37574-48-4Relevant academic research and scientific papers
Influence of Na+ on DNA reactions with aromatic epoxides and diol epoxides: Evidence that DNA catalyzes the formation of benzo[a]pyrene and benz[a]anthracene adducts at intercalation sites
Fernando, Harshica,Huang, Chao-Ran,Milliman, Ann,Shu, Luchuan,LeBreton, Pierre R.
, p. 1391 - 1402 (2007/10/03)
Reactions of the benzo[a]pyrene (BP) and benz[a]anthracene (BA) metabolites, (±)-trans-7, 8-dihydroxy-anti-9, 10-epoxy-7, 8, 9, 10- tetrahydro-BP (BPDE), (±)-trans-3, 4-dihydroxy-anti-1, 2-epoxy-1, 2, 3, 4- tetrahydro-BA (BADE), (±)-BP-4, 5-oxide (BPO), and (±)-BA-5, 6-oxide (BAO), were examined under pseudo-first-order conditions at varying Na+ (2.0-100 mM) and native, calf thymus DNA (ctDNA) concentrations. In 0.2 mM ctDNA and 2.0 mM Na+, at a pH of 7.3, most BPDE, BADE, BPO, and BAO (87-95%) undergo DNA catalyzed hydrolysis or rearrangement. For BPDE and BPO, overall, pseudo- first-order rate constants, k, in 2.0 mM Na+ and 0.2 mM ctDNA are 21-72 times larger than values obtained without DNA. For BADE and BAO, the rate constants are less strongly influenced by DNA; k values in 0.2 mM ctDNA are only 9-12 times larger than values obtained without DNA. Kinetic data for BPDE, BPO, BADE, and BAO and DNA intercalation association constants (K(A)) for BP and BA diols which are model compounds indicate that K(A) values for BPDE and BPO in 2.0 mM Na+ are 6.6-59 times larger than those of BADE and BAO. The greater DNA enhancement of rate constants for BPDE and BPO, versus BADE and BAO, correlates with the larger K(A) values of the BP metabolites. DNA adducts, which account for less than 10% of the yields, also form. For BPDE in 0.20 mM ctDNA, k decreases 5.1 times as the Na+ concentration increases from 2.0 to 100 mM. Nevertheless, the DNA adduct level remains constant over the range of Na+ concentrations examined. These results provide evidence that, for BPDE in 0.20 mM DNA and 2.0 mM Na+, ctDNA adduct formation follows a mechanism which is similar to that for DNA catalyzed hydrolysis. The pseudo-first-order rate constant for adduct formation, k(Ad), given approximately by k(Ad) ? (k(cat, Ad)K(A)[DNA])/(1 + K(A)[DNA]), where k(cat, Ad) is a catalytic rate constant. For BADE, BPO, and BAO, the influence of varying DNA and Na+ concentrations on k values is similar to that for BPDE, and provides evidence that the formation of adducts follows the same rate law.
Photoemission probes of catalysis of benzo[a]pyrene epoxide reactions in complexes with linear, double-stranded and closed-circular, single-stranded DNA
Huang, Chao-Ran,Milliman, Ann,Price, Harry L.,Urano, Shigeyuki,Fetzer, Sharon M.,LeBreton, Pierre R.
