Functionalized fullerenes in water: a closer look

The excellent photophysical properties of C60 fullerenes have spurred much research on their application to aqueous systems for biological and environmental applications. Spontaneous aggregation of C60 in water and the consequent diminution of photoactivity present a significant challenge to aqueous applications. The mechanisms driving the reduction of photoactivity in fullerene aggregates and the effects of functionalization on these processes, however, are not well understood. Here, we take a closer look at the molecular phenomena of functionalized fullerene interactions in water utilizing simulation and experimental tools. Molecular dynamic simulations were performed to investigate time-evolved molecular interactions in systems containing fullerenes with water, oxygen, and/or neighboring fullerene molecules, complimented by physical and chemical characterizations of the fullerenes pre- and postaggregation. Aggregates with widely different photoactivities exhibit similar fullerene-water interactions as well as surface and aggregation characteristics. Photoactive fullerene aggregates had weaker fullerene-fullerene and fullerene-O2 interactions, suggesting the importance of molecular interactions in the sensitization route.

Interaction of C₆₀ with water: first-principles modeling and environmental implications

The nature of fullerene-water interactions has been the subject of much research and debate. Specifically, the presence of a stabilizing, negative surface potential on colloidal aggregates of C60 in water is unexpected, given the neutral nature of pure carbon, and is not well understood. Previous simulation efforts have focused on the C60-water interaction using molecular dynamics simulations that lacked the ability to account for charge transfer and distribution interactions. In this study, first-principles density functional theory was used to analyze the fundamental electronic interactions to elucidate the polarization and charge transfer between water and C60. Simulations show that charge is inductively transferred to the C60 from water molecules, with subsequent polarization of the C60 molecule. In a case with two neighboring C60 molecules, the charge polarization induces a charge onto the second C60. Simulation suggests that this charge transfer and polarization may contribute at least partly to the observed negative surface potential of fullerene aggregates and, combined with hydrogen bonding network formation around C60, provides a fundamental driving force for aggregate formation in water.

Water distribution in dentin matrices: bound vs. unbound water

OBJECTIVE

This work measured the amount of bound versus unbound water in completely-demineralized dentin.

METHODS

Dentin beams prepared from extracted human teeth were completely demineralized, rinsed and dried to constant mass. They were rehydrated in 41% relative humidity (RH), while gravimetrically measuring their mass increase until the first plateau was reached at 0.064 (vacuum) or 0.116 gH2O/g dry mass (Drierite). The specimens were then exposed to 60% RH until attaining the second plateau at 0.220 (vacuum) or 0.191 gH2O/g dry mass (Drierite), and subsequently exposed to 99% RH until attaining the third plateau at 0.493 (vacuum) or 0.401 gH2O/g dry mass (Drierite).

RESULTS

Exposure of the first layer of bound water to 0% RH for 5 min produced a -0.3% loss of bound water; in the second layer of bound water it caused a -3.3% loss of bound water; in the third layer it caused a -6% loss of bound water. Immersion in 100% ethanol or acetone for 5 min produced a 2.8 and 1.9% loss of bound water from the first layer, respectively; it caused a -4 and -7% loss of bound water in the second layer, respectively; and a -17 and -23% loss of bound water in the third layer. Bound water represented 21-25% of total dentin water. Chemical dehydration of water-saturated dentin with ethanol/acetone for 1 min only removed between 25 and 35% of unbound water, respectively.

SIGNIFICANCE

Attempts to remove bound water by evaporation were not very successful. Chemical dehydration with 100% acetone was more successful than 100% ethanol especially the third layer of bound water. Since unbound water represents between 75 and 79% of total matrix water, the more such water can be removed, the more resin can be infiltrated.

Li adsorption on a graphene–fullerene nanobud system: density functional theory approach

The adsorption mechanism of Li onto the graphene–C₆₀ nanobud structure.

Controlling the physicochemical state of carbon on graphene using focused electron-beam-induced deposition

Focused electron-beam-induced deposition (FEBID) is a promising nanolithography technique using "direct-write" patterning by carbon line and dot deposits on graphene. Understanding interactions between deposited carbon molecules and graphene enables highly localized modification of graphene properties, which is foundational to the FEBID utility as a nanopatterning tool. In this study, we demonstrate a unique possibility to induce dramatically different adsorption states of FEBID-produced carbon deposits on graphene, through density functional theory calculations and complementary Raman experiments. Specifically, an amorphous carbon deposit formed by direct irradiation of high energy primary electrons exhibits unusually strong interactions with graphene via covalent bonding, whereas the FEBID carbon formed due to low-energy secondary electrons is only weakly interacting with graphene via physisorption. These observations not only are of fundamental importance to basic physical chemistry of FEBID carbon-graphene interactions but also enable the use of selective laser-assisted postdeposition ablation to effectively remove the parasitically deposited, physisorbed carbon films for improving FEBID patterning resolution.

