Spectroscopic and computational studies on the adenosylcobalamin-dependent methylmalonyl-CoA mutase: evaluation of enzymatic contributions to Co-C bond activation in the Co3+ ground state

Methylmalonyl-CoA mutase (MMCM) is an enzyme that utilizes the adenosylcobalamin (AdoCbl) cofactor to catalyze the rearrangement of methylmalonyl-CoA to succinyl-CoA. Despite many years of dedicated research, the mechanism by which MMCM and related AdoCbl-dependent enzymes accelerate the rate for homolytic cleavage of the cofactor's Co-C bond by approximately 12 orders of magnitude while avoiding potentially harmful side reactions remains one of the greatest subjects of debate among B(12) researchers. In this study, we have employed electronic absorption (Abs) and magnetic circular dichroism (MCD) spectroscopic techniques to probe cofactor/enzyme active site interactions in the Co(3+)Cbl "ground" state for MMCM reconstituted with both the native cofactor AdoCbl and its derivative methylcobalamin (MeCbl). In both cases, Abs and MCD spectra of the free and enzyme-bound cofactor are very similar, indicating that replacement of the intramolecular base 5,6-dimethylbenzimidazole (DMB) by a histidine residue from the enzyme active site has insignificant effects on the cofactor's electronic properties. Likewise, spectral perturbations associated with substrate (analogue) binding to holo-MMCM are minor, arguing against substrate-induced enzymatic Co-C bond activation. As compared to the AdoCbl data, however, Abs and MCD spectral changes for the sterically less constrained MeCbl cofactor upon binding to MMCM and treatment of holoenzyme with substrate (analogues) are much more substantial. Analysis of these changes within the framework of time-dependent density functional theory calculations provides uniquely detailed insight into the structural distortions imposed on the cofactor as the enzyme progresses through the reaction cycle. Together, our results indicate that, although the enzyme may serve to activate the cofactor in its Co(3+)Cbl ground state to a small degree, the dominant contribution to the enzymatic Co-C bond activation presumably comes through stabilization of the Co(2+)Cbl/Ado. post-homolysis products.     

Correlation between molecular orbitals and doping dependence of the electrical conductivity in electron-doped metal-phthalocyanine compounds

We have performed a comparative study of the electronic properties of six different electron-doped metal-phthalocyanine (MPc) compounds (ZnPc, CuPc, NiPc, CoPc, FePc, and MnPc), in which the electron density is controlled by means of potassium intercalation. Despite the complexity of these systems, we find that the nature of the underlying molecular orbitals produces observable effects in the doping dependence of the electrical conductivity of the materials. For all the MPc's in which the added electrons are expected to occupy orbitals centered on the ligands (ZnPc, CuPc, and NiPc), the doping dependence of the conductivity has an essentially identical shape. This shape is different from that observed in MPc materials in which electrons are also added to orbitals centered on the metal atom (CoPc, FePc, and MnPc). The observed relation between the macroscopic electronic properties of the MPc compounds and the properties of the molecular orbitals of the constituent molecules clearly indicates the richness of the alkali-doped metal-phthalocyanines as a model class of compounds for the investigation of the electronic properties of molecular systems.     

Supramolecular structures of coronene and alkane acids at the Au(111)-solution interface: a scanning tunneling microscopy study

Scanning tunneling microscopy (STM) is utilized to study the solution-solid interface formed between Au(111) and solutions of coronene in hexanoic, heptanoic, and octanoic acids. In all three cases adsorbed coronene is observed and lays flat on the metal surface. Heptanoic and hexanoic acid solutions produce a hexagonal symmetry monolayer. For the heptanoic and hexanoic cases, dipole-image dipole interactions and H bonding stabilize a surface structure in which 12 acid molecules surround each coronene and produce a coronene spacing of 1.45 nm. In the case of octanoic acid as solvent, the incorporation of the solvent into the monolayer is not as strongly favored. The coronene spacing can range from close-packed (1.2 nm) with no solvent presumed present in the monolayer, to 1.50 nm with up to 12 solvent molecules surrounding each coronene. The close-packed regions have hexagonal symmetry, as do those with the largest (1.5 nm) spacing. Heptanoic acid solutions give the clearest STM images and are associated with the most stable two-component monolayer. The present paper demonstrates that non-covalent interactions at the solution-metal interface can lead to complex multicomponent monolayer structures.     

Direct observation of cations and polynucleotides explains polyion adsorption to like-charged surfaces

We show an experimental approach for directly observing the condensation of polynucleotides and their electrolyte counterions at a liquid/solid interface. X-ray standing waves (XSW) generated by Bragg diffraction from a d = 20 nm Si/Mo multilayer substrate are used to measure the distinct distribution profiles of the polyanions and simple cations along the surface normal direction with subnanometer resolution. The 1D spatial sensitivity of this approach is enhanced by observing the XSW induced fluorescence modulations over multiple orders of Bragg peaks. We study the interesting divalent cation driven adsorption of anionic polynucleotides to anionic surfaces by exposing a hydroxyl-terminated silica surface to an aqueous solution with ZnCl2 and mercurated poly-uridylic acid (a synthetic RNA molecule). The in situ long-period XSW measurements are used to follow the evolution of both the Zn and Hg distribution profiles during the adsorption process. The conditions and physical mechanisms that govern the observed divalent cation adsorption and subsequent polynucleotide adsorption to an anionic surface are explained by a thermodynamic model that incorporates nonlinear electrostatic effects.     

