The passage of homopolymeric RNA through small solid-state nanopores

Solid-state nanopores are widely acknowledged as tools with which to study local structure in biological molecules. Individual molecules are forced through a nanopore, causing a characteristic change in an ionic current that depends on the molecules' local diameter and charge distribution. Here, the translocation measurements of long (~5-30 kilobases) single-stranded poly(U) and poly(A) molecules through nanopores ranging from 1.5 to 8 nm in diameter are presented. Individual molecules are found to be able to cause multiple levels of conductance blockade upon traversing the pore. By analyzing these conductance blockades and their relative incidence as a function of nanopore diameter, it is concluded that the smallest conductance blockades likely correspond to molecules that translocate through the pore in predominantly head-to-tail fashion. The larger conductance blockades are likely caused by molecules that arrive at the nanopore entrance with many strands simultaneously. These measurements constitute the first demonstration that single-stranded RNA can be captured in solid-state nanopores that are smaller than the diameter of double-stranded RNA. These results further the understanding of the conductance blockades caused by nucleic acids in solid-state nanopores, relevant for future applications, such as the direct determination of RNA secondary structure.     

Effects of Perinatal Exposure to PCBs on Neuropsychological Functions in the Rotterdam Cohort at 9 Years of Age.

PCBs are known for their neurotoxic properties, especially on the developing brain. To increase insight into the neurotoxic effects of PCB exposure, the authors studied the effects of perinatal exposure to environmental levels of these compounds on different neuropsychological domains. In 9-year-old children of the Rotterdam PCB--dioxin cohort, higher prenatal PCB levels were associated with longer response times (RTs), more variation in RTs, and lower scores on the Tower of London (TOL; Shallice, 1982). A longer breast-feeding duration was associated with lower TOL scores and with better spatial organizational skills. There was some evidence of negative effects of lactational exposure to PCBs on scores on the TOL. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Predicting adult fish acute lethality with the zebrafish embryo: relevance of test duration, endpoints, compound properties, and exposure concentration analysis

The zebrafish embryo toxicity test has been proposed as an alternative for the acute fish toxicity test, which is required by various regulations for environmental risk assessment of chemicals. We investigated the reliability of the embryo test by probing organic industrial chemicals with a wide range of physicochemical properties, toxicities, and modes of toxic action. Moreover, the relevance of using measured versus nominal (intended) exposure concentrations, inclusion of sublethal endpoints, and different exposure durations for the comparability with reported fish acute toxicity was explored. Our results confirm a very strong correlation of zebrafish embryo to fish acute toxicity. When toxicity values were calculated based on measured exposure concentrations, the slope of the type II regression line was 1 and nearly passed through the origin (1 to 1 correlation). Measured concentrations also explained several apparent outliers. Neither prolonged exposure (up to 120 h) nor consideration of sublethal effects led to a reduced number of outliers. Yet, two types of compounds were less lethal to embryos than to adult fish: a neurotoxic compound acting via sodium channels (permethrin) and a compound requiring metabolic activation (allyl alcohol).     

Salt dependence of ion transport and DNA translocation through solid-state nanopores

We report experimental measurements of the salt dependence of ion transport and DNA translocation through solid-state nanopores. The ionic conductance shows a three-order-of-magnitude decrease with decreasing salt concentrations from 1 M to 1 muM, strongly deviating from bulk linear behavior. The data are described by a model that accounts for a salt-dependent surface charge of the pore. Subsequently, we measure translocation of 16.5-mum-long dsDNA for 50 mM to 1 M salt concentrations. DNA translocation is shown to result in either a decrease ([KCl] > 0.4 M) or increase of the ionic current ([KCl] < 0.4 M). The data are described by a model where current decreases result from the partial blocking of the pore and current increases are attributed to motion of the counterions that screen the charge of the DNA backbone. We demonstrate that the two competing effects cancel at a KCl concentration of 370 +/- 40 mM.     

Influence of albumin on sorption kinetics in solid-phase microextraction: consequences for chemical analyses and uptake processes

Because of its simplicity, solid-phase microextraction (SPME) is an increasingly popular technique to use in experiments measuring freely dissolved concentrations of compounds in biological and environmental samples. However, a number of studies have shown that sorption kinetics of compounds in such SPME systems is dependent on the presence of a binding matrix. This affects the interpretability of nonequilibrium SPME data. In this study, this phenomenon was investigated by measuring the rate of depletion of pyrene from a "loaded" poly(dimethylsiloxane) fiber into surrounding cell culture medium containing different concentrations of bovine serum albumin (BSA). The rate of depletion was found to steadily increase with increasing concentrations of BSA. It was postulated that BSA facilitated the transport of pyrene through the medium. This phenomenon was modeled by considering diffusion of BSA-bound pyrene in addition to diffusion of unbound pyrene in the aqueous boundary layer (BL) around the fiber. The model closely fit the experimental data and illustrated that diffusion in the BL was rate limiting because the analyte's affinity for the fiber was high and the BL thickness significant. The concentration of binding matrix and the analyte's affinity for the matrix further determined the extent to which BSA-facilitated transport contributed to the kinetics of the system.     

Nanopore tomography of a laser focus

We demonstrate that the ionic current through a solid-state nanopore can be used to measure at single nanometer resolution the three-dimensional intensity profile of a laser directly in the focus of a microscope objective. We find a linear dependence of the ionic current on the incident laser power since the laser-induced heat increases the temperature locally in the solution. Our data show a temperature increase of up to 20 K in the center of the focus for a laser wavelength of 1064 nm. Measurements of the two-dimensional temperature profiles at different positions along the optical axis allow us to reconstruct the three-dimensional temperature profile of the laser focus, similar to tomography. Our new technique does not rely on the help of any optical elements and allows quantitative measurement of optical intensity or temperature distributions in aqueous environments with nanometer resolution.     

The Effect of Social Comparison Information on the Life Satisfaction of Frail Older Persons.

In this study, the authors examined the effects of social comparison on the life satisfaction of 455 community-dwelling older persons. These older persons were confronted with a fictitious interview with either an upward or a downward target. After downward comparison, older persons felt more satisfied with their lives than after upward comparison, especially those who had higher levels of frailty. These effects were only found with lower levels of identification. Apparently, downward comparison only serves its self-enhancing function on life satisfaction among frail older persons when they perceive the comparison target as different from themselves. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Electromagnetic torque tweezers: a versatile approach for measurement of single-molecule twist and torque

The well-established single-molecule force-spectroscopy techniques have recently been complemented by methods that can measure torque and twist directly, notably magnetic torque tweezers and the optical torque wrench. A limitation of the current torque measurement schemes is the intrinsic coupling between the force and torque degrees of freedom. Here we present electromagnetic torque tweezers (eMTT) that combine permanent and electromagnets to enable independent control of the force and torsional trap stiffness for sensitive measurements of single molecule torque and twist. Using the eMTT, we demonstrate sensitive torque measurements on tethered DNA molecules from simple tracking of the beads' (x,y)-position, obviating the need for any angular tracking algorithms or markers. Employing the eMTT for high-resolution torque measurements, we experimentally confirm the theoretically predicted torque overshoot at the DNA buckling transition in high salt conditions. We envision that the flexibility and control afforded by the eMTT will enable a range of new torque and twist measurement schemes from single-molecules to living cells.