Magnetism in dopant-free ZnO nanoplates

It is known that bulk ZnO is a nonmagnetic material. However, the electronic band structure of ZnO is severely distorted when the ZnO is in the shape of a very thin plate with its dimension along the c-axis reduced to a few nanometers while keeping the bulk scale sizes in the other two dimensions. We found that the chemically synthesized ZnO nanoplates exhibit magnetism even at room temperature. First-principles calculations show a growing asymmetry in the spin distribution within the distorted bands formed from Zn (3d) and O (2p) orbitals with the reduction of thickness of the ZnO nanoplates, which is suggested to be responsible for the observed magnetism. In contrast, reducing the dimension along the a- or b-axes of a ZnO crystal does not yield any magnetism for ZnO nanowires that grow along c-axis, suggesting that the internal electric field produced by the large {0001} polar surfaces of the nanoplates may be responsible for the distorted electronic band structures of thin ZnO nanoplates.

Mechanism of Li adsorption on carbon nanotube-fullerene hybrid system: a first-principles study

The lithium (Li) adsorption mechanism on the metallic (5,5) single wall carbon nanotube (SWCNT)-fullerene (C(60)) hybrid material system is investigated using first-principles method. It is found that the Li adsorption energy (-2.649 eV) on the CNT-C(60) hybrid system is lower than that on the peapod system (-1.837 eV) and the bare CNT (-1.720 eV), indicating that the Li adsorption on the CNT-C(60) hybrid system is more stable than on the peapod or bare CNT system. This is due to the C(60) of high electron affinity and the charge redistribution after mixing CNT with C(60). In order to estimate how efficiently Li can utilize the vast surface area of the hybrid system for increasing energy density, the Li adsorption energy is calculated as a function of the adsorption positions around the CNT-C(60) hybrid system. It turns out that Li preferably occupies the mid-space between C(60) and CNT and then wraps up the C(60) side and subsequently the CNT side. It is also found that the electronic properties of the CNT-C(60) system, such as band structure, molecular orbital, and charge distribution, are influenced by the Li adsorption as a function of the number of Li atoms. From the results, it is expected that the CNT-C(60) hybrid system has enhanced the charge transport properties in addition to the Li adsorption, compared to both CNT and C(60).

Interfacial reactions of ozone with surfactant protein B in a model lung surfactant system

Oxidative stresses from irritants such as hydrogen peroxide and ozone (O(3)) can cause dysfunction of the pulmonary surfactant (PS) layer in the human lung, resulting in chronic diseases of the respiratory tract. For identification of structural changes of pulmonary surfactant protein B (SP-B) due to the heterogeneous reaction with O(3), field-induced droplet ionization (FIDI) mass spectrometry has been utilized. FIDI is a soft ionization method in which ions are extracted from the surface of microliter-volume droplets. We report structurally specific oxidative changes of SP-B(1-25) (a shortened version of human SP-B) at the air-liquid interface. We also present studies of the interfacial oxidation of SP-B(1-25) in a nonionizable 1-palmitoyl-2-oleoyl-sn-glycerol (POG) surfactant layer as a model PS system, where competitive oxidation of the two components is observed. Our results indicate that the heterogeneous reaction of SP-B(1-25) at the interface is quite different from that in the solution phase. In comparison with the nearly complete homogeneous oxidation of SP-B(1-25), only a subset of the amino acids known to react with ozone are oxidized by direct ozonolysis in the hydrophobic interfacial environment, both with and without the lipid surfactant layer. Combining these experimental observations with the results of molecular dynamics simulations provides an improved understanding of the interfacial structure and chemistry of a model lung surfactant system subjected to oxidative stress.

Charge transport through polyene self-assembled monolayers from multiscale computer simulations

We combine first-principles density-functional theory with matrix Green's function calculations to predict the structures and charge transport characteristics of self-assembled monolayers (SAMs) of four classes of systems in contact with Au(111) electrodes: conjugated polyene chains (n = 4, 8, 12, 16, and 30) thiolated at one or both ends and saturated alkane chains (n = 4, 8, 12, and 16) thiolated at one or both ends. For the polyene SAMs, we find no decay in the current as a function of chain length and conclude that these 1-3 nm long polyene SAMs act as metallic wires. We also find that the polyene-monothiolate leads to a contact resistance only 2.8 times higher than that for the polyene-dithiolate chains, indicating that the device conductance is dominated by the properties of the molecular connector with less importance in having a second molecule-electrode contact. For the alkane SAMs, we observe the normal exponential decay in the current as a function of the chain length with a decay constant of beta(n) = 0.82 for the alkane-monothiolate and 0.88 for the alkane-dithiolate. We find that the contact resistance for the alkane-monothiolate is 12.5 times higher than that for the alkane-dithiolate chains, reflecting the extra resistance due to the weak contact on the nonthiolated end. These contrasting charge transport characteristics of alkane and polyene SAMs and their contact dependence are explained in terms of the atomic projected density of states.