Der mikrochemische Nachweis des Anabasins und seine Unterscheidung von Nikotyrin und Nikotin

Zusammenfassung Das Problem des Nachweises von Anabasin und seiner Unterscheidung von den Alkaloiden Nikotin und Nikotyrin wurde untersucht. In der Goldbromwasserstoffsäure, Platinbromwasserstoffsäure, Wismutbromwasser-stoffsäure und im Cadmiumbromid wurden Reagenzien gefunden, die für den Nachweis und die Unterscheidung dieser Alkaloide sehr gut geeignet sind.Das Verhalten des Nikotins mit zahlreichen Reagenzien wurde eingehend untersucht. Einige der hierbei erhaltenen Verbindungen wurden beschrieben.

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Summary The problem of detecting anabasin and its differentiation from the alkaloids nicotine and nicotyrine was studied. Auribromhydric acid, bromoplatinic acid, bromobismuthic acid and cadmium bromide were found to be reagents suitable for the detection and differentiation of these alkaloids.An extended study was made of the behavior of nicotine toward numerous reagents. Some of the compounds obtained in this investigation are described.

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Rèsumè Etude du problème de l'identification de l'anabasine et de sa différenciation des alcaloïdes nicotine et nicotyrine. On a constaté que les acides aurichlorhydrique, platibromhydrique, bismuthobromhydrique, et le bromure de cadmium étaient des réactifs convenant parfaitement à l'identification et la différenciation de ces alcaloïdes. Le comportement de la nicotine en présence de nombreux réactifs a été soigneusement étudié. L'auteur décrit les combinaisons obtenues lors de ces réactions.

     

Styrene copolymers as pollutant adsorbents : Safe sampling volume determination

The determination of safe sampling volumes of benzene and acetone on air samplers filled with Porapak Q, Chromosorb 101, 102 and 103 is described. Two direct methods are employed; one method is based on frontal chromatography, while the second one is a concentration method followed by thermal desorption. The results are compared to those determined by the indirect pulse chromatographic method. The safe sampling volumes determined by the direct methods were in accordance. The indirect method gives slightly different values; however, this procedure is carried out at "infinite dilution". The effect of the pollutant concentration on the safe sampling volumes was also examined. The values decrease with increasing pollutant concentration.     

Analysis and interpretation of excess molar properties of amphiphile + water systems : Part 3. Excess molar volumes of isopropoxyethanol + water and isobutoxyethanol + water

Excess molar volumes at 25°C, are reported for the systems isoPrOEtOH + water and isoBuOEtOH + water across their entire composition ranges. These data have been compared to those for their normal alkyl analogs in order to assess the effects of both - and β-chain branching.

The results are discussed in terms of the relative abilities of the amphiphiles to form structured aggregates.     

Quantum chemical calculations and mutational analysis suggest heat shock protein 90 catalyzes trans-cis isomerization of geldanamycin

The affinity of geldanamycin (GA) for binding to heat shock protein 90 (HSP90) is 50- to 100-fold weaker than is the affinity of the structurally distinct natural product radicicol. X-ray crystallography shows that although radicicol maintains its free conformation when bound to HSP90, the conformation of GA is dramatically altered from an extended conformation with a trans amide bond to a kinked shape in which the amide group in the ansa ring has the cis configuration. We have performed ab initio quantum chemical calculations to demonstrate that the trans-cis isomeriztion of GA in solution is both kinetically and thermodynamically unfavorable. Thus, we propose that HSP90 catalyzes the isomerization of GA. We identify Ser113, a conserved residue outside the ATP binding pocket, as essential for the isomerization of GA. In support of this model, we show that radicicol binds equally well to both wild-type HSP90 and the Ser113 mutant, whereas the binding of GA to the Ser113 mutant is decreased significantly from its binding to wild-type HSP90. Based on this finding, a mechanism of keto-enol tautomerization of GA catalyzed by HSP90 is proposed. The added requirement of isomerization prior to tight binding may explain the enhanced binding affinity of GA for HSP90 in a cell extract versus in a purified form.     

Studies towards the total synthesis of contignasterol

The (17R,20S,22S,24S) C₂₀-C₂₉ segment of contignasterol has been stereoselectively prepared in 8 steps and 40% overall yield from (S)-carvone. Synthetic studies towards contignasterol's C/D ring functionalization/isomerization are also reported.     

Calculated structures and relative stabilities of furoxan,some 1,2-dinitrosoethylenes and other isomers

The structures and relative stabilities of furoxan and some of its isomers, e.g., the 1,2-dinitrosoethylenes, have been determined by means of ab initio Hartee–Fock and Møller–Plesset calculations. Geometries were optimized at the HF/3-21G, HF/6-31G* and MP2/6-31G* levels, and subsequently used for computing MP2/6-31G*, MP3/6-31G*, and MP4/6-31G* energies. The results are markedly affected by the inclusion of electronic correlation, which renders three of the isomers unstable. It also emphasizes the importance of a zwitterionic contribution to the structure of furoxan, which promotes ring-opening through a cis 1,2-dinitrosoethylene intermediate/transition state that has an MP4/6-31G*//MP2/6-31G* energy that is 31.6 kcal/mol above furoxan.