Reductive coupling of α,β-enones I: Reduction of methyl-vinyl ketone and mesityl oxide.

Reductive coupling of methyl-vinyl ketone with TiCl₄-Mg gives pinacol $\text{1}$ (25%). According to the reducing agents, mesityl oxide yields 2,4,5,7-tetramethyl-octa-2,4,6-triene $\text{3}$ (with 4TiCl₃-LialH₄), triene $\text{3}$ or 2,4,5,7-tetramethyl-octa-2,6-dien-4,5-diol $\text{5}$ (with TiCl₄-Mg), pinacol $\text{5}$ (with VCl₃-Mg), and 2-acetyl-1,3,3,4,4-pentamethyl-cyclopentane $\text{7}$ (with CrCl₃-Mg or FeCl₃-Mg or ZrCl₄ or ZrCl₄-Mg) as major products.     

The Origin of115–130°C Thermoluminescence Bands in Chlorophyll-Containing Material

High-temperature thermoluminescence (TL) emitted in the temperature region from +50 to +150°C has been studied in a variety of chlorophyll-containing samples that were allowed to dry during the TL measurement. Analysis of the recorded traces by a multicomponent-fit-ting procedure revealed the existence of up to three bands of nonphotosynthetic origin with peak positions at62–75,114–128 and151–157°C and apparent activation energies of 27.0-28.8, 14.1-15.4 and 22.1-23.3 kcal/mol (the bands are denoted as HT₁ HT₂ and HT₃, respectively). Low-temperature treatment of leaves, incubation of algae in the presence of paraquat, exposure of algae or isolated thylakoids to a strong light, all conditions known to stimulate oxidative damage to membrane lipids, caused appearance of a small HT₁, band and significant rise in the intensity of the HT₂ band. The increase in the HT₂ component correlated positively with accumulation of conjugated dienes and malondialdehyde in thylakoids illuminated with a strong light. Different quenchers of active oxygen species and scavengers of free radicals added to preilluminated thylakoids or thylakoid lipid extracts before the TL measurements, as well as injection of argon into the TL measuring chamber, caused no changes in the intensity of the HT₂ emission. The HT₂ band in the thylakoids increased strongly upon addition of linoleate peroxidized by hydroxyradicals generated in the Fenton reaction but remained unchanged if the linoleate was oxidized with the use of lipoxygenase. We suggest that the HT₂ band arises due to thermal decomposition of lipid cyclic peroxides present in the samples. In turn, the decomposition reaction leads to formation of carbonyls in triplet state with following migration of excitation energy toward chlorophyll. Contrary to the HT₁, and HT₂ bands, the HT₃ band of TL cannot be associated with the thermolysis of lipid peroxidation products already present in the samples before starting the TL gradient.     

Atomistic simulation of the homogeneous nucleation and of the growth of N2 crystallites

We report on a computer simulation study of the early stages of the crystallization of molecular nitrogen. First, we study how homogeneous nucleation takes place in supercooled liquid N(2) for a moderate degree of supercooling. Using the umbrella sampling technique, we determine the free energy barrier of formation for a critical nucleus of N(2). We show that, in accord with Ostwald's rule of stages, the structure of the critical nucleus is predominantly that of a metastable polymorph (alpha-N(2) for the state point investigated). We then monitor the evolution of several critical nuclei through a series of unbiased molecular dynamics trajectories. The growth of N(2) crystallites is accompanied by a structural evolution toward the stable polymorph beta-N(2). The microscopic mechanism underlying this evolution qualitatively differs from that reported previously. We do not observe any dissolution or reorganization of the alpha-like core of the nucleus. On the contrary, we show that alpha-like and beta-like blocks coexist in postcritical nuclei. We relate the structural evolution to a greater adsorption rate of beta-like molecules on the surface and show that this transition actually starts well within the precritical regime. We also carefully investigate the effect of the system size on the height of the free energy barrier of nucleation and on the structure and size of the critical nucleus.     

Kinetic model of energy transfer processes between low-coordinated ions on MgO by photoluminescence decay measurements.

Photoluminescence decay studies of emitting species on MgO nanocubes at room temperature provide evidence of three surface species characterized by an excitation and emission wavelength couple {lambda(exc);lambda(em)}. Species A corresponds to {lambda(exc)=240 nm; lambda(em)=380 nm}, whereas the couple {lambda(exc)=280 nm; lambda(em)=470 nm} is assigned to two species: B and B', the former is involved in energy transfer from excited state A* and the latter in direct emission from excited state B'*. A simple model for energy transfer from species A* to B is proposed. The numerical resolution of equations corresponding to this model is in good agreement with experimental data. This method quantifies the kinetics of intrinsic emission and energy transfer processes. Lifetime values indicate that phosphorescence is taking place, and species A, B and B' are identified as edge O(2-) (4 C), corner O(2-) (3 C) and kink O(2-) (3 C) oxide ions respectively.     

