Transformation of dinitrosyl iron complexes [(NO)2Fe(SR) 2]- (R = Et, Ph) into [4Fe-4S] clusters [Fe 4S4(SPh)4]2-: Relevance to the repair of the nitric oxide-modified ferredoxin [4Fe-4S] clusters

Article abstract of DOI:10.1021/ja806050x

Transformation of dinitrosyl iron complexes (DNICs) [(NO) ₂Fe(SR)₂]^- (R = Et, Ph) into [4Fe-4S] clusters [Fe₄S₄(SPh)₄]^2- in the presence of [Fe(SPh)₄]^2-/1- and S-donor species S₈ via the reassembling process ([(NO)₂Fe(SR)₂]^- → [Fe₄S₃(NO)₇]^- (1)/[Fe ₄S₃(NO)₇]^2- (2) → [Fe ₄S₄(NO)₄]^2- (3) → [Fe ₄S₄(SPh)₄]^2- (5)) was demonstrated. Reaction of [(NO)₂Fe(SR)₂]^- (R = Et, Ph) with S₈ in THF, followed by the addition of HBF₄ into the mixture solution, yielded complex [Fe₄S₃(NO) ₇]^- (1). Complex [Fe₄S₃(NO) ₇]^2- (2), obtained from reduction of complex 1 by [Na][biphenyl], was converted into complex [Fe₄S₄(NO) ₄]^2- (3) along with byproduct [(NO)₂Fe(SR) ₂]^- via the proposed [Fe₄S₃(SPh)(NO) ₄]^2- intermediate upon treating complex 2 with 1.5 equiv of [Fe(SPh)₄]^2- and the subsequent addition of 1/8 equiv of S₈ in CH₃CN at ambient temperature. Complex 3 was characterized by IR, UV-vis, and single-crystal X-ray diffraction. Upon addition of complex 3 to the CH₃CN solution of [Fe(SPh)₄] ^- in a 1:2 molar ratio at ambient temperature, the rapid NO radical-thiyl radical exchange reaction between complex 3 and the biomimetic oxidized form of rubredoxin [Fe(SPh)₄]^- occurred, leading to the simultaneous formation of [4Fe-4S] cluster [Fe₄S ₄(SPh)₄]^2- (5) and DNIC [(NO) ₂Fe(SPh)₂]^-. This result demonstrates a successful biomimetic reassembly of [4Fe-4S] cluster [Fe₄S ₄(SPh)₄]^2- from NO-modified [Fe-S] clusters, relevant to the repair of DNICs derived from nitrosylation of [4Fe-4S] clusters of endonuclease III back to [4Fe-4S] clusters upon addition of ferrous ion, cysteine, and IscS.

Full text of DOI:10.1021/ja806050x

Published on Web 11/17/2008
Transformation of Dinitrosyl Iron Complexes [(NO)2Fe(SR)2]-
(R ) Et, Ph) into [4Fe-4S] Clusters [Fe4S4(SPh)4]2-: Relevance
to the Repair of the Nitric Oxide-Modified Ferredoxin [4Fe-4S]
Clusters
Chih-Chin Tsou, Zong-Sian Lin, Tsai-Te Lu, and Wen-Feng Liaw*
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013,
Taiwan, Republic of China
Received August 1, 2008; E-mail: wfliaw@mx.nthu.edu.tw
Abstract: Transformation of dinitrosyl iron complexes (DNICs) [(NO)2Fe(SR)2]- (R ) Et, Ph) into [4Fe-4S]
clusters [Fe4S4(SPh)4]2- in the presence of [Fe(SPh)4]2-/1- and S-donor species S8 via the reassembling
process ([(NO)2Fe(SR)2]- f [Fe4S3(NO)7]- (1)/[Fe4S3(NO)7]2- (2) f [Fe4S4(NO)4]2- (3) f [Fe4S4(SPh)4]2-
(5)) was demonstrated. Reaction of [(NO)2Fe(SR)2]- (R ) Et, Ph) with S8 in THF, followed by the addition
of HBF4 into the mixture solution, yielded complex [Fe4S3(NO)7]- (1). Complex [Fe4S3(NO)7]2- (2), obtained
from reduction of complex 1 by [Na][biphenyl], was converted into complex [Fe4S4(NO)4]2- (3) along with
byproduct [(NO)2Fe(SR)2]- via the proposed [Fe4S3(SPh)(NO)4]2- intermediate upon treating complex 2
with 1.5 equiv of [Fe(SPh)4]2- and the subsequent addition of 1/8 equiv of S8 in CH3CN at ambient
temperature. Complex 3 was characterized by IR, UV-vis, and single-crystal X-ray diffraction. Upon addition
of complex 3 to the CH3CN solution of [Fe(SPh)4]- in a 1:2 molar ratio at ambient temperature, the rapid
NO radical-thiyl radical exchange reaction between complex 3 and the biomimetic oxidized form of
rubredoxin [Fe(SPh)4]- occurred, leading to the simultaneous formation of [4Fe-4S] cluster [Fe4S4(SPh)4]2-
(5) and DNIC [(NO)2Fe(SPh)2]-. This result demonstrates a successful biomimetic reassembly of [4Fe-4S]
cluster [Fe4S4(SPh)4]2- from NO-modified [Fe-S] clusters, relevant to the repair of DNICs derived from
nitrosylation of [4Fe-4S] clusters of endonuclease III back to [4Fe-4S] clusters upon addition of ferrous ion,
cysteine, and IscS.
