V-Nitrosamines

V-Nitrosamines typically are light yellow volatile soHds or oils. The electron delocalization in the NNO functionaUty sufficiendy restricts rotation around the N—N bond that the E (4) and Z (5) isomers of unsymmetrically substituted examples can often be separated (43). 

The spectroscopic properties of the /V-nitrosamines, especially the nmr and mass spectra, vary widely depending on the substituents on the amine nitrogen (44—47). The nmr spectra are affected by the E—Z isomerism around the N—N partial double bond and by the axial—equatorial geometry resulting from conformational isomerism in the heterocycles (44,45). Some general spectral characteristics for typical dialkylnitrosamines and simple heterocycHc nitrosamines are given in Table 1. 

Synthesis. The classic laboratory synthesis of /V-nitrosamines is the reaction of a secondary amine with acidic nitrite [14797-65-0] at ca pH 3. The primary nitrosating intermediate is N2O2 arising from nitrous acid [7782-77-6] (48). 

Inhibition of nitrosation is generally accompHshed by substances that compete effectively for the active nitrosating iatermediate. /V-Nitrosamine formation in vitro can be inhibited by ascorbic acid [50-81-7] (vitamin C) and a-tocopherol [59-02-9] (vitamin E) (61,62), as well as by several other classes of compounds including pyrroles, phenols, and a2iridines (63—65). Inhibition of iatragastric nitrosation ia humans by ascorbic acid and by foods such as fmit and vegetable juices or food extracts has been reported ia several instances (26,66,67). 

Reactions. The chemistry of the /V-nitrosamines is extensive and will be only summarized here (8,35,42). Most of the reactions of the nitrosamines, with respect to thek biological or environmental behavior, involve one of two main reactive centers, either the nitroso group itself or the C—H bonds adjacent (a) to the amine nitrogen. The nitroso group can be removed readily by a reaction which is essentially the reverse of the nitrosation reaction, or by oxidation or reduction (68,69). 

The effects of uv radiation on V/-nitroso compounds depend on the pH and the medium. Under neutral conditions and ia the absence of radical scavengers, these compounds often appear chemically stable, although the E—Z equiUbrium, with respect to rotation around the N—N bond, can be affected (70). This apparent stabiUty is due to rapid recombination of aminyl radicals and nitric oxide [10102-43-9] formed duting photolysis. In the presence of radical scavengers nitrosamines decay rapidly (71). At lower pH, a variety of photoproducts are formed, including compounds attributed to photoelimination, photoreduction, and photo-oxidation (69). Low concentrations of most nitrosamines, even at neutral pH, can be eliminated by prolonged kradiation at 366 nm. This technique is used ki the identification of /V-nitrosamines that are present ki low concentrations ki complex mixtures (72). 

This property has been exploited in syntheses of /V-nitrosamine derivatives by the reaction of electrophiles (E) with a-Hthiated intermediates. These intermediates are prepared by hydrogen—Hthium exchange using Hthium dusopropylamide [4111-54-0] (LDA) (76,77). 

V-Nitrosamines are potentially hazardous and should be handled in designated hoods and with protective clothing. Nitrosamines can be destroyed by treatment with aluminum—nickel alloy under basic conditions (78,79). 

Toxicity. Many /V-nitrosamines are toxic to animals and cells in culture (4,6—8,88). /V-Nitrosodimethy1amine [62-75-9] (NDMA) is known to be acutely toxic to the Hver in humans, and exposure can result in death (89). Liver damage, diffuse bleeding, edema, and inflammation are toxic effects observed in humans as a result of acute and subacute exposure to NDMA. These effects closely resemble those observed in animals dosed with NDMA (89,90). 

Mutagenicity. The AJ-nitrosamines, in general, induce mutations in standard bacterial-tester strains (117). As with carcinogenicity, enzymatic activation, typically with Hver microsomal preparations, is required. Certain substituted A/-nitrosamine derivatives (12) induce mutations without microsomal activation (31,33,34). Because the a-acetoxy derivatives can hydroly2e to the corresponding a-hydroxy compounds, this is consistent with the hypothesis that enzymatic oxidation leads to the formation of such unstable a-hydroxy intermediates (13) (118). However, for simple /V-nitrosamines, no systematic relationship has been found between carcinogenicity and mutagenicity (117,119—123). 

Wet chemical methods determining titratable amine ate reported for products entering urethane (amine number as meq/g) or epoxy (AHEW = amine hydrogen equivalent weight) trade appHcations. For secondary amines /V-nitrosamine contaminants are reportable down to ppb using Thermoelectron Corporation thermal energy analy2er techniques. 

Secondary aromatic amines form /V-nitrosamines and tertiary aromatic amines undergo ring nitrosation to yield C-nitroso products. 

