed that complete vaporization of metallic mercury and mercury(II) chloride were achieved below the boiling point of water. The effect of soil compositions on mercury removal was also evaluated to better understand thermal remediation process. Higher vapor pressures expected both from soil pore water and inorganic carbonate minerals in soil as well as creation of permeability were significant for complete vaporization and removal of mercury.

Method for predicting photocatalytic oxidation rates of organic compounds.

In designing a photocatalytic oxidation (PCO) system for a given air pollution source, destruction rates for volatile organic compounds (VOCs) are required. The objective of this research was to develop a systematic method of predicting PCO rate constants by correlating rate constants with physical-chemical characteristics of compounds. Accordingly, reaction rate constants were determined for destruction of volatile organics over a titanium dioxide (TiO2) catalyst in a continuous mixed-batch reactor. It was found that PCO rate constants for alkanes and alkenes vary linearly with gas-phase ionization potential (IP) and with gas-phase hydroxyl radical reaction rate constant. The correlations allow rates of destruction of compounds not tested in this research to be predicted based on physical-chemical characteristics.

Effect of water content on transient nonequilibrium NAPL-gas mass transfer during soil vapor extraction.

The effect of water content on the volatilization of nonaqueous phase liquid (NAPL) in unsaturated soils was characterized by one-dimensional venting experiments conducted to evaluate the lumped mass transfer coefficient. An empirical correlation based upon the modified Sherwood number, Peclet number, and normalized mean grain size was used to estimate initial lumped mass transfer coefficients over a range of water content. The effects of water content on the soil vapor extraction SVE process have been investigated through experimentation and mathematical modeling. The experimental results indicated that a rate-limited NAPL-gas mass transfer occurred in water-wet soils. A severe mass transfer limitation was observed at 61.0% water saturation where the normalized effluent gas concentrations fell below 1.0 almost immediately, declined exponentially from the initiation of venting, and showed long tailing. This result was attributed to the reduction of interfacial area between the NAPL and mobile gas phases due to the increased water content. A transient mathematical model describing the change of the lumped mass transfer coefficient was used. Simulations showed that the nonequilibrium mass transfer process could be characterized by the exponent beta, a parameter which described the reduction of the specific area available for NAPL volatilization. The nonequilibrium mass transfer limitations were controlled by the soil mean grain size and pore gas velocity, were well described by beta values below 1.0 at low water saturation, and were well predicted with beta values greater than 1.0 at high water saturation.

Development of a fluorescence model for the determination of constants associated with binding, quenching, and Förster resonance energy transfer efficiency.

Determining accurate dissociation constants for equilibrium processes involving a fluorescent mechanism can prove to be quite challenging. Typically, titration curves and nonlinear least squares fitting of the data using computer programs are employed to obtain such constants. However, these approaches only consider the total fluorescence signal and often ignore other energy transfer processes within the system. The current model considers the impact on fluorescence from equilibrium binding (viz., metal-ligand, ligand-substrate, etc.), quenching, and resonance energy transfer. This model should provide more accurate binding constant as well as insights into other photonic processes. The equations developed for this model are discussed and are applied to experimental data from titrimetric experiments. Since the experimental data are generally in excess of the number of parameters that are needed to define the system, fitting is operated in an overdetermined mode and employs error minimization (either absolute or relative) to define goodness of fit. Examples of how changes in certain parameters affect the shape of the titrimetric curve are also presented. The current model does not consider chelation-enhanced fluorescence.

Surface complexation modeling of Hg(II) adsorption at the goethite/water interface using the charge distribution multi-site complexation (CD-MUSIC) model.

HYPOTHESIS: Previous attempts to describe Hg(II) adsorption onto mineral surfaces using surface complexation models (SCMs) have proven unsuccessful and/or require the use of hypothetical surface species. Given that metal ion adsorption at the mineral-water interface is greatly influenced by mineral surface heterogeneity and the presence of competing adsorbates in solution, it stands to reason that estimating the crystal face composition (CFC) of the mineral surface and the extent of carbonate contamination in the experimental system will improve SCM predictions. EXPERIMENTS: The Charge Distribution Multi-Site Complexation (CD-MUSIC) model was used to simulate experimental Hg(II) adsorption data, collected on the iron hydroxide mineral goethite, in the presence and absence of competing adsorbates and complexing ligands as a function of pH and ionic strength. The CFC of each goethite sample studied was predicted using a newly discovered relationship between goethite's proton reactive site density (N(H)) and specific surface area (SSA). Carbonate's presence in the experimental systems was determined utilizing a novel methodology developed in this work. FINDINGS: The CD-MUSIC model developed in this study accurately predicted Hg(II) adsorption onto goethite over the entire range of experimental conditions investigated while only employing surface species consistent with spectroscopic evidence.

Temperature effects on alkaline earth metal ions adsorption on gibbsite: approaches from macroscopic sorption experiments and molecular dynamics simulations.

Two approaches, macroscopic adsorption experiments and molecular dynamics simulations, were employed to study the effect of temperature on alkaline earth metals adsorption on gibbsite surfaces. Increased reaction temperature enhanced the extent of metal ion adsorption for all of the alkaline earth metals studied. Whereas Mg(2+) and Sr(2+) adsorption displayed dependence on ionic strength, Sr(2+) adsorption exhibited less dependence on background ionic strength regardless of temperature. The ionic strength dependence was attributed to outer-sphere complexation reactions. The ionic strength effect on metal ion removal decreased with increasing temperature for both metals. Ba(2+) removal by gibbsite, on the other hand, was not affected by ionic strength. Results from molecular dynamics simulations were in agreement with the findings of the experimental study. The amount of thermal energy required to remove waters of hydration from the metal cation and the ratio of outer-sphere to inner-sphere complexation decreased with increasing ionic radii. It was observed from both macroscopic and molecular approaches that the tendency to form inner-sphere complexes on gibbsite decreased in the order: Ba(2+)>Sr(2+)>Mg(2+) and that the common assumption that alkaline earth metal ions form outer-sphere complexes appears to be dependent on ionic radius and temperature.