ReaxFF reactive force field for solid oxide fuel cell systems with application to oxygen ion transport in yttria-stabilized zirconia

We present the ReaxFF reactive force field developed to provide a first-principles-based description of oxygen ion transport through yttria-stabilized zirconia (YSZ) solid oxide fuel cell (SOFC) membranes. All parameters for ReaxFF were optimized to reproduce quantum mechanical (QM) calculations on relevant condensed phase and cluster systems. We validated the use of ReaxFF for fuel cell applications by using it in molecular dynamics (MD) simulations to predict the oxygen ion diffusion coefficient in yttria-stabilized zirconia as a function of temperature. These values are in excellent agreement with experimental results, setting the stage for the use of ReaxFF to model the transport of oxygen ions through the YSZ electrolyte for SOFC. Because ReaxFF descriptions are already available for some catalysts (e.g., Ni and Pt) and under development for other high-temperature catalysts, we can now consider fully first-principles-based simulations of the critical functions in SOFC, enabling the possibility of in silico optimization of these materials. That is, we can now consider using theory and simulation to examine the effect of materials modifications on both the catalysts and transport processes in SOFC.

Mechanical and Transport Properties of the Poly(ethylene oxide)−Poly(acrylic acid) Double Network Hydrogel from Molecular Dynamic Simulations

We used atomistic molecular dynamics (MD) simulations to investigate the mechanical and transport properties of the PEO-PAA double network (DN) hydrogel with 76 wt % water content. By analyzing the pair correlation functions for polymer-water pairs and for ion-water pairs and the solvent accessible surface area, we found that the solvation of polymer and ion in the DN hydrogel is enhanced in comparison with both PEO and PAA single network (SN) hydrogels. The effective mesh size of this DN hydrogel is smaller than that of the SN hydrogels with the same water content and the same molecular weight between the cross-linking points (Mc). Applying uniaxial extensions, we obtained the stress-strain curves for the hydrogels. This shows that the DN hydrogel has a sudden increase of stress above approximately 100% strain, much higher than the sum of the stresses of the two SN hydrogels at the same strain. This arises because PEO has a smaller Mc value than PAA, so that the PEO in the DN reaches fully stretched out at 100% strain that corresponds to 260% strain in the PEO SN (beyond this point, the bond stretching and the angle bending increase dramatically). We also calculated the diffusion coefficients of solutes such as D-glucose and ascorbic acid in the hydrogels, where we find that the diffusion coefficients of those solutes in the DN hydrogel are 60% of that in the PEO SN and 40% of that in the PAA SN due to its smaller effective mesh size.

Frequency and risk factors for urinary tract infection in cats with diabetes mellitus

BACKGROUND

Identification and control of infections are important in the management of diabetic cats. Urinary tract infections have not been well characterized in diabetic cats. This retrospective study was performed to review and characterize urinary tract infections in diabetic cats.

HYPOTHESIS

Urinary tract infections are common in diabetic cats.

ANIMALS

A review was made of the medical records of 141 diabetic cats that had had urine obtained for culture by antepubic cystocentesis and that had not been treated with antibiotics, undergone urinary tract catheterization or urinary tract surgery within 2 weeks of urine collection or had urethral obstruction at the time of urine collection.

METHODS

A review of medical records.

RESULTS

Urinary tract infection was identified in 18 of 141 diabetic cats. Escherichia coli was the most common isolate (67%). Female cats were at increased risk (prevalence odds ratios [POR], 3.7; 95% confidence interval [CI], 1.3 to 10.2; P = .013). Clinical signs of lower urinary tract disease and findings on urine sediment examination were good predictors of positive urine cultures.

CONCLUSIONS AND CLINICAL IMPORTANCE

Urinary tract infections are common in diabetic cats regardless of status of diabetic control, suggesting routine monitoring with urine sediment exams or urine culture is warranted.

