Voltammetric techniques for low-cost on-site routine analysis of thymol in the medicinal plant Ocimum gratissimum

The composition of essential oils varies according to culture conditions and climate, which induces a need for simple and inexpensive characterization methods close to the place of extraction. This appears particularly important for developing countries. Herein, we develop an analytical strategy to determine the thymol content in Ocimum Gratissimum, a medicinal plant from Benin. The protocol is based on electrochemical techniques (cyclic and square wave voltammetry) implemented with a low cost potentiostat. Thymol is a phenol derivative and was directly oxidized at the electrode surface. We had to resort to submillimolar concentrations (25-300 μM) in order to minimize production of phenol oligomers that passivate the electrode. We worked first on two essential oils and realized that in one of them the thymol concentration was below our detection method. These results were confirmed by gas chromatography - mass spectrometry. Furthermore, we optimized the detection protocol to analyze an infusion made directly from the leaves of the plant. Finally, we studied whether the cost of the electrochemical cell may also be minimized by using pencil lead as working and counter electrodes.

Evidence of conformational landscape alteration and macromolecular complex formation in the early stages of in vitro human prion protein oxidation

Oxidative stress is proposed to be one of the major causes of neurodegenerative diseases. Cellular prion protein (PrP) oxidation has been widely studied using chemical reagents such as hydrogen peroxide. However, the experimental conditions used do not faithfully reflect the physiological environment of the cell. With the goal to explore the conformational landscape of PrP under oxidative stress, we conducted a set of experiments combining the careful control of the nature and the amount of ROS produced by a⁶⁰Co γ-irradiation source. Characterization of the resulting protein species was achieved using a set of analytical techniques. Under our experimental condition hydroxyl radical are the main reactive species produced. The most important findings are i) the formation of molecular assemblies under oxidative stress, ii) the detection of a majority of unmodified monomer mixed with oxidized monomers in these molecular assemblies at low hydroxyl radical concentration, iii) the absence of significant oxidation on the monomer fraction after irradiation. Molecular assemblies are produced in small amounts and were shown to be an octamer. These results suggest either i) an active recruitment of intact monomers by molecular assemblies' oxidized monomers then inducing a structural change of their intact counterparts or ii) an intrinsic capability of intact monomer conformers to spontaneously associate to form stable molecular assemblies when oxidized monomers are present. Finally, abundances of the intact monomer conformers after irradiation were modified. This suggests that monomers of the molecular assemblies exchange structural information with intact irradiated monomer. All these results shed a new light on structural exchange information between PrP monomers under oxidative stress.

Remote oxidative modifications induced by oxygen free radicals modify T/R allosteric equilibrium of a hyperthermophilic lactate dehydrogenase

L-Lactate dehydrogenase (LDH) is a model protein allowing to shed light on the fundamental molecular mechanisms that drive the acquisition, evolution and regulation of enzyme properties. In this study, we test the hypothesis of a link between thermal stability of LDHs and their capacity against unfolding induced by reactive oxygen species (ROS) generated by γ-rays irradiation. By using circular dichroism spectroscopy, we analysed that high thermal stability of a thermophilic LDH favours strong resistance against ROS-induced unfolding, in contrast to its psychrophilic and mesophilic counterparts that are less resistant. We suggest that a protein's phenotype linking strong thermal stability and resistance against ROS damages would have been a selective evolutionary advantage. We also find that the enzymatic activity of the thermophilic LDH that is strongly resistant against ROS-unfolding is very sensitive to inactivation by irradiation. To address this counter-intuitive observation, we combined mass spectrometry analyses and enzymatic activity measurements. We demonstrate that the dramatic change on LDH activity was linked to remote chemical modifications away from the active site, that change the equilibrium between low-affinity tense (T-inactive) and high-affinity relaxed (R-active) forms. We found the T-inactive thermophilic enzyme obtained after irradiation can recover its LDH activity by addition of the allosteric effector 1, 6 fructose bis phosphate. We analyse our data within the general framework of allosteric regulation, which requires that an enzyme in solution populates a large diversity of dynamically-interchanging conformations. Our work demonstrates that the radiation-induced inactivation of an enzyme is controlled by its dynamical properties.

Guanosine radical reactivity explored by pulse radiolysis coupled with transient electrochemistry

We follow the reactivity of a guanosine radical created by a radiolytic electron pulse both by spectroscopic and electrochemical methods. This original approach allows us to demonstrate that there is a competition between oxidation and reduction of these intermediates, an important result to further analyse the degradation or repair pathways of DNA bases.

Transient electrochemistry: beyond simply temporal resolution

Transient electrochemistry is a powerful method to solve many physicochemical issues.