1. Introduction
N-Nitrosamines are well-known pro-mutagens that can react with DNA following metabolism to produce DNA adducts, such as O
6-alkyl-guanine. These adducts can result in DNA replication miscoding errors, leading to GC > AT mutations and an increased risk of genomic instability and carcinogenesis [
1]. In 2018, N-nitrosodimethylamine (NDMA,
1), a genotoxic carcinogen, was detected as a synthesis impurity in some valsartan drugs, while other N-nitrosamines, such as N-nitrosodiethylamine (NDEA,
2), were later detected in other sartan products. In September 2019, the FDA stated that a low amount of NDMA had been detected in ranitidine. The FDA also announced that it had found excessive levels of NDMA in metformin in February 2022. Some N-nitrosamines, such as N-nitrososarcosine (NSAR,
3), N-nitrosomethylvinylamine (
4), 3-(methylnitrosamino)propionitrile (MNPN,
5), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK,
6), N-nitrosornicotine (NNN,
7) and N-methyl-N-nitrosourea (MNU,
8), occur not only in drugs but also in pickled foods and tobacco (
Figure 1). Therefore, controlling their concentrations in drugs, foods and tobacco is very important.
When we performed HPLC or GC analyses of certain asymmetrical N-nitrosamines, we often observed two peaks for one nitrosamine. This finding was attributed to the fact that asymmetrical N-nitrosamine may have configurational isomers due to the hindered rotation of a single bond (N‒N), resulting in strong variations in the anisotropic effects. The two conformers have features similar to those of the
E/
Z isomers relative to a double bond (
Figure 2). A similar phenomenon has been reported, in which the stereospecific response of the
E/Z isomers of NSAR (
3) was determined by LC–ESI–MS/MS [
2]. NSAR (
3) and MNPN (
5) have also been shown to produce two isomer peaks in the UPLC–MS/MS assay [
3]. In this paper, we report a series of asymmetrical N-nitroamines (
3–
7) displaying two groups of NMR signals. However, few studies have reported the NMR assignment of asymmetrical N-nitroamines isomers. Inspired by the above phenomenon, variable-temperature (VT)
1H-NMR experiments were carried out to determine the percentage changes of the two configurational isomers, which revealed the configurational isomerism phenomenon. As density functional theory (DFT) calculations are widely used to determine NMR assignments for the characterization of complex structures [
4,
5], we performed DFT calculations to assign the NMR signals of these conformers. To our knowledge, this is the first report of the NMR assignment of configurational isomers of N-nitrosamines.
2. Results and Discussion
As shown in
Figures S1–S12, the
1H-NMR spectrum of N-nitrososarcosine
3 showed one group of major signals (
δH 3.79 and 4.28) and a set of minor signals (
δH 3.01 and 5.01). In addition, the major carbon signals of
3 were observed at
δC 40.0, 47.3 and 167.6, and the minor signals were observed at
δH 33.0, 54.6 and 170.3. Similarly, the
13C-NMR spectrum of N-nitrososarcosine
4 showed two sets of different carbon signals. However, some of the
1H-NMR signals had differences, such as signals of ‒NCH
3 (
δH 3.15 vs.
δH 3.89) and H-1 (
δH 7.89 vs.
δH 7.58). Two conformers of
5 showed two groups of distinct 1D NMR signals, of which the maximum difference in the
1H-NMR and
13C-NMR spectra between the two isomers was 0.77 ppm for ‒NCH
3 (3.03 vs. 3.80 ppm) and 8.6 ppm for C-3 (49.1 vs. 40.5 ppm), respectively. Differences in the
1H-NMR spectra between the two isomers of
6 were present in the alkyl chain, including H-2, H-3 and H-4. Furthermore, the differences in their
13C-NMR spectra were associated with ‒NCH
3 and the chain from C-2 to C-4. Two groups of NMR signals in the spectrum of
7 can be easily distinguished. Above all, the major and minor signals were also assigned based on the peak integration (
Table 1 and
Table 2). Furthermore, the configurational exchange and conformer ratios of
3–
7 were investigated via a VT
1H-NMR experiment.
