A mechanism from quantum chemical studies for methane formation in methanogenesis

The mechanism for methane formation in methyl-coenzyme M reductase (MCR) has been investigated using the B3LYP hybrid density functional method and chemical models consisting of 107 atoms. The experimental X-ray crystal structure of the enzyme in the inactive MCR(ox1)(-)(silent) state was used to set up the initial model structure. The calculations suggest a mechanism not previously proposed, in which the most remarkable feature is the formation of an essentially free methyl radical at the transition state. The reaction cycle suggested starts from a Michaelis complex with CoB and methyl-CoM coenzymes bound and with a squareplanar coordination of the Ni(I) center in the tetrapyrrole F(430) prosthetic group. In the rate-limiting step the methyl radical is released from methyl-CoM, induced by the attack of Ni(I) on the methyl-CoM thioether sulfur. In this step, the metal center is oxidized from Ni(I) to Ni(II). The resulting methyl radical is rapidly quenched by hydrogen-atom transfer from the CoB thiol group, yielding the methane molecule and the CoB radical. The estimated activation energy is around 20 kcal/mol, which includes a significant contribution from entropy due to the formation of the free methyl. The mechanism implies an inversion of configuration at the reactive carbon. The size of the inversion barrier is used to explain the fact that CF(3)-S-CoM is an inactive substrate. Heterodisulfide CoB-S-S-CoM formation is proposed in the final step in which nickel is reduced back to Ni(I). The suggested mechanism agrees well with experimental observations.     

Design,synthesis, and evaluation of 6-carboxyalkyl and 6-phosphonoxyalkyl derivatives of 7-oxo-8-ribitylaminolumazines as inhibitors of riboflavin synthase and lumazine synthase

A series of 6-carboxyalkyl and 6-phosphonoxyalkyl derivatives of 7-oxo-8-D-ribityllumazine were synthesized as inhibitors of both Escherichia coli riboflavin synthase and Bacillus subtilis lumazine synthase. The compounds were designed to bind to both the ribitylpurine binding site and the phosphate binding site of lumazine synthase. In the carboxyalkyl series, maximum activity against both enzymes was observed with the 3'-carboxypropyl compound 22. Lengthening or shortening the chain linking the carboxyl group to the lumazine by one carbon resulted in decreased activity. In the phosphonoxyalkyl series, the 3'-phosphonoxypropyl compound 33 was more potent than the 4'-phosphonoxybutyl derivative 39 against lumazine synthase, but it was less potent against riboflavin synthase. Molecular modeling suggested that the terminal carboxyl group of 6-(3'-carboxypropyl)-7-oxo-8-D-ribityllumazine (22) may bind to the side chains of Arg127 and Lys135 of the enzyme. A hypothetical molecular model was also constructed for the binding of 6-(2'-carboxyethyl)-7-oxolumazine (15) in the active site of E. coli riboflavin synthase, which demonstrated that the active site could readily accommodate two molecules of the inhibitor.     

Dual quartz crystal microbalance compensation using a submerged reference crystal. Effect of surface roughness and liquid properties

Absolute resonant frequency measurements were made on gold-coated AT-cut quartz crystals with one face in contact with a series of liquids. The effect of surface roughness and liquid properties (viscosity and density) was analyzed in terms of a “trapped liquid” model. In this model, liquid present in surface imperfections is viewed as rigidly coupled mass. In some of the literature this density-dependent, but not viscosity-dependent, term is viewed as being additive to the hydrodynamic shift seen for a smooth surface. Data obtained using 1 μm and 5 μm surface finish crystals are inconsistent with the predictions of the trapped liquid model. This suggests hydrodynamic coupling between liquid internal and external to the crevices. Despite the lack of a theoretical model for the liquid motion, it is possible to compensate for frequency variations resulting from changing liquid properties and for roughness effects by making direct measurements of the resonant frequency difference between two crystals exposed to the same solution. This novel procedure works to the extent that the two crystals have similar surface topographies. Compensation is excellent for 1 μm finish crystals and good for 5 μm finish crystals.     

A convenient synthesis of the thieno[3,2-c]pyridine nucleus

The unsubstituted thieno[3,2-c]pyridine ring system was prepared from thiophene-3-carboxaldehyde in 4 steps. The sequence is suitable for scale-up.     

The Influence of pH and Temperature on the Equilibrium and Dynamic Surface Tension of Aqueous Solutions of Sodium Oleate

For aqueous solutions of sodium oleate, the dynamic surface tension (using the maximum bubble pressure technique) and the equilibrium surface tensions (using the du Noüy ring method) were studied. Experiments were carried out within the pH range 7 to 13 and at temperatures from 20 to 60 degrees C. From the equilibrium surface tension studies at 25 degrees C, a wide variation in surface activity was found to occur in the premicellar concentration range, depending on pH and oleate concentration, and this was explained on the basis of the formation of strongly surface-active premicellar species. It was also shown that the critical micelle concentration (cmc) of the system increased with pH within the pH range 7-12. Dynamic surface tension experiments were carried out from 20 to 60 degrees C at concentrations beyond the cmc in alkaline solutions, and the adsorption kinetics at the air/solution interface were analyzed using established theoretical diffusion models. From these data, the effective diffusion coefficients (D(eff)) for the oleate species were determined and the presence of an interfacial activation barrier in the diffusion process was confirmed. It was found that the D(eff) values obtained within the temperature range from 40 to 60 degrees C at 2-3 mM were in general agreement with previously reported values for the oleate dimer obtained by pulse-gradient FT-NMR. Furthermore, D(eff) significantly increased with temperature and also increased with concentration at higher temperatures (>40 degrees C). Copyright 2001 Academic Press.     

