Christophe Mavon

Reductive unfolding of serum albumins uncovered by Raman spectroscopy

The reductive unfolding of bovine serum albumin (BSA) and human serum albumin (HSA) induced by dithiothreitol (DTT) is investigated using Raman spectroscopy. The resolution of the S-S Raman band into both protein and oxidized DTT contributions provides a reliable basis for directly monitoring the S-S bridge exchange reaction. The related changes in the protein secondary structure are identified by analyzing the protein amide I Raman band. For the reduction of one S-S bridge of BSA, a mean Gibbs free energy of -7 kJ mol(-1) is derived by studying the reaction equilibrium. The corresponding value for the HSA S-S bridge reduction is -2 kJ mol(-1). The reaction kinetics observed via the S-S or amide I Raman bands are identical giving a reaction rate constant of (1.02 +/- 0.11) M(-1) s(-1) for BSA. The contribution of the conformational Gibbs free energy to the overall Gibbs free energy of reaction is further estimated by combining experimental data with ab initio calculations.

Dissociative electron attachment to DNA-diamine thin films: Impact of the DNA close environment on the OH− and O− decay channels

We measure the desorption of anions stimulated by the impact of 0-20 eV electrons on highly uniform thin films of plasmid DNA-diaminopropane. The results are accurately correlated with film thickness and composition by AFM and XPS measurements, respectively. Resonant structures in the H(-), O(-), and OH(-) yield functions are attributed to the decay of transient anions into the dissociative electron attachment (DEA) channel. The diamine induces ammonium-phosphate bridges along the DNA backbone, which suppresses the DEA O(-) channel and in counter-part increases considerably the desorption of OH(-). The close environment of the phosphate groups may therefore play an important role in modulating the rate and type of DNA damages induced by low energy electrons.

Conception and realization of a parallel-plate free-air ionization chamber for the absolute dosimetry of an ultrasoft X-ray beam

We report the design of a millimeter-sized parallel plate free-air ionization chamber (IC) aimed at determining the absolute air kerma rate of an ultra-soft X-ray beam (E = 1.5 keV). The size of the IC was determined so that the measurement volume satisfies the condition of charged-particle equilibrium. The correction factors necessary to properly measure the absolute kerma using the IC have been established. Particular attention was given to the determination of the effective mean energy for the 1.5 keV photons using the PENELOPE code. Other correction factors were determined by means of computer simulation (COMSOL™ and FLUKA). Measurements of air kerma rates under specific operating parameters of the lab-bench X-ray source have been performed at various distances from that source and compared to Monte Carlo calculations. We show that the developed ionization chamber makes it possible to determine accurate photon fluence rates in routine work and will constitute substantial time-savings for future radiobiological experiments based on the use of ultra-soft X-rays.

Measurement of tritium in the free water of milk spotting and quantifying some biases and proposing ways of improvement

As one of the three natural isotopes of hydrogen, tritium is ubiquitous and may potentially be present in any water or organic molecule that constitutes a biological matrix. Milk is one of the most frequently monitored foodstuffs in the vicinity of chronic release of radionuclides, as it is a very common food product and also because it integrates deposition on large areas of grass or crops at a local scale. Different parameters have been studied to assess their impact on the reliability of tritium measurements in the free water of milk. The volume of the sample, the technique used to extract the water and the level of dehydration modulate the results but in different ways: dispersion of results and under- or over-estimation of the tritium activity. The influence of sample storage and preparation has also been investigated. Methodological improvements of tritium measurements in the free water of milk are proposed. An original fractionation effect during distillation of milk is also described.

Materials for surface insulation: influence of physical and chemical properties

Epoxy resin is a widely used material in medium and high voltage apparatus either for bulk or surface insulation. If the last steps of life of outdoor insulation are well known under humidity with the ignition of corona discharges and flashover, the first step of ageing is still not understood. To help in understanding ageing mechanisms, various materials with different properties were tested and compared to epoxy resin. The main parameters that are studied are the diffusion of the water in the materials and the hydrolysis of the organic matrix. The performed tests are humidity cycles for 8 hours with an alternative voltage, the comparison of the materials is based on the 50 Hz currents, temperature and light emission for samples containing various water contents.

Quartz resonator based low-energy ionizing radiation detection

Our work aims at the development of a wireless, remote sensing method for quantitatively measuring radiation doses. We have selected high-frequency (several 100 MHz to 1 GHz) quartz crystal resonators as probes of the ionizing radiation field. We here focus on the effects of low energy X ray and electron beams on these sensors. As opposed to classical work concerned with eliminating the disturbances induced by radiations, we here attempt to characterize the response of the resonators as a function of dose and radiation energy in order to be able to use the frequency signal as a remotely interrogated radiation probe. We present results of bulk acoustic wave resonators, surface acoustic wave resonators and surface acoustic wave delay lines under X-rays and electrons irradiation: we interpret all observed signals (frequency or phase shift during electron irradiation) as thermal effects. Hence, surface acoustic wave resonators can make suitable calorimeters, compatible with wireless interrogation, and high sensitivity if a quartz cut with high temperature sensitivity (Z-cut) is used.