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Selenol Nitrosation and Se-Nitrososelenol Homolysis A Reaction Path with Possible Biochemical Implications.

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Communications
Bioinorganic Chemistry
Selenol Nitrosation and Se-Nitrososelenol
Homolysis: A Reaction Path with Possible
Biochemical Implications
Cathleen Wismach, Wolf-Walther du Mont,*
Peter G. Jones, Ludger Ernst, Ulrich Papke,
Govindasamy Mugesh, Wolfgang Kaim,
Matthias Wanner, and Klaus D. Becker
Dedicated to Professor Reinhard Schmutzler
on the occasion of his 70th birthday
The biological role of nitric oxide (NO) as a secondary
messenger, for instance in the cardiovascular system, has been
known for more than a decade. Nitrosation of the cysteine
[*] Dipl.-Chem. C. Wismach, Prof. Dr. W.-W. du Mont,
Prof. Dr. P. G. Jones, Prof. Dr. L. Ernst, Dr. U. Papke,
Prof. Dr. K. D. Becker
Fachbereich Chemie und Pharmazie
Technische Universit)t Braunschweig
Postfach 3329, 38023 Braunschweig (Germany)
Fax: (+ 49) 531-391-5387
E-mail: w.du-mont@tu-bs.de
Prof. Dr. W. Kaim, Dr. M. Wanner
Institut f<r Anorganische Chemie der Universit)t Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Prof. Dr. G. Mugesh
Department of Inorganic & Physical Chemistry
Indian Institute of Science
Bangalore 560 012 (India)
3970
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200453872
Angew. Chem. Int. Ed. 2004, 43, 3970 –3974
Angewandte
Chemie
residues of oxyhemoglobin leading to S-nitrosothiol functions
(RSNO) supports the active transport of NO, which can be
liberated enzymatically from RSNO. Accordingly, S-nitrosothiols are efficient vasodilators, as is NO, but also intrinsically toxic.[1–3] Se-nitrososelenols (RSeNO) are unknown;
however, there may be factors common to the biological
cycles of NO and those of selenoproteins.[4] For example,
selenoproteins are efficient reagents for peroxynitrite reduction,[5] and S-nitrosothiols can be substrates of mammalian
selenoenzymes such as GPx (glutathione peroxidases) and
TrxR (thioredoxin reductases).[6–10] Whether the selenocysteine moieties (Sec) in the active centers of TrxR and GPx
play a particular role in the course of the enzymatic NO
liberation from S-nitrosothiols is still unknown. Wang et al.
proposed recently that in course of the in vitro cleavage of Snitrosoglutathione (GSNO), catalyzed by GPx or by bis(4chlorophenyl)diselenide, selenols could play a crucial role.[10]
In our opinion the key steps of this kind of catalysis may be
selenol nitrosation with subsequent Se-nitrososelenol homolysis, a yet-unknown type of reaction in selenoprotein chemistry. To test this hypothesis, we studied model reactions for the
nitrosation of various selenols and thiols.
Our intention was to generate Se-nitrososelenols that
would be as stable as possible. This led us to choose selenols
with organic substituents that had helped previously in
stabilizing (typically labile) compounds with covalent selenium–iodine bonds: compounds that are designed for intramolecular coordination,[11, 12] and compounds with extremely
bulky alkyl substituents.[13, 14]
A thiol designed for intramolecular coordination at the N
atom of the heterocyclic ring, 2-(4,4-dimethyl-2-oxazolinyl)phenylthiol (1 a), reacted with tert-butylnitrite at temperatures above about 20 8C furnishing the red nitrosothiol 1 b,
which was characterized in solution by NMR spectroscopy. At
room temperature the compound decomposes forming NO
and the parent disulfide 2 c [Eq. (1)]. The related selenol 2 a,
tBuOH
REH þ tBuON¼O ƒƒƒ
ƒ! fREN¼Og ! 1=2 R2 E2 þ NO
E ¼ S: 1 a
1b
1c
E ¼ Se : 2 a
2b
2c
3a
ð2Þ
3b
tBuOH
TsiSeH þ tBuON¼O ƒƒƒƒ!