Introduction
in vitro has been made possible via their distinctive EPR signal
at g ) 2.03.1-4
Nitric oxide (NO) regulates various physiological reactions
including vasodilation, neuronal transmission, inflammation,
immune function, and cancer killing.1,2 DNICs and RSNO are
known to be two possible forms for storage and transport of
NO in biological systems.3 DNICs are intrinsic NO-derived
species that can exist in various NO-overproducing tissues. In
vivo, nitric oxide can be stabilized and stored in the form of
dinitrosyl iron complexes with proteins (protein-bound DNICs)
and is probably released from cells in the form of low-molecular-
weight dinitrosyl iron complexes (LMW-DNICs).4 Characteriza-
tion of both protein-bound and low-molecular-weight DNICs
Iron-sulfur [Fe-S] clusters are ubiquitous and evolutionary
prosthetic groups that are required to sustain fundamental life
processes.5 Three different types of [Fe-S] cluster biosynthetic
systems (Nif (nitrogen fixation specific), Isc (iron sulfur cluster),
and Suf (sulfur utilization factor)) have been discovered/
proposed. Importantly, the biosynthesis of [Fe-S] clusters,
constructed by the requirement of cysteine desulfurase and the
participation of an [Fe-S] cluster scaffold protein, includes the
following steps: (i) mobilizing iron and sulfur atoms from their
storage sources in nontoxic forms, (ii) assembling them into an
[Fe-S] core, and (iii) insertion of the assembled [Fe-S] core into
apoprotein.6 However, design and interpretation of such in vitro
“cluster transfer” experiments are difficult because some apo-
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10.1021/ja806050x CCC: $40.75
2008 American Chemical Society

Nitric Oxide-Modified Ferredoxin [4Fe-4S] Clusters
A R T I C L E S
[Fe-S] proteins can be spontaneously activated by the simple
addition of Fe2+ and SH-/S.7 A rational mechanism is that the
[Fe-S] core is delivered to apo-[Fe-S] protein by specific carrier
proteins.7
in the presence of NO(g), were elucidated.16,17 In addition, we
also demonstrated that nitrosylation of the [2Fe-2S] cluster
[S5Fe(µ-S)2FeS5]2- yielding [(NO)2FeS5]- and the reversible
transformation of complex [(NO)2FeS5]- to the [S5Fe(µ-
S)2FeS5]2- by photolysis in the presence of the NO-acceptor
reagent [(C4H8O)Fe(S,S-C6H4)2]- are consistent with reports of
the in vitro degradation of ferredoxin [2Fe-2S] clusters to DNICs
and the repair of NO-modified [2Fe-2S] ferredoxin by cysteine
desulfurase and L-cysteine.18 In this report, an exciting advance
in this topic, the transformation of DNICs into [4Fe-4S] clusters
[Fe4S4(SR)4]2- in the presence of the reduced/oxidized-form
[Fe(SPh)4]2-/1- and sulfur-donor species, was demonstrated.
NO has been demonstrated to react with the [Fe-S] clusters
of several proteins including succinate dehydrogenase, nitro-
genase, succinate-Q reductase, mitochondrial aconitase, cytosolic
aconitase, HiPIP, endonuclease III, mammalian ferrochelatase,
and SoxR to form the monomeric, EPR-active DNICs.5a,8-14
Recently, Ding and co-workers showed that when Escherichia
coli (E. coli) cells are exposed to nitric oxide, the ferredoxin
[2Fe-2S] clusters are modified to form protein-bound dinitrosyl
iron complexes. In the repair of NO-modified ferredoxin [2Fe-
2S] clusters, the dinitrosyl iron complexes can be directly
transformed back to the ferredoxin [2Fe-2S] cluster by cysteine
desulfurase (IscS) and L-cysteine in vitro with no need of the
addition of iron or any other protein components.14a,b Removal
of the dinitrosyl iron complex from ferredoxin preventing
reassembly of the [2Fe-2S] cluster suggests the iron in the
dinitrosyl iron complex may be recycled for the reassembly of
an iron-sulfur cluster in the protein.14a,b Drapier and co-workers
demonstrated that nitric oxide converts IRP-1 (iron regulatory
proteins) from a [4Fe-4S] aconitase to a trans-regulatory protein
through [Fe-S] cluster disassembly. Reversibly, NO-modified
aconitases repairing of the [4Fe-4S] clusters was also
observed.14c In addition, the [4Fe-4S] cluster of the E. coli
endonuclease III modified by NO forming the protein-bound
dinitrosyl iron complex in vitro/in vivo and the NO-modified
[4Fe-4S] cluster being efficiently repaired in aerobically growing
E. coli cells were also reported.12
Results and Discussion
Transformation of [(NO)2Fe(SR)2]- (R ) Et, Ph) into
[Fe4S3(NO)7]- (1). Consistent with the facile conversion of
DNICs [(NO)2Fe(SR)2]- into Roussin’s black salt (RBS)
[Fe4S3(NO)7]- (1) in the presence of S8 via the intermediate
[(NO)2FeS5]- observed in the previous study,18-20 quantitative
transformation of DNICs [(NO)2Fe(SEt)2]- to complex 1 was
displayed and monitored by IR νNO spectra. The shift of the
NO stretching frequency from 1715 s, 1673 s cm-1 to 1790 w,
1740 s, 1707 m cm-1 is consistent with the formation of complex
[Na-18-crown-6-ether][Fe4S3(NO)7] (1) when a THF solution
of [(NO)2Fe(SEt)2]- was reacted with S8 in a 1:1 molar ratio,
followed by the addition of HBF4 into the mixture solution
(Scheme 1a).18-20
Conversion of Complex [Fe4S3(NO)7]2- (2) into
[Fe4S4(NO)4]2- (3). The conversion of complex [Fe4S3(NO)7]2-
(2),21 obtained from reduction of complex 1 by [Na][biphenyl],
into complex [Fe4S4(NO)4]2- (3), characterized by IR, UV-vis,
and single-crystal X-ray diffraction, was carried out when
complex 2 was treated with 1.5 equiv of [Fe(SPh)4]2- and the
subsequent addition of 1/8 equiv of S8 in CH3CN at ambient
temperature (Scheme 1b).22 Quantitative conversion of complex
2 to complex 3 and [(NO)2Fe(SPh)2]- in the presence of
[Fe(SPh)4]2- and S8 was monitored by IR. The shift of IR νNO
stretching frequencies from 1748 m, 1690 s, 1660 sh cm-1 to
1743 s, 1708 s, 1697 sh cm-1 implicated the formation of the
known [(NO)2Fe(SPh)2]- accompanied by the proposed inter-
mediate [Fe4S3(SPh)(NO)4]2- (A) when complex 2 was reacted
with 1.5 equiv of [Fe(SPh)4]2- in CH3CN at room temperature
In chemistry, synthetic routes to the [2Fe-2S] clusters
[(SR)2Fe(µ-S)2Fe(SR)2]2- (R ) alkyl, aryl) were discovered
shortly after the preparation of the first [4Fe-4S] cluster, and
the isolation/characterization of mononuclear rubredoxin
[Fe(SR)4]2-/1- subsequently followed.15 Recently, the formation
pathway of the anionic {Fe(NO)2}9 DNICs [(NO)2Fe(SR)2]-
from nitrosylation of the biomimetic oxidized and reduced form
of rubredoxin [Fe(SR)4]2-/1- (R ) Ph, Et) and the reactivity of
the mononitrosyl tris(thiolate) complexes [(NO)Fe(SR)3]-, an
intermediate for the conversion of [Fe(SR)4]2-/1- into DNICs
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J. AM. CHEM. SOC. VOL. 130, NO. 50, 2008 17155

A R T I C L E S
Tsou et al.