, p. 7794 - 7805 (2007/10/02)
Fluorescence intensity measurements of overall, pseudo-first-order rate constants for two epoxide-containing metabolites of benzo[a]pyrene (BP) were carried out in Tris, EDTA buffer (pH 7.3) without DNA, and in buffer with double-stranded calf thymus DNA (DS ctDNA) and with closed-circular, single-stranded viral M13mp19 DNA (SS M13 DNA). Highly purified SS M13 DNA was employed in order to avoid polymeric contamination which is present in DNA samples obtained using a standard preparation method relying on phenol extraction and which influences results from measurements of DNA-ligand interactions. The BP metabolites examined were highly carcinogenic (±)-trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[e]pyrene (BPDE) and less genotoxic benzo[a]pyrene 4,5-oxide (BPO). Without DNA, BPDE hydrolyzes to 7,8,9,10-tetrahydroxytetrahydro-BP, while BPO hydrolyzes to trans-4,5-dihydroxy-4,5-dihydro-BP (BP45D) and rearranges to 4-hydroxy-BP. With DNA, BPDE and BPO hydrolysis and rearrangement are catalyzed, and DNA modification occurs. In DS ctDNA, previous kinetic and binding measurements indicate that catalysis occurs primarily at intercalation sites. In SS M13 DNA (0.20 mM), BPDE has overall, pseudo-first-order rate constants (k) of (12 ± 1) × 10-3 and (2.8 ± 0.5) × 10-3 s-1, at Na+ concentrations of 2.0 and 100 mM, respectively. At these Na+ concentrations, values of k measured in SS M13 DNA are 3-16 times larger than values measured without DNA, but smaller than values measured in DS ctDNA. For BPO, the ordering of k values without DNA, with SS M13 DNA, and with DS ctDNA is the same as for BPDE. At 2.0 mM Na+, the nonreactive diols, trans-7,8-dihydroxy-7,8-dihydro-BP (BP78D) and BP45D, which are model compounds for BPDE and BPO, respectively, have SS M13 DNA association constants [(7.2 ± 0.5) × 103 and (2.7 ± 0.5) × 103 M-1] that are 2.3 times smaller than DS ctDNA association constants. In contrast, at 100 mM Na+, association constants for SS M13 DNA are 2.9-3.1 times larger than for DS ctDNA. Fluorescence lifetime measurements indicate that, in SS M13 DNA, reversible binding involves intercalation into local duplex regions. Estimated catalytic rate constants (kcat) for BPDE hydrolysis in SS M13 DNA, obtained from BP78D association constants and from k values measured with and without DNA, are (22.8 ± 2.5) × 10-3 and (3.5 ± 0.7) × 10-3 s-1, at 2.0 and 100 mM Na+, respectively. For this Na+ concentration range, the ratio of kcat values for DS ctDNA versus SS M13 DNA is almost constant (1.7 ± 0.6) even though the absolute kcat values vary by more than a factor of 5. The similar magnitudes of kcat values for SS M13 DNA and DS ctDNA provide evidence that catalytic sites in SS M13 DNA are similar to intercalated catalytic sites in DS ctDNA.
Acid-catalyzed rearrangement of K-region arene oxides: Observation of ketone intermediates and a sterically induced change in rate-determining step
Nashed, Nashaat T.,Sayer, Jane M.,Jerina, Donald M.
, p. 1723 - 1730 (2007/10/02)
K-region arene oxides rearrange to phenols in acetonitrile in two acid-catalyzed steps: rapid rearrangement of the arene oxide to positionally isomeric keto tautomers of K-region phenols, followed by slow enolization. Accumulation of the ketones, proposed intermediates in the acid-catalyzed solvolyses of arene oxides in aqueous solution, allowed their direct spectroscopic observation and characterization for the first time under solvolytic conditions. Rate constants and products are reported for the K-region arene oxides of benz[a]anthracene, its 1-, 4-, 7-, 11-, 12-methyl, and 7,12-dimethyl substituted derivatives, benzo[a]pyrene, benzo[c]phenanthrene, 3-bromophenanthrene, chrysene, dibenz[a,h]anthracene, phenanthrene, and pyrene. No primary kinetic isotope effect is observed for ketone formation from phenanthrene 9,10-oxide. A linear correlation with a slope of 1.07 is observed between the logarithm of the second-order rate constants for acid-catalyzed reaction of the arene oxide at each K-region position in acetonitrile (first step) and in methanol (where ketone does not accumulate). Negative deviations from this correlation are observed for the formation of ketones in which the carbonyl oxygen is peri to a methyl substituent. These results are discussed in terms of a mechanism in which pseudoaxial opening of the epoxide gives an initial carbocation that must undergo conformational isomerization in order to produce phenolic products by migration of a pseudoaxial hydrogen. For compounds that follow the correlation, the rate-determining step in both methanol and acetonitrile is formation of the carbocation. For those compounds that deviate from the correlation and whose carbocations have their hydroxyl group in a peri position to a methyl ring substituent, the rate-determining step changes from formation of the carbocation (methanol) to its conformational inversion (acetonitrile). With few exceptions, NMR and kinetic evidence show that the regioisomeric K-region keto tautomers from a given arene oxide enolize with very similar rates (kslow). Rates of enolization are decreased by electron withdrawing groups and by steric factors that favor nonplanarity of the ring system. A large primary kinetic isotope effect (kH/kD = 4.4) is observed for the acid-catalyzed enolization of the K-region ketone derived from phenanthrene. Slow abstraction of a proton by the solvent acetonitrile from the α-methylene group of the O-protonated ketone is proposed to account for these results and for the fact that ketone does not accumulate in more basic solvents. The major driving force for enolization (kslow) is development of aromaticity in the phenol. For unsubstituted keto tautomers, a linear relationship, log kslow = 31.8-39.2 (X), is observed, where X is the Hu?ckel π-bond character for the K-region bond of the parent hydrocarbon.
