Showing papers on "Bond cleavage published in 1997"

Derivatization Followed by Reductive Cleavage (DFRC Method), a New Method for Lignin Analysis: Protocol for Analysis of DFRC Monomers

Abstract: A new method for selective and efficient cleavage of arylglycerol−β-aryl (β-O-4) ether linkages in lignins is introduced. The acronym “DFRC” relates to the reactions involved, derivatization followed by reductive cleavage. Derivatization, accompanied by cell wall solubilization, is accomplished with acetyl bromide in acetic acid; reductive cleavage of resulting β-bromo ethers utilizes zinc in an acidic medium. Following acetylation, degradation monomers (4-acetoxycinnamyl acetates) are quantified by GC, providing data analogous to those from analytical thioacidolysis. Keywords: Acetyl bromide; lignin; β-aryl ether; thioacidolysis; reductive elimination

DNA Strand Scission by Polycyclic Aromatic Hydrocarbon o-Quinones: Role of Reactive Oxygen Species, Cu(II)/Cu(I) Redox Cycling, and o-Semiquinone Anion Radicals†,‡

Abstract: In previous studies, benzo[a]pyrene-7,8-dione (BPQ), a polycyclic aromatic hydrocarbon (PAH) o-quinone, was found to be 200-fold more potent as a nuclease than (+/-)-anti-7,8-dihydroxy-9,10-epoxy-7,8,9, 10-tetrahydrobenzo[a]pyrene, a suspect human carcinogen. The mechanism of strand scission mediated by naphthalene-1,2-dione (NPQ) and BPQ was further characterized using either phiX174 DNA or poly(dG).poly(dC) as the target DNA. Strand scission was extensive, dependent on the concentration of o-quinone (0-10 microM), and required the presence of NADPH (1 mM) and CuCl2 (10 microM). The production of reactive species, i.e., superoxide anion radical, o-semiquinone anion (SQ) radical, hydrogen peroxide (H2O2), hydroxyl radical (OH.), and Cu(I), was measured in the incubation mixtures. The formation of SQ radicals was measured by EPR spectroscopy under anaerobic conditions in the presence of NADPH. A Cu(II)/Cu(I) redox cycle was found to be critical for DNA cleavage. No strand scission occurred in the absence of Cu(II) or when Cu(I) was substituted, yet Cu(I) was required for OH* production. Both DNA strand scisson and OH. formation were decreased to an equal extent, albeit not completely, by the inclusion of OH. scavengers (mannitol, soduim benzoate, and formic acid) or Cu(I) chelators (bathocuproine and neocuproine). In contrast, although the SQ radical signals of NPQ and BPQ were quenched by DNA, no strand scission was observed. When calf thymus DNA was treated with PAH o-quinones, malondialdehyde (MDA) was released by acid hydrolysis. The formation of MDA was inhibited by OH. scavengers suggesting that OH* cleaved the 2'-deoxyribose moiety in the DNA to produce base propenals. These studies indicate that for PAH o-quinones to act as nucleases, NADPH, Cu(II), Cu(I), H2O2, and OH*, were necessary and that the primary species responsible for DNA fragmentation was OH., generated by a Cu(I)-catalyzed Fenton reaction. The genotoxicity of PAH o-quinones may play a role in the carcinogenicity and mutagenicity of the parent hydrocarbons.

Kinetic isotope effects and mechanism of biomimetic oxidation of methane and benzene on FeZSM-5 zeolite

Abstract: Earlier, iron complexes stabilized in a ZSM-5 zeolite matrix have been shown to produce a new form of surface oxygen (called α-oxygen) upon decomposition of N2O. α-Oxygen exhibits a very high reactivity typical for oxygen of monooxygenases (MO) and mimics its unique ability in selective oxidation of hydrocarbons at room temperature. Kinetic isotope effect (KIE) measurements reported here reveal additional similarities between MO and the model. Depending on the temperature, the value of KIE for oxidation of methane with α-oxygen ranges from 1.9 to 5.5. For the oxidation of benzene the value of KIE is 1.0. This indicates that both biological and chemical oxidation of methane involves a rate limiting CH bond cleavage, whereas the reaction with benzene is probably limited by the formation of an epoxy-type intermediate. The assumed structure of the active sites as well as some features of the oxidation mechanism allow one to consider FeZSM-5-N2O system as a new and successful model for methane monooxygenase.