Scheme 1
Scheme 2
Figure 1. ORTEP drawing and labeling scheme of the [Fe4S4(NO)4]2- unit
in the [Me4N]+ salt with thermal ellipsoids drawn at 50% probability.
Selected bond distances (Å) and angles (deg): Fe(1) · · ·Fe(2) 2.6681(15);
Fe(1)· · ·Fe(1A) 2.795(2); Fe(1) · · ·Fe(2A) 2.7147(15); Fe(2) · · ·Fe(1A)
2.7147(15); Fe(2) · · · Fe(2A) 2.771(2); Fe(1)-S(1) 2.228(2); Fe(1)-S(2)
2.250(2); Fe(1)-S(2A) 2.253(2); Fe(2)-S(2) 2.234(2); Fe(2)-S(1) 2.239(2);
Fe(2)-S(1A) 2.255(2); S(1)-Fe(2A) 2.255(2); S(2)-Fe(1A) 2.253(2);
Fe(1)-N(1) 1.666(7); Fe(2)-N(2) 1.665(7); O(1)-N(1) 1.180(8); O(2)-N(2)
1.193(8); S(1)-Fe(1)-S(2) 105.72(8); S(1)-Fe(1)-S(2A) 103.93(8);
S(2)-Fe(1)-S(2A)100.07(8);S(2)-Fe(2)-S(1)105.86(9);S(2)-Fe(2)-S(1A)
103.66(8); S(1)-Fe(2)-S(1A) 100.77(8); Fe(1)-S(1)-Fe(2) 73.35(7);
Fe(1)-S(1)-Fe(2A)74.55(7);Fe(2)-S(1)-Fe(2A)76.12(7);Fe(2)-S(2)-Fe(1)
73.01(7); Fe(2)-S(2)-Fe(1A) 74.45(7); Fe(1)-S(2)-Fe(1A) 76.74(7);
O(1)-N(1)-Fe(1) 176.5(6); O(2)-N(2)-Fe(2) 176.7(7).
for 3 h, and thereafter, the appearance of νNO stretching
frequencies (1743 s, 1697 s, 1660 s cm-1) confirmed the
formation of complex 3 and [(NO)2Fe(SPh)2]- upon subsequent
addition of 1/8 equiv of S8 into the mixture solution and stirring
at room temperature overnight. The conversion of complex 2
into complex 3 in the presence of [Fe(SPh)4]2- and S8 may be
accounted for by the following reaction sequence, as shown in
Scheme 2a,b: nitrosylation of [Fe(SPh)4]2- by complex 2 leads
to [(NO)2Fe(SPh)2]- along with [SPh]- 16,17 and the buildup of
the proposed intermediate A. The subsequent reductive elimina-
tion of diphenyl disulfide of intermediate A accompanied by
the concurrent oxidative addition of S atom yields complex 3
(Scheme 2b). This rationalizes the absence of mononitrosyl iron
complex [(NO)Fe(SPh)3]- observed spectrally in this transfor-
mation process.16,17 It is presumed that the rapid intermolecular
the cubane-like geometry and triggers the insertion/oxidative
addition of S to yield complex 3.
Single-Crystal X-ray Structure of Complex [Me4N]
[Fe4S4(NO)4]2- (3-Me4N). Compared to the known complex 4,
displaying an IR νNO stretching frequency at 1729 cm-1
(CH3CN),22a complex 3-Me4N exhibits a diagnostic IR νNO
stretching frequency at 1660 cm-1 (CH3CN). Presumably,
complex 3-Me4N, containing {Fe(NO)}8-{Fe(NO)}8/{Fe(NO)}7-
{Fe(NO)}7 coupling, may rationalize the absence of paramag-
netism and the EPR signal. An ORTEP diagram of
[Fe4S4(NO)4]2- unit in the [Me4N]+ salt is depicted in Figure
1, and selected bond dimensions are presented in the figure
caption. As shown in Table 1, the apparently longer Fe · · ·Fe
distance (average 2.722(2) Å) of complex 3-Me4N, compared
addition of
a generated thiyl radical to the adjacent
[Fe4S3(NO)4]2- core occurred to yield intermediate A (Scheme
2a), since two NO radicals binding to an electron-rich iron(II)
center of complex [Fe(SPh)4]2-, triggering displacement of
thiolates ([SPh]-), and the subsequent reductive elimination of
thiyl radical, yielding diphenyl disulfide, are known.17 Presum-
ably, this highly reactive intermediate A, generated in the course
of NO-radical transfer from complex 2 to [Fe(SPh)4]2-, retains
9
17156 J. AM. CHEM. SOC. VOL. 130, NO. 50, 2008

Nitric Oxide-Modified Ferredoxin [4Fe-4S] Clusters
A R T I C L E S
a 22
Table 1. Selected Metric Data for [Fe4S4(NO)4]0/1-/2-
tial reductive elimination of diphenyl disulfide of intermediate
b
c
A and the concomitant oxidative addition of S, as observed in
[Fe4S4(NO)4]
[Fe4S4(NO)4]-
[Fe4S4(NO)4]2-
the reaction of complex 2 and [Fe(SPh)4]2-
.