To further investigate the configurational behavior of asymmetrical N-nitrosamines
3–
7, DFT quantum chemical calculations were conducted [
6]. Because the hindered rotation of the nitryl formed
E and
Z configurations, resembling the
Z/
E isomers relative to the double bond, two configurational isomers (a/b) were converted to
Z/
E for further calculations (
Figure 2).
Compound
3 may contain 4 isomers
3a–
3d (
Figure 3). The DFT calculations showed that the Gibbs free energies of isomers
3c and
3d are higher than those of
3a and
3b (
Figure 3), suggesting that they are more unstable than
3a and
3b; thus, we mainly considered the contributions of
3a and
3b to the NMR data.
Compounds
4 and
7 have
sp2 CH or CH
2, which could affect the stability of the isomers. For compound
4, we considered four possible stable conformers, and their energies were calculated. As shown in
Figure 4, the interaction of N=O and
sp2 CH can be represented through the energy difference between E
1 and E
2. Similarly, the interaction between N=O and
sp2 CH
2 can be shown through the energy difference of E
4–E
1. In addition, the interaction between the nitrogen atoms of N=O and
sp2 CH
2 can be interpreted by the energy calculation of E
3 and E
1. On the basis of their energy differences, the closer
sp2 values of CH or CH
2 and N=O are, the more unstable they are. Thus, for compound
7, the pyridine ring is rich in electrons, similar to the double bond in compound
4, which repels the N=O-containing electrons. Meanwhile, considering the steric hindrance of pyridine, the
E-configuration of
7 is more stable than the
Z-configuration, which is consistent with the energy calculation.
Intriguingly, N-nitrososarcosine
8 showed only one set of NMR signals, suggesting that only one optimized conformer was present in
8, which was caused by the key hydrogen bond between the oxygen or nitrogen in the nitryl moiety and the hydrogen in urea, restricting its configurational exchange. The presence of hydrogen bonds was established by energy calculations at the M062X/Def2TZVP level of theory. Both the
E configuration (
8a) and the
Z configuration (
8b) might form a hydrogen bond. The
E configuration (
8a) was predicted to be 4.97 Kcal/mol lower in energy than the
Z configuration (
8b), indicating that the
E configuration (
8a) may be the stable configuration, with an intermolecular hydrogen bond of approximately 2.225 Å (
Figure 5A). The DFT quantum chemical calculations showed that the calculated
13C NMR data for the
E configuration (
8a) were less different from the experimental data. Based on the above evidence, one set of NMR signals was concluded to be from the
E configuration (
8a).
A summary of these calculated NMR data and their comparisons with experimental values are presented in
Table 3 and
Table 4, and the correlation coefficients are presented in
Table 5; these data were used to assign the NMR signals for the
Z and
E configurations of N-nitrososarcosine
3–
7.
Table 6 shows the Gibbs free energy values (G, Kcal/mol) of
Z/
E isomers for compounds
3–
7 at the M062X/Def2TZVP level of theory with Grimme’s D3 correction. The major calculated molecular models of
3–
7 are shown in
Figure 6.
To quantify the ratios of isomers and the changes in the ratio at different temperatures, we carried out VT NMR spectroscopic studies (
Figure 7). All ratios of the
3–
7 isomers were changed in the VT-NMR experiments. To determine whether these changes were affected by temperature or time, control NMR experiments were performed at room temperature (RT).
Figure 7A shows that the
Z/E ratio of
3 quickly increased in the VT-NMR experiment. This ratio was maintained at 120~130% even though the NMR probe temperature changed from 90 °C to 30 °C. In the control RT-NMR experiment of
3, the
Z/E ratio increased slowly from 2 to 12% within seven hours. A similar phenomenon was also observed in the VT/RT-NMR experiments of
6 (
Figure 7D). The
Z/E ratios of
4 and
5 exhibited small changes of approximately 12 and 24%, respectively (
Figure 7B, C). In addition,
7 was shown to exhibit different changes in the
Z/E ratios in the VT/RT-NMR experiment, but they ultimately showed a similar
Z/E ratio of approximately 50%. Based on these VT/RT-NMR experiments, the rapid changes in the
Z/E ratios of isomers
3–
7 were temperature-dependent. To our surprise, when the NMR probe temperature returned to 30 °C from higher temperatures, the
Z/E ratios did not show a significant decrease. This means there might be a balance between the two isomers in solvents.