Alzheimer's disease neuropathology is characterized by neuronal death, amyloid beta-peptide deposits and neurofibrillary tangles composed of paired helical filaments of tau protein. Although crucial for our understanding of the pathogenesis of Alzheimer's disease, the molecular mechanisms linking amyloid beta-peptide and paired helical filaments remain unknown. Here, we show that amyloid beta-peptide-induced nitro-oxidative damage promotes the nitrotyrosination of the glycolytic enzyme triosephosphate isomerase in human neuroblastoma cells. Consequently, nitro-triosephosphate isomerase was found to be present in brain slides from double transgenic mice overexpressing human amyloid precursor protein and presenilin 1, and in Alzheimer's disease patients. Higher levels of nitro-triosephosphate isomerase (P < 0.05) were detected, by Western blot, in immunoprecipitates from hippocampus (9 individuals) and frontal cortex (13 individuals) of Alzheimer's disease patients, compared with healthy subjects (4 and 9 individuals, respectively). Triosephosphate isomerase nitrotyrosination decreases the glycolytic flow. Moreover, during its isomerase activity, it triggers the production of the highly neurotoxic methylglyoxal (n = 4; P < 0.05). The bioinformatics simulation of the nitration of tyrosines 164 and 208, close to the catalytic centre, fits with a reduced isomerase activity. Human embryonic kidney (HEK) cells overexpressing double mutant triosephosphate isomerase (Tyr164 and 208 by Phe164 and 208) showed high methylglyoxal production. This finding correlates with the widespread glycation immunostaining in Alzheimer's disease cortex and hippocampus from double transgenic mice overexpressing amyloid precursor protein and presenilin 1. Furthermore, nitro-triosephosphate isomerase formed large beta-sheet aggregates in vitro and in vivo, as demonstrated by turbidometric analysis and electron microscopy. Transmission electron microscopy (TEM) and atomic force microscopy studies have demonstrated that nitro-triosephosphate isomerase binds tau monomers and induces tau aggregation to form paired helical filaments, the characteristic intracellular hallmark of Alzheimer's disease brains. Our results link oxidative stress, the main etiopathogenic mechanism in sporadic Alzheimer's disease, via the production of peroxynitrite and nitrotyrosination of triosephosphate isomerase, to amyloid beta-peptide-induced toxicity and tau pathology.

A theoretical density functional theory (DFT, B3LYP) investigation has been carried out on the catalytic cycle of the carbonic anhydrase. A model system including the Glu106 and Thr199 residues and the "deep" water molecule has been used. It has been found that the nucleophilic attack of the zinc-bound OH on the CO2 molecule has a negligible barrier (only 1.2 kcal mol -1 ). This small value is due to a hydrogenbond network involving Glu106, Thr199, and the deep water molecule. The two usually proposed mechanisms for the internal bicarbonate rearrangement have been carefully examined. In the presence of the two Glu106 and Thr199 residues, the direct proton transfer (Lipscomb mechanism) is a two-step process, which proceeds via a proton relay network characterized by two activation barriers of 4.4 and 9.0 kcal mol -1 . This pathway can effectively compete with a rotational mechanism (Lindskog mechanism), which has a barrier of 13.2 kcal mol -1 . The fast proton transfer found here is basically due to the effect of the Glu106 residue, which stabilizes an intermediate situation where the Glu106 fragment is protonated. In the absence of Glu106, the barrier for the proton transfer is much larger (32.3 kcal mol -1 ) and the Lindskog mechanism becomes favored.

A theoretical investigation has been carried out at the DFT (B3LYP) level on the mechanism of the metathesis reaction catalyzed by Grubbs' complexes. Two model systems have been used:  (a) The first model is formed by one ethylene molecule and the Cl2(PH3)2RuCH2 complex (Grubbs' catalyst). (b) In the second model the Cl2(PPh3)2RuCH2 species has been considered. The following results are relevant:  (i) The “primary” active catalytic species is a metal-carbene (PR3)2Cl2RuCH2. The corresponding carbenoid complex (PR3)2ClRu−CH2Cl is significantly higher in energy (18.45 and 19.26 kcal mol-1 for the two model systems) and thus cannot represent the starting active species of the process. (ii) The existence of three different reaction pathways has been demonstrated. One of the two most likely reaction channels is characterized by the presence of “secondary” active species of carbenoid type. These species, after olefin coordination, become slightly more stable than the corresponding carbenic forms and play a key role in the formation of the metallacyclobutane intermediates. Their stability further increases when phenyl rings replace the phosphine hydrogens. (iv) The cyclopropanation is disfavored since it requires the overcoming of larger activation barriers than those found for the metathesis.

Enantioselective gold-catalysis is emerging as a powerful tool in organic synthesis for the stereoselective manipulation of unfunctionalized unsaturated hydrocarbons. Despite the exponential growth, the molecular complexity of common chiral gold complexes generally prevents a complete description of the mechanism steps and activation modes being documented. In this study, we present the results of a combined experimental-computational (DFT) investigation of the mechanism of the enantioselective gold-catalyzed allylic alkylation of indoles with alcohols. A stepwise S(N)2'-process (i.e. anti-auroindolination of the olefin, proton-transfer, and subsequent anti-elimination [Au]-OH) is disclosed, leading to a library of tricyclic-fused indole derivatives. The pivotal role played by the gold counterion, in terms of molecular arrangement (i.e. "folding effect") and proton-shuttling in restoring the catalytic species, is finally documented.

In this paper we have used a DFT (B3LYP) approach to investigate the potential energy surface for the reaction between ethylene and (chloromethyl)zinc chloride (ClCH2ZnCl), which represent a model system for the Simmons−Smith cyclopropanation reaction. Two reaction channels have been found:  one leads to the cyclopropane product (addition channel) and the other to the propene product (insertion channel). The addition reaction has an activation energy of 24.7 kcal mol-1 and, as experimentally found, is favored with respect to the insertion, which is characterized by a larger activation energy (36.0 kcal mol-1). The addition transition state corresponds to a three-centered structure which explains the stereochemical features which have been experimentally observed for this reaction. A simple diabatic model is used to rationalize the reactivity pattern that characterizes the Simmons−Smith cyclopropanation and the different behavior observed for the reaction between singlet methylene 1CH2 and olefins.