Synthesis and inhibition properties of conformational probes for the mutase-catalyzed UDP-galactopyranose/furanose interconversion

UDP-galactose mutase is a flavoenzyme that catalyzes the isomerization of UDP-galactopyranose into UDP-galactofuranose, a key step in the biosynthesis of important bacterial oligosaccharides. Several mechanisms for this unique ring-contraction have been proposed, one of them involving a putative 1,4-anhydrogalactopyranose as an intermediate in the reaction. The purpose of this study was to probe the mutase binding site with conformationally restricted analogues of its substrate. Thus, we describe the straightforward synthesis of two C-glycosidic UDP-galactose derivatives: analogue 1, presenting a galactose moiety locked in a bicyclic (1,4)B boat conformation, and UDP-C-Galf 2, where the galactose residue is locked in the conformation of the mutase substrate. The two molecules were found to be inhibitors of UDP-galactose mutase at levels depending on the redox state of the enzyme. Strong inhibition of the native enzyme, but a low one of the reduced mutase, were observed with UDP-C-Galf 2, whereas 1 displayed intermediate inhibition levels under both native and reducing conditions. These data provide evidence of a significant conformational difference of the mutase binding pocket in the reduced enzyme and in the native one, the enzyme switching from a low Galf-affinity state (reduced enzyme) to a very strong one (native enzyme). It is remarkable that the mutase binds the boat-locked analogue 1 with similar affinities in both its conformational states. These results support a mechanism involving the formation of 1,4-anhydrogalactopyranose as a low-energy intermediate. An alternative explanation would be that the distortion of the galactose moiety during the cycle contraction transiently brings the carbohydrate into a conformation close to a (1,4)B boat.     

Bovine α-Lactalbumin: Identification of two metal-ion-binding sites using the europium (III) luminescent probe

The luminescent Eu^III ion has been used to probe the metal-binding sites of bovine α-lactalbumin (BLA) in D₂O. Upon addition of apo-BLA to an Eu^III-containing solution, the intrinsic luminescence of the protein is quenched, and the Eu^III luminescence is enhanced. Luminescent titrations point to there being at least two different metal-binding sites in the apo-protein. Curve analysis of the high resolution ⁵D₀←⁷F₀ excitation spectra reveals the existence of three different environments for the bonded Eu^III ions. Two environments, labelled I_a and I_b, give ⁵D₀←⁷F₀ bands very close in energy; they contain four negatively charged groups and are assigned to one site we identify as the calcium-binding site. Site I is protected from solvent influences and is somewhat rigid, since it displays selectivity towards lanthanide ions. The origin of the two similar environments I_a and I_b could not be determined unambiguously. The third environment is ascribed to a nonspecific metal-binding site in which the Eu^III ion is more exposed to the solvent (site II). It is sequentially populated after saturation of site I, and its population is pH-dependent. The affinity constant of Eu^III for this site was estimated from the excitation spectra: log K₂^app ? 3.5(1). Assignment of the metal binding sites has been facilitated by comparison with model compounds, [Eu(dota)]^? (dota ? 1,4,7,10-tetraazacyclododecane N,N′,N″, N?-tetraacetate), [Eu(dtpa)]^2? (dtpa ? diethylenetriamine tetraacetate), and [Eu(bsa)] (bsa ? bovine serum albumin). The usefulness and limits of the use of curve-analysis procedures to unravel the various components of ⁵D₀←⁷F₀ excitation spectra in biological materials are also discussed.     

Charge assisted chiral hybrid H-bonded molecular networks

The co-crystallisation of [Fe(CN)5NO]2- and cyclic achiral 1(2+) and chiral 2(2+) or 3(2+) bisamidinium tectons leads in the crystalline phase to the formation of 1-D H-bonded achiral and chiral molecular networks respectively. In all cases, the network is formed by mutual bridging of the anionic and cationic units through a chelate mode of H-bonding.     

Mössbauer study and molecular orbital calculations on some bimetallic derivatives of ferrocene and ferricinium

We have studied by absorption Mössbauer spectroscopy, some bridged derivatives of biferrocene, unoxidized (Fe^II– Fe^II) and mono-oxidized (Fe^II – Fe^III). The mixed-valence species exhibits a quasi-delocalized behavior at the Mössbauer timescale (～ 10^?8 s), with a splitting of the lines which is interpreted in terms of a partial valence trapping. This in turn reveals intramolecular properties at variance from those of the unbridged cation. SCC-Xα MO calculations have been performed; they correctly reproduce the measured quadruple splittings and provide evidence that the HOMO of the mixed-valence complex is of \(d_{xy} ,d_{x^2 - y^2 } \) type. Some consequences on the mixed-valence properties are discussed.