Inhibition of Nitrosamine Formation. Nitrites can react with secondary amines and A/-substituted amides under the acidic conditions of the stomach to form /V-nitrosamines and A/-nitrosamides. These compounds are collectively called N-nitroso compounds. There is strong circumstantial evidence that in vivo A/-nitroso compounds production contributes to the etiology of cancer of the stomach (135,136), esophagus (136,137), and nasopharynx (136,138). Ascorbic acid consumption is negatively correlated with the incidence of these cancers, due to ascorbic acid inhibition of in vivo A/-nitroso compound formation (139). The concentration of A/-nitroso compounds formed in the stomach depends on the nitrate and nitrite intake. 

Extensive developments in the preparation of the enantiomers of chiral nitroso and nitro compounds have not appeared since the earlier review1, and thus no additional discussion of the sources of nitroso and nitro compounds is made in this supplement. However, because of the continuing interest in carcinogenic properties of /V-nitrosamines and related compounds24, there has been a number of reports concerning the ECD and VCD of such compounds, and the ECD and VCD of nitrosamines are discussed as derivatives of chiral amines. 

Various gas chromatographic techniques combined with plentiful detection methods were used to separate and quantify volatile V-nitrosamines. Preconcentration methods were usually applied for separating these compounds. Thus, a method was developed for determination of V-nitrosodimethylamine (278a) in minced fish or frankfurters, based on SPE followed by GC-CLD-TEA RSD was 0.56 to 2.25%569. This method has been adopted by AOAC. A similar GC method using NPD was described for the determination of 278a in fish products570. Steam distillation can also be used to isolate volatile... 

The diffusion-limited electrochemical oxidation of V-nitrosamines in an aqueous pH 1.5 buffer was demonstrated at a GCE coated with a film of mixed valence ruthenium oxides, stabilized by cyano crosslinks. This electrode was used in a potentiostatic amperometric detector for FIA and HPLC, to allow the determination of representative N-nitrosamines (278a, 278c and 278d) for 278c, LOD was 10 nM and RSD 2% at 0.80 pM... 

Tobacco smoke and iV-nitrosation are the focus of intense research activity. Workers in the field use the following concepts Tobacco-specific /V-nitrosamines (TSNA) mainstream tobacco smoke (MSTS), smoke inhaled in a puff sidestream tobacco smoke (SSTS), smoke evolved by smoldering cigarettes between puffs nitroso organic compounds (NOC), referring especially to IV-nitrosamines volatile NOC (VNOC) and iV-nitroso amino acids (NAA). 

Hoffmann D, Djordjevic MV, Fan J, Zang E, Glynn T, Connolly GN (1995) Five leading U.S. commercial brands in moist snuff in 1994 assessment of carcinogenic V-nitrosamines. J Natl Cancer Inst 87 1862-1869... 

Berger, M.R., Schmahl, D. Zerban, H. (1987) Combination experiments with very low doses of three genotoxic V-nitrosamines with similar organotrophic carcinogenicity in rats. Carcinogenesis, 8, 1635-1643... 

Exercise 23-27 a. Write two valence-bond structures for /V-nitroso-/V-methylmethan-amine and show how these structures explain the fact that the /V-nitrosamine is a much weaker base than A/-methylmethanamine. 

Organic hydrazines or diazanes are substitution products of NH2—NH2 and have many properties similar to those of amines in being basic and forming acyl derivatives as well as undergoing alkylation and condensations with carbonyl compounds (Section 16-4C). Unsymmetrical hydrazines can be prepared by careful reduction of /V-nitrosamines. l,l-Dimethyldiazane is prepared in this way for use as a rocket fuel ... 

Broadly, NOC are divided into two groups (a) /V-nitrosamines and (b) N-nitrosamides. In jV-nitrosamines, the two remaining substituent groups on the >N—N=0 atom can be either alkyl or aryl, or they can form the part of a heterocyclic ring (as in N-nitrosopyrrolidine). On the other hand, in N-nitrosamides, more correctly called N-acyl-N-nitroso compounds, one of these... 

V-Nitrosamines undergo efficient photodecomposition, especially in acidic (pH < 4) aqueous solution. Polo and Chow (27) have proposed the following mechanism for photodecomposition of NDMA ... 

Recent advances in chromatography have made it possible to employ microbore HPLC for the determination of NOC. Its main advantage is that it uses a very low mobile-phase flow (20-100 /rl/min). This might make the TEA compatible with a reversed-phase system. Massey et al. (73), in fact, have successfully used reversed-phase chromatography for the HPLC-TEA determination of V-nitroso-V, 7V -di methylpiperazinium iodide. A 500-mm X 1-mm microbore ODS column and a mobile phase consisting of 0.1 M ammonium heptane-sulfonate in methanol water (70 30) (flow rate 20 /zl/min) was used for the HPLC separation. In another study, Riihl and Reusch (74) used a microbore Spherisorb 3 SW column for HPLC-TEA determination of volatile V-nitrosamines. The mobile phase was a mixture of 2-propanol and n-hexane (2.5 97.5). Further application of such techniques for the determination of various polar NOC, especially A-nitrosamides, in foods is desirable. 

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