Environmental implications and applications of carbon nanomaterials in water treatment.

Carbon nanomaterials have been proposed as a basis for developing new technologies for photocatalytic oxidation and disinfection, improved membrane processes, adsorbents, and biofilm-resistant surfaces. This study details recent progress towards the development of these proposed applications. We explored the use of carbon nanomaterials such as fullerene C60, single-wall carbon nanotubes (SWCNTs), and multi-wall carbon nanotubes (MWCNTs) for a range of new technologies including, degradation of a probe organic compound by in situ generation of reactive oxygen species (ROS), new strategies for microbial disinfection, and the inhibition of biofilm development on membrane surfaces. The results show that the degradation of 2-chlorophenol by ROS produced microbial inactivation, and the mobility of the nanoparticle aggregates of the carbon nanomaterials all increased as suspensions were fractionated to enrich with smaller aggregates with sonication followed by successive membrane filtration.

Nanomaterials as possible contaminants: the fullerene example.

An assessment of the potential risks posed by nanomaterials will require case-by-case evaluations of the processes controlling exposure and hazards such as toxicity. Factors that control fullerene transport and transformation in aqueous environments and their relationship to toxicity are discussed. Natural organic matter is observed to either increase or decrease nanoparticle stability while trends in reactive oxygen generation run counter to proposed mechanisms of possible fullerene toxicity.

C60 in water: nanocrystal formation and microbial response.

Upon contact with water, under a variety of conditions, C60 spontaneously forms a stable aggregate with nanoscale dimensions (d = 25-500 nm), termed here "nano-C60". The color, hydrophobicity, and reactivity of individual C60 are substantially altered in this aggregate form. Herein, we provide conclusive lines of evidence demonstrating that in solution these aggregates are crystalline in order and remain as underivatized C60 throughout the formation/stabilization process that can later be chemically reversed. Particle size can be affected by formation parameters such as rates and the pH of the water addition. Once formed, nano-C60 remains stable in solution at or below ionic strengths of 0.05 I for months. In addition to demonstrating aggregate formation and stability over a wide range of conditions, results suggest that prokaryotic exposure to nano-C60 at relatively low concentrations is inhibitory, indicated by lack of growth (> or = 0.4 ppm) and decreased aerobic respiration rates (4 ppm). This work demonstrates the fact that the environmental fate, distribution, and biological risk associated with this important class of engineered nanomaterials will require a model that addresses not only the properties of bulk C60 but also that of the aggregate form generated in aqueous media.

Arresting pore formation of a cholesterol-dependent cytolysin by disulfide trapping synchronizes the insertion of the transmembrane beta-sheet from a prepore intermediate.

Perfringolysin O (PFO), a member of the cholesterol-dependent cytolysin family of pore-forming toxins, forms large oligomeric complexes comprising up to 50 monomers. In the present study, a disulfide bridge was introduced between cysteine-substituted serine 190 of transmembrane hairpin 1 (TMH1) and cysteine-substituted glycine 57 of domain 2 of PFO. The resulting disulfide-trapped mutant (PFO(C190-C57)) was devoid of hemolytic activity and could not insert either of its transmembrane beta-hairpins (TMHs) into the membrane unless the disulfide was reduced. Both the size of the oligomer formed on the membrane and its rate of formation were unaffected by the oxidation state of the Cys(190)-Cys(57) disulfide bond; thus, the disulfide-trapped PFO was assembled into a prepore complex on the membrane. The conversion of this prepore to the pore complex was achieved by reducing the C190-C57 disulfide bond. PFO(C190-C57) that was allowed to form the prepore prior to the reduction of the disulfide exhibited a dramatic increase in the rate of PFO-dependent hemolysis and the membrane insertion of its TMHs when compared with toxin that had the disulfide reduced prior mixing the toxin with membranes. Therefore, the rate-limiting step in pore formation is prepore assembly, not TMH insertion. These data demonstrate that the prepore is a legitimate intermediate during the insertion of the large transmembrane beta-sheet of the PFO oligomer. Finally, the PFO TMHs do not appear to insert independently, but instead their insertion is coupled.

Mechanism of membrane insertion of a multimeric beta-barrel protein: perfringolysin O creates a pore using ordered and coupled conformational changes.

Perfringolysin O, a bacterial cytolytic toxin, forms unusually large pores in cholesterol-containing membranes by the spontaneous insertion of two of its four domains into the bilayer. By monitoring the kinetics of domain-specific conformational changes and pore formation using fluorescence spectroscopy, the temporal sequence of domain-membrane interactions has been established. One membrane-exposed domain does not penetrate deeply into the bilayer and is not part of the actual pore, but is responsible for membrane recognition. This domain must bind to the membrane before insertion of the other domain into the bilayer is initiated. The two domains are conformationally coupled, even though they are spatially separated. Thus, cytolytic pore formation is accomplished by a novel mechanism of ordered conformational changes and interdomain communication.