Structures and Properties of Newton Black Films Characterized Using Molecular Dynamics Simulations

We used molecular dynamics (MD) simulations to investigate the structures and properties of Newton black films (NBF) for several surfactants: sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (C16TAB), and surfactin using film thicknesses up to 10 nm. By calculating the interface formation energy for various packing conditions on the surface pressure-area isotherm, we found that the most probable surface concentration is approximately 42 A(2)/molecule for SDS and C16TAB and approximately 170 A(2)/molecule for surfactin. We then used this most probable concentration of each surfactant to simulate NBF with various film thicknesses. From analyzing the disjoining pressure-film thickness isotherms with the density profiles and the solvation coordination number, we found that the increase of the disjoining pressure during the film thinning was coupled with the change in inner structure of the NBF (i.e., density profile and the solvation of ionic entities). In the range of film thicknesses less than approximately 30 A, the disjoining pressures for the SDS and C16TAB were found to be larger than that of the surfactin. We predicted the Gibbs elasticity (175 dyn/cm for surfactin; 109 dyn/cm for C16TAB; 38 dyn/cm for SDS) required to assess the stability of NBF against surface concentration fluctuations, and the shear modulus (6.5 GPa for the surfactin; 6.1 GPa for the C16TAB; 3.5 GPa for the SDS) and the yield stress (approximately 0.8 GPa for surfactin; approximately 0.8 GPa for C16TAB; approximately 0.4 GPa for the SDS) to assess the mechanical stability against the externally imposed mechanical perturbation.

Molecular Dynamics Simulation of Amphiphilic Bistable [2]Rotaxane Langmuir Monolayers at the Air/Water Interface

Bistable [2]rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttling motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), between a monopyrrolotetrathiafulvalene (mpTTF) unit and a 1,5-dioxynaphthalene (DNP) unit located along a dumbbell component. The most stable state of the rotaxane (CBPQT(4+)@mpTTF) is that in which the CBPQT(4+) ring encircles the mpTTF unit, but a second less favored metastable co-conformation with the CBPQT(4+) ring surrounding the DNP (CBPQT(4+)@DNP) can be formed experimentally. For both co-conformations of an amphiphilic bistable [2]rotaxane, we report here the structure and surface pressure-area isotherm of a Langmuir monolayer (LM) on a water subphase as a function of the area per molecule. These results from atomistic molecular dynamics (MD) studies are validated by comparing with experiments based on similar amphiphilic rotaxanes. For both co-conformations, we found that as the area per molecule increases the thickness of the LM decreases while the molecular tilt increases. Both co-conformations led to similar LM thicknesses at the same packing area. From the simulated LM systems, we calculated the electron density profiles of the monolayer as a function of area per molecule, which show good agreement with experimental analyses from synchrotron X-ray reflectivity measurements of related systems. Decomposing the overall electron density profiles into component contributions, we found distinct differences in molecular packing in the film depending upon the co-conformation. Thus we find that the necessity of allowing the tetracationic ring to become solvated by water leads to differences in the structures for the two co-conformations in the LM. At the same packing area, the value of the overall tilt angle does not seem to be sensitive to whether the CBPQT(4+) ring is encircling the mpTTF or the DNP unit. However, the conformation of the dumbbell does depend on the location of the CBPQT(4+) ring, which is reflected in the segmental tilt angles of the mpTTF and DNP units. Using the Kirkwood-Buff formula in conjunction with MD calculations, we find the surface pressure-area isotherms for each co-conformation in which the CBPQT(4+)@mpTTF form has smaller surface tension and therefore larger surface pressure than the CBPQT(4+)@DNP at the same packing area, differences that decreases with increasing area per molecule, which is verified experimentally.

Conformations and charge transport characteristics of biphenyldithiol self-assembled-monolayer molecular electronic devices: A multiscale computational study

We report a computational study of conformations and charge transport characteristics of biphenyldithiol (BPDT) monolayers in the (sqrt.3 x sqrt.3)R30 degrees packing ratio sandwiched between Au(111) electrodes. From force-field molecular-dynamics and annealing simulations of BPDT self-assembled monolayers (SAMs) with up to 100 molecules on a Au(111) substrate, we identify an energetically favorable herringbone-type SAM packing configuration and a less-stable parallel packing configuration. Both SAMs are described by the (2sqrt.3 x sqrt.3)R30 degrees unit cell including two molecules. With subsequent density-functional theory calculations of one unit cell of the (i) herringbone SAM with the molecular tilt angle theta approximately 15 degrees , (ii) herringbone SAM with theta approximately 30 degrees , and (iii) parallel SAM with theta approximately 30 degrees, we confirm that the herringbone packing configuration is more stable than the parallel one but find that the energy variation with respect to the molecule tilting within the herringbone packing is very small. Next, by capping these SAMs with the top Au(111) electrode, we prepare three molecular electronic device models and calculate their coherent charge transport properties within the matrix Green's function approach. Current-voltage (I-V) curves are then obtained via the Landauer-Buttiker formula. We find that at low-bias voltages (|V| < or = 0.2 V) the I-V characteristics of models (ii) and (iii) are similar and the current in model (i) is smaller than that in (ii) and (iii). On the other hand, at higher-bias voltages (|V| > or 0.5 V), the I-V characteristics of the three models show noticeable differences due to different phenyl band structures. We thus conclude that the BPDT SAM I-V characteristics in the low-bias voltage region are mainly determined by the -Au [corrected] interaction within the individual molecule-electrode contact, while both intramolecular conformation and intermolecular interaction can affect the BPDT SAM I-V characteristics in the high-bias voltage region.