Fe· · ·Fe (Å)
Fe-S (Å)
2.651(1)
2.217(2)
1.663(6)
1.155(7)
3.503
73.43(6)
104.37(7)
177.6(5)
2.693(1)
2.231(2)
1.659(5)
1.168(6)
3.510
74.25(6)
103.76(7)
177.5(6)
2.722(2)
2.243(2)
1.666(7)
1.187(8)
3.512
74.70(7)
103.34(9)
176.6(7)
These results implicate that complexes 2/1 optimize the
electronic property of the [Fe4S3(NO)7]2-/1- core to serve as a
good NO-donor species, and the highly reactive intermediate
A optimizes the electronic property of [Fe4S3(SPh)(NO)4]2- core
for its required S insertion/oxidative addition function.
Fe-N (Å)
N-O (Å)
S· · ·S (Å)
Fe-S-Fe (deg)
S-Fe-S (deg)
Fe-N-O (deg)
Conversion of Complex 3 into [Fe4S4(SPh)4]2- (5). To follow
up on our earlier study with a more complete understanding of
the conversion of DNICs into [2Fe-2S] clusters and their
relevance as a model of the transformation of NO-modified
ferredoxin [2Fe-2S] and [4Fe-4S] clusters,18 the reactivity of
complex 3 toward [Fe(SPh)4]- was investigated. In contrast to
the inertness of complex 4 toward the biomimetic oxidized-
form rubredoxin [Fe(SPh)4]- (Scheme 1f), the direct transforma-
tion of complex 3 to the known complex [Fe4S4(SPh)4]2- (5)
accompanied by the formation of byproduct [(NO)2Fe(SPh)2]-
was displayed when a CH3CN solution of complex 3 was treated
with [Fe(SPh)4]- in a 1:2 molar ratio for 10 min at ambient
temperature (Scheme 1d). The conversion of complex 3 to 5
was monitored and characterized by IR, 1H NMR, and UV-vis
spectrometry; the formation of one absorption band at 447 nm
and the shift of the νNO stretching frequencies from 1660 s cm-1
to 1743 s, 1697 m cm-1 confirmed the formation of complex 5
and [(NO)2Fe(SPh)2]-.15,20 Complex 5 was isolated in 45% yield
after the solution mixture was separated from [(NO)2Fe(SPh)2]-
and identified by 1H NMR.15 The transformation of complex 3
into complex 5 in the presence of [Fe(SPh)4]- may be accounted
for by the following reaction sequences: the process for the rapid
conversion of complex 3 into complex 5 along with
[(NO)2Fe(SPh)2]- is a mechanism of NO radical-thiyl radical
([PhS]·) exchange reaction. NO scavenging by the NO-acceptor
of the biomimetic oxidized-form rubredoxin [Fe(SPh)4]-, trig-
gering the reductive elimination of the coordinated phenylthi-
olate of [Fe(SPh)4]- to generate thiyl radical ([PhS] ·), with
concurrent binding to the [Fe4S4]2- core, rationalizes the
simultaneous formation of complex 5 and [(NO)2Fe(SPh)2]-
(Scheme 1d). Thus, the presence of biomimetic oxidized-form
rubredoxin [Fe(SPh)4]- appears to be crucial in triggering the
conversion of complex 3 to complex 5 (E1/2 ) -0.50 V vs SCE
for [Fe(SPh)4]2-/1- and E1/2 ) -0.65 V vs SCE for [Fe4S4-
(NO)4]2-/1-).15d,22a
a Average bond distances and angles. b [K(2,2,2-crypt)]+ salt.
c [Me4N]+ salt.
to those of 2.651(1) and 2.693(1) Å in complex [Fe4S4(NO)4]
and complex 4,22 respectively, indicates the weaker Fe· · ·Fe
interaction in complex 3-Me4N. Presumably, the electronic
perturbation caused by the reduction of complex 4 reduces the
Fe· · ·Fe interaction to relieve the richness of electron density
surrounding the delocalized [{Fe(NO)}8-{Fe(NO)}7] [Fe4(µ-S)4]
core. Compared to the average Fe-S bond distances of 2.217(2)
and 2.231(2) Å, respectively, found in complex [Fe4S4(NO)4]
and complex 4,22 the longer Fe-S distances of 2.243(2) Å
(average) observed in complex 3-Me4N indicate that reduction
of complex 4 yielding 3-Me4N results in the elongation of the
Fe-S distances.