Probe metal binding mode of imine covalent organic frameworks: cycloiridation for (photo)catalytic hydrogen evolution from formate

Metalation of covalent organic frameworks (COFs) is a critical strategy to functionalize COFs for advanced applications yet largely relies on the pre-installed specific metal docking sites in the network, such as porphyrin, salen, 2,2′-bipyridine, etc. We show in this study that the imine linkage of simple imine-based COFs, one of the most popular COFs, readily chelate transition metal (Ir in this work) via cyclometalation, which has not been explored before. The iridacycle decorated COF exhibited more than 10-fold efficiency enhancement in (photo)catalytic hydrogen evolution from aqueous formate solution than its molecular counterpart under mild conditions. This work will inspire more functional cyclometallated COFs to be explored beyond catalysis considering the large imine COF library and the rich metallacycle chemistry.

This study describes cyclometallation as a new metal binding mode for imine-based COFs. The iridacycle decorated COF could be used for catalytic hydrogen evolution from aqueous formate solution with high stability and high efficacy.     

Preparative and electrochemical investigations on the electron sponge behavior of cobalt telluride clusters: CO substitution in [Co11Te7(CO)10]n- ions (n=1, 2) by PMe2Ph and crystal structure of [Co11Te7(CO)5(PMe2Ph)5

The reaction of the cluster salts [Cp(2*) Nb(CO)(2)](n)[Co(11)Te(7)(CO)(10)] (Cp*=C(5)Me(5); n=1, 2) with excess PMe(2)Ph gave the neutral, dark brown clusters [Co(11)Te(7)(CO)(6)(PMe(2)Ph)(4)] (5) and [Co(11)Te(7)(CO)(5)(PMe(2)Ph)(5)] (6) with 147 metal valence electrons. The new compounds were characterized by IR spectroscopy, elemental analyses, and mass spectrometry. The molecular structure of 6 was determined by X-ray crystallography. Like its precursor anion, it consists of a pentagonal-prismatic [Co(11)Te(7)] core, but with a ligand sphere composed of five CO and five PMe(2)Ph ligands. Detailed electrochemical studies of both reactions reveal that a stepwise substitution of CO ligands in the initial cluster anions takes place leading to intermediate [Co(11)Te(7)(CO)(10-m)(PMe(2)Ph)(m)](n-) ions (m=1-5; n=1, 2). Each of these intermediates is distinguished by at least one oxidation and two reduction waves, giving rise to a total of 21 redox couples and 27 electroactive species. The electron sponge character of the new compounds is particularly pronounced in 5, which exhibits charges n between +1 and -4 corresponding to metal valence electron counts of between 146 and 151.     

Selective oxidation of methane by the bis(mu-oxo)dicopper core stabilized on ZSM-5 and mordenite zeolites

This work reports on the capability of the O2-activated Cu-ZSM-5 and Cu-MOR zeolites to selectively convert methane into methanol at a temperature of 398 K. A strong correlation between (i) the activity and (ii) the intensity of the 22 700 cm-1 UV-vis band, assigned to the bis(mu-oxo)dicopper core, is found (i) as a function of the reaction temperature, (ii) as a function of the Cu loading of the zeolite, and (iii) in comparison to other Cu materials. These three lines of evidence firmly support the key role of the bis(mu-oxo)dicopper core in this selective, low-temperature hydroxylation of methane.     

Synthesis of spiro-fused (C5)-isoxazolino-(C4)-pyrazolones (1-oxa-2,7,8-triazaspiro[4,4]-2,8-dien-6-ones) via 1,3-dipolar cycloaddition and cycloelimination

An efficient and selective method for the synthesis of spiro-fused (C5)-isoxazolino-(C4)-pyrazolones (C) is reported. The process consists of utilizing the Baylis-Hillman reaction-or a quicker, stepwise MAC procedure-to give I followed by 1,3-dipolar cycloaddition and Swern oxidation to give beta-ketoesters H, which were condensed with hydrazine derivatives to provide hydrazones that underwent cycloelimination. These novel spiro-fused (C5)-isoxazolino-(C4)-pyrazolones were confirmed by spectroscopic analysis as well as single-crystal X-ray of 5. We also concluded that all condensations/cycloeliminations, except with hydrazine itself, were more effective with catalysts or higher reaction temperatures. For example, TiCl(4) was an efficient catalyst for hydrazone formation and cycloelimination with methylhydrazine, while phenyl-, benzyl-, and (4-methoxyphenyl)hydrazine reacted effectively without catalyst in refluxing xylene.     

Polymorphic forms of cilostazol

Two unique conformational polymorphic forms of the compound 6‐[4‐(1‐cyclo­hexyl‐1H‐tetrazol‐5‐yl)­butoxy]‐3,4‐di­hydro­quinolin‐2(1H)‐one (cilostazol), C₂₀H₂₇N₅O₂, have been discovered and characterized using single‐crystal X‐ray structural analysis. A third polymorph also exists, but acceptable crystals could not be obtained. Features of both reported polymorphic structures include a chair conformation of the cyclo­hexyl ring and puckering in the quinolinone ring. The major feature distinguishing the two polymorphic forms is a rotational twisting of the butoxy chain between the tetrazole and quinolinone rings. This difference in conformation influences the intermolecular forces, and hence the packing of the two mol­ecules during crystallization.