Bifunctional Complex Research Articles

Dissolution of fe(iii)(hydr)oxides in natural waters; laboratory assessment on the kinetics controlled by surface coordination

Dissolution of Fe (III) (hydr)oxides, of importance in oceanographic and limnological cycles of iron, occurs (1) in anoxic environments, (2) at the oxic-anoxic boundary of the water-sediment column, and (3) as a light-induced process in oxic surface waters. The various pathways of dissolution of Fe (III) (hydr) oxides, especially by reductants in thermal and in photochemical processes, are discussed and assessed on the basis of laboratory experiments. The relevance of these mechanisms for the transformation of iron in natural waters is discussed. It is shown that surface processes and not transport processes control the dissolution kinetics and that all the dissolution reactions are critically dependent on the type of complexes formed on the surface of the Fe (III) (hydr) oxides. Thus, a reductant such as ascorbate exchanges electrons with a surface Fe (III) ion subsequent to its inner-sphere coordination to the oxide surface. The Fe (II) thereby formed becomes more easily detached from the surface, because of the larger lability in the crystalline lattice surface of the Fe(II)-O bond than of the Fe(III)-O bond. A catalytic dissolution that appears to be important in natural waters is accomplished by Fe (II) in the presence of a bifunctional complex former (dicarboxylic acid, hydroxycarboxylic acid, diphenol). The latter is able to form on the surface of Fe (III)(hydr)oxides a ternary complex with Fe(II): >Fe III−X−Fe II (X≠bridging ligand). Electron transfer from Fe (II) to the surface Fe (III) ions occurs through the bridging ligand. In light-induced reductive dissolution of Fe (III) (hydr) oxides as well as in thermal reductive dissolution, the inner-sphere surface coordination of the electron donor to the oxide surface is essential for the efficient occurrence of the photochemical (or thermal) redox reaction.

Point mutations in the dihydrofolate reductase-thymidylate synthase gene as the molecular basis for pyrimethamine resistance in Plasmodium falciparum

The dihydrofolate reductase-thymidylate synthase (DHFR-TS) bifunctional complex from pyrimethamine-sensitive (3D7) and drug-resistant (HB3 and 7G8) clones from Plasmodium falciparum was purified to homogeneity. A modified sequence of purification steps with a 10-formylfolate affinity column at its center, allows the isolation of the enzyme complex with a 10-fold higher yield than previously reported, irrespective of the pyrimethamine resistance of the parasites. Titration of the homogeneous DHFR-TS complex with the inhibitor revealed a 500-fold lower affinity of the enzyme from clone 7G8 for the drug than found with the enzyme from clone 3D7. Direct comparison of the homogeneous enzyme preparations on SDS-PAGE revealed no difference in the molecular mass of the DHFR-TS from the 3 clones, nor could a reproducible difference be detected in the peptide patterns obtained after digesting the DHFR-TS complex with various proteases. The amplification of segments from the DHFR-TS coding region of the 3 clones and 7 isolates of P. falciparum by polymerase chain reaction resulted in fragments of the predicted length without any size heterogeneity. The DNA sequence of the DHFR coding region from FCR-3, 3D7, HB3 and 7G8 differs in a total of 4 nucleotides. One point mutation changes amino acid residue 108 from threonine (FCR-3) or serine (3D7) to asparagine (HB3 and 7G8). The presence of asparagine-108 appears to be the molecular basis of pyrimethamine resistance of HB3 and 7G8. The degree of resistance is associated with a point mutation affecting the codon for amino acid 51 in 7G8.

Novel reactions of the [BPh4]- salt of the bifunctional complex [(triphos)Rh(S2CO)]+ (triphos = MeC(CH2PPh2)3): one-electron reduction, cleavage of B-H and B-C bonds, and metathesis of CS2-like heteroallene molecules

Etude des proprietes oxydoreductrices du complexe du titre au moyen de la voltammetrie cyclique. Reactivite vis a vis de CS 2 , SCNR ou R=Ph,Et

Molecular Organometallic Chemistry on Surfaces: Reactivity of Metal Carbonyls on Metal Oxides