Nanophase Segregation and Water Dynamics in the Dendrion Diblock Copolymer Formed from the Fréchet Polyaryl Ethereal Dendrimer and Linear PTFE

We propose a new material consisting of a dendrion copolymer formed from (a) a water-soluble dendritic polymer and (b) a hydrophobic backbone. Using molecular dynamics simulations techniques, we determine the structure and dynamics of the dendrion formed by second-generation Fréchet polyaryl ethereal dendrimer as the hydrophilic component and linear polytetrafluoroethylene (PTFE) as the hydrophobic polymer, with 5 and 10 wt % of water. We find that this material produces a well-developed nanoscale structure in which water forms a continuous nanophase, making this new family of compounds promising candidates for applications in fuel cell membranes. We find that the water molecules are incorporated into the dendrimer block of the copolymer to form a nanophase-segregated structure. The well-developed nanophase-segregated structures rendered by this material have characteristic dimensions of segregation ( approximately 30 Angstrom) and dendrimer conformational properties that are independent of water content. Calculations of water dynamics and proton transport in these nanophase-segregated structures indicate that the dendrion copolymer membrane with 10 wt % of water content has a water structure and transport properties equivalent to that of the hydrated Nafion membrane with 20 wt % of water content.

First-principles study of the switching mechanism of [2]catenane molecular electronic devices

We present a first-principles study of the coherent charge transport properties of bistable [2]catenane molecular monolayers sandwiched between Au(111) electrodes. We find that conduction channels around the Fermi level are dominated by the two highest occupied molecular orbital levels from tetrathiafulvalene (TTF) and dioxynaphthalene (DNP) and the two lowest unoccupied molecular orbital levels from tetracationic cyclophane (CBPQT4+), and the OFF to ON switching results from the energetic shifts of these orbitals as CBPQT4+ moves from TTF to DNP. We show that the superposition principle can be adopted for predicting the function of the composite device.

Molecular Dynamics Simulation Study on a Monolayer of Half [2]Rotaxane Self-Assembled on Au(111)

The self-assembled monolayer (SAM) structure of the tetrathiafulvalene-side half of the Stoddart-Heath type [2]rotaxane on Au(111) surface was investigated using molecular dynamics (MD) simulations. We find that the orientation of the cyclobis(paraquat-p-phenylene) (CBPQT) ring depends dramatically on the coverage, changing in order to obtain highly packed SAMs. The ring lies with its large hollow parallel to the surface at lower coverage (up to one CBPQT per 27 surface Au atoms with a footprint of 1.9 nm(2); 1/27) when free space is available around it, but as the coverage increases (up to one CBPQT per 12 surface Au atoms with a footprint of 0.9 nm(2); 1/12), it tilts completely around its axis and lies with its smaller side (paraquat or phenyl ring) parallel to the surface to accommodate the reduced area available. We find that the best packing densities correspond to one CBPQT per 12-18 surface Au atoms (1/18-1/12) with footprints in the range between 0.9 nm(2) and 1.3 nm(2).

Structures and Properties of Self-Assembled Monolayers of Bistable [2]Rotaxanes on Au (111) Surfaces from Molecular Dynamics Simulations Validated with Experiment