4a
fTsiSeN¼Og ! 1=2 Tsi2 Se2 þ NO þ Tsi2 Se3
4b
4c
ð3Þ
4d
Selenol 4 a reacted with tert-butylnitrite immediately even
at 78 8C.[15, 16] The deep-red nitrosation product is persistent
at 78 8C, and its IR spectrum displays the band (n(N=O) =
1459 cm1) expected for the Se-nitrososelenol 4 b . The lithium
derivative LiSeTsi was nitrosated in a similar way at 78 8C,
and even NaNO2 attacked selenol 4 a immediately giving a
red solution (THF, 20 8C). Decomposition of the red reaction
product at temperatures above 78 8C was followed by GCMS (static headspace-MS), which confirmed release of of NO
from the mixture.[16] During this decomposition 77Se NMR
signals could not be observed and 1H NMR peaks appeared
broadened, which can be explained by the presence of
paramagnetic species in solution. EPR spectra recorded a
few minutes after 4 a was mixed with tert-butylnitrite showed
the intense signal of an apparently selenium-centered radical
5.
The splitting pattern (Figure 1) can be simulated reasonably well if coupling to selenium (a(77Se) = 30 G) and to two
nitrogen atoms (a(14N) = 7.6 G) is assumed.[16, 17] The valency
of selenium in 5 may be compared to that in [C6H4N2SeW(CO)5]C ,[18] the {W(CO)5} complex of the cyclic radical anion
C6H4N2SeC (Figure 2) After about 20 min at room temper-
ð1Þ
however, reacted immediately with tert-butylnitrite or with
NOCl even at 78 8C furnishing deep-red solutions that
become brownish within 1–2 h at 78 8C or within a few
minutes at room temperature; this decomposition led to
diselenide 2 c. It turns out that nitrosation products 3 b and 4 b
carrying the extremely bulky trisyl substituents are significantly more stable than 1 b and 2 b [Eqs. (2) and (3), Tsi =
(Me3Si)3C].
Angew. Chem. Int. Ed. 2004, 43, 3970 –3974
tBuOH
TsiSH þ tBuON¼O ƒƒƒƒ! TsiSN¼O
Figure 1. EPR spectra of species 5; simulation (above) and experimental spectrum (below).
www.angewandte.org
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3971
Communications
Figure 2. Structure and EPR data of a radical with NSeN connectivity.[18a]
ature the EPR signal of 5 vanished and a broad long-lived
signal appeared consisting of a quintet (4 H) of triplets (1 N)
and arising from a dialkylnitroxide (RCH2)2NOC (a(14N) =
15.2 G, a(1H) = 10.5 G). For comparison, the values for
diethylnitroxide are a(14N) = 16.7 G and a(1H) = 11.2 G.[18b]
When isopentylnitrite was used in place of tert-butylnitrite,
a similar secondary radical was observed. The final products
from the nitrosation of 4 a are diselenide 4 c, the related
triselenide 4 d, trisylmethane (TsiH), and small amounts of an
insoluble solid that contains (according to elemental analysis)
C, H, N, and Se.
In contrast to selenol 4 a, trisylthiol 3 a was not attacked by
tert-butylnitrite or by isopentylnitrite at 78 8C. Nitrosation at
5–10 8C led to relatively stable S-nitrosothiol 3 b [Eq. (3)].
In a competition experiment selenol 4 a was found to be
much more reactive towards tert-butylnitrite than thiol 3 a is.
tert-Butylnitrite was added to an equimolar mixture of 3 a, 4 a,
and diselenide 4 c (77Se NMR signals of equal strength). The
reaction mixture immediately turned an intense red, and 77Se
NMR signals could not be detected. Only after a while at
room temperature were the 77Se NMR resonances for
diselenide 4 c and later for small amounts of 4 d detected.
The typical decomposition products of 3 b were not detected.
A control experiment showed that no reaction can be
observed between tert-butylnitrite and pure diselenide 4 c
(unchanged 77Se NMR signal of 4 c).
