Solvolysis of N-nitroso-N-(1-acetoxyalkyl)alkylamines in phosphate buffer : Characterization and mutagenicity of N-nitroso-N-(1-phosphonooxyalkyl)alkylamines.
“…α-Hydroxydialkylnitrosamines subsequently give rise to electrophiles that covalently modify DNA. The mechanism by which α-acetoxynitrosamines decompose has been the subject of differing reports, − but it has most recently been claimed that the simplest acyclic members of the family undergo S N 1 solvolysis to give N -nitrosiminium ions that subsequently hydrate, as in eq 2. …”
Section: Introductionmentioning
confidence: 99%
“…Mainly on the basis of differences in the rates of hydrolysis, the mechanism of eq 3the more “typical” carbonyl attack mechanism of ester hydrolysishas been proposed in the case of α-acetoxynitrosopyrrolidine . A mixture of the two mechanisms, eqs 2 and 3, has also been suggested …”
A study of the kinetics and mechanism of the decay of R-acetoxy-N-nitrosopyrrolidine and R-acetoxy-N-nitrosopiperidine are reported. The compounds differ in reactivity by more than 2 orders of magnitude at physiological pH. On the basis of thermodynamic parameters, common ion inhibition and azide ion trapping experiments, both compounds appear to decompose under these conditions by the formation of N-nitrosiminium ion intermediates. The differences in reactivity are rationalized on the basis of results from an ab initio study, described in the accompanying paper. The first direct study of the kinetics of decay of cyclic R-hydroxynitrosamines of nitrosopiperidine and nitrosopyrrolidine and a third compound, 2-hydroxy-2-methylnitrosopyrrolidine is also summarized. These prove to be highly unstable reactive intermediates, in contrast to what might be expected on the basis of earlier reports concerning cyclic R-hydroxynitrosamines.
“…α-Hydroxydialkylnitrosamines subsequently give rise to electrophiles that covalently modify DNA. The mechanism by which α-acetoxynitrosamines decompose has been the subject of differing reports, − but it has most recently been claimed that the simplest acyclic members of the family undergo S N 1 solvolysis to give N -nitrosiminium ions that subsequently hydrate, as in eq 2. …”
Section: Introductionmentioning
confidence: 99%
“…Mainly on the basis of differences in the rates of hydrolysis, the mechanism of eq 3the more “typical” carbonyl attack mechanism of ester hydrolysishas been proposed in the case of α-acetoxynitrosopyrrolidine . A mixture of the two mechanisms, eqs 2 and 3, has also been suggested …”
A study of the kinetics and mechanism of the decay of R-acetoxy-N-nitrosopyrrolidine and R-acetoxy-N-nitrosopiperidine are reported. The compounds differ in reactivity by more than 2 orders of magnitude at physiological pH. On the basis of thermodynamic parameters, common ion inhibition and azide ion trapping experiments, both compounds appear to decompose under these conditions by the formation of N-nitrosiminium ion intermediates. The differences in reactivity are rationalized on the basis of results from an ab initio study, described in the accompanying paper. The first direct study of the kinetics of decay of cyclic R-hydroxynitrosamines of nitrosopiperidine and nitrosopyrrolidine and a third compound, 2-hydroxy-2-methylnitrosopyrrolidine is also summarized. These prove to be highly unstable reactive intermediates, in contrast to what might be expected on the basis of earlier reports concerning cyclic R-hydroxynitrosamines.
“…The acid- and base-catalyzed reactions of α-acetoxynitrosamines have been less well studied, but here too, the morpholine derivative does not seem unusual. The observation that acid catalyzes group exchange α to the ring nitrogen suggests leaving-group protonation as in the mechanism of eq 4. − The morpholine derivative is more sluggish than the piperidine derivative, consistent with the diminution of equilibrium constant for protonation and the increase in the barrier for formation of the N -nitrosiminium ion, because of the electronegative oxygen atom in the morpholine ring. There is more evidence that the mechanism of the k OH reaction entails carbonyl group attack by hydroxide ion .…”
[reaction: see text] Alpha-acetoxy-N-nitrosomorpholine (7) has been synthesized starting by the anodic oxidation of N-acetylmorpholine in methanol. The 55% yield of N-nitrosomorpholinic acid, after cyanide-for-methoxy group exchange and hydrolysis, is an improvement of approximately 10-fold over our original 10-step method, and this is readily converted to 7. A study of the kinetics of decomposition of 7 in aqueous media at 25 degrees C and 1 M ionic strength was conducted over the pH range from 1 to 12. The reaction exhibited good first-order kinetics at all values of pH, and a plot of the log of k0, the buffer-independent rate constant for decomposition, against pH indicated that a pH-independent reaction dominates in the neutral pH region whereas acid- and base-catalyzed reactions dominate in the low and high pH regions, respectively. Reaction at neutral pH in the presence of increasing concentrations of acetate ion results in a decrease in the value of k(obsd), to an apparent limiting value consistent with a common-ion inhibition by the capture, and competing base-catalyzed hydration of, an N-nitrosiminium ion intermediate. The 100-fold smaller reactivity of 7 at neutral pH compared with its carbon analogue, alpha-acetoxy-N-nitrosopiperidine, is also consistent with the electronic effects expected for such a reaction. The dinitrophenylhydrazones derived from pH-independent and acid-catalyzed reactions are identical in kind and quantity, within experimental error, to those observed in the decay of alpha-hydroxy-N-nitrosomorpholine. Decay of 7 in the presence of benzimidazole buffer results in the formation of 2-(2-(1H-benzo[d]imidazol-1-yl)ethoxy)acetaldehyde (12) and 2-(1H-benzo[d]imidazol-1-yl)ethanol (13). Independent synthesis and study of 12 indicates that it is stable at 80 degrees C in 0.1 M DCl, but it slowly decomposes to 13 in neutral and basic media in a reaction that is stimulated by primary and secondary amines, but not by tertiary amines and carbonate buffer. The benzimidazole trapping studies and those of the stability of 12 indicate the possibility that metabolic activation of N-nitrosomorpholine by hydroxylation alpha to the nitroso nitrogen can result in the deposition of a metastable ethoxyacetaldehyde adduct on the heteroatoms of DNA.
“…was purchased from Kanto Chemicals (Tokyo). The authentic N-nitroso-1-phosphonooxypyrrolidine (NPYR-phosphate) disodium salt was synthesized by the method of Mochizuki et al [1986]. Other reagents were commercial products of reagent grade.…”
It is known that N-nitrosopyrrolidine (NPYR), a carcinogen in rodents, is metabolically activated by microsomal cytochrome P450 to form an alpha-hydroxylated derivative, which induces mutations. The mutations have been demonstrated by use of Salmonella typhimurium and Escherichia coli. We discovered directly acting mutagenicity of NPYR plus ultraviolet light-A (UVA) in bacteria and phage. With an O(6)-alkyltransferase-deficient strain of S.typhimurium, the NPYR plus UVA treatment gave greater mutation frequencies compared to those found with the parent strain. We identified the structure of the direct-acting mutagen as N-nitroso-1-phosphonooxypyrrolidine, and analyzed the spectrum of mutations induced in the DNA of M13mp2 phage. The basepair substitutions GC to TA and GC to AT appear to occur predominantly. Several hotspots were observed. In the conditions where SOS response was induced in the host E.coli, greater varieties of mutations were observed in phage DNA compared to those without the SOS response induction. These results suggest that alkylations of DNA occur by the photoactivated NPYR. The roles of the produced damage to the mutations are discussed.
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