Reaction of arylnitrenium ions with guanine derivatives: N1-methylguanosine and N2,N2-dimethylguanosine

2001 ◽  
Vol 79 (12) ◽  
pp. 1881-1886 ◽  
Author(s):  
Bernice Cheng ◽  
Robert A McClelland
Keyword(s):  
Rate Constants ◽  
Flash Photolysis ◽  
Initial Step ◽  
Direct Reaction ◽  
Ph Profile ◽  
Nitrenium Ion ◽  
Base Form ◽  
Guanine Derivatives ◽  
Arylnitrenium Ions ◽  
Conjugate Base

A prior flash photolysis study of the direct reaction of arylnitrenium ions with 2'-deoxyguanosine identified a second intermediate that grew in as the transient nitrenium ion reacted with the nucleoside. This intermediate was identified as the the product of the addition of the nitrenium ion to the C-8 position of guanine prior to loss of the C-8 proton — the C-8 intermediate. A feature of the C-8 intermediate is that it exists in acid–base forms. This behavior was evident in both a spectroscopic analysis as well as in the rate–pH profile, which showed a break around pH 4 from a pH-independent reaction to a reaction that was first-order in H+. The present study was designed to identify the structure of the conjugate base form. This involved a kinetic study of the decay of the C-8 intermediate derived from the reaction of the 2-fluorenylnitrenium ion with N1-methylguanosine and N2,N2-dimethylguanosine. The rationale was that the former is unable to lose the N-1 proton, while the latter cannot deprotonate at the NH2 group. The rate–pH profiles clearly show that it is the N-1 proton that is acidic. The rate constants for the C-8 intermediate of N2,N2-dimethylguanosine show the same downward break observed with 2'-deoxyguanosine and guanosine associated with conversion to the conjugate base form. In contrast, the rate constants for the N1-methylguanosine intermediate are independent of pH. Rate constants for the reaction forming the C-8 intermediate are also reported. These show that the reaction of nitrenium ions with the N2,N2-dimethylguanine derivative is significantly faster (except where the reactions are diffusion controlled). This is consistent with the initial step of the reaction of an arylnitrenium ion and guanine occurring by direct addition at C-8. The developing positive charge in such a reaction can be delocalized to the C-2 position where π donors such as NH2 and NMe2 can exert a stabilizing effect.Key words: nitrenium, arylnitrenium, guanosine, DNA adduct.

Kinetic stabilizing effect of the 4-N-methylacetamido substituent on the phenylnitrenium ion

2001 ◽  
Vol 79 (12) ◽  
pp. 1875-1880 ◽  
Author(s):  
Patrick H Ruane ◽  
Robert A McClelland
Keyword(s):  
Positive Charge ◽  
Flash Photolysis ◽  
Ph Profile ◽  
Phenyl Azide ◽  
Acidic Solutions ◽  
Nitrenium Ion ◽  
Azide Ion ◽  
Methoxy Substituent ◽  
Long Lifetime ◽  
Arylnitrenium Ions

Photolysis of 4-(N-methylacetamido)phenyl azide in aqueous solution results in quantitative formation of the 4-(N-methylacetamido)phenylnitrenium ion, this cation arising from solvent protonation of an initially formed singlet arylnitrene. The cation is observed by flash photolysis, and is identified through characteristic quenching by azide ion and by 2'-deoxyguanosine, both excellent nucleophiles for arylnitrenium ions in water. The nitrenium ion is protonated in acidic solutions to form the 4-(N-methylacetamido)aniline dication, whose pKa is determined to be 1.5 based on the rate–pH profile. This means that the nitrenium ion is relatively basic, which suggests that there is significant positive charge on the N-methylacetamido group. Further evidence for this is seen in the remarkably long lifetime (5 ms) of the nitrenium ion in water. In fact, the 4-(N-methylacetamido)phenylnitrenium ion is 5000-fold longer-lived than the 4-methoxy-substituted analog. A 4-methoxy substituent on a phenyl ring is more electron donating according to σ+ values (–0.78 for MeO vs. –0.60 for NMeAc). The dramatic reversal in the arylnitrenium ions is another example of the failure of these to follow the carbocation scale.Key words: nitrenium ion, flash photolysis, amide substituent.

1997 Alfred Bader Award Lecture Reactivities of arylnitrenium ions with guanine derivatives and other nucleophiles

1998 ◽  
Vol 76 (10) ◽  
pp. 1327-1337 ◽  
Author(s):  
Robert A McClelland ◽  
Timothy A Gadosy ◽  
Daniel Ren
Keyword(s):  
Rate Constants ◽  
Flash Photolysis ◽  
Lone Pair ◽  
Primary Amines ◽  
Diffusion Limit ◽  
Nitrenium Ions ◽  
Carbenium Ions ◽  
Alkyl Amines ◽  
Guanine Derivatives ◽  
Arylnitrenium Ions