Conversion of Complex 1 into [Fe4S4(NO)4]2- (3). In contrast
to the inertness of complex 1 toward the oxidized-form
[Fe(SPh)4]- (Scheme 1e), addition of 1.5 equiv of [Fe(SPh)4]2-
to a CH3CN solution of complex 1 (E1/2 ) -0.50 V vs SCE for
[Fe(SPh)4]2-/1-;E1/2 )-0.68VvsSCEfor[Fe4S3(NO)7]2-/1-),15d,21
and the subsequent addition of 1/8 equiv of S8 powder into the
mixture solution yielded the cubane cluster [Fe4S4(NO)4]2- (3)
and [(NO)2Fe(SPh)2]-, characterized by IR, when the mixture
solution was stirred at ambient temperature for 36 h (Scheme
1b).20,22 The IR νNO stretching frequencies 1743, 1697, 1660
cm-1, shifting from 1800, 1747, 1707 cm-1, via the appearance
of IR νNO 1743 s, 1708 s, 1697 sh cm-1, are in accordance with
the formation of [(NO)2Fe(SPh)2]- and complex 3. This result
suggests that the electron-deficient [Fe4S3(NO)4]- core, resulting
from trans- nitrosylation of [Fe(SPh)4]2- by complex 1, prefers
the coordination of the released [SPh]- from trans-nitrosylation
between [Fe(SPh)4]2- and complex 1 to yield intermediate A
(as shown in Scheme 2c), in contrast to the coordination of
[·SPh] to the [Fe4S3(NO)4]2- core, yielding intermediate A,
observed in the reaction of complex 2 and [Fe(SPh)4]2-. Thus,
the absence of the released [SPh]- from reaction of [Fe(SPh)4]-
Conclusion and Comments
and complex
1
coordinated to the electron-deficient
Studies on the transformation of dinitrosyl iron complexes
[(NO)2Fe(SR)2]- back to the known [4Fe-4S] cluster 5 have
led to the following results.
[Fe4S3(NO)4]- core yielding intermediate A may explain the
observed inertness of complex 1 toward the oxidized-form
[Fe(SPh)4]-. That is, the [SPh]--coordinated ligand may promote
the stability of the electron-deficient [Fe4S3(NO)4]- core, in
contrast to the thiyl radical [ · SPh] coordinated to the
[Fe4S3(NO)4]2- core, yielding intermediate A.
(1) This study demonstrates that DNICs [(NO)2Fe(SR)2]- can
be converted back to [4Fe-4S] cluster 5 in the presence of
[Fe(SPh)4]2-/1- and S-donor species S8 via a reassembling
process ([(RS)2Fe(NO)2]- f [Fe4S3(NO)7]- (1)/[Fe4S3(NO)7]2-
(2) f [Fe4S4(NO)4]2- (3) f [Fe4S4(SPh)4]2- (5)).
Alternatively, addition of 1 equiv of the oxidized-form
[Fe(SPh)4]- into a CH3CN solution of complex 2 and the
subsequent addition of 1/8 equiv of S8 powder into the mixture
solution also led to the formation of complex 3,
[(NO)2Fe(SPh)2]-, and the remaining complex 1, characterized
by IR and UV-vis, when the mixture solution was stirred at
ambient temperature for 36 h. Presumably, complex 3 was
produced upon the initial reduction of complex [Fe(SPh)4]- by
complex 2, yielding [Fe(SPh)4]2- and complex 1, the subsequent
nitrosylation of [Fe(SPh)4]2- by complex 1, yielding
[(NO)2Fe(SPh)2]- and intermediate A, followed by the sequen-
(2) In contrast to complexes 1 and 2 serving as NO-radical-
donor species toward [Fe(SPh)4]2-, the proposed degradation
species [Fe4S3(NO)4]2- and [Fe4S3(NO)4]- derived from NO
release of complexes 2 and 1 act as a thiyl-radical acceptor and
phenylthiolate acceptor, respectively, to yield the proposed
[Fe4S3(SPh)(NO)4]2- (A) intermediate.
(3) The electronic richness of the proposed intermediate
[Fe4S3(SPh)(NO)4]2- (A), derived from trans-nitrosylation be-
tween 2 and [Fe(SPh)4]2- or trans-nitrosylation between complex
1 and [Fe(SPh)4]2-, triggers reductive elimination of (PhS)2
9
J. AM. CHEM. SOC. VOL. 130, NO. 50, 2008 17157

A R T I C L E S
Tsou et al.
Scheme 3
accompanied by the concurrent oxidative addition of S atom to
yield complex 3.
ligands for cysteinate groups of rubredoxin (Scheme 3). That
is, protein-bound DNICs derived from nitrosative stress-
damaged [4Fe-4S] iron sulfur clusters can be converted into
RBS [Fe4S3(NO)7]- and the subsequent reduced-form RBS2-
[Fe4S3(NO)7]2-. The reduced-form [1Fe-0S] rubredoxin [Fe(S-
cys)4]2- then triggers trans-nitrosylation of RBS/RBS2-, and the
sequential insertion/oxidative addition of S derived from S-
donor species results in the formation of the preassembled cluster
[Fe4S4(NO)4]2-. The [Fe4S4(NO)4]2- cluster can be captured by
the [Fe(Scys)4]--containing holoprotein to yield [Fe4S4]-cluster-
binding protein, as shown in Scheme 3. The [Fe4S4]-cluster-
binding protein was then delivered to apoprotein to complete
the cycle of the degradation and reassembly of the NO-modified
[Fe-S] clusters (Scheme 3).6,7 On the basis of this study, IscA
protein, proposed to exist as a rubredoxin-containing protein,23
not only triggers the NO-cysteinate exchange reaction but also
delivers the assembled [Fe4S4] cluster to apo-[Fe-S] proteins
under nitrosative stress-damaged conditions.24 Also, DNICs
(4) The effective NO-accepting functionality of the biomi-
metic oxidized-form rubredoxin [Fe(SPh)4]- is responsible for
the NO radical-thiyl radical exchange reaction in the transfor-
mation of complex 3 to complex 5. These results illustrate that
complex 2 acts as a pure NO-donor species, whereas complex
3 prefers NO radical-thiyl radical exchange reaction to yield
complex 5.
