How Does CuII Convert into CuI? An Unexpected Ring‐Mediated Single‐Electron Reduction

Cu(x)O (x=1,2) nanomaterials with tailored composition and properties-a hot topic in sustainable technologies-may be fabricated from molecular sources through bottom-up processes that involve unexpected changes in the metal oxidation state and open intriguing challenges on the copper redox chemistry. How copper(II) sources may lead to copper(I) species in spite of the absence of any explicit reducing agent, and even in the presence of oxygen, is one such question-to date unanswered. Herein, we study copper "reduction without reductants" within one molecule and reveal that the actual reducing agent is abstracted atomic hydrogen. By investigating the fragmentation of a copper(II) precursor for copper oxide nanostructures by combined ESI-MS with multiple collisional experiments (ESI/MS(n)) and theoretical calculations, we highlight a copper-promoted C-H bond activation, leading to reduction of the metal center and formation of a Cu(I)-C-NCCN six-membered ring. Such a novel ring system is the structural motif for a new family of cyclic copper(I) adducts, which show a bonding scheme, herein reported for the first time, that may shed unprecedented light on copper chemistry. Beyond the relevance for the preparation of copper oxide nanostructures, the hydrogen-abstraction/proton-delivery/electron-gain mechanism of copper(II) reduction disclosed herein appears to be a general property of copper and might help to understand its redox reactivity.     

Synthetic studies toward 1,2-dioxanes as precursors of potential endoperoxide-containing antimalarials

Introduction in therapy of the naturally occurring trioxane artemisinin opened a new era in malaria treatment and prompted the development of semisynthetic and synthetic derivatives characterized by the presence of the key peroxide bridge. The 1,2-dioxane ring is present in some natural endoperoxides such as plakortin and dihydroplakortin, which are endowed with interesting antimalarial properties. Here we describe the development of a versatile stereocontrolled synthetic strategy to 1,2-dioxanes functionalized at the critical C3, C4, and C6 positions, potentially useful for the development of innovative antimalarials.     

Circularly‐Polarized Electrochemiluminescence from a Chiral Bispyrene Organic Macrocycle

The first observation of circular polarization of electrochemiluminescence (ECL) from a purely organic derivative is reported. A bispyrene scaffold mounted on a constrained polyether macrocycle displaying intense excimer fluorescence and highly circularly‐polarized (CP) photoluminescence has been selected for this purpose. The compound displays an ECL dissymmetry factor of about |8×10^?3|, which is in good agreement with the corresponding photoluminescence value. This observation is the first step towards the molecular engineering of tailored dyes that can act as both ECL and CP‐ECL reporters for (bio)analysis by bringing a new level of information when dealing with chiral environments. Additionally, it provides an extra dimension to the ECL phenomenon and opens the way to chiral detection and discrimination.     

A new synthesis of alkyl and aryl 2-aminoimidazoles

The condensation of cyanamide with α-aminocarbonyl compounds has been studied as a method of synthesizing alkyl and aryl 2-aminoimidazoles. Starting from N-alkylamino-aldehydes, 1,5-dialkyl-2-aminoimidazoles have been prepared. Starting from suitable aminoketones a variety of monosubstituted and disubstituted derivatives were obtained.     

Influence of solvent on temperature and thermal peak broadening in capillary zone electrophoresis

The present paper deals with the role of the solvent on thermal peak broadening. One main solvent property that determines the magnitude of the temperature gradient due to the generation of Joule heat in capillary zone electrophoresis is the thermal conductivity. As organic solvents have lower thermal conductivity than water (methanol and acetonitrile, e.g., nearly by a factor of 3) it can be hypothesized that the temperature gradient inside the capillary is more pronounced in organic solvents compared to an aqueous solution. On the other hand, the temperature dependence of the ion mobility (which is responsible for the velocity profile and thus for thermal peak broadening) is smaller in organic solvents. To get insight into the thermal effect of the solvent, first the temperature of a solution in a cylindrical tube was calculated utilizing the heat balance equation. It was shown that the two theoretical models most common in the literature (based on the analytical solution or on an assumption of the parabolic temperature profile in the tube, respectively) give the same results. The latter model was chosen for the further calculations, adding a quadratic term to express the electric conductivity as a function of the temperature. The temperature at the inner capillary wall and center as function of the capillary dimensions and the electric power was computed for electrolytes with a given conductivity at 25.0 degrees C with water, methanol, and acetonitrile as solvents. Capillary cooling systems used were circulating liquid cooling, enforced air-cooling, and natural convection in still air. The mean temperature (averaged over the cross section) resulting from Joule heating was compared with experimentally determined temperatures established upon application of an electric field; the latter temperature was derived from the measurement of the electric conductance of the background electrolyte solution and its (measured) temperature dependence. All investigations were carried out with solutions of the same initial electric conductivity (about 0.5 S.m(-1) at 25.0 degrees C). Agreement is found for natural convection conditions, and the deviation between theoretical and experimental results for the forced air and circulated liquid cooling systems can be related to the poorly defined thermal conditions of the capillaries in commercial instrumentation (with a part in a thermostated cassette and a part outside). For given conditions the temperature gradients in the organic solvents exceed largely those in water, independent of the type of cooling. As a consequence, the thermal plate height is significantly larger in organic solvents, at least under conditions where the deviation from the Nernst-Einstein limiting case is not too high. However, even for the maximum applicable field strengths the thermal plate height contributions are negligible compared to longitudinal diffusion in all solvents.     