The carcinogens 4-aminobiphenyl and 2-aminofluorene are metabolized to hydroxylamine esters that undergo N-O heterolysis to produce arylnitrenium ions that react with DNA, especially at guanine residues. These nitrenium ions and a number of their derivatives have been studied by the laser flash photolysis technique, to provide direct kinetic information about the lifetimes of the electrophiles in water and their reactivities with added nucleophiles. Arylnitrenium ions ArNH+ are longer-lived in water than arylcarbenium analogs ArCH2+, in some cases significantly longer-lived. The nitrenium ions do react with azide ion at the diffusion limit (providing the cation is not highly stabilized). This behaviour completely parallels that of carbenium ions. The biphenylyl- and fluorenylnitrenium ions react with guanine derivatives such as 2-deoxyguanosine (dG) with rate constants that are close to or at the diffusion limit (2 × 109 M-1 s-1) for the more reactive cations. Thus, in spite of cation lifetimes of the order of 100 ns to a millisecond in water, dG effectively competes with the solvent. The product is a C8 adduct, the same adduct observed with carcinogenic arylamines and DNA. With delocalized carbenium ions that have similar lifetimes, guanine derivatives compete very poorly with water. Thus, arylnitrenium ions have high dG:water selectivities; arylcarbenium ions have low selectivities. Nitrenium ions and carbenium ions do have parallel reactivities with primary alkyl amines. More reactive cations show a greater reactivity with less basic amines and the rate constants level below the diffusion limit. This can be explained by hydrogen bonding of the amine lone pair. Using the NH2 group of the alkyl amines as a model for the C2-NH2 group of guanine shows why nitrenium ions show no detectable reactivity at this site. The rate constant for the nitrenium-guanine reaction that forms the C8 adduct is at least an order of magnitude greater than that of a nitrenium-NH2 reaction. Nitrenium ions do form a guanine-NH2 adduct in DNA, suggesting that incorporation into the polymer changes reactivity patterns. With imidazoles, nitrenium ions show reactivity trends that parallel the nitrenium-dG reaction, with rate constants levelling at the 2 × 109 limit for the more reactive cations. Imidazole itself and 1-methylimidazole are generally less reactive than dG, while 2-methylimidazole and 1,2-dimethylimidazole are very similar. A Brönsted-like plot incorporating points for dG, primary amines and imidazoles shows no correlation of nitrenium rate constants with nitrogen basicity. This is true even if only dG and imidazoles are considered. Thus a previous correlation of nitrenium reactivity with purine N7 basicity is suspect. The conclusion is that there is some feature of guanine that makes its reaction with nitrenium ions unusually fast. The reasons for this are not immediately apparent, especially since there is conflicting evidence as to the detailed nature of the mechanism of the reaction that forms the C8 adduct. Key words: nitrenium, aryl azide, guanine, DNA, carcinogen.

Flash photolysis production of the tetrahedral intermediate of the methanolysis of methyl benzoate. Direct measurement of the rate of breakdown of the conjugate base form

1990 ◽  
Vol 68 (3) ◽  
pp. 375-382 ◽  
Author(s):  
Robert A. McClelland ◽  
V. M. Kanagasabapathy ◽  
Steen Steenken
Keyword(s):  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Carbonyl Oxygen ◽  
Methyl Benzoate ◽  
Ion Concentration ◽  
Hydroxide Ion ◽  
Tetrahedral Intermediate ◽  
Conjugate Base

Laser flash photolysis in aqueous basic solutions of the ortho acid derivatives 1-(phenyldimethoxymethyl)benzimidazole 2 and 4-bromo-1-(phenyldimethoxymethyl)imidazole 3 results in production of the phenyldimethoxymethyl cation, which has λmax at 260 nm. The cation decays in reactions with water (k = 9.9 × 104 s−1) and hydroxide ion (2.5 × 108 M−1 s−1) to finally yield methyl benzoate, whose formation was monitored at 234 nm. In solutions with pH 10–12, rate constants measured at this wavelength are the same as those obtained at 260 nm, but with pH > 13 and pH < 9, rate constants at 234 nm are smaller. With pH 9–10 and pH 12–13, single exponential kinetics are not observed at 234 nm. This behavior is interpreted in terms of a scheme where at each pH there are two consecutive first-order reactions, cation → phenyldimethoxyhydroxymethane (5) → ester, and the pH dependencies of the rate constants are such that they cross twice over the pH range of this study. The intermediate 5 is the tetrahedral intermediate formed in the methanolysis of methyl benzoate, and the 234-nm buildup at pH > 13 and pH < 9 directly measures its breakdown. At pH > 13 the rate constant is independent of pH with k = 9 × 106 s−1. This represents the rapid expulsion of methoxide from the conjugate base of 5. At pH < 9 the rate constants are proportional to hydroxide ion concentration, with [Formula: see text]. In these solutions the neutral intermediate predominates and the dependence on [OH−] of its rate of conversion to ester is interpreted in terms of breakdown of the anion and protonation of this species by water occurring at comparable rates. Thus, [Formula: see text] represents a situation where there is partial rate-limiting deprotonation of the neutral intermediate by hydroxide. The intermediate of this study bears a close resemblance to the tetrahedral intermediate of the hydrolysis of methyl benzoate. The observation that the anionic forms of such intermediates undergo breakdown at rates similar to those associated with the establishment of proton transfer equilibrium explains why the ester undergoes carbonyl oxygen exchange in base at a rate slower than hydrolysis. Keywords: tetrahedral intermediate, flash photolysis, ester hydrolysis.

