Quantifying screening ion excesses in single-molecule force-extension experiments.

We derive a thermodynamic identity that allows one to infer the change in the number of screening ions that are associated with a charged macromolecule as the macromolecule is continuously stretched. Applying this identity to force-extension data on both single-stranded and double-stranded DNA, we find that the number of polymer-associated ions depends nontrivially on both the bulk salt concentration and the bare rigidity of the polymer, with single-stranded DNA exhibiting a relatively large decrease in ion excess upon stretching. We rationalize these observations using simple models for polyelectrolyte extension.

On chemical structures with potent antiepileptic/anticonvulsant profile.

Epilepsy is a common chronic neurological disorder characterized by recurrent unprovoked seizures. There has been a considerable interest in the development of many antiepileptic and anticonvulsant agents for controlling epilepsy with fewer side effects and improvement of quality of life. Since the terms antiepileptics /anticonvulsants are used interchangeably, this article reviews their classification according to the chemical structure into: hydantoins, oxazolidinediones, succinimides, barbiturates, amides, benzodiazepines, valproic acid and its derivatives, GABA-analogues, cycloalkanes, semicarbazones, γ butyrolactones (GBLs), imidaquinazolines and pyrrolidine derivatives as well as miscellaneous compounds. In addition, the review discusses the different mechanisms of action of antiepileptic and anticonvulsant agents.

Detailed scaling analysis of low-force polyelectrolyte elasticity.

Single-molecule force-extension data are typically compared to ideal models of polymer behavior that ignore the effects of self-avoidance. Here, we demonstrate a link between single-molecule data and the scaling pictures of a real polymer. We measure a low-force elasticity regime where the extension L of chemically denatured single-stranded DNA grows as a power law with force f : L approximately f;{gamma} , with gamma approximately 0.60-0.69 . This compares favorably with the "tensile-blob" model of a self-avoiding polymer, which predicts gamma=2/3 . We show that the transition out of the low-force regime is highly salt dependent, and use the tensile-blob model to relate this effect to the salt dependence of the polymer's Kuhn length and excluded-volume parameter. We find that, contrary to the well-known Odijk-Skolnick-Fixman theory, the Kuhn length of single-stranded DNA is linearly proportional to the Debye length of the solution. Finally, we show that the low-force elasticity becomes linear (gamma=1) at approximately 3 M salt, and interpret this as a Theta point of the polymer. At this point, the force-extension data is best described by the wormlike chain model, from which we estimate the bare (nonelectrostatic) persistence length of the polymer to be approximately 0.6 nm .

Nonlinear low-force elasticity of single-stranded DNA molecules.

We reconcile single-molecule force-extension data with scaling theories of polymer elasticity: measurements of denatured single-stranded DNA show a regime where the extension grows as a nonlinear power law with force, in accordance with "tensile blob" models. Analysis of the salt dependence of this regime indicates that the polymer's Kuhn length is proportional to the Debye length; this contradicts the Odijk-Skolnick-Fixman theory, but agrees with other predictions. Finally, we identify a Theta condition of the polymer, and find that the wormlike chain model best describes the polymer's elasticity at this point.

Statistical determination of the step size of molecular motors.

Molecular motors are enzymatic proteins that couple the consumption of chemical energy to mechanical displacement. In order to elucidate the translocation mechanisms of these enzymes, it is of fundamental importance to measure the physical step size. The step size can, in certain instances, be directly measured with single-molecule techniques; however, in the majority of cases individual steps are masked by noise. The step size can nevertheless be obtained from noisy single-molecule records through statistical methods. This analysis is analogous to determining the charge of the electron from current shot noise. We review methods for obtaining the step size based on analysing, in both the time and frequency domains, the variance in position from noisy single-molecule records of motor displacement. Additionally, we demonstrate how similar methods may be applied to measure the step size in bulk kinetic experiments.

Biological sensing with an on-chip resistive pulse analyzer.

Resistive pulse sensors (or Coulter counters) detect the conductance change caused by single fluid-borne particles transiting a pore. Their simplicity in design and use, along with their capability for single-molecule sensitivity, make them well-suited to the analysis of biological particles. Here, we use standard methods of micro- and nanolithography to construct resistive-pulse devices that combine microfluidics with electronic sensing. We use the devices to detect single latex colloids, single DNA molecules, and specific antibody/antigen binding. We discuss the advantages of our design, and prospects for future applications.

Role of the Raf signal transduction cascade in the in vitro resistance to the anticancer drug doxorubicin.

The precise molecular events involved in the development of drug resistance (DR) remain largely unknown. Raf is an intermediate in the signal transduction cascades initiated by growth factors. The hypothesis behind the following studies is that deregulated Raf-1 expression plays a role in the development of drug resistance. A positive correlation was observed between increased Raf-1 activity and increased values for IC50 for doxorubicin in established cell lines. The National Cancer Institute/Adriamycin Resistant (NCI/ADR-RES) cell line exhibited both the highest Raf-1 activity and the highest IC50 values for doxorubicin (Adriamycin). In contrast, the MCF-7 cell line exhibited both lower Raf activity and lower IC50 values for doxorubicin. While MCF-7 cells transfected with either constitutively active DeltaRaf-1 or conditionally active DeltaRaf-1:AR demonstrated increased IC50 values for doxorubicin and a reduced capacity to undergo apoptosis after doxorubicin treatment as compared with parental cell lines. Moreover, growth curves performed show that both the constitutively and conditionally active forms of Raf-1 do not increase growth as compared with the parental MCF-7 cell line. This is important because it implies that higher cell counts between Raf transfectants and the parental MCF-7 cell line are attributable to differences in DR, not growth rates. These observations suggest a role for the Raf-1 protooncogene in the regulation of DR.

Capacitance cytometry: measuring biological cells one by one.

Measuring the DNA content of eukaryotic cells is a fundamental task in biology and medicine. We have observed a linear relationship between the DNA content of eukaryotic cells and the change in capacitance that is evoked by the passage of individual cells across a 1-kHz electric field. This relationship is species-independent; consequently, we have developed a microfluidic technique-"capacitance cytometry"-that can be used to quantify the DNA content of single eukaryotic cells and to analyze the cell-cycle kinetics of populations of cells. Comparisons with standard flow cytometry demonstrate the sensitivity of this new technique.