AbstractMetal carbonyls react on metal oxide surfaces to give a wide range of structures analogous to those of known compounds. The reactions leading to formation of surface‐bound metal carbonyls are explained by known molecular organometallic chemistry and the functional group chemistry of the surfaces. The reaction classes include formation of acid‐base adducts as the oxygen of a carbonyl group donates an electron pair to a Lewis acidic center; nucleophilic attack at CO ligands by basic surface hydroxyl groups or O2⊖ ions; ion‐pair formation by deprotonation of hydrido carbonyls to give carbonylate ions; interaction of bifunctional complexes with surface acid‐base pair sites such as [Mg2⊕O2⊖]; and oxidative addition of surface hydroxyl groups to metal clusters. The reactions of surface‐bound organometallic species include redox condensation and cluster formation on basic surfaces (paralleling the reactions in basic solution) as well as oxidation of mononuclear metal complexes and oxidative fragmentation of metal clusters by reaction with surface hydroxyl groups. Most supported metal carbonyls are unstable at high temperatures, but some, including osmium carbonyl cluster anions on the basic MgO surface, are strongly stabilized in the presence of CO and are precursors of catalysts for CO hydrogenation at 550 K.

Production of higher alkylaromatic hydrocarbons from ethylene and toluene or o-xylene in the presence of bifunctional complex catalysts

Production of higher alkylaromatic hydrocarbons from ethylene and toluene or o-xylene in the presence of bifunctional complex catalysts

Preparation and partial characterization of a soluble site-to-site directed enzyme complex composed of alcohol dehydrogenase and lactate dehydrogenase

A soluble, bifunctional enzyme complex has been prepared by crosslinking lactate dehydrogenase and alcohol dehydrogenase with glutaraldehyde. The crosslinking was performed on a solid phase while the active sites of alcohol dehydrogenase and lactate dehydrogenase were held adjacent to one another with the aid of a bis-NAD analog. Subsequently, the enzyme complex was released from the solid phase. The soluble enzyme complex was then purified by using NAD-Sepharose as an affinity adsorbent. Based on gel filtration experiments, the complex was estimated to consist of one of each dehydrogenase.

Does the bifunctional uridylate synthase channel orotidine 5'-phosphate? Kinetics of orotate phosphoribosyltransferase and orotidylate decarboxylase activities fit a noninteracting sites model

Uridylate synthase is a bifunctional protein that first forms orotidine 5'-phosphate (OMP) from orotate via its orotate phosphoribosyltransferase activity (EC 2.4.2.10) and then converts OMP to uridine 5'-phosphate (UMP) via the OMP decarboxylase activity (EC 4.1.1.23). A computer modeling analysis of the experiments that led to the proposal [Traut, T.W., & Jones, M.E. (1977) J. Biol. Chem. 252, 8374-8381] that uridylate synthase channels intermediate OMP suggests that the experimental results do not demonstrate preferential use of OMP generated in the bifunctional complex as against exogenous OMP. This analysis shows that the experimentally observed amounts of [6-14C]UMP from [6-14C]orotate in the presence of various amounts of exogenous [7-14C]OMP agree well with the amounts predicted by the computer simulations. Thus we conclude that uridylate synthase does not channel OMP. Additionally, the subsequent suggestion that channeling of OMP occurs to protect the intermediate from degradation by a nucleotidase [Traut, T.W. (1980) Arch. Biochem. Biophys. 200, 590-594] seems unlikely. The appropriate computer simulation demonstrates that low transient levels of OMP and protection of the intermediate are provided for strictly by the kinetic parameters of orotate phosphoribosyltransferase, OMP decarboxylase, and the nucleotidase. Additionally, calculations show that, in both sets of published experiments, the concentration of transient OMP greatly exceeded the concentration of OMP decarboxylase active sites. Thus, channeling of OMP by the bifunctional complex cannot be invoked to explain the evolution of uridylate synthase, and that event must be the result of some other selective pressure.

L-cysteine biosynthesis in Escherichia coli: nucleotide sequence and expression of the serine acetyltransferase (cysE) gene from the wild-type and a cysteine-excreting mutant.

Serine acetyltransferase (SAT) from Escherichia coli is subject to feedback inhibition by L-cysteine. A mutant was isolated which excretes L-cysteine because of a lesion in cysE, the structural gene for SAT, rendering the enzyme less feedback sensitive. To analyse the structural basis for this mutation the cysE genes both from wild-type E. coli and the mutant strain were cloned and their nucleotide sequences determined. The cysE gene contained an open reading frame consisting of 819 bp, equivalent to a protein of 273 amino acids. The mutant gene showed a single base change in position 767 resulting in a methionine to isoleucine substitution. A causal connection between this SAT sequence alteration, feedback insensitivity and L-cysteine excretion was demonstrated. The SAT from the wild-type strain was purified. It was composed of a single polypeptide chain migrating in SDS gels according to an Mr of 34,000. As in Salmonella typhimurium, the enzyme was associated in a bifunctional complex with O-acetylserine (thiol)-lyase.

