Biomimetic alcohol oxidations by an iron(III) porphyrin complex: relevance to cytochrome P-450 catalytic oxidation and involvement of the two-state radical rebound mechanism

The systematic oxidation reactions of a wide range of alcohols have been carried out by using an iron porphyrin complex in order to understand their relation to cytochrome P-450 enzymes and to have a practical application to organic synthesis. The iron porphyrin complex catalyzed efficiently alcohol oxidation to the respective carbonyl compound via a high-valent iron-oxo porphyrin intermediate ((Porp)Fe=O+). Several mechanistic studies such as isotope 18O labeling, deuterium isotope effect, linear free energy relationship, and ring-opening of radical clock substrate, have suggested that the alcohol is oxidized by a sequence of reactions involving an alpha-hydroxyalkyl radical intermediate and oxygen rebound to form the gem-diol, dehydration of which yields the carbonyl compounds. Moreover, it has been proposed that a two-state reactivity mechanism can also be adopted for alcohol oxidation reactions in iron porphyrin model systems as exhibited by P-450 enzymes.     

Amphiphilic treelike rods at interfaces: layered stems and circular aggregation

Amphiphilic dendron-rod molecules with three hydrophilic poly(ethylene oxide) (PEO) branches attached to a hydrophobic octa-p-phenylene rod stem were investigated for their ability to form two-dimensional micellar structures on a solid surface. A treelike shape of the molecules was reported to be a major factor in the formation of nonplanar micellar structures in solution and in the bulk state (cylindrical and spherical). We observed that in these treelike amphiphilic molecules the hydrophilic terminated dendron branches assemble themselves in surface monolayers with the formation of two-dimensional layered or circular micellar structures. We suggested the formation of the planar ribbon-like structures with interdigitated layering within the loosely packed monolayers and circular, ringlike structures (2D circular aggregates) in the precollapsed state.     

Ligand dynamics and reactivity in trimetallic clusters

The general relationships between trinuclear cluster reactivity and the ligand dynamical processes in these systems are explored. Three specific mechanistic studies are presented: (1) the rate and stereochemistry of ligand addition to ₃-imidoyl complexes, (2) the factors influencing the rate of carbon-hydrogen bond activation in ₃-alkyne complexes, and (3) the origin of anomalously large kinetic deuterium isotope effect in metal to ligand and ligand to metal hydrogen transfer in trinuclear and binuclear complexes. In all three cases, the current state of the mechanistic studies are summarized and the possible rate of specific ligand dynamical processes in controlling the mechanism are put forth.     

Ion-selective electrodes based on tetraphenyl-phosphonium 12-tungstosilicate and on crystal violet tetraphenylborate in the potentiometric titration of acid and basic dyes

Poly(vinyl chloride) membrane electrodes containing the specified substances are suitable as indicator electrodes in titrations of basic dyes with sodium tetraphenylborate solution and of acidic dyes with crystal violet solution. Useful curves are obtained even at the 10^-3 M level.     

The Tolman length: is it positive or negative?

By means of a large-scale molecular dynamics simulation, we show that the Tolman length, although positive, is much smaller in magnitude than previously reported. We found that the range of interparticle interaction can significantly affect the magnitude of the Tolman length. When the range of interaction is longer than five molecular diameters, the Tolman length is on the order of a few hundredths of the molecular diameter, rather than a few tenths known previously.     

Maleation of poly(3,4‐epoxy‐1‐butene) for accelerated crosslinking in the presence of a redox catalyst

Accelerated crosslinking of novel poly(3,4‐epoxy‐1‐butene) (3,4‐PEPB) oligomers in the presence of a cobalt‐based redox catalyst was investigated. Previous studies using model compounds, 3,4‐dimethoxy‐1‐butene and 1,4‐dimethoxy‐2‐butene, suggested that maleation of hydroxyl‐terminated 3,4‐PEPB oligomers would result in more rapid crosslinking in thin films. Novel maleated oligomers offered a unique combination of both electron‐rich and electron‐poor olefinic sites, and quantitative maleation significantly increased the crosslinking rate of 3,4‐PEPB. Efficient copolymerization between terminal maleate groups and olefinic groups in the repeating unit was proposed to account for accelerated crosslinking rates. Furthermore, the addition of novel reactive diluents, such as maleic acid mono‐ethyl ester, also effectively improved the 3,4‐PEPB crosslinking rate. Sol fraction measurements as a function of coating thickness revealed that the crosslinking rate versus oxygen diffusion was less significant for the maleated oligomers because of the presence of reactive electron‐poor olefins. Sol fractions were constant for catalyst concentrations greater than 0.25–0.50 wt % (as compared with oligomer feed). This observation suggested that a redox process was not a dominant factor in determining crosslinking rates at various experimental conditions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2789–2798, 2002     

