Another Unprecedented Wieland Mechanism Confirmed: Hydrogen Formation from Hydrogen Peroxide,Formaldehyde, and Sodium Hydroxide

In 1923, Wieland and Wingler reported that in the molecular hydrogen producing reaction of hydrogen peroxide with formaldehyde in basic solution, free hydrogen atoms (H^.) are not involved. They postulated that bis(hydroxymethyl)peroxide, HOCH₂OOCH₂OH, is the intermediate, which decomposes to yield H₂ and formate, proposing a mechanism that would nowadays be considered as a “concerted process”. Since then, several other (conflicting) “mechanisms” have been suggested. Our NMR and Raman spectroscopic and kinetic studies, particularly the determination of the deuterium kinetic isotope effect (DKIE), now confirm that in this base‐dependent reaction, both H atoms of H₂ derive from the CH₂ hydrogen atoms of formaldehyde, and not from the OH groups of HOCH₂OOCH₂OH or from water. Quantum‐chemical CBS‐QB3 and W1BD computations show that H₂ release proceeds through a concerted process, which is strongly accelerated by double deprotonation of HOCH₂OOCH₂OH, thereby ruling out a free radical pathway.     

Field induced formation of mesoscopic polymer chains from functional ferromagnetic colloids

The assembly and direct imaging of ferromagnetic nanoparticles into one-dimensional mesostructures (1-D) are reported. Polymer-coated ferromagnetic colloids (19 nm, 24 nm) were assembled at a crosslinkable oil-water interface under both magnetic field induced and zero-field conditions and permanently fixed into 1-D mesoscopic polymer chains (1-9 mum) in a process referred to as Fossilized Liquid Assembly (FLA). In the FLA process, nanoparticle chains were fixed at the oil interface through photopolymerization, enabling direct visualization of organized mesostructures using atomic force microscopy. Using the FLA methodology, we systematically investigated different conditions and demonstrated that dispersed ferromagnetic colloids possess sufficient dipolar interactions to organize into mesoscopic assemblies. Application of an external magnetic field during assembly enabled the formation of micron-sized chains which were aligned in the direction of the applied field. This universal methodology is an attractive alternative technique to cryogenic transmission electron microscopy (cryo-TEM) for the visualization of nanoparticle assembly in dispersed organic media.     

Hydrogen peroxide decomposition by a non-heme iron(III) catalase mimic: a DFT study

Non-heme iron(III) complexes of 14-membered tetraaza macrocycles have previously been found to catalytically decompose hydrogen peroxide to water and molecular oxygen, like the native enzyme catalase. Here the mechanism of this reaction is theoretically investigated by DFT calculations at the (U)B3LYP/6-31G* level, with focus on the reactivity of the possible spin states of the FeIII complexes. The computations suggest that H2O2 decomposition follows a homolytic route with intermediate formation of an iron(IV) oxo radical cation species (L.+FeIV==O) that resembles Compound I of natural iron porphyrin systems. Along the whole catalytic cycle, no significant energetic differences were found for the reaction proceeding on the doublet (S=1/2) or on the quartet (S=3/2) hypersurface, with the single exception of the rate-determining O--O bond cleavage of the first associated hydrogen peroxide molecule, for which reaction via the doublet state is preferred. The sextet (S=5/2) state of the FeIII complexes appears to be unreactive in catalase-like reactions.     

Inhibition of peroxynitrite-induced nitration of tyrosine by glutathione in the presence of carbon dioxide through both radical repair and peroxynitrate formation

Peroxynitrite (ONOO-/ONOOH) is assumed to react preferentially with carbon dioxide in vivo to produce nitrogen dioxide (NO2*) and trioxocarbonate(1-) (CO3*-) radicals. We have studied the mechanism by which glutathione (GSH) inhibits the NO2*/CO3*--mediated formation of 3-nitrotyrosine. We found that even low concentrations of GSH strongly inhibit peroxynitrite-dependent tyrosine consumption (IC50 = 660 microM) as well as 3-nitrotyrosine formation (IC50) = 265 microM). From the determination of the level of oxygen produced or consumed under various initial conditions, it is inferred that GSH inhibits peroxynitrite-induced tyrosine consumption by re-reducing (repairing) the intermediate tyrosyl radicals. An additional protective pathway is mediated by the glutathiyl radical (GS*) through reduction of dioxygen to superoxide (O2*-) and reaction with NO2* to form peroxynitrate (O2NOOH/O2NOO-), which is largely unreactive towards tyrosine. Thus, GSH is highly effective in protecting tyrosine against an attack by peroxynitrite in the presence of CO2. Consequently, formation of 3-nitrotyrosine by freely diffusing NO2* radicals is highly unlikely at physiological levels of GSH.     

Comment on “Persistent Room-Temperature Radicals from Anionic Naphthalimides: Spin Pairing and Supramolecular Chemistry” and on “Persistent Radical Pairs between N-Substituted Naphthalimide and Carbanion Exhibit pKa-Dependent UV/Vis Absorption”

EPR spectroscopic evidence for intramolecular electron transfer in anionic N-substituted naphthalimides to yield persistent diradical anions and intermolecular electron transfer from a variety of carbanions to 6-bromo-N-phenyl-naphthalimide to yield persistent radical–radical anion pairs was recently claimed in two papers by Zhang et al. In this comment, it is shown that the EPR spectra published in both papers do not agree with the proposed triplet-state species. Rather, the spectra are due to various doublet-state radicals, deriving from minor side reactions. The misinterpretations invalidate the general conclusions of the papers.