Bistable [2]rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttling motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT(4+)) (denoted as the ring), between a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) ring system located along a dumbbell component. When the ring is encircling the TTF unit, this co-conformation of the rotaxane is the most stable and thus designated the ground-state co-conformer (GSCC), whereas the other co-conformation with the ring surrounding the DNP ring system is less favored and so designated the metastable-state co-conformer (MSCC). We report here the structure and properties of self-assembled monolayers (SAMs) of a bistable [2]rotaxane on Au (111) surfaces as a function of surface coverage based on atomistic molecular dynamics (MD) studies with a force field optimized from DFT calculations and we report several experiments that validate the predictions. On the basis of both the total energy per rotaxane and the calculated stress that is parallel to the surface, we find that the optimal packing density of the SAM corresponds to a surface coverage of 115 A(2)/molecule (one molecule per 4 x 4 grid of surface Au atoms) for both the GSCC and MSCC, and that the former is more stable than the latter by 14 kcal/mol at the optimum packing density. We find that the SAM retains hexagonal packing, except for the case at twice the optimum packing density (65 A(2)/molecule, the 3 x 3 grid). For the GSCC and MSCC, investigated at the optimum coverage, the tilt of the ring with respect to the normal is theta = 39 degrees and 61 degrees, respectively, while the tilt angle of the entire rotaxane is psi = 41 degrees and 46 degrees , respectively. Although the tilt angle of the ring decreases with decreasing surface coverage, the tilt angle of the rotaxane has a maximum at 144 A(2)/molecule (the 4 x 5 grid/molecule) of 50 degrees and 51 degrees for the GSCC and MSCC, respectively. The hexafluorophosphate counterions (PF(6)(-)) stay localized around the ring during the 2 ns MD simulation. On the basis of the calculated density profile, we find that the thickness of the SAM is 40.5 A at the optimum coverage for the GSCC and 40.0 A for MSCC, and that the thicknesses become less with decreasing surface coverage. The calculated surface tension at the optimal packing density is 45 and 65 dyn/cm for the GSCC and MSCC, respectively. This difference suggests that the water contact angle for the GSCC is larger than for the MSCC, a prediction that is verified by experiments on Langmuir-Blodgett monolayers of amphiphilic [2]rotaxanes.

Density Functional Theory Studies of the [2]Rotaxane Component of the Stoddart−Heath Molecular Switch

The central component of the programmable molecular switch recently demonstrated by Stoddart and Heath is [2]rotaxane, which consists of a cyclobis(paraquat-p-phenylene) shuttle (CBPQT(4+))(PF(6)(-))(4) (the ring) encircling a finger and moving between two stations, tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP). As a step toward understanding the mechanism of this switch, we report here its electronic structure using two flavors of density functional theory (DFT): B3LYP/6-31G and PBE/6-31G. We find that the electronic structure of composite [2]rotaxane can be constructed reasonably well from its parts by combining the states of separate stations (TTF and DNP) with or without the (CBPQT)(PF(6))(4) shuttle around them. That is, the "CBPQT@TTF" state, (TTF)(CBPQT)(PF(6))(4)-(DNP), is described well as a combination of the (TTF)(CBPQT)(PF(6))(4) complex and free DNP, and the "CBPQT@DNP" state, (TTF)-(DNP)(CBPQT)(PF(6))(4), is described well as a combination of free TTF and the (DNP)(CBPQT)(PF(6))(4) complex. This allows an aufbau or a "bottom-up" approach to predict the complicated [n]rotaxanes in terms of their components. This should be useful in designing new components to lead to improved properties of the switches. A critical function of the (CBPQT(4+))(PF(6)(-))(4) shuttle in switching is that it induces a downshift of the frontier orbital energy levels of the station it is on (TTF or DNP). This occurs because of the net positive electrostatic potential exerted by the CBPQT(4+) ring, which is located closer to the active station than the four PF(6)(-)'s. This downshift alters the relative position of energy levels between TTF and DNP, which in turn alters the electron tunneling rate between them, even when the shuttle is not involved directly in the actual tunneling process. Based on this switching mechanism, the "CBPQT@TTF" state is expected to be a better conductor since it has better aligned levels between the two stations. A second potential role of the (CBPQT(4+))(PF(6)(-))(4) shuttle in switching is to provide low-lying LUMO levels. If the shuttle is involved in the actual tunneling process, the reduced HOMO-LUMO gap (from 3.6 eV for the isolated finger to 1.1 eV for "CBPQT@TTF" or to 0.6 eV for "CBPQT@DNP" using B3LYP) would significantly facilitate the electron tunneling through the system. This might occur in a folded conformation where a direct contact between free station and the shuttle on the other station is possible. When this becomes the main switching mechanism, we expect the "CBPQT@DNP" state to become a better conductor because its HOMO-LUMO gap is smaller and because its HOMO and LUMO are localized at different stations (HOMO exclusively at TTF and LUMO at CBPQT encircling DNP) so that the HOMO-to-LUMO tunneling would be through the entire molecule of [2]rotaxane. Thus an essential element in designing these switches is to determine the configuration of the molecules (e.g., through self-assembled monolayers or incorporation of conformation stabilizing units).