It has been known that repair of DNICs derived from
nitrosylation of a [4Fe-4S] cluster of endonuclease III back to
[4Fe-4S] was achieved by addition of ferrous ion, cysteine, and
IscS.12 Also, in vitro repair of DNICs to [2Fe-2S] protein
directed by IscS as well as L-cysteine and in vivo [2Fe-2S]
protein construction from assembly of cysteine, sulfur derived
from IscS, and free iron released from DNICs were proposed.14b
The biomimetic study of the degradation of [Fe-S] clusters and
repair of NO-modified [Fe-S] clusters (DNICs) that lead to initial
insights concerning the formation mechanism of [Fe-S] clus-
ters may lead to an understanding of the fundamental features
of [Fe-S] cluster biogenesis. This study may decipher that the
mechanism for the [4Fe-4S] iron-sulfur protein biosynthesis
is as follows: a preassembled [Fe4S4(NO)4]2- cluster derived
from the conversion of protein-bound DNICs is transferred from
solution to a site in a protein by exchanging its simple NO
(23) (a) Ding, H.; Clark, R. J. Biochem. J. 2004, 379, 433–440. (b) Ding,
H.; Clark, R. J.; Ding, B. J. Biol. Chem. 2004, 279, 37499–37504.
(24) (a) Krebs, C.; Agar, J. N.; Smith, A. D.; Frazzon, J.; Dean, D. R.;
Huynh, B. H.; Johnson, M. K. Biochemistry 2001, 40, 14069–14080.
(b) Wollenberg, M.; Berndt, C.; Bill, E.; Schwenn, J. D.; Seidler, A.
Eur. J. Biochem. 2003, 270, 1662–1671.
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17158 J. AM. CHEM. SOC. VOL. 130, NO. 50, 2008

Nitric Oxide-Modified Ferredoxin [4Fe-4S] Clusters
A R T I C L E S
for 30 min under nitrogen at ambient temperature. The reaction
was monitored by FTIR. The IR νNO shifting from 1715 s, 1673 s
cm-1 (R ) Et); 1737 s, 1693 s cm-1 (R ) Ph) to 1738 s, 1696 s
cm-1 was assigned to the formation of the known complex [Na-
18-crown-6-ether][(NO)2FeS5]. Hexane (15 mL) was then added
to precipitate the dark green solid [Na-18-crown-6-
ether][(NO)2FeS5] (yield 0.52 g, 90%), characterized by IR and
UV-vis spectrum.18
Reaction of [Na-18-crown-6-ether][(NO)2FeS5] and Fluoro-
boric Acid. A THF solution (10 mL) of fluoroboric acid (17.8 µL,
1 mmol) was added dropwise by cannula into a THF solution (5
mL) of [Na-18-crown-6-ether][(NO)2FeS5] (0.578 g, 1 mmol), and
the mixture solution was then stirred overnight under nitrogen at
ambient temperature. The reaction was monitored by FTIR. The
IR νNO shifting from 1738 s, 1696 s cm-1 to 1790 w, 1740 s, 1707 m
cm-1 was consistent with the formation of complex [Na-18-crown-
6-ether][Fe4S3(NO)7] (1).19 The mixture solution was filtered
through Celite to remove the insoluble solid, and hexane was then
added to precipitate the dark brown solid complex 1 (0.1491 g,
73%), characterized by IR and UV-vis spectrum.19
function not only as NO storage and transporter but also as a
nitrosative stress-damaged “descendant”, the precursor of [Fe-
S] clusters.
Experimental Section
Manipulations, reactions, and transfers were conducted under
nitrogen according to Schlenk techniques or in a glovebox (N2 gas).
Solvents were distilled under nitrogen from appropriate drying
agents (diethyl ether from CaH2; acetonitrile from CaH2-P2O5;
methylene chloride from CaH2; methanol from Mg/I2; hexane and
tetrahydrofuran (THF) from sodium benzophenone) and stored in
dried, N2-filled flasks over 4 Å molecular sieves. Nitrogen was
purged through these solvents (including dimethyl formamide
(DMF)) before use. Solvent was transferred to the reaction vessel
via stainless steel cannula under positive pressure of N2. The
reagents ferric trichloride (Aldrich), elemental sulfur (Showa),
sodium (Nihon Seiyaku Industries), bis(triphenylphosphorany-
lidene)ammonium chloride, and 18-crown-6-ether (TCI) were used
as received. Complexes [cation][(NO)2Fe(SR)2] (R ) Et, Ph),17,20
[cation]2[Fe(SPh)4],15c and [cation][Fe(SPh)4] (cation ) Na-18-
crown-6-ether) were synthesized and characterized by published
literatures.15e Infrared spectra of the νNO stretching frequencies were
recorded on a Perkin-Elmer model Spectrum One B spectrometer
with sealed solution cells (0.1 mm, CaF2 windows). UV-vis spectra
Reaction of [cation][Fe4S3(NO)7] (cation ) Na-18-crown-
6-ether (1), Me4N (1-Me4N)) and [Na-18-crown-6-ether]2-
[Fe(SPh)4]. To a CH3CN solution (5 mL) of complex [Na-18-crown-
6-ether]2[Fe(SPh)4] (0.1599 g, 0.15 mmol) was added in a dropwise
manner a CH3CN solution (5 mL) of complex 1 (0.0817 g, 0.1
mmol) at ambient temperature.15 The reaction solution was stirred
overnight at room temperature and monitored by FTIR. The IR
1
were recorded on a Jasco V-570 spectrometer. H NMR spectra
were obtained on a Varian Unity-500 spectrometer. Analyses of
carbon, hydrogen, and nitrogen were obtained with a CHN analyzer
(Heraeus).