A 'turn-on' FRET peptide sensor based on the mercury binding protein MerP.

A new fluorescent peptidyl chemosensor based on the mercury binding MerP protein with fluorescence resonance energy transfer (FRET) capabilities has been synthesized via Fmoc solid-phase peptide synthesis. The metal chelating unit, which is flanked by the fluorophores tryptophan (donor) and dansyl (acceptor), contains amino acids from MerP's metal binding loop (sequence: dansyl-Gly-Gly-Thr-Leu-Ala-Val-Pro-Gly-Met-Thr-Cys-Ala-Ala-Cys-Pro-Ile-Thr-Val-Lys-Lys-Gly-Gly-Trp-CONH(2)). A FRET enhancement or 'turn-on' response was observed for Hg(2+) as well as for Zn(2+), Cd(2+) and Ag(+) in a pure aqueous solution at pH 7.0. The emission intensity of the acceptor was used to monitor the concentration of these metals ions with detection limits of 280, 6, 103 and 496 microg L(-1), respectively. No response was observed for the other transition, alkali and alkaline earth metals tested. The fluorescent enhancement observed is unique for Hg(2+) since this metal generally quenches fluorescence. The acceptor fluorescence increase resulting from metal binding-induced FRET suggests a sensor that is inherently more sensitive than one based on quenching by the binding event.

Partitioning thermodynamics of selected endocrine disruptors between water and synthetic membrane vesicles: effects of membrane compositions.

The thermodynamics of partitioning of selected endocrine disruptors between water and synthetic membrane vesicles were investigated. For most of the chemicals investigated, partitioning is dominated by the enthalpy change for unsaturated lipid membrane vesicles and by the entropy contribution for saturated lipid membrane vesicles. The contribution of the entropy terms in determining the free-energy change becomes more important compared with the enthalpy terms with increased branching of p-substituted phenols. These results suggest that the thermal energy required for and the entropy gain associated with the creation of the cavity in the lipid bilayer is of critical importance in differentiating the process from 1-octanol/water partitioning. In addition, partitioning thermodynamics are significantly influenced by cholesterol content in the lipid membranes. Results of the present study and those in the literature suggest that partitioning processes significantly depend on the physical state of the lipid membranes and log K(ow)-based quantitative structure-activity relationships should be carefully applied for predicting bioconcentration by considering lipid compositions.

Partitioning of selected estrogenic compounds between synthetic membrane vesicles and water: effects of lipid components.

Partition coefficients of the steroid estrogens 17beta-estradiol, estriol, estrone, and 17alpha-ethynylestradiol and the industrial estrogenic compounds p-nonylphenol, p-(tert)octylphenol, bisphenol A, butylbenzylphthalate, and dibutylphthalate between liposome membrane vesicles and water (K(lipw) values) were determined using equilibrium dialysis. A moderate linear correlation with R2 values of as low as 0.679 were found for the relationship between log K(lipw) and log K(ow) for these compounds. Effects of lipid components used to prepare the membrane vesicles on the partitioning of 17beta-estradiol and p-nonylphenol were also evaluated. For both, K(lipw) values were larger for the vesicles prepared from phospholipids composed of shorter acyl chains such as dilauroyl-phosphatidylcholine than those composed of longer acyl chains such as distearoylphosphatidylcholine. Partition coefficients were higher for the vesicles prepared from phospholipids including unsaturated acyl chains such as dioleoyl-phosphatidylcholine than those solely composed of saturated acyl chains such as distearoyl-phosphatidylcholine. Both shorter acyl chains and double bonds resulted in a more fluid conformation of the lipid bilayer with the liquid crystalline phase rather than the gel phase and greater partitioning. In contrast, higher cholesterol contents reduced the partitioning coefficient. The presence of cholesterol in the void of the lipid bilayer possibly led to the stabilization of the bilayer and the decrease in the partitioning of 17beta-estradiol or p-nonylphenol molecules. These results suggest that phase transition is of critical importance in selecting lipid components to evaluate the bioconcentration for these compounds.

Effects of dissolved organic matter surrogates on the partitioning of 17beta-estradiol and p-nonylphenol between synthetic membrane vesicles and water.

Partition coefficients of two estrogenic compounds, 17beta-estradiol (E2) and p-nonylphenol (NP), between synthetic membrane vesicles (K'lipw values) and water were determined using equilibrium dialysis to evaluate the potential biological uptake of these compounds with and without the presence of dissolved natural organic matter (DOM) surrogates, Suwannee River fulvic acid, dialyzed Aldrich humic acid, and polyphenolic tannic acid. Overall, K'lipw values for E2 and NP reduced by 20-30% with the increase of DOM concentration from 0 to approximately 4 mg of C/L, a typical DOM concentration in the aquatic environment. This trend for E2 and NP is similar to that obtained by other researchers for moderately hydrophobic compounds including polycyclic aromatic hydrocarbons with three or four rings. A three-phase compartment model with three independent interactions, the sorption of E2 or NP by DOM surrogate (Koc), the sorption of DOM surrogates by membrane vesicles (KlipDOM), and the partition of E2 or NP by membrane vesicles (Klipw), was proposed, and K'lipw values for E2 or NP in the presence of DOM were calculated. The model predicted the decrease in Klipw values with the increase of DOM concentration, and the predictions using the three linear interactions agreed satisfactorily with the experimental results at relatively lower concentration of DOM.