Synthesis and characterization of NiIIIN3S2 complexes as active site models for the oxidized form of nickel superoxide dismutase

Article abstract of DOI:10.1002/chem.201304543

Nickel complexes, [Ni(H₂BA^RTPP)](ClO ₄)₂ (R=Ph for 1 or iPr for 2), supported by a pentadentate ligand H₂BA^RTPP were synthesized and oxidized to form Ni^III species having a N3S2

Full text of DOI:10.1002/chem.201304543

DOI: 10.1002/chem.201304543
Communication
&
Biomimetics
Synthesis and Characterization of NiIIIN3S2 Complexes as Active
Site Models for the Oxidized Form of Nickel Superoxide
Dismutase
Chien-Wei Chiang,[a] Yun-Li Chu,[a] Hong-Ling Chen,[a] Ting-Shen Kuo,[b] and Way-Zen Lee*[a]
Abstract: Nickel complexes, [Ni(H2BARTPP)](ClO4)2 (R=Ph
for 1 or iPr for 2), supported by a pentadentate ligand
H2BARTPP were synthesized and oxidized to form NiIII spe-
cies having a N3S2 coordination environment to mimic
the active site of the oxidized form of nickel superoxide
dismutase (NiSODox). The NiIII species 2+ exhibited a rhom-
bic signal with g values at 2.15, 2.12 and 2.02 similar to
that of NiSODox. DFT calculations revealed that 2+ has an
À
Scheme 1. The disproportionation of O2 into O2 and H2O2 through a cycle
of NiII and NiIII oxidation states.
unpaired electron primarily located in the dz2 orbital of the
NiIII center, which strongly overlaps with the pz orbital of
the axial pyridine nitrogen of H2BAPrTPP.
a low-spin (S=1/2) NiIII signal (gx =2.30, gy =2.24, and gz =2.01
with Azz =24.9 G).
Recently, several model compounds with N2S2 square
planar geometry for the active site of NiSODred have been syn-
thesized.[8] Maroney et al. have demonstrated that the axial imi-
Nickel superoxide dismutase (NiSOD), a new and distinct class
of SOD, was discovered from Streptomyces[1] and cyanobacte-
ria.[2] The enzyme can catalyze the disproportionation of super-
oxide anion (O2À) to dioxygen (O2) and hydrogen peroxide
(H2O2) by cycling between NiII and NiIII oxidation states. Numer-
ous studies have shown that O2À, a cellular toxin produced by
NADPH oxidase, can cause irreversible cellular damage[3] result-
ing in neurodegenerative diseases (e.g., Parkinson’s[4] and Alz-
heimer’s[5]), as well as cardiovascular diseases[6] and some can-
cers. The protein structure of the reduced form of NiSOD (Ni-
SODred), which has been recently determined, reveals the nickel
active site containing one bound nitrogen of the N-terminal
amine of His1, the other bound nitrogen from the backbone
amide of Cys2, two bound cysteinate (SÀ) residues from Cys2
and Cys6 on the equatorial plane, and the imidazole residue of
His1 at the position proximal to the nickel center (Scheme 1).[7]
The oxidized form of NiSOD (NiSODox) possesses an axial nitro-
gen from His1 that is coordinated to the NiIII center to form
a distorted square pyramidal active site (t=0.20). The NiSODox
has a rhombic electron spin resonance (EPR) spectrum with
À
dazole plays an essential role in optimizing the rate of O2 dis-
proportionation.[9] Therefore, mimics with an N3S2 coordina-
tion environment of the active site of NiSOD will be critical for
the study of the role of axial bound histidine in NiSODox.
Herein, we present two novel NiIII complexes, supported by an
N3S2 ligand, which closely mimic the active site of NiSODox.
The structures and electronic properties of these two model
compounds provide insight into the active site of NiSODox.
