Showing papers on "Bond cleavage published in 1983"

Pb(II)-catalysed cleavage of the sugar-phosphate backbone of yeast tRNAPhe--implications for lead toxicity and self-splicing RNA.

Abstract: Pb(II) is extremely efficient at depolymerizing RNA1–5 and studies on tRNAs have shown that site-specific cleavages in these molecules can be brought about by the action of Pb(II)6. We have observed, by difference Fourier analysis7,8, sugar–phosphate strand scission between residues 17 and 18 in crystals of yeast tRNAPhe soaked in dilute Pb(II) solution at pH 7.4. We have also deduced the structure of the Pb(II)–tRNAPhe derivative at pH 5.0 where this cleavage reaction is considerably slower and report that, in this structure, the sugar–phosphate backbone remains intact. We have, therefore, a picture of the reactants (at pH 5.0) and products (at pH 7.4) of this cleavage reaction. From this crystallographic study, and associated biochemical work, we have formulated a possible mechanism for the cleavage reaction and also present here some general ideas on the action of metal ions on nucleic acids.

DNA- and protein-scission activities of ascorbate in the presence of copper ion and a copper-peptide complex.

Abstract: L-Ascorbic acid, when combined with either copper(II) ion or a copper(II)-tripeptide complex, extensively cleaved several viral DNAs and proteins under in vitro conditions. Neither ascorbate nor copper tripeptide (Cu2+-diglycyl-L-histidine) alone caused any apparent changes on these molecules. Various transition metal ions and reducing agents were examined under comparable conditions to determine the basic requirements for both DNA degradation and protein scission activities. Copper and iron are the two most effective transition metal ions examined that exhibit these activities in the presence of ascorbate. The addition of catalase, but not superoxide dismutase, can partially inhibit the scission of DNA in vitro, suggesting that H2O2 may be involved in these activities. Among the various reducing agents tested, ascorbate was most effective in causing DNA scission and protein cleavage, corroborating the possible role of H2O2 in the cleavage reactions. One of the products of the reactions of copper/ascorbate is probably the hydroxyl radical generated from H2O2, which can be formed from the oxidation of ascorbate.

Relative importance of metal-metal bond scission and loss of carbon monoxide from photoexcited dimanganese decacarbonyl: spectroscopic detection of a coordinatively unsaturated, carbonyl-bridged dinuclear species in low-temperature alkane matrices

Abstract: En milieu rigide (alcane a 77 K) dissociation de Mn 2 (CO) 10 en Mn 2 (CO) 9 +CO. En solution fluide a 298 K, scission de la liaison Mn-Mn selon Mn 2 (CO) 10 →2 Mn(CO) 5 . Detection par spectres IR

Kinetics of acid-catalyzed cleavage of procyanidins

Abstract: Comparisons of the rates of cleavage of isomeric procyanidin dimers in the presence of excess phenylmethane thiol and acetic acid showed that compounds with a C(4)–C(8) interflavanoid bond were cleaved more rapidly than their C(4)–C(6) linked isomers, that 2,3-cis isomers with an axial flavan substituent were cleaved more rapidly than a 2,3-trans isomer with an equatorial substituent, and that cleavage rate was independent of the stereochemistry in the terminal unit. Cleavage rate constants followed Arrhenius temperature dependence. The C(4)–C(8) interflavanoid bond in 2,3-cls procyanidins should be labile to cleavage at 20°C over a pH range of 3.6 to 5.4. Reaction of loblolly pine bark tannins in the presence of excess (+)-catechin with acetic acid rapidly produced oligomeric procyanidins. The ratio of the procyanidin Bl to B7 decreased from 2.9:1 after 4 hours to 1.3:1 after 50–100 hours at 90°C. The structure and reactions of condensed tannins are interpreted in light of these results.

Magnitude of intrinsic isotope effects in the dopamine beta-monooxygenase reaction.

