Classification of Poincaré duality algebras over the rationals

The Poincaré duality algebras over Q play a key role in the rational homotopy classification of closed manifolds [3]. In this paper we give a way of classifying general Poincaré duality algebras and then specialize to the case of algebras which are generated by some homogeneous component and show how the classification reduces to the linear classification of certain homogeneous polynomials and exterior forms.     

Efficient loading of sulfonamide safety-catch linkers by Fmoc amino acid fluorides

[reaction: see text] Fmoc-protected amino acid fluorides were found to be excellent reagents for the acylation of sulfonamide safety-catch linkers (SCL) suitable for the subsequent preparation of peptide C-terminal thioesters. High loadings were obtained on different types of resins with low levels of epimerization.     

Ab initio and DFT calculations on the initial step in thiamin catalysis

The diphosphate ester (ThDP) of thiamin (vitamin B₁) is an important cofactor of enzymes within the carbohydrate metabolism. The initial reaction step shared by all ThDP-dependent enzymes is the deprotonation of the C2–H of the thiazolium ring. The replacement of the 4′-amino group by a hydroxyl one in the pyrimidine ring leads to the oxy-ThDP analogue which is known as an antagonist in thiamin catalysis.

Ab initio and DFT calculations on the MP2/6-31G* and B3LYP/6-31G* level were performed to study the proton relay function in thiamin and oxythiamin systems. Both MP2 and B3LYP calculations show significant differences of the reaction coordinate of the ylide formation in the systems. Tautomers, protonated and deprotonated species of both systems show different trends regarding their stability. The influence of correlation effects on the results is discussed by comparison with the HF-SCF/6-31G* data. Frequency calculations on the B3LYP/6-31G* level were performed to characterize the minima and transition state structures, respectively.     

Polymer dispersions from catalytic polymerization in aqueous systems

By contrast to traditional free radical emulsion polymerization, catalytic polymerization allows for polymer microstructure control. In terms of polymerizable monomers, both techniques are largely complementary. Since the beginning of this decade, an increasing number of reports on polyolefin, polybutadiene, polyalkenamer, polynorbornene, polyketone, and polyacetylene dispersions prepared by catalytic polymerization in disperse aqueous systems has appeared. This contribution reviews the preparation of these dispersions, their colloidal properties, particle formation mechanisms, particle morphologies, and polymer microstructures.     

Synthesis, Characterization, and Electrochemistry of sigma-Bonded Cobalt Corroles in High Oxidation States

The synthesis, electrochemistry, spectroscopy, and structural characterization of two high-valent phenyl sigma-bonded cobalt corroles containing a central cobalt ion in formal +IV and +V oxidation states is presented. The characterized compounds are represented as phenyl sigma-bonded cobalt corroles, (OEC)Co(C(6)H(5)) and [(OEC)Co(C(6)H(5))]ClO(4), where OEC is the trianion of 2,3,7,8,12,13,17,18-octaethylcorrole. The electronic distribution in both molecules is discussed in terms of their NMR and EPR spectroscopic data, magnetic susceptibility, and electrochemistry.     

IBX-mediated alpha-hydroxylation of alpha-alkynyl carbonyl systems. A convenient method for the synthesis of tertiary alcohols

[reaction: see text] IBX (o-iodoxybenzoic acid) is an excellent reagent for the alpha-hydroxylation of alpha-alkynyl carbonyl compounds without giving dehydrogenation products. The convenient procedure proves to be useful for the construction of a variety of tertiary alcohols (55-91%) under mildly acidic conditions.     

Absorbance Changes by Aromatic Amino Acid Side Chains in Early Rhodopsin Photointermediates

Abstract— Absorbance changes were monitored from 250 to 650 nm during the first microsecond after photolysis of detergent suspensions of bovine rhodopsin at 20°C. Global analysis of the resulting data produced difference spectra for bathorhodopsin, BSI and lumirhodopsin which give the change in absorbance of the aromatic amino acid side chains in these photointermediates relative to rhodopsin. These spectra show that the significant bleaching of absorbance near 280 nm, which has been seen previously for the lumirhodopsin, metarhodopsin I and metarhodopsin II intermediates, extends to times as early as bathorhodopsin. Because no corresponding absorbance increase is observed in the 250-275 nm region, the earliest bleaching of the 280 nm absorbance in rhodopsin is attributed to disruption of a hyperchromic interaction affecting Trp²⁶⁵. Partial decay of this 280 nm bleaching as bathorhodopsin converts to BSI takes place maximally near 290 nm, where Trp²⁶⁵ has been shown to absorb, and could be due to the ring of the retinylidene chromophore resuming a position at the BSI stage that reestablishes the hyperchromic interaction with Trp²⁶⁵. A subsequent change in the 250-300 nm region, which has no counterpart in the visible chromophore bands, indicates the possible presence of a protein-localized process as lumirhodopsin is formed.     

A Short Synthesis of (−)-Carbovir

(–)-Carbovir ((–)- 1 ) was synthesized via the cyclic carbonate 2 in four steps starting from enantiomerically enriched (–)-(S)-(cyclopent-2-enyl)methanol ((–)- 3 ).     

Phosphonio‐benzophospholide Zwitterions as Bridging 8e‐Donor Ligands: Synthetic and Mechanistic Studies

Reactions of the phosphonio‐benzophospholide π‐complexes 3a, b[Cr] with [M(CO)₅(olefin)] or of the σ‐complexes 2a, b[M] (M = Cr, Mo, W) with [M(CO)₃(aren)] lead to the first binuclear complexes 4a, b[CrM] featuring phosphonoio‐benzophospholides as μ‐bridging 8e‐donor ligands to two group 6 metal atoms. The constitution of the products was determined by spectroscopic and X‐ray diffraction studies. Mixed complexes with both group 6 and 7 metals were not accessible. Mechanistic studies showed that the reactions follow a complicated mechanism whose single steps may involve transfer of either M(CO)_n fragments or single CO ligands between complexes; the latter are associated with a σ/π‐coordination isomerization of the benzophospholide unit. Competition between both reaction channels can lead to the formation of product mixtures whose composition is controlled by the relative thermodynamic stabilities of the products. Computational studies suggest that in the more stable isomer of heterobimetallic complexes 4a, b[MM′] end‐on coordination to the heavier and side‐on coordination to the lighter metal atom is preferred.     

Reactivation pathway of the hydrogenase H‐cluster: Density functional theory study

This work puts forth a reaction pathway for the reactivation of exogenous ligand inhibited H‐cluster, the active site of Fe‐only hydrogenases. The H‐cluster is a dimetal complex, Fe–Fe, with the metal centers bridged by di(thiomethyl)amine. Exogenous ligands, H₂O, and OH^?, are bound to the distal iron (Fe_d). Density functional theory (DFT) calculations on the native and ruthenium‐modified H‐cluster have been performed using the B3LYP functional with 6‐31+G** and 6‐311+G** basis sets. We have ascertained that there is a thermodynamically favorable pathway for the reactivation of the OH^? inhibited H‐cluster, which proceeds by an initial protonation of the Fe_d–OH^? complex. The proposed reaction pathway has all its intermediate reactions ensue exothermically. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007