Homolytic C−S Bond Scission in the Desulfurization of Aromatic and Aliphatic Thiols Mediated by a Mo/Co/S Cluster: Mechanistic Aspects Relevant to HDS Catalysis

Abstract: The kinetics of the reaction of a series of aromatic and aliphatic thiols with cluster 1 were determined. These reactions form cluster 2 and the arene or alkane corresponding to the thiol: Cp‘2Mo2Co2S3(CO)4 (1) + RSH → Cp‘2Mo2Co2S4(CO)2 (2) + RH + 2CO. These reactions are first order in thiol and first order in cluster 1 with appreciable negative entropies of activation. These data suggest that the rate determining step of the desulfurization reaction is the initial association of the thiol to the cluster. The more nucleophilic thiolate anions react with 1 at −40 °C to form an adduct in which the thiolate anion is bound η1 to the Co atom. At −25 °C, the initial adduct rearranges to a fluxional μ2, η1-bound thiolate. The fluxional process is proposed to involve a concerted “walking” of the thiolate and a μ2-bound sulfide ligand on the surface of the cluster. Near 35 °C, the thiolate−cluster adduct undergoes C−S bond homolysis to give the paramagnetic anion of cluster 1 and the phenyl or alkyl radical. The...

Absolute Binding Energies of Lithium Ions to Short Chain Alcohols, CnH2n+2O,n= 1-4, Determined by Threshold Collision-Induced Dissociation

Abstract: Collision-induced dissociation of Li+(ROH) with xenon is studied using guided ion beam mass spectrometry. ROH includes the following eight short chain alcohols: methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-methyl-1-propanol, 2-butanol, and 2-methyl-2-propanol. In all cases, the primary product is endothermic loss of the neutral alcohol, with minor products that include those formed by ligand exchange, alkene and water loss, and C−C bond cleavage. The cross-section thresholds are interpreted to yield 0, 298, and 373 K bond energies for Li+−ROH after accounting for the effects of multiple ion−molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. The experimental bond energies determined here show a fairly constant deviation from previous experimental measurements (as obtained by equilibrium studies in an ion cyclotron resonance mass spectrometer). This discrepancy is discussed in some detail because it affects the absolute Li+ affinity scale used extensively in t...

Protecting Groups That Can Be Removed through Photochemical Electron Transfer: Mechanistic and Product Studies on Photosensitized Release of Carboxylates from Phenacyl Esters

Abstract: Photolysis of electron-donating photosensitizers in the presence of various phenacyl esters (PhCOCH2−OCOR) results in C−O bond scission leading to the formation of acetophenone (PhCOCH3) and the co...

Evidence for the Transition between Concerted and Stepwise Heterogeneous Electron Transfer−Bond Fragmentation Mechanisms

Abstract: The electrochemical reduction of tert-butyl p-cyanoperbenzoate in DMF leads to the cleavage of the oxygen−oxygen bond. A subsequent reaction yielding tert-butyl p-cyanobenzoate could be avoided by the addition of a mild acid, with the result that the main reduction peak was affected only by the kinetics of the electron transfer bond fragmentation process. Unusual features of the heterogeneous electron transfer mechanism were evidenced by the conventional voltammetric criteria, which pointed to a nonlinear potential dependence of the transfer coefficient α. The fine details of the electron transfer mechanism were obtained by convolution analysis of the voltammetric curves, a procedure that led to a bell-shaped potential dependence of α. The observed behavior is explained by the assumption that when the electrode potential is decreased, the process changes from a concerted electron transfer bond cleavage to a partially stepwise mechanism. The observed potential dependence of α is in agreement with the resul...

