Polarography of B(12) coenzyme

The polarography of B(12) coenzyme (5,6-dimethylbenzimidazolylcobamide, DBC coenzyme) has been investigated at the dropping mercury electrode. Exposure of solutions of the coenzyme to light and then to oxygen give polarograms comparable to those of B(12r) and hydroxocobalamin (B(12a)) respectively. At pH 11.6 the coenzyme gives two reduction waves, at -1.43 and -1.62 V vs. the S.C.E.; in less basic solutions the two waves merge to give one multielectron wave.     

Interaction of porphyrins with a dendrimer template: self-aggregation controlled by pH

The interaction between self-aggregated porphyrins such as 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and 5,10,15,20-tetrakis(4-phosphonatophenyl)porphyrin (TPPP), and a generation 5 (G5) PAMAM dendrimer template is governed by minute differences of porphyrin acido-basic properties. While at neutral pH both monomeric TPPS and TPPP form complexes with G5, decreasing pH did not lead to porphyrin ring protonation (pK(a) approximately 5) but rather to the preferential formation of H-aggregates (probably H-dimers), most likely due to protonation of the G5. Upon further acidification of the solution, this face-to-face orientation of the porphyrin units is being converted to edge-to-edge aligned J-aggregates with a tightly defined structure. This process starts by protonation of the porphyrin ring at pH below 2.3 and 2.8 for TPPS and TPPP, respectively. The AFM imaging of porphyrin/G5 nanostructures obtained at pH 0.7 shows the formation of long nanorods of TPPS with partially aggregated G5 and small aggregates of TPPP connected to individual G5 molecules.     

Characteristics of organic transformations in a confined dendritic core: studies on the AIBN-initiated reaction of dendrimer cobalt(II) porphyrins with alkynes

Cobalt(II) complexes of poly(aryl ester) dendrimer porphyrins [(m-[Gn]TPP)Co(II)] (generation number n=0-4), in the presence of azobisisobutyronitrile (AIBN) at 60 degrees C, underwent alkenylation with several alkynes at the metal center. A complete inhibition of double-bond migration (secondary transformation) was observed for [(m-[Gn]TPP)Co(II)] (n=3 and 4), which gave [(m-[Gn]TPP)Co(III)-C(=CH(2))R] (n=3 and 4) exclusively. Overall reaction rates for [(m-[Gn]TPP)Co(II)] (n=0-3) were hardly dependent on the size of the dendritic substituents, while a notable retardation was observed for the largest dendrimer, [(m-[G4]TPP)Co(II)]. Mechanistic studies on double-bond migration with pure [(m-[Gn]TPP)Co(III)-C(=CH(2))Bu] (n=0-4) demonstrated that the secondary transformation involves participation of [(m-[Gn]TPP)Co(III)H] (n=0-4), derived from [(m-[Gn]TPP)Co(II)] and AIBN, rather than [(m-[Gn]TPP)Co(II)] alone. Crossover experiments using [(m-[Gn]TPP)Co(III)-C(=CH(2))Bu] (n=2-4), in combination with nondendritic [(m-[G0]TPP)Co(II)] and AIBN, indicated a high level of steric protection of the active center by a robust [G4]-dendritic cage, as suggested by a (1)H NMR pulse relaxation time profile of m-[G4]TPPH(2).     

Synthesis of a novel star-shaped dendrimer by radial-growth polymerization of sarcosine N-carboxyanhydride initiated with poly(trimethyleneimine) dendrimer

A novel AB_n-type dendrimer/linear polymer block copolymer, i.e., poly(trimethyleneimine) dendrimer-block-(polysarcosine)₆₄ ( 1 ), was synthesized by ring-opening polymerization of sarcosine N-carboxyanhydride initiated with the 64-NH₂-terminal poly(trimethyleneimine) dendrimer as a macroinitiator. 1 has narrow molecular weight distributions (M_w/M_n = 1.0₁–1.0₃, by size exclusion chromatography) and controlled polysarcosine chain lengths (by varying the monomer/dendrimer feed molar ratios). Small-angle neutron scattering (SANS) data obtained in D₂O solution of 1 (DP's of polysarcosine = 2.0 and 24) fitted well with a Guinier plot of a spherical particle, and gave diameters of 44 and 100 Å, respectively.     

