A long-standing pathomechanistic model proposes that the polyglutamine (polyQ)-length-dependent toxicity threshold observed in all polyQ diseases is triggered by a conformational change within the monomer that occurs only above a certain polyQ length. If true, this yet undefined and elusive mutant-specific toxic conformation would constitute a direct therapeutic target. Three anti-polyQ antibodies-MW1, 1C2 and 3B5H10-have been extensively used to probe the conformation of polyQ. The crystal structure of the MW1 epitope reveals a linear, non-pathogenic polyQ. In contrast, although the detailed structure of its epitope is unknown, the 3B5H10 antibody is widely advertised and used as a conformational antibody that recognizes the toxic conformation of expanded polyQ. We solved the crystal structure of the 1C2 antigen-binding domain (1C2-Fab) and performed a direct comparison between the 1C2, MW1 and 3B5H10 structures. The MW1 and 1C2 antibodies have similar sequences and structures, consistent with their binding to short polyQ and their polyQ length-discrimination properties. Unexpectedly, the 3B5H10 antibody also shares striking features with MW1 and 1C2, which prompted us to revisit its binding properties. We show that the 3B5H10 epitope is actually a short, non-pathogenic polyQ. All three antibodies MW1, 1C2 and 3B5H10 interact similarly with polyQ of various lengths, and bind small polyQ epitopes in similar linear and extended conformations. Together with studies published during the recent years, our work argues against the hypothesis that a mutant-specific conformation in monomeric polyQ molecules is the toxic entity responsible for polyQ diseases.

A simple composite scheme is presented and benchmarked against the 38 reactions in Truhlar's HTBH38/08 and NHTBH38/08 databases. Mean unsigned deviation (MUD) for the complete set of 68 independent barriers is 0.40 kcal mol À1 , compared to 1.31 kcal mol À1 for G4 and 1.62 kcal mol À1 for the M06-2X-D3 method. The MUD of the new scheme on the barriers in the DBH24/08 subset (12 out of the 38 reactions in the other sets) is 0.27 kcal mol À1 , better than that obtained at the expensive CCSD(T,full)/aug-cc-pCV(T+d)Z level (0.46 kcal mol À1 ) and comparable to the most exact (and costly) Wn calculations (MUD = 0.14 kcal mol À1 ). The method was further tested against a subset of reactions, for which the geometry and energies of all species were determined at the much more demanding CCSD(T)-F12//pVQZ-F12 level. The SVECV-f12 procedure on this database results in RMSE and MUD values of only 0.21 and 0.16 kcal mol À1 .

Thione S-methylide, parent species of the thiocarbonyl ylide family, is a 1,3dipolar species on the [C 2 SH 4 ] potential energy surface, not so much studied as its isomers, thiirane, vinyl thiol, and thioacetaldehyde. The conrotatory ring-closure reaction toward thiirane was studied in the 90s, but no complete analysis of the potential energy surface is available. In this paper, we report a computational study of the reaction scheme linking all species. We employed several computational methods (density functional theory, CCSD(T) composite schemes, and CASSCF/CASPT2 multireference procedures) to find the best description of thione S-methylide, its isomers, and transition states. The barrier from thiirane to thione Smethylide amounts to 52.2 kcal mol −1 (against 17.6 kcal mol −1 for the direct one), explaining why thiocarbonyl ylides cannot be prepared from thiiranes. Conversion of thiirane to vinyl thiol implies a large barrier, supporting why the reaction has been observed only at high temperatures. Fragmentations of thiirane to S( 3 P) or S( 1 D) and ethylene as well as decomposition to hydrogen sulfide plus acetylene were also explored. Triplet and singlet open-shell species were identified as intermediates in the fragmentations, with energies lower than the transition state between thiirane and vinyl thiol, explaining the preference of the latter at low temperatures.

A new homology model of human microsomal epoxide hydrolase was derived based on multiple templates. The model obtained was fully evaluated, including MD simulations and ensemble-based docking, showing that the quality of the structure is better than that of only previously known model. Particularly, a catalytic triad was clearly identified, in agreement with the experimental information available. Analysis of intermediates in the enzymatic mechanism led to the identification of key residues for substrate binding, stereoselectivity, and intermediate stabilization during the reaction. In particular, we have confirmed the role of the oxyanion hole and the conserved motif (HGXP) in epoxide hydrolases, in excellent agreement with known experimental and computational data on similar systems. The model obtained is the first one that fully agrees with all the experimental observations on the system. Proteins 2017; 85:720-730. © 2016 Wiley Periodicals, Inc.

Methods rooted in the density functional theory and in the coupled cluster ansatz were employed to investigate the cycloaddition reactions to ethylene and acetylene of 1,3-dipolar species including ozone and the derivatives issued from replacement of the central oxygen atom by the valence-isoelectronic sulfur atom, and/or of one or both terminal oxygen atoms by the isoelectronic CH 2 group. This gives rise to five different 1,3-dipolar compounds, namely ozone itself (O 3 ), sulfur dioxide (SO 2 ), the simplest Criegee intermediate (CH 2 OO), sulfine (CH 2 SO), and thioformaldehyde Smethylide (CH 2 SCH 2 , TSM). The experimental and accurate theoretical data available for some of those molecules were employed to assess the accuracy of two lastgeneration composite methods employing conventional or explicitly correlated post-Hartree-Fock contributions (jun-Cheap and SVECV-f12, respectively), which were then applied to investigate the reactivity of TSM. The energy barriers provided by