To construct nickel complexes with an N3S2 coordination
environment that mimic the active site of NiSODox, two ligands,
2,6-bis((2-(N-anilinylthiocarbonyl)pyrrolidin-1-yl)methyl)pyridine
(H2BAPhTPP) and 2,6-bis((2-(N-isopropylaminothiocarbonyl)pyr-
rolidin-1-yl)methyl)pyridine (H2BAPrTPP) were designed and
synthesized partially using a method found in the literature
(Scheme 2).[10] The reaction of H2BAPhTPP with [Ni(CH3CN)6]-
(ClO4)2 in CH3CN gave
a
five-coordinate NiII complex,
[a] C.-W. Chiang, Y.-L. Chu, H.-L. Chen, Prof. W.-Z. Lee
Department of Chemistry, National Taiwan Normal University
88, Sec. 4, Ting-Chow Rd., 11677 Taipei (Taiwan)
E-mail: wzlee@ntnu.edu.tw
[b] T.-S. Kuo
Instrumentation Center, Department of Chemistry
National Taiwan Normal University
No. 88, Sec. 4, Ting-Chow Rd., Taipei 11677, Taiwan (R.O.C.)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304543.
Scheme 2. Synthetic procedure of H2BARTPP (R=Ph or iPr).
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Chem. Eur. J. 2014, 20, 6283 – 6286

Communication
[Ni(H2BAPhTPP)](ClO4)2 (1). An analogue, [Ni(H2BAPrTPP)](ClO4)2
(2), was also synthesized using H2BAPrTPP as the supporting
ligand. Crystals of 1 and 2 were obtained by slow diffusion
over two days at ambient temperature. The molecular struc-
tures of 1 and 2 determined by X-ray crystallography reveal
that the NiII center of 1 and 2 is coordinated by the pentaden-
tate H2BARTPP (R=Ph or iPr) forming a NiN3S2 arrangement in
a C2 symmetric fashion (Figure 1). The two bound thioamide
The UV/Vis spectrum of 1 (Figure S1 in the Supporting Infor-
mation) has two characteristic absorption bands at 310 and
355 nm (15800 and 9220mÀ1 cmÀ1) and two d–d transition
bands at 470 (shoulder, 520mÀ1 cmÀ1) and 645 nm
(200mÀ1 cmÀ1). The UV/Vis spectrum of 2 (Figure S2 in the Sup-
porting Information), which is similar to the spectrum of our
previously reported NiII(BDPP) complex,[12] exhibits a characteris-
tic absorption band at 360 nm (3600mÀ1 cmÀ1) and two d–d
transition bands at 460 and 655 nm (90 and 120mÀ1 cmÀ1).
Cyclic voltammograms of 1 and 2 exhibit quasireversible and
reversible curves with E1/2 values of 0.837 and 0.836 V versus
Fc/Fc+ in CH2Cl2, respectively (Figure 2, inset). The quasireversi-
ble behavior of 1 can be attributed to the partial dissociation
of thioamide protons of H2BAPhTPP, due to the increase in acid-
ity upon the oxidation of 1. The electrochemical study sug-
gests that complexes 1 and 2 can be oxidized to NiIII species
mimicking the oxidized form of NiSOD.
Notably, treatment of 2 with the oxidant (NH4)2Ce(NO3)6 in
THF immediately changed its color from green to brown, indi-
cating the formation of 2+. The UV/Vis spectrum of 2+, ob-
tained from the reaction solution (Figure 2), displayed two CT
bands at 310 and 360 nm (7000 and 3200mÀ1 cmÀ1) and a d–d
transition band at 480 nm (700mÀ1 cmÀ1). The UV/Vis absorp-
tion pattern of the oxidized product 2+ is comparable to those
of as-isolated NiSODox (372 and 507 nm)[13] and reported [NiIII-
(BDPP)](PF6) (300, 380, and 450 nm)[12] However, treatment of
1 with (NH4)2Ce(NO3)6 in THF resulted in a decomposition reac-
tion observed by UV/Vis spectroscopy.