Interrelations of organism prevalence, specimen collection method, and host age, sex, and breed among 8,354 canine urinary tract infections (1969-1995)

Selected information was compiled from canine urinalyses and urine cultures conducted between January 1969 and December 1995. Eight thousand three hundred fifty-four microbial isolates (bacteria and fungi) included 4,873 isolates from females and 3,481 from males. Ten bacterial genera accounted for 96.3% of the urinary isolates, including Escherichia coli (44.1%), Staphylococcus spp. (11.6%), Proteus spp. (9.3%), Klebsiella spp. (9.1%), Enterococcus spp. (8.0%), and Streptococcus spp. (5.4%) as the 6 most common isolates in both genders of dogs. Among these 6 genera, female dogs were generally predisposed over males, although males had more urinary tract infections (UTIs) caused by Klebsiella spp. Distributions of ages at UTI diagnosis tended to be similar between genders. Infection with a single microbial species was responsible for >72% of UTIs in both genders. Among females, 40 breeds and a mixed-breed group represented 90.2% of all positive urine cultures, 88.4% of the individual dogs with UTIs. and 88.2% of the microbial isolations. Among males, these same 41 breed groups represented 87.9% of all positive urine cultures, 87.6% of the individual dogs, and 88.2% of the microbial isolations.

Isolation of Clostridium innocuum from cases of recurrent diarrhea in patients with prior Clostridium difficile associated diarrhea

Clostridium innocuum isolates resistant to vancomycin (MIC values of 16-24 microg/mL) were isolated from three patients with recurrent Clostridium difficile -associated diarrhea (CDAD). We discuss the clinical significance and problems associated with the identification and differentiation of these two clostridial species, which may result in misdiagnosis of patients.

Taylorella asinigenitalis sp. nov., a bacterium isolated from the genital tract of male donkeys (Equus asinus)

Three bacterial isolates that were phenotypically indistinguishable from Taylorella equigenitalis were obtained from the urethral fossae of three male donkeys (Equus asinus), one located in the state of California and the other two in the state of Kentucky, USA. Based on results of pulsed-field gel electrophoresis, the isolate from California differed from the two Kentucky isolates, which were the same. Mares bred artificially (California) or naturally (Kentucky) did not show signs of disease, even though infection with the organism was established in those bred naturally. Mares and, uncharacteristically, all three jacks produced antibodies that reacted in the complement fixation test utilized to identify mares recently infected with T. equigenitalis. Sequence analysis of DNA encoding the 16S rRNA revealed that the gene sequences of these isolates were virtually identical to each other (>99.8% similarity), but different (97.6% similarity) from those of several confirmed isolates of T. equigenitalis. The 16S rDNA sequences of the latter were 100% identical. DNA-DNA hybridization studies revealed a mean hybridization level of 89% between the donkey isolate from California and the donkey isolate from Kentucky. On the other hand, the mean DNA-DNA hybridization level from the donkey isolates with DNA from a strain of T. equigenitalis was 23%. The DNA G+C composition was 37.8 mol% for the two donkey isolates, as well as the strain of T. equigenitalis used in the hybridization studies. These data support our opinion that micro-organisms isolated from the male donkeys are different from T. equigenitalis and it is proposed that they be considered a new species within the genus Taylorella and named Taylorella asinigenitalis sp. nov. The type strain is strain UCD-1T (= ATCC 700933T = LMG 19572T).

Multiplex polymerase chain reaction for distinguishing Taylorella equigenitalis from Taylorella equigenitalis-like organisms

It is difficult to distinguish isolates of Taylorella equigenitalis, the cause of contagious equine metritis, from a T. equigenitalis-like organism isolated from asymptomatic donkeys and horses. Although T. equigenitalis is responsible for a severe, contagious disease of the reproductive tract of equids, the T. equigenitalis-like organism, although contagious, does not appear to produce disease. Because of the economic consequences of correctly distinguishing isolates of these 2 microorganisms, a polymerase chain reaction (PCR)-based assay was developed that will distinguish isolates of T. equigenitalis from the T. equigenitalis-like microorganism. The primers used in the PCR assay were designed to amplify unique regions of the gene encoding the 16S ribosomal RNA.

Bacteria associated with pyothorax of dogs and cats: 98 cases (1989-1998)

OBJECTIVE

To determine the organisms most commonly isolated from pleural fluid from dogs and cats with pyothorax.

DESIGN

Retrospective study.

ANIMALS

51 dogs and 47 cats.

