Nitrenium ions from food-derived heterocyclic arylamine mutagens [13]

Full text of DOI:10.1021/ja974222h

J. Am. Chem. Soc. 1998, 120, 1643-1644
1643
4-aminobiphenyl.5a All three heterocyclic esters show pH de-
pendent hydrolysis kinetics (Figure 1) consistent with spontaneous
uncatalyzed hydrolysis of the conjugate base of a hydrolytically
Nitrenium Ions from Food-Derived Heterocyclic
Arylamine Mutagens
unreactive acid. The apparent ionization constant, K
limiting hydrolysis rate constant, k , can be obtained from fits of
the kinetic data to eq 1. The pH dependence of initial UV
a
, and the
Michael Novak,* Lulu Xu, and Rebecca A. Wolf
o
Department of Chemistry and Biochemistry
Miami UniVersity, Oxford, Ohio 45056
-pH
kobsd ) k K /(K + 10
)
(1)
o
a
a
ReceiVed December 15, 1997
absorbance (Figures S1-S3 of the Supporting Information)
Heterocyclic arylamines, the products of the pyrolysis of
proteins and amino acid mixtures, are known mutagens to
Salmonella in the presence of rat liver homogenates.1 These
confirms the presence of an ionization equilibrium. Since this
was not observed for 1d,5 the pK values are assigned to the
a
a
deprotonation of the endocyclic pyridyl nitrogens of the conjugate
materials are carcinogenic in laboratory animals, and are assumed
to be carcinogenic in humans.2 Similar to their carbocyclic
acids of 1a-c. The pK and kinetic parameters for 1a-c are
a
collected in Table 1. There is good agreement between the pK
a
analogues, they are promutagens and procarcinogens requiring
metabolism into hydroxylamines and subsequently into carboxylic
values determined from the UV titrations and the kinetic fits. The
endocyclic nitrogens of 1a-c reduce the magnitude of the rate
o
constant k compared to that for 1d, with o-pyridyl N (1a,b)
or sulfuric acid esters of the hydroxylamines.1
-3
Because of this,
and similarities in DNA adducts derived from the two classes of
4
having a larger effect than m-pyridyl N (1c).
amines, it is believed that esters of heterocyclic N-arylhydroxy-
-
N
3
has no significant effect on the hydrolysis rate constants
lamines yield N-arylnitrenium ions that are responsible for the
carcinogenic effects of these compounds. Although the inter-
mediacy of arylnitrenium ions in the hydrolysis of the carbocyclic
for 1a-c (Table S1 in the Supporting Information), but has a
marked effect on product distributions. Figure 2 shows that for
8
8
5
1c the hydrolysis product 2 is replaced by the azide adduct 3 as
esters is well established, as is the involvement of these cations
-
6
the N3 concentration increases, but the yield of the rearrangement
product 4 is unaffected by N
in the formation of DNA adducts, no reports of the generation
8
-
up to 0.20 M. The adduct 3 is
or chemistry of heterocyclic nitrenium ions derived from these
3
mutagens have appeared.7
the tetrazole tautomer of the likely initial adduct 5. In solution
The esters 1a-c were synthesized to test the hypothesis that
heterocyclic nitrenium ions can be generated from their decom-
position.8 The esters 1a and 1b are derivatives of the mutagen
,9
2-amino-5-phenylpyridine (Phe-P-1), a pyrolysis product of
phenylalanine, and the synthetic mutagen 2-amino-3-methyl-5-
phenylpyridine (MePhe-P-1).4
a,9a
These esters are heterocyclic
or in the solid state most 2-azidopyridines are less stable than
their tetrazole tautomers.10 These results are consistent with the
mechanism of Scheme 1 that is similar to that used to explain
the hydrolysis behavior of 1d and other esters of carbocyclic
5
N-arylhydroxylamines and N-arylhydroxamic acids.
s
The rate constant ratio kaz/k can be determined by fits of the
product yield data to eq 2 or 3, where [S] is the yield of solvent-
analogues of the carbocyclic ester 1d derived from the carcinogen
-
f ) [S]/([S] + [Az]) ) 1/(l + (k /k )[N ])
(2)
s
az
s
3
*
Phone: (513) 529-2813. Fax: (513) 529-5715. E-mail: minovak@
miamiu.muohio.edu.
1) Sugimura, T. EnViron. Health Perspect. 1986, 67, 5-10. Hatch, F. T.;
Knize, M. G.; Felton, J. S. EnViron. Mol. Mutagen. 1991, 17, 4-19.
2) Ohgaki, H.; Matsukura, N.; Morino, K.; Kanachi, T.; Sugimura, T.;
Takayama, S. Carcinogenesis 1984, 5, 815-819. Sugimura, T. Mutat. Res.
985, 150, 33-41. Eisenbrand, G.; Tang, W. Toxicology 1993, 84, 1-82.