The EPR spectrum of 2+, generated in situ in THF at 77 K,
exhibited a rhombic signal that confirmed the formation of
a square pyramidal NiIII species with g values of 2.15, 2.12 and
2.02 (Figure 3). The observed EPR signal was similar to that of
NiSODox (gx =2.30, gy =2.24, and gz =2.01). Notably, only few
NiIII species in a distorted square pyramidal ligand environment
have been reported in the literature.[14] The EPR spectrum of
the reaction mixture of 1 and (NH4)2Ce(NO3)6 was also ob-
tained, but only a very weak NiIII signal was observed (Fig-
ure S5 in the Supporting Information). Recently, Harrop et al.
reported a NiSODox active site mimic that contained an N3S2
ligand. The EPR spectrum of their oxidized products, generated
in situ, revealed a thiyl radical signal with a g value of 2.00 and
an anisotropic signal (g=2.26, 2.17, ~2.00) for NiIII species.[15]
Complexes 1 and 2 were individually added to CH3CN sus-
pensions of KO2 to evaluate the probability of the oxidation of
Figure 1. Thermal ellipsoid representation of 1 and 2 at 50% probability
level. Hydrogen atoms excepting thioamide protons and counter ions of
1 and 2 are omitted for clarity.
S atoms of H2BARTPP on complexes 1 and 2 are trans to each
other with a S-Ni-S angle of 136.38 and 143.48, respectively.
The t values (0.41 and 0.29) of five-coordinate complexes
1 and 2 suggest that the geometry of 1 and 2 is distorted
square pyramidal.[11] The metric parameters of 1 and 2 are pre-
sented in Table 1.
À
1 and 2 by O2 anion. Unfortunately, neither NiIII species nor
À
Table 1. Selected bond lengths [ꢁ] and angles [8] for 1 and 2.
disproportionation products of O2 were formed. Instead, de-
protonated products Ni(BAPhTPP) (3) and Ni(BAPrTPP) (4) were
obtained. Complexes 3 and 4 could also be obtained by
adding NEt3 to 1 and NaH to 2, respectively, to remove the
thioamide protons of the corresponding ligand to form imido-
thiolate groups of BARTPP2À (R=Ph or iPr) in 3 and 4
(Scheme 3). Recrystallization of 3 and 4 by slow diffusion gave
green needle-like crystals. X-ray analysis revealed that com-
plexes 3 and 4 are five-coordinate NiII species (Figure S6 in the
Supporting Information) with t values (0.27 for 3 and 0.32 for
4) comparable to that of NiSODox (t=0.20). With a dianionic
supporting ligand, BARTPP2À (R=Ph or iPr), complexes 3 and 4
1
2
Ni(1)-S(1)
Ni(1)-S(2)
Ni(1)-N(1)
Ni(1)-N(2)
2.3215(11)
2.3152(12)
2.171(4)
1.949(4)
2.150(4)
2.333(3)
2.325(3)
2.155(8)
1.953(8)
2.146(8)
80.2(3)
Ni(1)-N(3)
N(2)-Ni(1)-N(1)
N(2)-Ni(1)-N(3)
N(2)-Ni(1)-S(1)
N(2)-Ni(1)-S(2)
S(1)-Ni(1)-S(2)
N(1)-Ni(1)-N(3)
80.80(16)
80.16(15)
113.04(12)
110.62(12)
136.34(5)
160.94(14)
81.1(4)
110.4(2)
106.2(3)
143.46(13)
161.4(3)
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4 observed in cyclic voltammetry
is similar to that of most model
[8f,16]
compounds for NiSODred
.
Consequently, no NiIII species
was observed from the reaction
of 3 or 4 with (NH4)2Ce(NO3)6.
A DFT calculation for 2+ was
performed to gain insight into
the geometric and electronic
structure of the NiIII center. The
geometry of the NiIII center in
the optimized structure of 2+ in
the gas phase reveals a distorted
square pyramidal ligand environ-
ment with S-Ni-S and Neq-Ni-Neq
angles of 137.738 and 165.718,
respectively, and a t value of
0.47. The average calculated
lengths of NiÀNeq and NiÀS are
2.053 and 2.359 ꢁ, respectively,
and that of NiÀNax is 1.929 ꢁ.