Novel Aspects in Metal Phosphorus Double Bond Chemistry: Simple Access to P-Arylated, P-Alkylated and Chiral Derivatives and their Conversion to Three Membered Metallacycles

Abstract Complexes CP(CO)2M=PR2 (M = Cr, Mo, W) with a metal phosphorus double bond can be prepared either via decarbonylation of the metallo-phosphanes CP(CO)3M-PR2 or dehydrdchlorination of the bifunctional complexes Cp(C0)2(R2PX)M-Y (X = C1, H, NMe2; Y = H, C1) respectively. These procedures are complementary and allow the introduction of diverse substituents at the phosphorus (R = alkyl, aryl, alkoxy, dialkylamino) as well as the synthesis of species Cp(C0) (Me3P)M=PR2 with a chiral metal centre. The high reactivity of the M=P-complexes offers easy access to diverse three membered metallaand dimetallacycles.

How a bifunctional complex drives reactivity of carbon dioxide-like molecules: sodium [N,N'-ethylenebis(salicylaldiminato)]cobaltate(I) promotion of the reductive coupling of molecules resembling carbon dioxide

How a bifunctional complex drives reactivity of carbon dioxide-like molecules: sodium [N,N'-ethylenebis(salicylaldiminato)]cobaltate(I) promotion of the reductive coupling of molecules resembling carbon dioxide

Unsymmetrisch substituierte alkin- und dialkinkomplexe von wolfram(IV) und molybdän(IV). Die kristallstruktur von [W 2Cl 8(PhCCCCSiMe 3)(pyridin) 2

The unsymmetrically substituted alkyne ClCCSiMe 3 and the dialkynes PhCCCCSiMe 3 and PhCCSiMe 2CCPh react with tungsten hexachloride in CCl 4, in the presence of C 2Cl 4 as a reducing agent, and with molybdenum pentachloride in CCl 4, respectively, to form the alkyne complexes [MoCl 4-(ClCCiMe 3)] 2 ( 1), [WCl 4(ClCCSiMe 3)] 2 ( 2), [WCl 4(PhCCCCSiMe 3)] 2 ( 3), [W 2Cl 8(PhCCCCSiMe 3)] x ( 4), [W 2Cl 8(PhCCSiMe 2CCPh)] x ( 5), [Mo 2Cl 8-(PhCCSiMe 2CCPh)] x ( 6). In all cases the alkyne ligand is side-on coordinated to the metal (η 2), the bonding features correspond to the cyclopropene type. The coordination number of the metal is 7; the complexes 1–3 are dimerised via MCl 2M bridges and the bifunctional complexes 4–6 are polymerised via analogous bridges. In 3, the tungsten atom is coordinatd to the alkyne group adjacent to the phenyl substituent. In the molybdenum compounds the alkyne ligand is cleaved with Lewis bases such as pyridine or diethyl ether forming solvate complexes of molybdenum(IV), whereas in the corresponding tungsten complexes only the chloro bridges are cleaved. The pyridine adducts [W 2Cl 8(PhCCCCSiMe 3)(pyridine) 2] ( 7) and [W 2Cl 8(PhCCSiMe 2CCPh)(pyridine) 2] ( 8) are described in further detail. All complexes are characterized by elemental analyses, mass and IR spectra, the tungsten compounds (as ether adducts) also by their 13C NMR spectra. 7 was obtained in single crystals and has been subjected to a X-ray diffraction study. It crystallises in the monoclinic space group P2 1/ c with four formula units per unit cell (4154 independent, observed reflexions, R = 0.096) and with the lattice constants a 880.0(6), b 2866.0(20), c 1501.2(10) pm; β 94.4(3)°. EAch tungsten atom is surrounded by four chlorine atoms in equatorial positions, both alkyne groups are bonded side-on (η 2) to the tungsten atoms, whereas the N atoms of the pyridine molecules are located trans to the alkyne groups. The CC bond lengths of the alkyne groups are 125 pm (adjacent to the phenyl substituent) and 140 pm (adjacent to the SiMe 3 group), respectively. A strong trans-effect of the two alkyne groups is observed, resulting in shifting the tungsten atoms out of the Cl 4 planes by about 33 pm. The dihedral angle of 94.9°, enclosed by the two WC 2 planes, is remarkable.