Formation of the dimethylbenzimidazole ligand of coenzyme B(12) under physiological conditions by a facile oxidative cascade

Dimethylbezimidazole, the axial ligand of vitamin B(12), is synthesized from riboflavin by a two-electron oxidation, a retro-aldol condensation, and a second two-electron oxidation. This oxidative cascade readily takes place nonenzymatically under physiological conditions. [reaction: see text]     

Stabilisation of methylene radicals by cob(II)alamin in coenzyme B12 dependent mutases

Coenzyme B12 initiates radical chemistry in two types of enzymatic reactions, the irreversible eliminases (e.g., diol dehydratases) and the reversible mutases (e.g., methylmalonyl-CoA mutase). Whereas eliminases that use radical generators other than coenzyme B12 are known, no alternative coenzyme B12 independent mutases have been detected for substrates in which a methyl group is reversibly converted to a methylene radical. We predict that such mutases do not exist. However, coenzyme B12 independent pathways have been detected that circumvent the need for glutamate, beta-lysine or methylmalonyl-CoA mutases by proceeding via different intermediates. In humans the methylcitrate cycle, which is ostensibly an alternative to the coenzyme B12 dependent methylmalonyl-CoA pathway for propionate oxidation, is not used because it would interfere with the Krebs cycle and thereby compromise the high-energy requirement of the nervous system. In the diol dehydratases the 5'-deoxyadenosyl radical generated by homolysis of the carbon-cobalt bond of coenzyme B12 moves about 10 A away from the cobalt atom in cob(II)alamin. The substrate and product radicals are generated at a similar distance from cob(II)alamin, which acts solely as spectator of the catalysis. In glutamate and methylmalonyl-CoA mutases the 5'-deoxyadenosyl radical remains within 3-4 A of the cobalt atom, with the substrate and product radicals approximately 3 A further away. It is suggested that cob(II)alamin acts as a conductor by stabilising both the 5'-deoxyadenosyl radical and the product-related methylene radicals.     

Growth of iron clusters on octahedral B12N12 cage: a time-dependent-DFT analysis

Structural Chemistry - Aiming to search for new sensors of drugs and vehicles for their transportation, in this work is studied the growth of iron clusters, Fen n?≤?4, on the...     

Steric inhibition of pi-stacking: 1,3,6,8-tetraarylpyrenes as efficient blue emitters in organic light emitting diodes (OLEDs)

The sterically congested tetraarylpyrenes 1-3, which can be readily accessed by Suzuki coupling, exhibit no-aggregation (pi-stacking) behavior in both solution and solid states. The indisposed tendency of 1-3 toward crystallization and their moderate molecular dimensions permit exploitation as blue light emitting materials in OLEDs with respectable device performances.     

The Co(I) induced methylmalonyl-succinyl rearrangement in a model for the coenzyme B12 dependent methylmalonyl-CoA mutase

The rearrangement of 2-bromomethyl-2-methylmonothiomalonates to succinyl derivatives was found to take place in quantitative yields in the presence of one molar equivalent of Co(I) generated by the reduction of heptamethyl Co(II)yrinate perchlorate with NaBH4 or electrochemically. The chiral thiomalonate gave racemic succinate.     

Steric control over hydrogen bonding in crystalline organic solids: a structural study of N,N'-dialkylthioureas

Hydrogen bonding in crystalline N,N'-dialkylthioureas was examined with the help of single-crystal X-ray diffraction, DFT calculations, and Cambridge Structural Database (CSD) analysis. A CSD survey indicated that unlike the related urea derivatives, which persistently self-assemble into one-dimensional hydrogen-bonded chains, the analogous thioureas can form two different hydrogen-bonding motifs in the solid state: chains, structurally similar with those found in ureas, and dimers, that further associate into hydrogen-bonded layers. The formation of one motif or another can be manipulated by the bulkiness of the organic substituents on the thiourea group, which provides a clear example of steric control over the hydrogen bonding arrangement in crystalline organic solids.     

Rotation of the exo-methylene group of (R)-3-methylitaconate catalyzed by coenzyme B(12)-dependent 2-methyleneglutarate mutase from Eubacterium barkeri