On the Simple Complexity of Carbon Monoxide on Oxide Surfaces: Facet‐Specific Donation and Backdonation Effects Revealed on TiO2 Anatase Nanoparticles

Atomic‐scale relationships between the structure of TiO₂ surfaces and the physicochemical properties of surface sites, functional for titania‐based applications, can be obtained from IR spectroscopy by using carbon monoxide (CO) as a molecular probe. In the literature, it is reported that strongly unsaturated cationic Ti sites (Lewis acid), which are important for reactivity, should cause a large upshift of the CO stretching frequency. By using IR spectroscopy of CO on TiO₂ nanomaterials and theoretical analyses, here this model is challenged. It is shown that the stretching frequency of adsorbed CO results from a facet‐dependent and synergic CO–surface donation (upshift) ‐ surface–CO backdonation (downshift) mechanism. These results imply that the interaction of adsorbed molecules with the Ti centers is tuned by the surface oxygen atoms of the first coordination sphere, which play an active role as indirect electron density donors (Lewis base).     

Effects of sugars and polyols on the stability of azurin in ice

This study represents the first attempt to gain a quantitative estimate of the protective influence of sugars (sucrose and trehalose) and polyols (sorbitol and glycerol) on the thermodynamic stability (DeltaG degrees ) of a protein in low-temperature part-frozen aqueous solutions. The method, based on guanidinium chloride denaturation of the azurin mutant C112S from Pseudomonas aeruginosa, distinguishes between the effects of cooling to subfreezing temperatures from those induced specifically by the formation of a solid ice phase. The results point out that in the liquid state the generally stabilizing effect (at molar concentrations) of these polyhydric compounds is markedly attenuated on cooling to subfreezing temperatures such that at -15 degrees C, only sucrose still exerts a significant increase in DeltaG degrees . At this temperature, and in the absence of additives, the formation of ice caused a progressive destabilization of the native fold, DeltaG degrees decreasing up to 3-4 kcal/mol as the fraction of liquid water in equilibrium with ice (V(L) was reduced to less than 1%. Unexpectedly, denaturation profiles in ice at selected V(L) demonstrate that none of the above sugars and polyols counters effectively the decrease in protein stability at small V(L). Only trehalose was able to partly attenuate the ice perturbation, raising DeltaG degrees by a modest 0.6-0.8 kcal/mol relative to the salt reference. In all cases the reduction in DeltaG degrees caused by the solidification of water correlates with the decrease in m-value. The implication is that DeltaASA of unfolding is smaller in ice because protein-ice interactions either increase the solvent-accessible surface area (ASA) of the native fold (partial unfolding) or reduce the ASA of the denatured state (compaction), or both. Information on the protein tertiary structure in ice, in the absence and in the presence of sucrose or glycerol, suggests that these osmolytes play an important role in maintaining a compact native state that in their absence is expanded and partly unfolded. Thus, it appears that the prevailing mechanism by which these osmolytes act as cryoprotectants is through preservation of the native conformation in the liquidus rather than by increasing the thermodynamic stability of the native fold.     

Semisynthesis of dimeric proteins by expressed protein ligation

A one-pot synthesis of homodimeric proteins is described. The synthetic strategy is based on a double expressed protein ligation reaction between thioester peptides and a new bis-cysteinyl linker. The protocol was also applied to the synthesis of heterodimers.