Isolation of the Products Resulting from the Reaction of cis and trans Diaminedichloroplatinum [II] with DNA and Chromatin on the Dowex 50 W Column

The production of platinated derivatives of nucleic acid bases resulting from the reaction of cis and trans DDP with DNA and chromatin was studied. Bifunctional complex of guanine appeared to be the major product of the interaction of cis isomer with both DNA and chromatin, although other bifunctional adducts of A-Pt-G and A-Pt-A were also isolated. The main product of the interaction of trans DDP with DNA was a monofunctional adduct of guanine. Small amounts of the bifunctional complexes were also isolated. When ssDNA was incubated with trans DDP more bifunctional complexes appeared, what suggests that geometric constrains of double helix prevent formation of these complexes. Trans isomer reacts more easily with chromosomal proteins than cis DDP does. Therefore after the reaction of trans DDP with chromatin less platination occurs on DNA moieties.

Isolation of the Adducts of Platinum Complexes and Nucleic Acid Bases on the Dowex 50 W Column

The method presented here enables the isolation of three monofunctional adducts. These are: platination of cytosine, adenine and guanosine. The three detected bifunctional complexes were: guanine N7 to N7, adenine N7 to N7 and adenine N7 to N1. A mixed bifunctional complex of guanine and adenosine and the product of polymerization of three adenines and two moieties of cis DDP were also detected. In the peak eluted separately from the guanine standard, no Pt was detected. The sample eluted in this peak had UV spectrum different from the standard and may represent the degraded product of guanine and polymerized Pt.

The Influence of Cysteine on the Reaction of d-Guanosine with cis -Diamminedichloroplatinum (II)

Mono- and bifunctional complexes of cis DDP and d-guanosine were isolated on Sephadex G-25. The presence of cysteine in incubation mixture resulted in the vanishing of bifunctional complex of d-guanosine and the appearance of d-guanosine-cis DDP-cysteine complex in the eluted peak. It is also possible that the eluted peak contained no homogeneous product of the reaction.

Interaction of small molecules with metal centers: the role of modeling studies

Abstract

Carbon dioxide fixation: bifunctional complexes containing acidic and basic sites working as reversible carriers

Carbon dioxide fixation in its intact form is achieved from ''bifunctional'' complexes containing in their structure a nucleophilic cobalt(I) and an alkali cation: Co(R-salen)M (R-salen = substituted salen ligand; salen = N,N'-ethylenebis(salicylideneaminato); M = Li, Na, K, Cs). The fixation of CO/sub 2/ by such systems displays different degrees of reversibility depending on the reaction solvent and the nature of the alkali cation. The X-ray analysis carried out on (Co(n-Pr-salen)K-(CO)/sub 2/(THF)) shows carbon dioxide anchored to the cobalt through a Co-C sigma bond, while the oxygens interact with the alkali cation, in a polymeric structure. IR spectra are diagnostic for the presence of CO/sub 2/ bonded as a bent molecule in a reduced form displaying three strong bands in the following ranges: 1700-1650, 1300-1250, and 1250-1200 cm/sup -1/. The binding of CO/sub 2/ is partially prevented in the presence of a crown ether, which can complex the alkali cation. The reaction of (Co(salen)Na(CO/sub 2/)) and (Co(salen)Na(THF)) with dicyclohexano-18 crown-6(DCHC) gave a complex, (Co(salen)Na)/sub 2/(DCHC), whose structure shows the presence of the intact bifunctional unit where, however, all the coordination sites around the alkali cation are filled by the oxygens from salen and DCHC and are not further available for bindingmore » the oxygen atoms of the CO/sub 2/ molecule. Crystallographic details and the final R factor for specific observed reflections are given for (Co(n-Pr-salen)K(CO/sub 2/)(THF))/sub n/, (CO(n-Pr-salen)K(CO/sub 2/)(THF))/sub n/, (Co(salen)Na)/sub 2/(DCHC).« less

Metal-carbon and carbon-carbon bond formation from small molecules and one carbon functional groups

Metal promoted elementary transformations on C1 molecules (C02, CH2O, CO), including their reversible fixation, are described. Utilization of monofunctional vs bifunctional (acidbase) complexes represents the main point in the strategy for the activation of a C1 molecule, the primary objective being the formation of a reactive metal-carbon bond. Simulation of CO2 by related molecules discloses general pathways leading to a C-C bond formation from one carbon functional groups. Transformations of a 2-C,O metal-bonded formaldehyde generated metal-bonded species supposed to be key intermediates in carbon monoxide reduction and in C2 units synthesis. A comparison is sketched on some extreme models [early transition metals vs copper(I)] to be used in carbon monoxide activation processes.