Solvolysis-decomposition of 2-adamantyl chloroformate: evidence for two reaction pathways

Reaction of 2-adamantyl chloroformate under a variety of solvolytic conditions leads to 2-adamantyl chloride accompanied by solvolysis products, some with and some without retention of the CO(2) unit. For example, in 100% ethanol, only 4.8% 2-adamantyl chloride is formed with the mixed carbonate (88%) being the dominant product, and in 100% 2,2,2-trifluoroethanol, the products are both formed with loss of CO(2), 59% of the chloride and 41% of the ether. With exclusion of the specific rates in 100% and 90% ethanol and methanol, a good Grunwald-Winstein plot against Y(Cl) values (solvent ionizing power) is obtained, with a slope of 0.47 +/- 0.03. The results are compared with those reported earlier for 1-adamantyl chloroformate and isopropyl chloroformate and mechanistic conclusions are drawn.     

Recyclable self-assembly-supported catalytic system for orthoalkylation

[reaction: see text] A new recyclable supported catalyst system for orthoalkylation was devised using a self-assembly consisting of the barbiturate and 2,4,6-triaminopyrimidine H-bonding motifs. At high temperature, the system is completely homogeneous so as to give an efficient catalytic activity, while it is heterogenized at room temperature to form an insoluble solid phase for the easy recovery of the catalyst after the reaction.     

Dynamic identification of phosphopeptides using immobilized metal ion affinity chromatography enrichment, subsequent partial beta-elimination/chemical tagging and matrix-assisted laser desorption/ionization mass spectrometric analysis

The enrichment of phosphopeptides using immobilized metal ion affinity chromatography (IMAC) and subsequent mass spectrometric analysis is a powerful protocol for detecting phosphopeptides and analyzing their phosphorylation state. However, nonspecific binding peptides, such as acidic, nonphosphorylated peptides, often coelute and make analyses of mass spectra difficult. This study used a partial chemical tagging reaction of a phosphopeptide mixture, enriched by IMAC and contaminated with nonspecific binding peptides, following a modified beta-elimination/Michael addition method, and dynamic mass analysis of the resulting peptide pool. Mercaptoethanol was used as a chemical tag and nitrilotriacetic acid (NTA) immobilized on Sepharose beads was used for IMAC enrichment. The time-dependent dynamic mass analysis of the partially tagged reaction mixture detected intact phosphopeptides and their mercaptoethanol-tagged derivatives simultaneously by their mass difference (-20 Da for each phosphorylation site). The number of new peaks appearing with the mass shift gave the number of multiply phosphorylated sites in a phosphopeptide. Therefore, this partial chemical tagging/dynamic mass analysis method can be a powerful tool for rapid and efficient phosphopeptide identification and analysis of the phosphorylation state concurrently using only MS analysis data.     

Origins and predictions of stereoselective antibody catalysis: theoretical analysis of Diels-Alder catalysis by 39A11 and its germ-line antibody

The binding of enantiomeric haptens and transition states by the Schultz Diels-Alderase antibody 39A11 and its germ-line antibody were studied theoretically. The mechanisms by which one hapten and one transition state stereoisomer is recognized selectively are explored with docking simulations and quantum mechanical models. Transition states of the relevant Diels-Alder reaction were located with density functional theory. A prediction is made that the stereoselectivity of 39A11 will be achieved by two strategically placed hydrogen bonds and pi-stacking interactions of the maleimide with a binding-site tryptophan, arranged so as to coordinate one enantiomeric transition state. Binding of other ligands by antibody 39A11 and the germ-line antibody has also been investigated. The polyspecific nature of 39A11 and its germ-line precursor was found to originate from the general ability of the binding pockets to achieve hydrophobic binding of small organic substrates. Comparison of the highly homologous progesterone and Diels-Alderase antibodies (DB3, 1E9, and 39A11) highlights the fact that differences of several key residues in the binding pockets are sufficient to confer selectivity for different antigens.