PROCEDURE

Results of bacteriologic culture of pleural fluid samples obtained by means of thoracentesis were obtained from medical records. To obtain information on in vitro antimicrobial susceptibility of organisms commonly isolated from dogs and cats, records of all dogs and cats examined during 1998 were reviewed, and information was obtained on identity and in vitro antimicrobial susceptibility of aerobic organisms isolated from samples other than urine or urinary tract samples.

RESULTS

Median ages of dogs and cats were 4 years. Bacteria were isolated from pleural fluid samples from 47 of 51 (92%) dogs and 45 of 47 (96%) cats. Obligate anaerobic bacteria were isolated from 28 dogs and 40 cats. A mixture of obligate anaerobic and facultative bacteria was isolated from 17 dogs and 20 cats. Samples from cats most often yielded a member of the nonenteric group (most commonly members of the genus Pasteurella), whereas those from dogs more often yielded a member of the family Enterobacteriaceae (most commonly E coli).

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that antimicrobial agents chosen for the initial treatment of dogs and cats with pyothorax should be active against a mixture of obligate anaerobic and facultative bacteria.

Bartonella spp. isolated from wild and domestic ruminants in North America

Bartonella species were isolated from 49% of 128 cattle from California and Oklahoma, 90% of 42 mule deer from California, and 15% of 100 elk from California and Oregon. Isolates from all 63 cattle, 14 deer, and 1 elk had the same polymerase chain reaction/restriction fragment length polymorphism profiles. Our findings indicate potential for inter- and intraspecies transmission among ruminants, as well as risk that these Bartonella spp. could act as zoonotic agents.

Evaluation of iodophor skin preparation techniques and factors influencing drainage from ventral midline incisions in horses

OBJECTIVE

To document natural bacterial flora on the ventral aspect of the equine abdomen, to compare 2 preparation techniques, and to identify potential risk factors that may contribute to incisional drainage.

DESIGN

Prospective study.

ANIMALS

53 horses undergoing exploratory celiotomy.

PROCEDURE

Group-1 horses (n = 26) were prepared with povidone-iodine and alcohol. Group-2 horses (27) were prepared with a film-forming iodophor complex. Numbers of bacterial colony-forming units (CFU) were measured before and after surgical scrub, following skin closure, and after recovery from general anesthesia. Swab specimens to identify normal skin bacterial flora and potential pathogens were obtained by swabbing a 4 x 4-cm area. Variables that might affect incisional drainage were also investigated.

RESULTS

For both techniques, there was a significant reduction in bacterial numbers after skin preparation. Incisional drainage was observed in 14 (26%) horses (8 group-1 and 6 group-2 horses). Preexisting dermatitis, poor intraoperative drape adherence, high number of bacterial CFU obtained after recovery from anesthesia, and high number of CFU obtained from the surgery room environment were the main risk factors associated with subsequent incisional drainage. Bacillus spp, nonhemolytic Staphylococcus spp, Micrococcus spp, Corynebacterium spp, Streptomyces spp, other nonenteric genera, and nonhemolytic Streptococcus spp were the most common isolates obtained before surgical scrub.

CONCLUSIONS AND CLINICAL RELEVANCE

Both skin preparation techniques were equally effective in reducing numbers of bacterial CFU by 99%, and a significant difference was not found in incisional drainage rate between groups. Protection of the wound during recovery from anesthesia and the immediate postoperative period may reduce incisional drainage after abdominal surgery in horses.

Antimicrobial susceptibility of Nocardia nova isolated from five cats with nocardiosis

From 1992 through 1997, 5 cats were admitted to the hospital because of chronic, nonhealing lesions containing draining tracts. Exudate from 2 of the 5 cats contained macroscopically visible granules. On the basis of cytologic findings, lesions were described as pyogranulomatous. Degenerative neutrophils and activated macrophages, along with slender, branching, gram-positive, partially acid-fast microorganisms, were observed in stained smears of exudates obtained from all 5 affected cats. Nocardia nova was found in pure culture from all affected sites. Most isolates were susceptible to ampicillin, aminoglycosides (ie, amikacin, kanamycin), tetracyclines (ie, doxycycline, minocycline), macrolides (ie, erythromycin, clarithromycin), imipenem, sulfisoxazole, and trimethoprim-sulfamethoxazole. Other antimicrobials were less effective, and these included amoxicillin-clavulanic acid, the cephalosporins (ie, cefotaxime, ceftizoxime, ceftriaxone), and some aminoglycosides (ie, gentamicin, tobramycin). Four of the 5 cats were successfully treated, 3 with a trimethoprim-sulfonamide combination, and 1 with clarithromycin. The outcome of treatment of the fifth cat is unknown. Findings in this report may be useful in diagnosis and treatment of nocardiosis caused by N nova in cats.