Methanolysis of K-region arene oxides: Comparison between acid-catalyzed and methoxide ion addition reactions
Nashed, Nashaat T.,Bax, Ad,Loncharich, Richard J.,Sayer, Jane M.,Jerina, Donald M.
, p. 1711 - 1722 (2007/10/02)
Reactions of K-region arene oxides of benz[a]anthracene (BA-O) and its 1-(1-MBA-O), 4- (4-MBA-O), 7-(7-MBA-O), 11- (11-MBA-O), 12-methyl- (12-MBA-O), 7,12-dimethyl-(DMBA-O), and 7-bromo- (7-BrBA-O) substituted derivatives, benzo[a]pyrene (BaP-O), benzo[c]phenanthrene (BcP-O), benzo[e]pyrene (BeP-O), benzo[g]chrysene (BgC-O), chrysene (Chr-O), dibenz[a,h]anthracene (DBA-O), phenanthrene (Phe-O), 3-bromophenanthrene (3-BrPhe-O), and pyrene (Pyr-O) with acid and methoxide ion in methanol, are compared. For the acid-catalyzed reaction, products consist of cis- and trans-methanol adducts and phenols. There is no isotope effect on the ratio of phenols to solvent adducts produced from Phe-O or BcP-O when deuterium is substituted for the hydrogen that migrates. This observation is consistent with a mechanism in which product distribution in acid is determined by the relative rates of solvent capture and conformational inversion of a carbocation intermediate. As expected, only trans-methanol adducts, consisting of regioisomeric mixtures for unsymmetrical arene oxides, are formed from the reaction of methoxide ion with K-region arene oxides. The use of methanol permits the identification of products formed at each benzylic position of unsymmetrical arene oxides. Rate data for reactivity at each position could be fitted to the equation log kMeO/kMeOPhe-O = m(log kH/kHPhe-O) + b, where kMeO and kH are the second-order rate constants of the methoxide ion addition and acid-catalyzed reaction, respectively, and kMeOPhe-O and kHPhe-O are the corresponding rate constants for the reference compound phenanthrene oxide. A plot of log kMeO/kMeOPhe-O vs log kH/kHPhe-O for the reaction of 1-MBA-O, 12-MBA-O, DMBA-O, BcP-O, and BgC-O, which have either a methyl substituent in the bay region or a sterically crowded fjord region which affects the planarity of the molecules, defined one line (m = 0.31 ± 0.02, b = 0.67 ± 0.09), whereas a plot of the data for the reaction of the nearly planar arene oxides BA-O, 4-MBA-O, 7-MBA-O, 11-MBA-O, BaP-O, BeP-O, Chr-O, DBA-O, Phe-O, and Pyr-O defined a different line (m = 0.33 ± 0.07, b = -0.05 ± 0.05). The nonzero intercept for the sterically crowded, nonplanar arene oxides indicates a steric acceleration of their rates of methoxide ion addition. The positive slopes of both lines are consistent with an SN2 mechanism with an unsymmetrical transition state in which the epoxide C-O bond breaking is more advanced than the formation of the new C-O bond to methoxide ion, such that a partial positive charge is developed on the aromatic moiety.
Photoemission probes of hydrocarbon-DNA interactions: A comparison of DNA influences on the reactivities of (±)-trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a] pyrene, benzo[a]pyrene 4,5-oxide, and benz[a]anthracene 5,6-oxide
Urano, Shigeyuki,Price, Harry L.,Fetzer, Sharon M.,Briedis, Anita V.,Milliman, Ann,LeBreton, Pierre R.