Activation of CO2-like molecules by a multifunctional complex: possible pathways leading to C–C bond formation involving functional groups simulating CO2

The bifunctional complex [Co(salen)Na(thf)][salen =N,N′-ethylenebis(salicylideneiminato) dianion, thf = tetrahydrofuran] promoted the reductive coupling of methyl pyruvate, while diphenylketen added to the salen ligand surrounding the cobalt ion to produce a complex which can be the precursor for ligand-assisted C–C bond formation involving CO2-like molecules.

Folate Metabolism in Filariae. Enzymes Associated with 5,10-Methenyltetrahydrofolate and 10-Formyltetrahydrofolate

Adult Dirofilaria immitis and Brugia pahangi were found to possess the following folate-related enzymes that catalyze the formation of 5,10-methenyltetrahydrofolate (methenylFH4) or 10-formylFH4 (f10FH4): f10FH4 synthetase, methenylFH4 cyclohydrolase, f5FH4 cyclodehydrase, and a bifunctional complex composed of formiminoglutamate: FH4 formiminotransferase and 5-fomiminoFH4 cyclodeaminase. The properties of these filarial enzymes were generally similar to those of their counterparts from invertebrate and vertebrate sources, although each possessed one or more distinctive characteristics.

Studies on a pyrimidine phosphoribosyltransferase from murine leukemia P1534J. Partial purification, substrate specificity, and evidence for its existence as a bifunctional complex with orotidine 5-phosphate decarboxylase.

A pyrimidine phosphoribosyltransferase, previously shown to utilize 5-fluorouracil and possibly also uracil and orotate (Reyes, P. (1969) Biochemistry 8, 2057-2062), has been purified about 100-fold from murine leukemia P1534J. Roughly 20% of the original activity was recovered to yield an enzyme preparation with a specific activity of 7.4 mumol of 5-fluorouracil utilized/hour/mg of protein. Disc gel electrophoresis of this preparation revealed the presence of a major band of protein accompanied by several trace contaminants. Emphasis was placed on a study of the substrate specificity of this enzyme. 5-Fluorouracil, uracil, and orotate phosphoribosyltransferase activities purified in parallel during fractionation with ammonium sulfate and protamine sulfate and eluted together from columns of Sephadex tG-150 and DEAE-cellulose. The three phosphoribosyltransferase activities eluted from the Sephadex columns with an apparent molecular weight of 55,000 to 60,000. In spite of this coordinate fractionation, preferential losses of orotate activity were experienced during DEAE-cellulose chromatography. Orotate activity continued to behave in a unique manner under other conditions, such as during proteolytic digestion. In the latter case, however, all three activities responded in parallel when digestion took place in the presence of 5mM UMP. The following results provided additional evidence to support the view that all three phosphoribosyltransferase activities may be catalyzed by the same enzyme: (a) the apparent Km for 5-phosphoribosyl 1-pyrophosphate (PP-ribose-P) did not change significantly when enzyme activity was measured with either 5-fluorouracil, uracil, or orotate; (b) 5-fluorouracil and uracil were found to be mutually competitive inhibitors; the effect of 5-fluorouracil on orotate activity was likewise competitive in nature; (c) in the absence of UMP, orotate was a noncompetitive inhibitor of 5-fluorouracil and uracil activities, but in the presence of 5mM UMP it became a competitive inhibitor of both of these activities; (d) 5-fluorouracil and orotate activities co-sedimented in 5 to 20% sucrose gradients (uracil activity was not examined); and (e) a wide variety of normal mouse tissues displayed virtually the same 5-fluorouracil to uracil to orotate activity ratio as found in P1534J enzyme preparations. The apparent Km and Ki values reported in this study indicate that the preferred pyrimidine substrate is orotate. It seems likely, therefore, that this enzyme functions in vivo as an orotate phosphoribosyltransferase. Orotate phosphoribosyltransferase and orotidine 5'-monophosphate (OMP) decarboxylase activities (a) eluted together during gel filtration on Sephadex G-150, (b) co-sedimented in 5 to 20% sucrose gradients, (c) remained associated during fractionation with ammonium sulfate and protamine sulfate, and (d) separated into a phosphoribosyltransferase and decarboxylase component when enzyme preparations previously subjected to limited proteolysis by elastase were sedimented in sucrose gradients...