Singlet Methylene Removal Rate Constants from the (0,1,0) Vibrational Level: Enhancement via Complex-Mediated Vibrational Relaxation

2001 ◽  
Vol 215 (6) ◽  
Author(s):  
M.A. Buntine ◽  
G.J. Gutsche ◽  
W.S. Staker ◽  
M.W. Heaven ◽  
K.D. King ◽  
...  
Keyword(s):  
Vibrational Level ◽  
Rate Constants ◽  
Vibrational Relaxation ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Removal Rate ◽  
Laser Flash ◽  
Laser Absorption ◽  
Photolysis Laser ◽  
Singlet Methylene

The technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene,

Remarkable kinetic stability of α-thiocarbamoyl substituted 4-methoxybenzylcations

1998 ◽  
Vol 76 (12) ◽  
pp. 1910-1915 ◽  
Author(s):  
Robert A McClelland ◽  
Victoria E Licence ◽  
John P Richard ◽  
Kathleen B Williams ◽  
Shrong-Shi Lin
Keyword(s):  
Key Words ◽  
Strong Interaction ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Visible Absorption ◽  
Methyl Group ◽  
Thioamide Group ◽  
Azide Ion ◽  
Uv Visible ◽  
Leaving Groups

4-Methoxybenzyl cations bearing α-(N,N-dimethylcarbamoyl) and α-(N,N-dimethylthiocarbamoyl) substituents have been generated photochemically upon irradiation of precursors with pentafluorobenzoate or 4-methoxybenzoate leaving groups. The ions have been observed with flash photolysis in 40:60 acetonitrile:water and in 50:50 methanol:water, and rate constants were measured for their decay in solvent alone and for their capture by azide ion. The cations so studied and their lifetimes in 40% acetonitrile are 6, ArC+H-CONMe2, 0.6 μs; 2, ArC+H-CSNMe2, 7 ms; and 4, ArC+(CH3)-CSMe2, 6 ms, where Ar = 4-MeOC6H4. The cation 4 reacts with solvent by elimination of a proton from the α-methyl group, and the rate constant for solvent addition must be less than 1 s-1. The CSNMe2 substituted cations are 105-107-fold longer lived than analogs where the thioamide group has been replaced with an α-methyl. The UV-visible absorption spectra of these two cations also show significant differences from those of typical 4-methoxybenzyl cations. Thus, both the lifetimes and spectra point to a strong interaction of the benzylic centre with the thioamide group. Key words: flash photolysis, thiocarbamoyl stabilized carbocation, photosolvolysis.

Mechanistic studies of aromatic ketone-sensitized photoreduction of bis(acetylacetonato)copper(II)

1983 ◽  
Vol 61 (5) ◽  
pp. 801-808 ◽  
Author(s):  
Yuan L. Chow ◽  
Gonzalo E. Buono-Core ◽  
Bronislaw Marciniak ◽  
Carol Beddard
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Decay Rates ◽  
Polynuclear Aromatic Hydrocarbons ◽  
Aromatic Ketone ◽  
Quantum Yields ◽  
Triplet States ◽  
Quenching Rate ◽  
Triplet Energy

Bis(acetylacetonato)copper(II), Cu(acac)2, quenches triplet excited states of ketones and polynuclear aromatic hydrocarbons efficiently, but only aromatic ketones with high triplet energy successfully sensitize photoreduction of Cu(acac)2 in alcohols under nitrogen to give derivatives of aeetylacetonatocopper(I), Cu(acac). For the triplet state benzophenone-sensitized photoreduction of Cu(acac)2, the quantum yields of photoreduction (ΦC) and those of benzophenone disappearance (ΦB) were determined in methanol with various concentrations of Cu(acac)2. The values of the quenching rate constant, kq, determined from these two types of monitors on the basis of the proposed mechanism were in good agreement (6.89 ~ 7.35 × 109 M−1 s−1). This value was higher, by a factor of about two, than that obtained from the monitor of the benzophenone triplet decay rates generated by flash photolysis in the presence of Cu(acac)2. The quenching rate constants of various aromatic ketone and hydrocarbon triplet states by Cu(acac)2 were determined by flash photolysis to be in the order of the diffusion rate constant and the quantum yields of these photoreductions were found to be far from unity. Paramagnetic quenching, with contributions of electron exchange and charge transfer, was proposed as a possible quenching mechanism. For a series of aromatic ketone sensitizers with higher triplet energy, this mechanism was used to rationalize the observed high quenching rate constants in contrast to the low quantum yields of photoreduction.

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