3) Shinohara, A.; Saito, K.; Yamazoe, Y.; Kamataki, T.; Kato, R. Cancer
(
-
3
-
f ) [Az]/([S] + [Az]) ) (k /k )[N ]/(1 + (k /k )[N ])
az
az
s
az
s
3
(
(3)
adducts.5,11 These
3
1
-
derived products and [Az] is the yield of N
(
Res. 1986, 46, 4312-4367. Snyderwine, E. G.; Wirth, P. J.; Roller, P. P.;
ratios are reported in Table 1 for 1a-c and 1d. The hydrolyses
of all three heterocyclic esters yield reactive electrophiles gener-
ated after the rate-limiting step of the reaction. These intermedi-
ates appear to be the nitrenium ions 6a-c that behave similarly
to the nitrenium ions derived from their carbocyclic analogues.
Adamson, R. H.; Sato, S.; Thorgiersson, S. S. Carcinogenesis 1988, 9, 411-
4
18. Meerman, J. H. N.; Ringer, D. P.; Coughtrie, M. W. H.; Bamforth, K.
J.; Gilissen, R. A. H. J. Chem.-Biol. Interact. 1994, 92, 321-328.
4) (a) Saris, C. P.; van Dijk, W. J.; Westra, J. G.; Hamzink, M. R. J.; van
de Werken, G.; Zomer, G.; Stavenuiter, J. F. C. Chem.-Biol. Interact. 1995,
(
9
5, 29-40. (b) Hashimoto, Y.; Shudo, K.; Okamoto, T. Biochem. Biophys.
Comparison of kaz/k for 6a-c and 6d show that the endocyclic
N considerably reduces the selectivity of these ions. If kaz is
s
Res. Commun. 1980, 96, 355-362. (c) Snyderwine, E. G.; Roller, P. P.;
Adamson, R. H.; Sato, S.; Thorgiersson, S. S. Carcinogenesis 1988, 9, 1061-
1
065.
(
5) (a) Novak, M.; Kahley, M. J.; Eiger, E.; Helmick, J. S.; Peters, H. E.
(9) (a) Lutgerink, J. T.; Stavenuiter, J. F. C.; Zomer, G.; Hamzink, M.;
van Dijk, P.; Westra, J. G.; Kriek, E. Carcinogenesis 1989, 10, 1957-1960.
(b) Stavenuiter, J. F. C.; Verrips-Kroon, M.; Bos, E. J.; Westra, J. G.
Carcinogenesis 1985, 6, 13-19. (c) Stavenuiter, J.; Hamzink, M.; van der
Hulst, R.; Zomer, G.; Westra, G.; Kriek, E. Heterocycles 1987, 26, 2711-
2716. (d) Setliff, F. L. J. Chem. Eng. Data 1970, 15, 590-591. (e) Novak,
M.; Lagerman, R. K. J. Org. Chem. 1988, 53, 4762-4769.
(10) Boyer, J. H.; Miller, E. J. J. Am. Chem. Soc. 1959, 81, 4671-4673.
Wentrup, C.; Winter, H.-W. J. Am. Chem. Soc. 1980, 102, 6161-6163.
(11) Richard, J. P.; Jencks, W. P. J. Am. Chem. Soc. 1982, 104, 4689-
4691; 1982, 104, 4691-4692; 1984, 106, 1383-1396. Kemp, D. S.; Casey,
M. L. J. Am. Chem. Soc. 1973, 95, 6670-6680. Rappoport, Z. Tetrahedron
Lett. 1979, 2559-2562.
J. Am. Chem. Soc. 1993, 115, 9453-9460. (b) Davidse, P. A.; Kahley, M. J.;
McClelland, R. A.; Novak, M. J. Am. Chem. Soc. 1994, 116, 4513-4514. (c)
Novak, M.; Kahley, M. J.; Lin, J.; Kennedy, S. A.; Swanegan, L. A. J. Am.
Chem. Soc. 1994, 116, 11626-11627. (d) McClelland, R. A.; Davidse, P. A.;
Hadzialic, G. J. Am. Chem. Soc. 1995, 117, 4173-4174.
(
6) Novak, M.; Kennedy, S. A. J. Am. Chem. Soc. 1995, 117, 574-575.
Kennedy, S. A.; Novak, M.; Kolb, B. A. J. Am. Chem. Soc. 1997, 119, 7654-
664.
7) The heterocyclic 2-imidazolylnitrenium ion has been characterized as
7
(
a very long-lived species: Bolton, J. L.; McClelland, R. A. J. Am. Chem.
Soc. 1989, 111, 8172-8181.
(8) See the Supporting Information.
S0002-7863(97)04222-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/07/1998

1644 J. Am. Chem. Soc., Vol. 120, No. 7, 1998
Communications to the Editor
Scheme 1
Figure 1. log kobs vs pH for 1a-c. Theoretical lines were obtained from
fits to eq 1. Kinetic parameters are reported in Table 1.