2-Methyleneglutarate mutase from the anaerobe Eubacterium (Clostridium) barkeri is an adenosylcobalamin (coenzyme B(12))-dependent enzyme that catalyzes the equilibration of 2-methyleneglutarate with (R)-3-methylitaconate. Two possibilities for the mechanism of the carbon skeleton rearrangement of the substrate-derived radical to the product-related radical are considered. In both mechanisms an acrylate group migrates from C-3 of 2-methyleneglutarate to C-4. In the "addition-elimination" mechanism this 1,2-shift occurs via an intermediate, a 1-methylenecyclopropane-1,2-dicarboxylate radical, in which the migrating acrylate is simultaneously attached to both C-3 and C-4. In the "fragmentation-recombination" mechanism the migrating group, a 2-acrylyl radical, becomes detached from C-3 before it starts bonding to C-4. In an attempt to distinguish between these two possibilities we have investigated the action of 2-methyleneglutarate mutase on the stereospecifically deuterated substrates (Z)-3-methyl[2'-(2)H(1)]itaconate and (Z)-3-[2'-(2)H(1),methyl-(2)H(3)]methylitaconate. The enzyme catalyzes the equilibration of both compounds with their corresponding E-isomers and with a 1:1 mixture of the corresponding (E)- and (Z)-2-methylene[2'-(2)H(1)]glutarates, as shown by monitoring of the reactions with (1)H and (2)H NMR. In the initial phase of the enzyme-catalyzed equilibration a significant excess (8-11%) of (E)-3-methyl[2'-(2)H(1)]itaconate over its equilibrium value was observed ("E-overshoot"). The E-overshoot was only 3-4% with (Z)-3-[2'-(2)H(1),methyl-(2)H(3)]methylitaconate because the presence of the deuterated methyl group raises the energy barrier from 3-methylitaconate to the corresponding radical. The overshoot is explained by postulating that the migrating acrylate group has to overcome an additional energy barrier from the state leading back to the substrate-derived radical to the state leading forward to the product-related radical. It is concluded that the fragmentation-recombination mechanism can provide an explanation for the results in terms of an additional energy barrier, despite the higher calculated activation energy for this pathway.     

"Squaring the clusters": a Mn(III)4Ni(II)4 molecular square from nickel(II)-induced structural transformation of a Mn(II/III/IV)12 cage

A Mn(III)(4)Ni(II)(4) molecular square exhibiting slow magnetization relaxation has been prepared from the reaction of a Mn(II)(4)Mn(III)(6)Mn(IV)(2) cluster and a simple Ni(II) source.     

Density functional theory study of anionic and neutral per-substituted 12-vertex boron cage systems, B(12)X(12)(n-) (n = 2, 1, 0)

The 12(12) closomers form a rapidly expanding class of compounds where a 12-vertex cage is surrounded by 12 identical substituents. Density functional theory (B3LYP/6-31G(d)) is used to study a number of these closomers in different states of oxidation (dianion, radical anion, and neutral cages). The cage stability increases as the group electronegativity of the substituent increases. Also, the 12(12) closomer becomes easier to oxidize as the Hammett sigma(p) parameter becomes more negative (electron-donating). As the closomer is oxidized, the size of the cage increases and the B-B distances become more asymmetric. The Raman-active breathing mode in the 404-434 cm(-1) range moves to lower frequency as the cage is oxidized, which is caused by removing one or two electrons from a cage-bonding molecular orbital.     

Synthesis and nonisothermal reaction of a novel acrylonitrile-capped poly(propyleneimine) dendrimer with epoxy resin

A novel acrylonitrile-capped poly(propylene imine) dendrimer (PAN4) was synthesized and characterized with FTIR and ¹H-NMR. PAN4 and its precursor (poly(propylene imine) dendrimer (1.0GPPI) were employed to cure bisphenol A epoxy resin (DGEBA), and the nonisothermal reaction kinetics of DGEBA/PAN4 and DGEBA/1.0GPPI was systematically investigated using a differential scanning calorimeter (DSC) in a comparative way. The apparent activation energies determined with the Kissinger method were 59.7 kJ/mol for DGEBA/1.0GPPI and 53.9 kJ/mol for DGEBA/PAN4. Applied the Málek method, it was found that a two-parameter autocatalytic model (SB(m, n)) could well simulate the reaction rates, and further analysis of the reaction rate constants showed PAN4 could cure DGEBA at a greatly decreased rate by a factor a more than ten compared with 1.0GPPI control.     

Twelvefold Functionalization of an Icosahedral Surface by Total Esterification of [B12(OH)12]2-: 12(12)-Closomers

No Abstract     

A novel cascade supramolecular complex with a reversible nanosized 18-component H-bonded {(C6H6)6(SbF6)12}12- cage unit templated by a chiral metal-organic complex

A novel cascade supramolecular complex [Ag(L)](SbF6).C6H6 with a reversible nanosized {(C6H6)6(SbF6)12}12- H-bonded cage unit was reported.     

Oxidative transformation of organic compounds using bis(1,10-phenanthroline)silver(II) peroxydisulfate as a twin catalyst/oxidant

The mild and efficient oxidation of a variety of organic functional groups with a twin catalyst/oxidant, bis(phenanthroline)silver(II) peroxydisulfate, [Ag(phen)₂]S₂O₈, is reported. This reagent shows high selectivity and the type of obtained products and the extent of oxidation are strongly dependent to the type of the functional groups and oxidant/substrate molar ratios.