Demonstration of the Role of Scission of the Proximal Histidine−Iron Bond in the Activation of Soluble Guanylyl Cyclase through Metalloporphyrin Substitution Studies§

Abstract: Activation of soluble guanylyl cyclase (sGC) by NO correlates with scission of the proximal iron−histidine bond, as demonstrated by the application of electronic absorption and resonance Raman spectroscopy to the study of metalloporphyrin-substituted enzymes. The non-native metalloporphyrins, Mn(II)PPIX and Co(II)PPIX, can be introduced into heme-deficient sGC forming five-coordinate complexes. The similarity among Mn(II)sGC, Co(II)sGC, and the corresponding metalloporphyrin-substituted derivatives of Mb and Hb provides confirming evidence for the presence of an axial histidine ligand in sGC. Upon addition of NO, Mn(II)sGC forms a six-coordinate species with the histidine ligand still bound to the Mn, and the enzyme is not activated. In contrast, the Co(II)sGC(NO) adduct is five-coordinate and the enzyme is activated. These data imply that the activated state of sGC is attained when the proximal histidine−metal bond is broken.

The Equilibrium Between 2-Lithium-Oxazole(-Thiazole, -Imidazole) Derivatives and Their Acyclic Isomers – A Structural Investigation†

Abstract: 2-Metallated oxazoles, thiazoles and imidazoles are of interest for the preparation of derivatives of the respective heterocycles, and – especially in the case of metallated thiazoles – as acyllithium equivalents in organic syntheses. Metallation at C2 of these compounds, however, often leads (more so with Li-oxazoles than with Li-thiazoles and Li-imidazoles) to products derived from the acyclic isomers of the metallated heterocycles. In order to obtain more information on the positions of the relevant equilibria, we have carried out a 13C-NMR study of substituted li- and ZnCl-oxazoles, Li-thiazoles and Li-imidazoles. In the case of Li-oxazoles, the equilibrium is completely on the side of the acyclic isomers. Addition of ZnCl2, however, leads to ring-closure, and, even in the case of 2-ZnCl-substituted benzoxazole, only the ring-closed isomer is observed. This demonstrates a dramatic gegenion effect on the equilibrium between two isomeric anions. In the case of Li-thiazoles, depending on the substitution pattern, either solely the ring-closed or an equilibrium with the ringopened isomer is observed. In the series of Li-imidazoles, only the 2-lithiated benzimidazole is in equilibrium with its ring-opened isomer. The tendency of these compounds to undergo ring-opening parallels the leaving group properties of the various subunits [OLi > SLi > R2NLi, and phenyl-O(S, NR)Li > vinyl-O(S, NR)Li]. The difference between the effects of Li+ vs. ZnCl+ in the (benz)oxazole series is in agreement with results of solid-state structure investigations of 2-lithiated and a 2-zincated thiazole: in the Li compound the C–S bond is 2.9 pm longer than in the ZnCl species, indicating a more facile C–S bond cleavage with Li+ as the gegenion.

Generation and Reactivity of Aromatic Thioether Radical Cations in Aqueous Solution As Studied by Pulse Radiolysis

Abstract: The radical cations of thioanisole (1), p-methylthioanisole (2), and the benzyl phenyl sulfides 3−5 have been produced by pulse radiolysis in aqueous solutions, using SO4•- or Tl2+ as oxidizing species. The radical cations 1•+−5•+ exhibit very similar UV spectra, with strong absorptions between 300−350 and 500−600 nm. In contrast to aliphatic thioether radical cations, 1•+−5•+ do not undergo dimerization (via formation of a three-electron bond with the parent thioethers). In the absence of bases, 1•+ is a long-lived species with a lifetime >30 ms, whereas 3•+, 4•+, and 5•+ decay rapidly by both C−S bond and C−H bond cleavage with kC-H = 1.3 × 103 s-1 and kC-S = 1.3 × 103 s-1 for 3•+and kC-H = 0.95 × 103 s-1 and kC-S = 2.65 × 103 s-1 for 4•+. In the presence of OH- or HPO42-, also 1•+ undergoes a deprotonation process, with a rate larger than those of the benzyl phenyl sulfide radical cations. For example, the rate constant for the OH--induced deprotonation is 3.4 × 107 M-1 s-1 for 1•+and 9.5 × 106 M-1 s-1...