ν
NO shifting from 1800 w, 1747 s, 1707 w cm-1 to 1743 s, 1708 s,
1697 sh cm-1 implicated the formation of the known [Na-18-crown-
6-ether][(NO)2Fe(SPh)2] (1743 s, 1697 s cm-1) and the proposed
intermediate [Na-18-crown-6-ether]2[Fe4S3(SPh)(NO)4] (1708 s
cm-1). The mixture solution was then added into a CH3CN solution
of sulfur powder S8 (0.0032 g, 0.0125 mmol) via cannula under
positive N2 pressure at ambient temperature. After the reaction
solution was stirred at ambient temperature for 24 h, the IR νNO
spectrum shifted from 1743 s, 1708 s, 1697 sh cm-1 to 1743 s,
1697 s, 1660 m cm-1, suggesting the formation of [Na-18-crown-
6-ether][(NO)2Fe(SPh)2] and [Na-18-crown-6-ether]2[Fe4S4(NO)4]
(3) (absorption spectrum (CH3CN) [λmax, nm (ꢀ, M-1 cm-1)]: 239
(30 405), 263 (21 374), 293 (10 663)).20,22 Hexane was added to
the mixture solution, extracting the byproduct (PhS)2, identified by
1H NMR. The red-brown CH3CN mixture solution was then filtered
through Celite to remove the insoluble solid, and diethyl ether was
added into the filtrate to separate the black precipitate complex 3
(0.057 g, 54%), characterized by IR, UV-vis, and single-crystal
X-ray diffraction, and the red-brown upper solution. The red-brown
upper solution was dried under vacuum and then redissolved in
THF-diethyl ether (1:1 volume ratio). The red-brown solution was
then filtered through Celite again to remove [Na-18-crown-6-
Preparation of [Na-18-crown-6-ether][Fe(SR)4] (R ) Et,
Ph). FeCl3 (0.1625 g, 1 mmol), NaSR (R ) Et (0.336 g, 4 mmol),
Ph (0.528 g, 4 mmol)), and 18-crown-6-ether (0.264 g, 1 mmol)
were dissolved in DMF (10 mL). The reaction mixture was stirred
for 30 min under nitrogen at 0 °C and then further stirred for 5 h
at ambient temperature. The reaction mixture was then filtered
through Celite to remove the insoluble solid, and diethyl ether was
added into the filtrate, leading to the precipitation of complex [Na-
18-crown-6-ether][Fe(SR)4] (R ) Et (0.299 g, 51%), Ph (0.335 g,
43%)), characterized by UV-vis spectrum (absorption spectrum
(DMF) [λmax, nm]: R ) Et (353, 488), R ) Ph (328, 554)).15e
Preparation of [Na-18-crown-6-ether]2[Fe(SPh)4]. A CH3CN
solution (10 mL) of FeCl2 (0.127 g, 1 mmol), NaSPh (0.607 g, 4.6
mmol), and 18-crown-6-ether (0.528 g, 2 mmol) was stirred for 30
min under nitrogen at ambient temperature. The reaction solution
was filtered through Celite to remove the insoluble solid, and diethyl
ether was then added into the filtrate, leading to the precipitation
of complex [Na-18-crown-6-ether]2[Fe(SPh)4] (0.65 g, 61%),
characterized by UV-vis spectrum (absorption spectrum (Ch3CN)
[λmax, nm]: 337, 390, 1700).15c
Reaction of [Na-18-crown-6-ether]2[Fe(SR)4] (R ) Et, Ph)
and NO(g). NO(g) (48 mL (10% NO + 90% N2), 0.2 mmol) was
added into a CH3CN solution (10 mL) of complex [Na-18-crown-
6-ether][Fe(SR)4] (R ) Et (0.059 g, 0.1 mmol), Ph (0.0786 g, 0.1
mmol)) via a gastight syringe at 0 °C. The reaction solution was
stirred for 10 min at ambient temperature and then monitored by
FTIR. The IR νNO spectrum, showing two strong absorption bands
at 1721 s, 1679 s cm-1 for R ) Et and 1743 s, 1697 s cm-1 for R
) Ph, was assigned to the formation of complex [Na-18-crown-
6-ether][(NO)2Fe(SR)2] (R ) Et, Ph), respectively. The solution
was then dried under vacuum, and the crude semisolid was
redissolved in THF. The mixture was filtered through Celite to
remove the insoluble solid, and hexane was added into the filtrate,
leading to the precipitation of complex [Na-18-crown-6-
ether][(NO)2Fe(SR)2] (R ) Et (0.0423 g, 81%), Ph (0.0513 g,
83%)), characterized by FTIR and UV-vis spectrum.17
1
ether][SPh], identified by H NMR, and addition of hexane into
the filtrate led to the precipitation of the known [Na-18-crown-6-
ether][(NO)2Fe(SPh)2] (0.083 g, 89%), characterized by FTIR and
UV-vis spectrum.20 In a similar fashion, reaction of complex
1-Me4N and [Na-18-crown-6-ether]2[Fe(SPh)4], followed by addi-
tion of S8, led to the formation of complex 3-Me4N. Recrystalli-
zation from a CH3CN solution of [Me4N]2[Fe4S4(NO)4] (3-Me4N)
layered with diethyl ether at -20 °C for 2 weeks led to dark crystals
suitable for X-ray crystallography.
Reaction of [Na-18-crown-6-ether][Fe4S3(NO)7] (1) and
[Na][biphenyl]. THF solution (5 mL) of sodium metal (0.023 g, 1
mmol) and biphenyl (0.019 g, 0.12 mmol) was stirred under nitrogen
for 2 h at room temperature. The purple sodium-biphenyl mixture
was then added into a THF solution (5 mL) of complex 1 (0.0817
g, 0.1 mmol) and 18-crown-6-ether (0.0264 g, 0.1 mmol) in a
dropwise manner at ambient temperature. After the reaction solution
was stirred for 12 h, solvent was removed under positive nitrogen
atmosphere to leave the black precipitation. THF was then added
to wash the precipitation twice, and the solid was dried under
vacuum to yield the black complex [Na-18-crown-6-
Reaction of [Na-18-crown-6-ether][(NO)2Fe(SR)2] (R ) Et,
Ph) and S8. Complexes [Na-18-crown-6-ether][(NO)2Fe(SR)2] (R
) Et (0.525 g, 1 mmol), Ph (0.622 g, 1 mmol)), and sulfur powder
S8 (0.026 g, 1 mmol) were dissolved in THF (10 mL) and stirred
9
J. AM. CHEM. SOC. VOL. 130, NO. 50, 2008 17159

A R T I C L E S
Tsou et al.