, p. 3881 - 3893 (2007/10/02)
Time-resolved fluorescence and UV photoelectron measurements have been employed to examine the influence of calf thymus DNA on the reactivities of epoxides derived from benzo[a]pyrene (BP) and benz[a]anthracene (BA). By monitoring the increase in fluorescence intensity, which accompanies reaction at 23°C, overall, pseudo-first-order rate constants have been measured for reactions of the highly carcinogenic bay region epoxide (±)-trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) and of two less carcinogenic K region epoxides benzo[a]pyrene 4,5-oxide (BPO) and benz[a]anthracene 5,6-oxide (BAO). Overall rate constants for hydrolysis and rearrangement reactions have been measured for BPDE, BPO, and BAO in buffer alone (1.0 mM sodium cacodylate, pH 7.1). The rate constants increase in the order BPO ((3.8 ± 0.1) × 10-6 s-1) -5 s-1) -4 s-1). These results have been compared with overall rate constants for reactions, carried out in calf thymus DNA, which result in catalyzed hydrolysis and rearrangement, as well as DNA adduct formation. In DNA, the ordering of the rate constants for BPO and BAO changes from that observed in buffer alone. The rate constants increase in the order BAO ((2.8 ± 0.1) × 10-3 s-1) -2 s-1) -1 s-1). This ordering is the same as the ordering of association constants for the reversible binding to DNA of the fluorescent diols trans-7,8-dihydroxy-7,8-dihydro-BP (BP78D), trans-4,5-dihydroxy-4,5-dihydro-BP (BP45D) and cis-5,6-dihydroxy-5,6-dihydro-BA (BAD), which are model compounds of BPDE, BPO, and BAO, respectively. For the model compounds, the association constants for intercalation increase in the order BAD ((3.6 ± 0.9(× 102 M-1) 3 M-1) 4 M-1). This ordering is consistent with the ordering of the association constants of BPDE ((2.5 ± 0.3) × 104 M-1) and of BPO ((6.0 ± 1.0) × 103 M-1). The temperature dependence of the association constants of the model compounds demonstrates that, for the intercalation of the BP diols into DNA, differences in the enthalpy of binding contribute significantly to differences in the free energy of binding. UV photoelectron data and results from ab initio molecular orbital calculations on BPDE, BPO, and BAO indicate that, for these three epoxides, the association constants increase as the ionization potentials decrease and the polarizabilities increase. The percentage of epoxide reaction that yields DNA adducts has been compared under varying conditions. For long reaction times (>1 h) in systems containing native, calf thymus DNA at low salt concentrations, the ordering of adduct yields is BPO (14.9 ± 1.1%) > BPDE (10.1 ± 3.0%) > BAO (3.6 ± 0.4%). For short reaction times (10 min) in systems containing native DNA stabilized with 0.10 mM Mg2+, the ordering of adduct yields is BPDE (7.3 ± 1.9%) > BPO (1.3 ± 0.1%) > BAO (0.1 ± 0.1%). These results suggest that the ability of an epoxide to form adducts with exposed DNA during long reaction times is less indicative of the genotoxic potency of the epoxide than its ability to form adducts with stabilized DNA during short reaction times.
Absolute Configurations of K-Region Epoxide Enantiomers of 3-Methylcholanthrene, Benzanthracene, and Benzopyrene
Weems, Henri B.,Mushtaq, Mohammad,Yang, Shen K.
, p. 2679 - 2688 (2007/10/02)
The absolute configurations of K-region epoxide enantiomers of 3-methylcholanthrene, benzanthracene, and benzopyrene have been determined via their monomethyl ether derivatives.Methoxylation of each racemic or enantiomeric epoxide by sodium methoxide resulted in a pair of monomethyl ether derivatives, which were separated by normal-phase high-performance liquid chromatography (HPLC).The position of the methoxy group was determined by products formed by acid-catalyzed dehydration and/or demethanolization of each monomethyl ether derivative.Enantiomers of each epoxide and its methoxylated derivatives were resolved by at least two of the four Pirkle chiral stationary phase HPLC columns utilized in this study.The absolute stereochemistries of enantiomeric monomethyl ether derivatives were established by comparing their circular dichroism spectra with those of enantiomeric monomethyl ether derivatives derived from trans dihydrodiol enantiomers of known absolute configurations.The absolute configuration of each epoxide enantiomer was deduced from the location of the methoxy group and the absolute configuration of enantiomeric monomethyl ether derivatives.Results indicate that the method described is useful in general for the determination of absolute configurations of K-region epoxide enantiomers of polycyclic aromatic hydrocarbons.