Dna damage induced via 5,6-dihydrothymid-5-yl in single-stranded oligonucleotides

Abstract: 5,6-Dihydrothymid-5-yl (4) is generated via Norrish type I cleavage of isopropyl ketone 7 Ketone 7 was site specifically incorporated into chemically synthesized polythymidylates and an oligonucleotide containing all four native deoxyribonucleotides No damage is induced in oligonucleotides containing 7 upon photolysis under anaerobic conditions In the presence of O2, strand breaks and alkaline labile lesions are formed at the original site of 7, and at nucleotides adjacent to the 5‘-phosphate of 7 Kinetic isotope effect experiments reveal that direct strand scission at the thymidine adjacent to the 5‘-phosphate of 4 arises from C1‘ hydrogen atom abstraction The observed KIE (∼39) is attributed to hydrogen atom abstraction from C1‘ by the peroxyl radical 35 derived from 4 Enzymatic end group analysis and measurement of free base release are consistent with a process involving C1‘ hydrogen atom abstraction Cleavage experiments carried out in the presence of t-BuOH (105 M) and NaN3 (10 mM) indicate

Direct Observation of Ultrafast Decarboxylation of Acyloxy Radicals via Photoinduced Electron Transfer in Carboxylate Ion Pairs.

Abstract: Charge-transfer (CT) photoactivation of the electron donor−acceptor salts of methylviologen (MV2+) with carboxylate donors (RCO2-) including benzilates [Ar2C(OH)CO2-] and arylacetates (ArCH2CO2-) leads to transient [MV•+, RCO2•] radical pairs. Femtosecond time-resolved spectroscopy reveals that the photogenerated acyloxy radicals (RCO2•) rapidly lose carbon dioxide by C−CO2 bond cleavage, in competition with back-electron transfer to restore the original ion pair, [MV2+, RCO2-]. The decarboxylation rate constants for ArCH2CO2• lie in the range (1−2) × 109 s-1, in agreement with previous reports. In striking contrast, the C−CO2 bond scission in Ar2C(OH)CO2• occurs within a few picoseconds (kCC = (2−8) × 1011 s-1). The rate constants for decarboxylation of these donors approach those of barrier-free unimolecular reactions. Thus, real-time monitoring of the decarboxylation of benziloxy radicals represents the means for the direct observation of the transition state for C−C bond scission.

Chemistry of C-2 Glyceryl Radicals: Indications for a New Mechanism of Lipid Damage

Abstract: Precursors for the selective generation of C-2 glyceryl radicals were synthesized, and the chemical behavior of the corresponding radicals was investigated by ESR spectroscopy, product analysis, and kinetic measurements. It was found that cleavage of the β-C,O bond proceeds rapidly, if a hydroxyl group is present at the radical carbon center. The rate constant for the elimination of a β-acetoxy group from radical 30 was dependent on the solvent (kE = 4 × 105 s-1 in methanol, kE = 2 × 107 s-1 in toluene). With these results and ab initio calculations a concerted elimination mechanism is suggested. α-Methoxy-substituted C-2 glyceryl radicals 42 and 43 showed heterolytic β-C,O bond cleavage under formation of radical cations. With ester-substituted radicals 24 and 35 no elimination could be observed. To demonstrate the biological significance of these findings, C-2 lysolecithin radical 53 was generated, which led to fast β-elimination.

Ligand Effects in the Models and Mimics of Oxyhemocyanin and Oxytyrosinase. A Density Functional Study of Reversible Dioxygen Binding and Reversible O-O Bond Cleavage.

Abstract: We studied the energetics of dioxygen binding and the core isomerization between the Cu2(μ-η2:η2-O2) and Cu2(μ-O)2 core structures of {[LCu]2O2}2+, L = 1,4,7-triazacyclononane (1) and L = hydrotris(pyrazolyl)borate (2), by gradient-corrected density functional methods. N-Substituted derivatives of 1 and 2 are synthetic inorganic mimics of oxyhemocyanin with similar physical and spectroscopic properties but different chemical behavior. The calculated dioxygen binding energies of 1 and 2 are −60 and −184 kJ/mol, respectively, in line with the observation that 1 binds oxygen reversibly and 2 does so irreversibly. Bond energy decomposition showed that orbital interactions contribute equally to the binding energy of 1 and 2, which explains the similarities in their spectral properties. Electrostatic interactions due to the negatively charged ligand are responsible for stronger binding in 2. The core isomerization energies for isomerization from the Cu2(μ-η2:η2-O2) to the Cu2(μ-O)2 isomers of 1 and 2 are +1 and...