ether]2[Fe4S3(NO)7] (2) (0.071 g, 64%), characterized by IR (1748
w, 1690 s, 1660 sh cm-1 (CH3CN)) and UV-vis spectrum.21
Reaction of Complex [Na-18-crown-6-ether]2[Fe4S3(NO)7]
(2) and [Na-18-crown-6-ether]2[Fe(SPh)4]. To a CH3CN solution
(5 mL) of complex [Na-18-crown-6-ether]2[Fe(SPh)4] (0.1599 g,
0.15 mmol) was added in a dropwise manner the CH3CN solution
(5 mL) of complex 2 (0.1104 g, 0.1 mmol) at ambient temperature.
The reaction solution was stirred at room temperature for 3 h, and
the IR νNO shifting from 1748 w, 1690 s, 1660 sh cm-1 to 1743 s,
1708 s, 1697 sh cm-1 implicated the formation of the known [Na-
18-crown-6-ether][(NO)2Fe(SPh)2] (1743 s, 1697 s cm-1) and the
proposed intermediate [Na-18-crown-6-ether]2[Fe4S3(SPh)(NO)4]
(1708 s cm-1). Then the mixture solution was added into a CH3CN
solution of sulfur powder (0.0032 g, 0.0125 mmol) via cannula
under positive N2 pressure at ambient temperature. After the reaction
solution was stirred overnight, the IR νNO spectrum shifted from
ether was added into the filtrate to separate the black precipitate
[Na-18-crown-6-ether]2[Fe4S4(SPh)4] (0.0617 g, 45%), identified by
15
1
UV-vis and H NMR, and the red-brown upper solution. The
filtrate (the red-brown upper solution) was dried under vacuum and
then redissolved in THF-diethyl ether. The red-brown solution was
then filtered through Celite, and addition of hexane into the filtrate
led to the precipitation of black solid [Na-18-crown-6-
ether][(NO)2Fe(SPh)2] (0.091 g, 73%), characterized by FTIR and
UV-vis.20
Crystallography. Crystallographic data and structure refine-
ment parameters of complex [Me4N]2[Fe4S4(NO)4] is summarized
in the Supporting Information. The crystal chosen for X-ray
diffraction studies measured 0.14 × 0.1 × 0.02 mm for complex
[Me4N]2[Fe4S4(NO)4]. The crystal was mounted on a glass fiber
and quickly coated in epoxy resin. Unit-cell parameters were
obtained by least-squares refinement. Diffraction measurements for
complex [Me4N]2[Fe4S4(NO)4] were carried out on a Kappa CCD
diffractometer with graphite-monochromated Mo KR radiation
(λ ) 0.7107 Å) and between 2.26° and 25.31° for complex
[Me4N]2[Fe4S4(NO)4]. Least-squares refinement of the positional
and anisotropic thermal parameters of all non-hydrogen atoms and
hydrogen atoms were fixed at calculated position and refined as
riding modes. A multiscan absorption correction was made. The
SHELXTL25 structure refinement program was employed.
1743 s, 1708 s, 1697 sh cm-1 to 1743 s, 1697 sh, 1660 m cm-1
,
implicating the formation of the known [Na-18-crown-6-
ether][(NO)2Fe(SPh)2] and complex 3.20,22 Hexane was added to
the mixture to extract the byproduct (PhS)2, identified by 1H NMR.
The red-brown CH3CN solution was filtered through Celite to
remove the insoluble solid, and then diethyl ether was added into
the filtrate to separate the black precipitate complex 3 (0.0516 g,
49%) and the red-brown upper solution. The red-brown upper
solution was dried under vacuum and then redissolved in
THF-diethyl ether. This red-brown solution was then filtered
through Celite to remove [Na-18-crown-6-ether][SPh], and then
addition of hexane into the filtrate resulted in the precipitation of
the known [Na-18-crown-6-ether][(NO)2Fe(SPh)2] (0.072 g, 77%),
characterized by IR and UV-vis spectrum.20
Acknowledgment. We gratefully acknowledge financial support
from the National Science Council of Taiwan. The authors thank
Mr. Ting-Shen Kuo for the single-crystal X-ray structural
determination.
Reaction of [Na-18-crown-6-ether]2[Fe4S4(NO)4] (3) and
[Na-18-crown-6-ether][Fe(SPh)4]. To a CH3CN solution (5 mL)
of complex [Na-18-crown-6-ether]2 [Fe4S4(NO)4] (0.1046 g, 0.1
mmol) was added in a dropwise manner a CH3CN solution (5 mL)
of [Na-18-crown-6-ether][Fe(SPh)4] (0.1558 g, 0.2 mmol) at
ambient temperature. After the reaction solution was stirred for 10
min at ambient temperature, the IR νNO shifted from 1660 s cm-1
to 1743 s, 1697 s cm-1, implicating the formation of the known
[Na-18-crown-6-ether][(NO)2Fe(SPh)2]. The red-brown solution was
filtered through Celite to remove the insoluble solid, and then diethyl
Supporting Information Available: X-ray crystallographic
files in CIF format for the structure determination of
[Me4N]2[Fe4S4(NO)4]. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA806050X
(25) Sheldrick, G. M. SHELXTL, Program for Crystal Structure Determi-
nation; Siemens Analytical X-ray Instruments Inc.: Madison, WI, 1994.
9
17160 J. AM. CHEM. SOC. VOL. 130, NO. 50, 2008