Ab initio electronic structure calculations of entire blue copper azurins.

We present a theoretical study of the blue-copper azurin extracted from Pseudomonas aeruginosa and several of its single amino acid mutants. For the first time, we consider the whole structure of this kind of protein rather than limiting our analysis to the copper complex only. This is accomplished by combining fully ab initio calculations based on density functional theory with atomic-scale molecular dynamics simulations. Beyond the main features arising from the copper complex, our study reveals the role played by the peripheral parts of the proteins. In particular, we find that oxygen atoms belonging to carboxyl groups which are distributed all over the protein contribute to electronic states near the HOMO. The contribution of the outer regions to the electronic structure of azurins had so far been overlooked. Our results highlight the need to investigate them thoroughly; this is especially important in prospect of understanding complex processes such as the electronic transport through metal-metalloprotein-metal junctions.

Electron transport in single molecules measured by a distance-modulation assisted break junction method.

We describe a method to determine whether a measured current in a break junction is due to electron tunneling via space or conduction through a molecule bridged between two electrodes. By modulating the electrode separation, we monitor both the DC and the AC components of the current. The AC component indicates if a molecule is connected to the electrodes while the DC component is the transport current through the molecule. This method allows us to remove the tunneling background from conductance histograms and unambiguously measure the I- V characteristic of single molecules. Furthermore, it provides valuable information about the electromechanical properties of single molecules.

Dielectric properties of self-assembled layers of octadecylamine on mica in dry and humid environments.

Atomic force microscopy operating in noncontact electrostatic force mode was used to study the interaction of water with films of alkylamines and alkylsilanes on mica. The films efficiently block water adsorption except in exposed mica areas, where it strongly modifies the mobility of surface ions. We also studied the molecular orientation of octadecylamine molecules forming monolayers and multilayer islands. In monolayer films the molecules bind to mica through the amino group, producing a positive contact potential relative to mica (dipole pointing up). In multilayer films the methyl and amino group terminations are exposed in alternating layers that give rise to alternating values of the contact potential. These findings correlate with low and high friction forces measured in the methyl termination and amino terminations.

Oxidative dissolution of chalcopyrite by Acidithiobacillus ferrooxidans analyzed by electrochemical impedance spectroscopy and atomic force microscopy.

The microbiological leaching of chalcopyrite (CuFeS(2)) is of great interest because of its potential application to many CuFeS(2)-rich ore materials. However, the efficiency of the microbiological process is very limited because this mineral is one of the most refractory to bacterial attack. Knowledge of bacterial role during chalcopyrite oxidation is very important in order to improve the efficiency of bioleaching operation. The oxidative dissolution of a massive chalcopyrite electrode by Acidithiobacillus ferrooxidans was evaluated by electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM). A massive chalcopyrite electrode was utilized in a Tait-type electrochemical cell in acid medium for different immersion times in the presence or absence of bacterium. The differences observed in the impedance diagrams were correlated with the adhesion process of bacteria on the mineral surface.

Formation of studtite during the oxidative dissolution of UO2 by hydrogen peroxide: a SFM study.

Understanding the formation of alteration phases on the surface of spent nuclear fuel, such as those observed during leaching experiments, is necessary in order to predict the concentration of radionuclides in the near-field of a final repository. Hydrogen peroxide has been identified as one of the oxidants formed by the radiolysis of water in the presence of spent nuclear fuel; especially due to alpha activity. The presence of this species in solution can contribute to the formation of uranium peroxide secondary phases. In this work, we have studied the oxidative dissolution of synthetic UO2 disks in hydrogen peroxide solutions of two different concentrations (5 x 10(-4) and 5 x 10(-6) mol dm(-3)), both at pH 5.8 +/- 0.1. The solid surface evolution of the disks has been followed by means of ex-situ scanning force microscope (SFM) measurements, and uranium concentration in solution has been determined by inductively coupled plasma mass spectrometry. During the first stage of the experiment, SFM images indicate that only UO2 dissolution is occurring. After 142 h, a secondary phase is observed on the surface of the solid at 5 x 10(-4) mol dm(-3) hydrogen peroxide concentration. This secondary phase has been identified by X-ray diffraction as studtite (UO4 x 4H2O). From the analysis of SFM topographic profiles at different elapsed times, a precipitation rate for the studtite has been estimated to be in the range of (8-32) x 10(-10) mol m(-2) s(-1).

Sequential atomic force microscopy imaging of a spontaneous nanoencapsulation process.

Since hydrophilic matrices were proposed for controlled drug delivery, many polymeric excipients have been studied in order to make drug release fit the desired profiles. It has been pointed out that lambda-carrageenan, a sulphated polymer from algae, can suitably control the release rate of basic drugs from hydrophilic matrices with no need for complex technological processes. In this work, we propose a method to monitor morphologically the interaction between lambda-carrageenan and dexchlorpheniramine maleate (D-CPM), in order to find out how the release profiles can be so easily controlled. To this end, solutions of both polymer and drug were prepared at very low concentration. Solutions were mixed and samples were taken every hour over a period of 20 h. The characterization technique employed, atomic force microscopy (AFM), provides a high resolution, allowing to show the three-dimensional morphology of the samples within the nanometric scale. The results demonstrate that lambda-carrageenan is able to nanoencapsulate spontaneously D-CPM molecules, which offers the possibility to easily control the release rate of the drug. This work has moreover demonstrated the suitability of AFM for the specific case of the on-time monitoring of interaction processes that happen in pharmaceutical systems.