These results suggest that formation of nitrososelenol 4 b
is followed by homolytic cleavage of the SeN bond.
Formation of the unusual Se-centered radical 5 with the
probable composition TsiSe(NO)2C can be understood as the
reversible addition of released NO to the remaining nitrososelenol 4 b or as the addition of two NO molecules to a
selenyl readical. Since attempts to isolate pure 4 b were
unsuccessful, it was desirable to find a suitable trapping
reaction characteristic for Se-nitrososelenols. Such a reaction
is obviously the 1,4-addition to 2,3-dimethylbutadiene. The
related nitrosothiol 3 b adds to 2,3-dimethylbutadiene with
formation of an a,b-unsaturated oxime 6, in which the
trisythio group is in the d-position [Eq. (4)].
When Motherwell and co-workers carried out the related
reaction with Ph3CSNO, they isolated a mixture of E/Z
isomers in which the E isomer was the major product.[19] In
our work the NMR spectrum of 6 indicates that only one
isomer is present, and the X-ray crystal structure determination confirms the E configuration (Figure 3).[20] Compound
6 crystallizes in the P1̄ space group with two independent, but
similar molecules (s-trans at C11-C12-C13-C14-N) that are
connected by hydrogen bonds.
The very characteristic 13C NMR resonance of the oxime
function of 6 (d = 149 ppm) should be especially helpful for
the detection of the related selenium compound 7, even when
only small amounts are present in reaction mixtures. Addition
of excess dimethylbutadiene to red solutions from the nitrosation of 4 a led to reaction mixtures from which very small
amounts of d-selenooxime 7 were obtained [Eq. (5)]. The
Figure 3. X-ray crystal structure of 6. Selected distances [E] and angles [8] (values of both independent molecules): S-C11 1.8321(12), 1.8337(12),
S-C10 1.8541(12), 1.8577(12), O-N 1.4086(14), 1.4117(13), N-C14 1.2794(16), 1.2800(16), C11-C12 1.5075(17), 1.5094(17), C12-C13 1.3505(17),
1.3513(17), C13-C14 1.4561(17), 1.4570(17); C11-S-C10 109.30(6), 108.44(6), C13-C12-C11 121.19(11), 121.73(11), C13-C12-C15 125.01(12),
125.29(11), C12-C13-C16 122.87(12), 123.68(11), C14-N-O 111.21(10), 111.25(10).
3972
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2004, 43, 3970 –3974
Angewandte
Chemie
mass spectrum of d-selenooxime 7 is analogous to that the
sulfur compound 6. NMR spectra (complete assignment
through 2D 1H/13C experiments) indicate that 7 is a mixture
of isomers (E/Z ca. 1:1) composition.
In summary, nitrosation of selenols 2 a, and 4 a occurs
significantly faster than that of the related thiols 1 a and 3 a.
The nitrosated selenium species are thermally much less
stable than the related S-nitrosothiols and release NO much
more readily.[22] We expect that the reaction sequence selenol
nitrosation followed by SeNO homolysis will give instigate
further studies on the possible interactions of selenoproteins[4]
with nitrosation agents.[1–3, 22]
Experimental Section
1 b: To a solution of 1 a (0.3 g, 1.46 mmol) in n-pentane (5 mL) was
added tert-butylnitrite (1.5 mL, 14.6 mmol) at 20 8C. In the
13
C NMR spectrum all signals except that of a quarternary C atom
(which was covered by a solvent peak) were observed. IR and UV
data were obtained from the reaction mixture. 1 b: 1H NMR (C6D6):
d = 1.19 (s, CH3), 3.63 (s, OCH2); 6.84–6.79 (m, 2 HAr), 7.94–8.00 ppm
(m, 2 HAr), 13C NMR (C6D6): d = 28.5 (s, CCH3), 69.0 (s, CCH3), 78.4
(s, OCH2), 125.3 (s, CH), 126.2 (s, CH), 126.4 (s, q), 130.1 (s, CH),
131.3 (s, CH), 160.5 ppm (s, C=N); UV/Vis (n-pentane, 20 8C) l (lg e):
218 (2.89), 304 (2.28); IR (film, n-pentane): ñ = 1648 (st), 1467 (m),
1364 (m), 1353 (m), 1314 (m), 1189 (m,), 1135 (m), 1080 (m), 1042 (m),
1033 (st), 965 (st), 770 (m), 736 (m), 698 (m), 683 (m), 651 cm1 (m).