Abstract: Intrinsic primary hydrogen isotope effects (kH/kD) have been obtained for the carbon-hydrogen bond cleavage step catalyzed by dopamine beta-monooxygenase. Irreversibility of this step is inferred from the failure to observe back-exchange of tritium from TOH into substrate under conditions of dopamine turnover; this result cannot be due to solvent inaccessibility at the enzyme active site, since we will demonstrate [Ahn, N., & Klinman, J. P. (1983) Biochemistry (following paper in this issue)] that a solvent-derived proton or triton must be at the enzyme active site prior to substrate activation. As shown by Northrop [Northrop, D. B. (1975) Biochemistry 14, 2644], for enzymatic reactions in which the carbon-hydrogen bond cleavage step is irreversible, comparison of D(V/K) to T(V/K) allows an explicit solution for kH/kD. Employing a double-label tracer method, we have been able to measure deuterium isotope effects on Vmax/Km with high precision, D(V/K) = 2.756 +/- 0.054 at pH 6.0. The magnitude of the tritium isotope effect under comparable experimental conditions is T(V/K) = 6.079 +/- 0.220, yielding kH/kD = 9.4 +/- 1.3. This result was obtained in the presence of saturating concentrations of the anion activator fumarate. Elimination of fumarate from the reaction mixture leads to high observed values for isotope effects on Vmax/Km, together with an essentially invariant value for kH/kD = 10.9 +/- 1.9. Thus, the large disparity between isotope effects, plus or minus fumarate, cannot be accounted for by a change in kH/kD, and we conclude a role for fumarate in the modulation of the partitioning of enzyme-substrate complex between catalysis and substrate dissociation. On the basis of literature correlations of primary hydrogen isotope effects and the thermodynamic properties of hydrogen transfer reactions, the very large magnitude of kH/kD = 9.4-10.9 for dopamine beta-monooxygenase suggests an equilibrium constant not very far from unity for the carbon-hydrogen bond cleavage step. This feature, together with the failure to observe re-formation of dopamine from enzyme-bound intermediate or product and overall rate limitation of enzyme turnover by product release, leads us to propose a stepwise mechanism for norepinephrine formation from dopamine in which carbon-hydrogen bond cleavage is uncoupled from the oxygen insertion step.

A very long cobalt to nitrogen bond in a coenzyme B12 model. Relevance to the role of the 5,6-dimethylbenzimidazole in cobalt-carbon bond cleavage in coenzyme B12

Abstract: Structure cristalline du complexe 2NH 2 pyCo (DH) 2 -i-C 3 H 7 . (groupe P2 1 2 1 2 1 avec a=27,83, b=8,564 et c=8,198 A, Z=4). La structure est affinee jusqu'a R=0,033. Le complexe est pseudooctaedrique avec les coordinats dimethylglyoxime en position equatorial et le groupe isopropyl en position axiale

Carbon-carbon bond formation via carbonyl-carbene complexes

Abstract: Abstract — The synthetic potential of carbonyl carbene complexes is demonstrated upon their reactions with alkynes. Both the electrophilicity of the carbene car— hon atom and the facile substitution of carbon monoxide are used for selective carbon carbon bond formation. Nucleophilic alkynes (ynamines) add to the carbene carbon atom and then undergo insertion into the metal carbene bond. Chromium(O) is effective as a template in the annulation of carbene ligands bearing aromatic or vinylic side chains: Metal—assisted carbene transfer or both carbene and carbonyl transfer to alkynes are observed to yield indenes or l,4—hydroquinones. The hydro— quinome formation occurs regioselectively with respect to the alkyne incorporation and the annulation of diarylcarbene ligands. The synthetic utility of this reaction is exemplified by the synthesis of vitamins K and E.

Mechanism of thermal depolymerization of trimethylsiloxy‐terminated polydimethylsiloxane

Abstract: “Catalyst-free” trimethylsilyl end-blocked polydimethylsiloxanes of Mn = 68,300 and 111,000 were isothermally depolymerized above 400°C. The volatilized products were a mixture of cyclic oligomers and trace quantities of Me3Si(OSiMe2)nOSiMe3 (n = 0,1). The relative degree of polymerization was followed as a function of fractional conversion. The data were consistent with the oretical curves for a mechanism involving initiation by random siloxane bond cleavage followed by a rapid and complete unzipping of the kinetically active fragments. It is suggested that catalysis may be responsible for the initiation process, thus accounting for the low activation energy (43 Kcal/mol) relative to the siloxane bond energy (108 kcal/mol).

