Magnetic tweezers measurements of the nanomechanical stability of DNA against denaturation at various conditions of pH and ionic strength.

The opening of DNA double strands is extremely relevant to several biological functions, such as replication and transcription or binding of specific proteins. Such opening phenomenon is particularly sensitive to the aqueous solvent conditions in which the DNA molecule is dispersed, as it can be observed by considering the classical dependence of DNA melting temperature on pH and salt concentration. In the present work, we report a single-molecule study of the stability of DNA against denaturation when subjected to changes in solvent. We investigated the appearance of DNA instability under specific external applied force and imposed twist values, which was revealed by an increase in the temporal fluctuations in the DNA extension. These fluctuations occur in the presence of a continuous interval of equilibrium states, ranging from a plectonemic state to a state characterized by denaturation bubbles. In particular, we observe the fluctuations only around a characteristic force value. Moreover, this characteristic force is demonstrated to be notably sensitive to variations in the pH and ionic strength. Finally, an extension of a theoretical model of plectoneme formation is used to estimate the average denaturation energy, which is found to be linearly correlated to the melting temperature of the DNA double strands.

Magnetic tweezers measurements of the nanomechanical properties of DNA in the presence of drugs.

Herein, we study the nanomechanical characteristics of single DNA molecules in the presence of DNA binders, including intercalating agents (ethidium bromide and doxorubicin), a minor groove binder (netropsin) and a typical alkylating damaging agent (cisplatin). We have used magnetic tweezers manipulation techniques, which allow us to measure the contour and persistence lengths together with the bending and torsional properties of DNA. For each drug, the specific variations of the nanomechanical properties induced in the DNA have been compared. We observed that the presence of drugs causes a specific variation in the DNA extension, a shift in the natural twist and a modification of bending dependence on the imposed twist. By introducing a naive model, we have justified an anomalous correlation of torsion data observed in the presence of intercalators. Finally, a data analysis criterion for discriminating between different molecular interactions among DNA and drugs has been suggested.

Ions Transport and Adsorption Mechanisms in Porous Electrodes During Capacitive-Mixing Double Layer Expansion (CDLE).

A model of the electro-diffusion of ions in porous electrodes is applied to analyze the dynamics of capacitive-mixing extraction of energy from salinity gradients with carbon porous electrodes. The complex time-evolution of the cell voltage observed in experiments is satisfactorily described. The asymmetry on the duration of the solution-change steps performed in open circuit is found to be due to the nonlinear voltage-concentration relationship of the electric double layers and to a current that redistributes the counterions along the depth of the electrode leading to nonuniform charge and salt adsorption. The validated model is an essential tool for the design and optimization of renewable energy extraction by this technique.