2 b: tert-Butylnitrite (0.11 mL, 1.06 mmol) was added to a solution
of 2 a (0.27, 1.06 mmol) in THF (5 mL) at 78 8C, and the solution
turned deep red instantaneously. 77Se NMR measurements at 78 8C
were unsuccessful. At room temperature, the signal of diselenide
2 c[11a] was detected. 77Se NMR (THF/[D8]THF): d = 434 [ref. [11a]:
d = 454.8 (CDCl3)]. In order to record EPR spectra the reaction was
carried out in n-pentane and the spectra of the reaction mixture were
measure at room temperature immediately. EPR (n-pentane): g =
2.005, a(N) = 15.4 G (1 N), a(Hb) = 10.1 G (4 H, the same quintet-oftriplets pattern as that for the secondary radical obtained by
nitrosation of 4 a).
3 b: A solution of the freshly sublimed thiol 3 a (1 g, 3.79 mmol) in
n-pentane (5 mL) at room temperature was treated with tertbutylnitrite (0.64 mL, 35.87 mmol). The resulting red–green solution
was stirred for 30 min, and subsequently all volatiles were removed
under reduced pressure to furnish 3 b as green solid (0.99 g, 89 %),
m.p. 81–82 8C. 1H NMR (C6D6): d = 0.14 ppm (s, CH3); 13C NMR
(C6D6): d = 1.5 [Si((CH3)3)], 3.4 ppm (CSi3); 29Si{1H} NMR (C6D6):
d = 3.1 ppm; MS (CI/isobutane): m/z = 294 [M+H]+; UV/Vis (npentane, 20 8C) l (lg e): 210 (3.74); 236 (3.41); 264 (3.00); 364 (2.84);
564 (1.14); 608 (1.09). IR (KBr): ñ = 612 (st), 621 (st), 652 (m), 677
(m), 720 (m), 803 (m), 821 (m), 853 (m), 1021 (m), 1098 (m), 1261 (m),
1412 (st), 2902 (m), 2961 cm1 (m). Elemental analysis (%) calcd for
C10H27NOSSi3 (293): C 40.96, H 9.22, N 4.78, S 10.92; found: C 40.83,
H 8.94, N 4.26, S 10.67.
4 b: tBuONO (0.29 mL, 2.0 mmol) was added to a solution of
selenol 4 a (0.62 g, 2.0 mmol) in n-pentane (5 mL) at 78 8C. The
solution turned red immediately. No signals could be detected by 77Se
NMR spectroscopy. Removal of the volatiles after 30 min gave a red
oil. IR: ñ = 1621 (s), 1459 (st), 1368 (st), 1255 (st), 1099 (m), 1024 (m),
954 (s), 836 (st), 680 (m), 615 cm1 (s).
MS: The reaction mixture was transferred into an argon-filled
headspace tube. The sample was kept at room temperature for 1 h
before a gaseous sample was removed. GC/MS [m/z = 28 (100), 30
(45), 32 (15)] indicated NO accompanied by some air.
EPR (n-pentane): The reaction mixture was transferred into an
EPR tube at room temperature under N2. First radical species 5: g =
2.011 (quintet, simulation with a(N) = 7.6 G (2 N) and a(Se) = 30 G);
Angew. Chem. Int. Ed. 2004, 43, 3970 –3974
second radical species : g = 2.005 (quintet of triplets, a(N) = 15.2 G
(1 N), a(Hb) = 10.5 G (4 H)).
77
Se NMR (C6D6) after decomposition of 4 b: d = 635 (4 d), 552
(4 d), 482, 396 (4 a), 332 ppm; 1H NMR (C6D6): d = 0.37, 0.32, 0.29,
0.13 ppm; IR (KBr): ñ = 1259 (st), 1098 (s), 1014 (s), 849 (st), 678 (s),
651 (2), 602 cm1(s).
