The authors review recent advances in the physics of strongly interacting charged systems functioning in water at room temperature. In these systems, many phenomena go beyond the framework of mean-field theories, whether linear Debye-Hückel or nonlinear Poisson-Boltzmann, culminating in charge inversion—a counterintuitive phenomenon in which a strongly charged particle, called a macroion, binds so many counterions that its net charge changes sign. The review discusses the universal theory of charge inversion based on the idea of a strongly correlated liquid of adsorbed counterions, similar to a Wigner crystal. This theory has a vast array of applications, particularly in biology and chemistry; for example, in the presence of positive multivalent ions (e.g., polycations), the DNA double helix acquires a net positive charge and drifts as a positive particle in an electric field. This simplifies DNA uptake by the cell as needed for gene therapy, because the cell membrane is negatively charged. Analogies of charge inversion to other fields of physics are also discussed.

Solid-state nanopores are sensors capable of analysing individual unlabelled DNA molecules in solution. Although the critical information obtained from nanopores (for example, DNA sequence) comes from the signal collected during DNA translocation, the throughput of the method is determined by the rate at which molecules arrive and thread into the pores. Here, we study the process of DNA capture into nanofabricated SiN pores of molecular dimensions. For fixed analyte concentrations we find an increase in capture rate as the DNA length increases from 800 to 8,000 base pairs, a length-independent capture rate for longer molecules, and increasing capture rates when ionic gradients are established across the pore. Furthermore, we show that application of a 20-fold salt gradient allows the detection of picomolar DNA concentrations at high throughput. The salt gradients enhance the electric field, focusing more molecules into the pore, thereby advancing the possibility of analysing unamplified DNA samples using nanopores.

The present state of the theory of volume effects in a polymer chain is reviewed. The theory of coils (i.e., chains with predominately repulsive volume interaction of monomers) and the analogy between the excluded volume problem and the theory of second-order phase transitions are briefly described. The theory of globules, formed by attractive interaction of monomers or by external attractive fields, is considered in greater detail. The coil-globule transition under various conditions is analyzed. A theory is constructed for the simplest model of the polymer chain—the model of "interacting beads on a flexible string." The connection of this model with more realistic ones is discussed. This review is based on the approach proposed by I. M. Lifshitz in 1968. While attention is focused on the physical aspects of the problems, some questions concerning biological applications are discussed in the conclusion.

We present measurements and theoretical modeling of the ionic conductance G of solid-state nanopores with 5-100 nm diameters, with and without DNA inserted into the pore. First, we show that it is essential to include access resistance to describe the conductance, in particular for larger pore diameters. We then present an exact solution for G of an hourglass-shaped pore, which agrees very well with our measurements without any adjustable parameters, and which is an improvement over the cylindrical approximation. Subsequently we discuss the conductance blockade ΔG due to the insertion of a DNA molecule into the pore, which we study experimentally as a function of pore diameter. We find that ΔG decreases with pore diameter, contrary to the predictions of earlier models that forecasted a constant ΔG. We compare three models for ΔG, all of which provide good agreement with our experimental data.

To understand the kinetics of protein folding, we introduce the concept of a “transition coordinate” which is defined to be the coordinate along which the system progresses most slowly. As a practical implementation of this concept, we define the transmission coefficient for any conformation to be the probability for a chain with the given conformation to fold before it unfolds. Since the transmission coefficient can serve as the best possible measure of kinetic distance for a system, we present two methods by which we can determine how closely any parameter of the system approximates the transmission coefficient. As we determine that the transmission coefficient for a short-chain heteropolymer system is dominated by entropic factors, we have chosen to illustrate the methods mentioned by applying them to geometrical properties of the system such as the number of native contacts and the looplength distribution. We find that these coordinates are not good approximations of the transmission coefficient and therefore, cannot adequately describe the kinetics of protein folding.

Journal de Physique, Journal de Physique Archives représente une mine d informations facile à consulter sur la manière dont la physique a été publiée depuis 1872.