Intramolecular Stereospecific Pummerer Reactions of Aryl (Substitutedmethyl) Sulfoxides Bearing Electron-withdrawing Groups with Acetic Anhydride

Abstract: The Pummerer reaction of optically active cyanomethyl aryl sulfoxides with acetic anhydride gave the corresponding α-acetoxy sulfides which were induced with a partial asymmetry nearly 30% at α-carbon, while the 18O-label of the original sulfoxides was retained in more than 85% in the resulting ester, the Pummerer reaction product. Kinetic experiments with cyanomethyl (p-substituted phenyl) sulfoxides and α,α-dideuterated cyanomethyl p-tolyl sulfoxide in the reaction with acetic anhydride containing a small amount of acetic acid revealed that the rates were correlated with Hammett σ-values and ρ-value of −0.70 was obtained, while the kinetic isotope effect was practically nil, i.e., kH⁄kD=1.01. These observations indicate clearly that the rearrangement is intramolecular and proceeds via forming a very intimate ion-pair and the rate-determining step is believed to be the S–O bond cleavage after the initial reversible acylation and deprotonation. Elimination of acetic acid proceeds through the Elcb type pro...

Silver halide photosensitive material

Abstract: A silver halide photosensitive material containing a compound selected from Types 1-4 below which is capable of undergoing a one-electron oxidation to form a one-electron oxidation product (OEOP), and a reducing compound (Type 1) the OEOP is capable of releasing further two or more electrons accompanying a subsequent bond cleavage reaction, (Type 2) the OEOP is capable of releasing further one electron accompanying a subsequent bond cleavage reaction, and the compound having, in its molecule, two or more groups adsorptive to silver halide, (Type 3) the OEOP is capable of releasing further one or more electrons after going through a subsequent bond forming process, and (Type 4) the OEOP is capable of releasing further one or more electrons after going through a subsequent intramolecular ring cleavage reaction.

Direct cleavage of the vicinal diol linkage of the lignin model compound dihydroanisoin by the basidiomycete Phanerochaete chrysosporium

Abstract: The white rot basidiomycete Phanerochaete chrysosporium metabolized dihydroanisoin (1,2-dianisylethane-1,2 diol) in low nitrogen stationary cultures, conditions under which the ligninolytic system is expressed. Anisyl alcohol was isolated as a metabolic product indicating an initial diol bond cleavage of the substrate. Use of 3H-labeled dihydroanisoin (1,2-dianisylethane-1,2-diol-1,2 3H) indicated that the diol bond was cleaved directly, yielding anisyl aldehyde as the initial product. The metabolically stable ketol anisoin was shown not be an intermediate in the metabolism of dihydroanisoin. The diol cleavage reaction was dependent on the concentration of molecular oxygen but O2 could be replaced by H2O2 under some conditions. The cleavage reaction was inhibited by exogenously-added tyrosine2-Cu2+ complex (TCC). The appearance of the fungal diol cleavage system parallels the appearance of the ligninolytic system under a variety of physiological conditions. In addition, preincubation of ligninolytic cultures with 2.5 mM l-glutamate represses both the ligninolytic and the diol cleavage activities.

Electrophilic attack on the [µ3-acetyl-C1(Fe1 : Fe2)O(Fe1 : Fe3)]nonacarbonyl-triangulo-triferrate(1–) anion by fluoroboric acid and methyl fluorosulphate. Carbon–oxygen bond cleavage to give µ3-ethylidyne and µ-methoxo-groups. X-Ray crystal structures of Fe3(CO)9(µ3-MeCO)(µ-H), Fe3(CO)9(µ3-CMe)(µ3-OMe), and Fe3(CO)9(µ3-CMe)(µ3-COMe)

Abstract: The (µ3-acetyl)-nonacarbonyltriferrate(1–) ion, [Fe3(CO)9(µ3-MeCO)]–, reacts with fluoroboric acid to give the neutral clusters Fe3(CO)9(µ3-MeCO)(µ-H)(1), Fe3(CO)10(µ-CMe)(µ-H)(2), and in small yield, Fe3(CO)9(µ3-MeCOH)(3). With methyl fluorosulphate the anion reacts to give either Fe3(CO)9(µ3-CMe)(µ3-OMe)(4) or Fe3(CO)9(µ3-CMe)(µ3-COMe)(5), depending upon the reaction conditions. The crystal structures of (1), (4), and (5) have been determined by X-ray diffraction studies. Mechanisms for the electrophilic attacks and for the C–O bond cleavage of the µ3-acetyl group are discussed.

Unprecedented Oxidative Addition of the P4S3 Cage Molecule with Phosphorus‐Phosphorus Bond Cleavage; Synthesis and Molecular Structure of [Ir(μ‐P4S3)(PPh3)Cl(CO)]2

Abstract: The complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 1066. DOI:10.1002/anie.198310660