6: A mixture of 3 b (1.1 g, 3.8 mmol) and 2,3-dimethylbuta-1,3diene (2 mL) was stirred for 3 d. The volatiles were removed to give a
yellow oil. On addition of n-pentane a colorless solid (1 g, 72 %)
separated, which was recrystallized from n-pentane (m.p. 145 8C
(decomp)). 1H NMR (C6D6): d = 0.24 (s, SiCH3), 1.67 (s, CH3), 2.10 (s,
CH3), 3.30 (s, CH2), 8.08 (br s, OH), 8.24 ppm (s, CH); 13C NMR
(C6D6): d = 2.78 (SiCH3), 11.44 (s, Si3CS), 13.68 (s, CH3), 18.10 (s,
CH3), 39.15 (s, CH2), 136 (s, q), 149.80 ppm (s, CH); MS (CI/
isobutane): m/z = 376 [M+H]+.
7: To a solution of 4 a (1.2 g, 3.8 mmol) in n-pentane (10 mL) at
78 8C was added tert-butylnitrite (1.3 mL, 38 mmol). After 5 min
2,3-dimethylbuta-1,3-diene (1 mL) was added to the red solution. The
solution was stirred at 50 8C for 5 d and then warmed to room
temperature. Removal of all volatiles gave a red oil, to which npentane (5 mL) was added. An insoluble white solid was removed and
the pentane-soluble fraction subjected to column chromatography
(SiO2, pentane/ethylacetate 95:5). 13C NMR spectra indicate that the
second fraction contained 7 (a few mg).
Important NMR data of 7 in C6D6 : (E)-oxime: 1H NMR: d =
8.20 ppm (s, 1 H, N=CH); 13C NMR: d = 149.9 ppm (CH, 1J(C, H) =
163 Hz, N=CH); (Z)-oxime: 1H NMR: d = 8.51 ppm (s, 1 H, N=CH);
13
C NMR: d = 149.8 ppm (CH, 1J(C, H) = 161 Hz, N=CH). 77Se NMR:
d = 223, 240 ppm. MS(CI/isobutane): m/z = 424 [M+H]+.
Received: January 28, 2004
Revised: May 6, 2004 [Z53872]
.
Keywords: bioinorganic chemistry · nitrogen oxides ·
nitrosation · oximes · selenium
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Communications
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[15] Another IR absorption at 1621 cm1 is apparently due to
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[20] X-ray structure determination of 6 (C16H37NOSSi3): Mr = 375.80,
triclinic, space group P1̄, a = 8.9455(6), b = 15.5786(10), c =
17.1565(10) O, a = 94.117(3)8, b = 91.435(3), g = 106.193(3)8,
V = 2287.6(3) O3, Z = 4, 1calcd = 1.091 Mg m3, MoKa radiation
(l = 0.71073 O3), T = 133 K; the crystal (0.45 P 0.28 P 0.08 mm3)
was mounted in inert oil on a Bruker Smart 1000 CCD area
detector. Intensities were collected in the 2q-range 4–608. From a
total of 32 682 reflections, 11 270 were independent (Rint =
0.0322). All non-hydrogen atoms were refined anisotropically
versus F2 with the full matrix least squares procedure: R1 =
0.0301, wR2 = 0.0896 (all data).[21] Methyl hydrogens were
refined as rigid groups, OH hydrogens freely, and the other
hydrogens with the riding model. CCDC-241350 contains the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (+
44) 1223-336-033; or deposit@ccdc.cam.ac.uk).
[21] G. M. Sheldrick, SHELXL-97, A Program for Crystal Structure
Refinement, GRttingen, 1997.
[22] Comment added after submission: The full characterization of a
stable aromatic Se-nitrososelenol was recently reported by K.
Goto, K. Shimada, and T. Kawashima at the 11th International
Conference on the Chemistry of Selenium and Tellurium, IIT
Bombay, February 23 – 27, 2004, Abstract IL-3.
3974
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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