Table 1. Kinetic Parameters for 1a-da
pKa
UV
data
kinetic
c
-1 c
kaz/ks (M-1
10 ( 2
ester
b
data
ko (s
)
)
-
-
-
-
6
1
1
1
1
a
b
c
2.83 ( 0.05 2.5 ( 0.3
(6.7 ( 0.4) × 10
4
3.34 ( 0.05 3.43 ( 0.04 (8.5 ( 0.1) × 10
3.95 ( 0.07 4.11 ( 0.09 (4.4 ( 0.2) × 10
(1.3 ( 0.2) × 10
300 ( 50
79 ( 11
3
complicated by the lack of sites on the proximal ring of the
1 d
3 e
d
(2.9 ( 0.2) × 10
O at µ ) 0.5 and T ) 20
nitrenium ion that can lead to aromatic products after initial
a
-
Determined in 5% vol/vol CH
C, unless otherwise indicated. Buffers used to maintain pH were 0.02
3
CN-H
2
nucleophilic attack. The products of N
3
and solvent reaction
-
°
with 6b are under investigation. All three initial N
adducts
3
M Na HPO /NaH PO , NaOAc/AcOH, NaHCO /HCO H, and HClO
2
4
2
4
2
2
4
decompose into new products during isolation.
The acetic acid esters analogous to 1a and 1b yield the adducts
b
solutions at pH e 2.5. The initial UV absorbance vs pH was fit to a
c
standard titration curve (see the Supporting Information). Obtained
4
a
7
and 8 upon reaction with 2′-deoxyguanosine (d-G). The
from a fit of kobs vs pH to eq 1. Measured at 0 °C; see ref 5a. e See
d
ref 5a.
adduct 8 is formed in much greater yield than 7 under identical
conditions (ca. 15-fold in DNA studies and ca. 5-fold in saturated
d-G solutions). The structure of 7 is unusual since most aromatic
amine carcinogens yield adducts similar to 8, but the relative
yields of 7 and 8 are consistent with our observation that 6b is
-
3
0-fold more selective toward N
3
than 6a. The selectivity of
, is smaller than
by a factor of ca. 3-8. For example, kd-G/k for 6d is 1.1
10 M . If that is true for heterocyclic arylnitrenium ions,
a-c will have small but measurable kd-G/k values. Since Phe-
carbocyclic arylnitrenium ions toward d-G, kd-G/k
s
6
k
az/k
s
s
3
-1 6
×
6
s
Figure 2. Product yields as a function of [N -
yield data for 2 and 3 were fit to eqs 2 and 3, respectively, to obtain the
theoretical lines for those compounds and the value of kaz/k . The
-] from 0.0
] at pH 7.0 for 1c. Product
3
1
P-1 and MePhe-P-1 are weakly mutagenic heterocyclic amines,
it is not surprising that 6a and 6b are relatively unselective ions.
We are preparing esters of a wide variety of heterocyclic
s
theoretical line for 4 is the average value of its yield for [N
3
arylhydroxylamines with mutagenicities toward Salmonella that
to 0.2 M.
4
vary over a factor of 10 . Measurement of kaz/k
s
and kd-G/k
s
for
nitrenium ions derived from these esters will show if there is a
correlation of mutagenicity with nitrenium ion selectivity as there
is for carbocyclic nitrenium ions.13
diffusion limited, as it is for 6d,5d these reduced selectivities are
due to increased k for the heterocyclic ions. This is consistent
with the expected effect on nitrenium ion stability of replacement
of aromatic ring carbons by the more electronegative nitrogen
atom. The lower selectivity of 6a compared to 6c is expected
because the o-pyridyl N will more strongly interact with the
positive charge of 6a than will the m-pyridyl N of 6c.
s
Acknowledgment. This work was supported by a grant from the
American Cancer Society (CN-172). R.A.W. acknowledges a Howard
Hughes Medical Institute Undergraduate Summer Research Internship
provided by a HHMI grant to Miami University (71195-520402).
The effect of the o-methyl in 6b (a 30-fold increase in kaz/k
compared to 6a) is considerably larger than the effect of two
s
Supporting Information Available: Synthesis of 1a-c, isolation and
characterization of 2-4, Table S1, and Figures S1-S3 (7 pages). See
any current masthead page for ordering information and Web access
instructions.
methyl groups in the (2,6-dimethylphenyl)nitrenium ion compared
12
to phenylnitrenium ion (ca. 7-fold increase in kaz/k
s
). This may
be due to greater localization of charge at the ortho carbon of 6a
-
and 6b. The reaction of 6b with both solvent and N
3
is
JA974222H
(
12) Fishbein, J. C.; McClelland, R. A. J. Am. Chem. Soc. 1987, 109, 2824-
(13) Novak, M.; Lin, J.; Kayser, K. J. To be submitted to J. Am. Chem.
Soc.
2
825.