Mechanistic Studies of Photoacid Generation from Substituted 4,6-Bis(trichloromethyl)-1,3,5-triazines

Abstract: The photochemistry and photophysics of 2-methyl- (1), 2-(2‘-furylethylidene)- (2), and 2-[(4‘-methoxy)styryl]-4,6-bis(trichloromethyl)-1,3,5-triazine (3), three compounds that find application as photoacid generators in photoresist formulations, have been investigated under conditions of direct excitation and using various phenothiazine derivatives as photosensitizers. C−Cl bond cleavage is confirmed by laser flash photolysis as the primary photochemical step in the direct photolysis of these compounds; the chlorine atoms formed in this reaction can be detected by complexation with chloride anions or benzene. In the case of 1, this photodissociation occurs with a lifetime of <20 ns. The quantum yield of bond breaking for 1 after direct excitation as determined by laser flash photolysis and steady-state irradiations was found to be 30−40 times greater than for 2 and 3. The mechanism under sensitized conditions involves electron transfer from the excited phenothiazines to the triazines as the primary photoc...

Carbon−Oxygen Bond Strength in Diphenyl Ether and Phenyl Vinyl Ether: An Experimental and Computational Study

Abstract: The thermal decomposition of gaseous diphenyl ether (DPE) and phenyl vinyl ether (PVE) has been studied, at atmospheric pressure in hydrogen and in a very low-pressure reactor, over a temperature range of 1050−1200 K. The high-pressure rate constant for homolytic bond cleavage C6H5O−C6H5 → C6H5O• + C6H5• (1) obeys k1 (s-1) = 1015.50 exp(−75.7/RT). Two pathways can be distinguished for C6H5OC2H3: C6H5• + C2H3O• (2) and C6H5O• + C2H3• (3). The overall rate constant follows k2+3 (s-1) = 1015.50 exp(−73.3/RT). The rate ratio, v2/v3, amounts to 1.8 and appears to be temperature independent. These findings result in bond dissociation energies (BDE) at 298 K for C6H5O−C6H5, C6H5−OC2H3, and C6H5O−C2H3 of 78.8, 75.9, and 76.0 kcal mol-1, respectively. The enthalpies for reactions 1−3 have been also determined at 298 and 1130 K by ab-initio calculations using the density functional theory formalism on the B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) level. Comparison between experiments and theoretical calculations reve...

Oxidation Mechanism of NAD Dimer Model Compounds

Abstract: The oxidation of a dimeric N-benzyldihydronicotinamide with various oxidants such as quinones, triphenyl carbenium ions and a triplet exited tris(bipyridine) ruthenium(II) complex occurs via initial outer-sphere electron transfer followed by fast C–C bond cleavage and second electron transfer. The kinetic studies allow the determination of the oxidation potential of this compound.

Degradation of multiply-chlorinated hydrocarbons on Cu(100)

Abstract: Dechlorination and hydrodechlorination of multiply-chlorinated ethanes and propanes on a clean Cu(100) surface have been studied by Auger electron spectroscopy, temperature-programmed desorption, and chemical displacement measurements. The rate-limiting step in degradation is dissociation of the first C−Cl bond in the molecule, and this process is more facile in CCl2 groups than in CCl groups. The activation energies for C−Cl bond scission on Cu(100) are 12−20% of the gas phase bond dissociation energies, and the extent of dissociation by the physisorbed molecules is a sensitive function of the relative rates of C−Cl bond scission and molecular desorption. The chlorinated hydrocarbon fragments generated on the surface by C−Cl bond cleavage undergo facile α- or β-chlorine elimination while all C−H bonds remain intact. β-Chlorine elimination dominates in cases where chlorine is present at both the α- and β-positions, and as a result of β-chlorine elimination, alkenes are generated and evolved to the gas pha...