Owing to the rigidity of
H2BAPrTPP in comparison with
Figure 2. Oxidation experiment of 2 in THF using (NH4)2Ce(NO3)6 monitored by UV/Vis spectroscopy. Inset: cyclic
voltammograms of 2 in CH2Cl2 with different scan rates.
the coordinated residues in the active site of NiSODox, the NiÀ
Nax length of 2+ is shorter than the NiÀNHis length of NiSODox
(2.35 ꢁ), as a result, the NiÀNeq and NiÀS lengths of 2+ slightly
exceed those of NiSODox (1.965 ꢁ for NiÀNeq and 2.175 ꢁ for
NiÀSCys). The spin density plot of 2+ shows that the NiIII center
has an unpaired electron primarily located in the dz2 orbital of
the NiIII center, which strongly overlaps the pz orbital of the
axial pyridine nitrogen of H2BAPrTPP (Figure 4b). The DFT calcu-
Figure 3. X-band EPR spectra of 2+ recorded in frozen THF at 77 K.
Figure 4. Optimized gas-phase structure (left) and spin density (right) for:
a) 1+, and b) 2+, derived from DFT calculations.
Scheme 3. Deprotonation of 1 and 2 by NEt3 and NaH, respectively, to form
3 and 4.
lation also reveals that complex 2 is readily oxidized to NiIII
species due to the strongly coordinated pyridyl group in the
axial position and the electron-rich isopropyl thioamide groups
on the equatorial plane. In contrast, the spin density plot of 1+
(Figure 4a) shows that an unpaired electron can delocalize to
the phenyl ring of H2BAPhTPP. The delocalization of the un-
paired electron could cause side reactions to occur on the
are suspected to be readily oxidized to NiIII species. However,
the cyclic voltammogram of 3 revealed two irreversible oxida-
tion events at 352 and 630 mV versus Fc/Fc+ in CH2Cl2, and
that of 4 revealed such events at 334 and 637 mV (Figure S7 in
the Supporting Information). The irreversible behavior of 3 and
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phenyl ring or the thioamide nitrogen of H2BAPhTPP leading to
the decomposition of 1+. Such decomposition was observed
upon the oxidation of 1 by (NH4)2Ce(NO3)6 by UV/Vis and EPR
spectroscopy. In addition, the cyclic voltammograms of 1 in
different scan rates exhibited that more oxidized 1+ would de-
compose when the scan rate was decreased (Figure S9 in the
Supporting Information).
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In conclusion, two NiII complexes 1 and 2, which can be oxi-
dized to NiIII species to mimic the active site of NiSODox, were
synthesized. With the axial pyridyl group and electron donat-
ing isopropyl thioamide groups, complex 2 was oxidized by
(NH4)2Ce(NO3)6 to form a metastable NiIII species, which exhibit-
ed a rhombic signal in its EPR spectrum. Also, DFT calculations
supported the formation of 2+. The NiIII complex 2+ has
a N3S2 ligand environment closely mimicking the first coordi-
nation sphere of the active site of NiSODox. Furthermore, our
results suggest that the axial His1 of NiSOD plays an important
role in stabilizing the NiIII center of the oxidized form of NiSOD.
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CH2Cl2) is much higher than that of wild-type NiSOD at
À0.365 V versus Fc/Fc+ in aqueous solution[17] lying between
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We thank the financial support from the Ministry of Science
and Technology of Taiwan (102-2113M-003-007-MY3 to W.-Z.L.)
and National Taiwan Normal University (T10207000288 and
T10207000332 to W.-Z.L.).
Keywords: biomimetics
·
catalysis
·
density functional
calculations · nickel(III) complexes · superoxide
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Published online on April 15, 2014
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