Ligand-dependent oxidation of copper bound α-amino-isobutyric acid as 1-aminocyclopropane-1-carboxylic acid oxidase mimics

Article abstract of DOI:10.1016/j.poly.2015.05.043

As a continuity of the [Cu^II(AIB)(bpy)(H₂O)](ClO₄)₂-containing (1) ACCO (1-aminocyclopropane-1-carboxylate oxidase) model, two new copper(II)-amino acid complexes [Cu^II₂(AIB)₂(PBI)₂(CH₃OH)(ClO₄)]ClO₄ (2) and [Cu^II₂(AIB)₂(PBT)₂(H₂O)](ClO₄)₂·CH₃OH (3) with a modified hetero-bidentate ligands (PBT = 2-pyridin-2-yl-benzothiazole and PBI = 2-pyridin-2-yl-1H-benzoimidazole) have been synthesized, characterized by various techniques including IR, UV-Vis, electrochemical and X-ray measurements, and investigated their reactivity toward H₂O₂ with respect to the redox behavior compare to 1.

Full text of DOI:10.1016/j.poly.2015.05.043

Polyhedron 98 (2015) 12–17
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Ligand-dependent oxidation of copper bound
a-amino-isobutyric acid as
1-aminocyclopropane-1-carboxylic acid oxidase mimics
a
a
a
b
a,
⇑
Dóra Lakk-Bogáth , Milán Molnár , Gábor Speier , Michel Giorgi , József Kaizer
a
Department of Chemistry, University of Pannonia, H-8200 Veszprém, Hungary
Aix-Marseille Université, FR1739, Spectropole, Campus St. Jérôme, Avenue Escadrille Normandie-Niemen, 13397 Marseille cedex 20, France
b
a r t i c l e i n f o
a b s t r a c t
II
Article history:
2 4 2
As a continuity of the [Cu (AIB)(bpy)(H O)](ClO ) -containing (1) ACCO (1-aminocyclopropane-1-
Received 20 January 2015
Accepted 31 May 2015
Available online 6 June 2015
II
carboxylate oxidase) model, two new copper(II)–amino acid complexes [Cu
2
(AIB)
OH (3) with a modified hetero-bidentate ligands
PBT = 2-pyridin-2-yl-benzothiazole and PBI = 2-pyridin-2-yl-1H-benzoimidazole) have been synthesized,
2 2 3
(PBI) (CH OH)
II
(ClO
4
)]ClO
4
(2) and [Cu
2
(AIB)
2
2
(PBT) (H
2
O)](ClO
4 2
) ꢀCH
3
(
characterized by various techniques including IR, UV–Vis, electrochemical and X-ray measurements, and
investigated their reactivity toward H with respect to the redox behavior compare to 1.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
2 2
O
Biomimetic oxidation
Oxidase activity
Kinetic studies
Amino acid
Copper(II)
1
. Introduction
Besides the Fe-based models [18–21], some Cu(II) complexes
of ACC and AIB with the NN supporting ligands 2,2 -bipyridine
0
Metal coordinated amino acids (AA) are important due to their
(bpy), 1,10-phenantroline and 2-picolylamine have also been
investigated in their reaction with hydrogen peroxide, where
Cu(I)–AA (brown species) and Cu(II)–OOH as possible active
species were proposed [22–25]. These complexes serve as
involvement in a number of biochemical and catalytic systems,
such as oxygen conveyer, electron transfer and oxidation. For exam-
ple, 1-aminocyclopropane-1-carboxylate (ACC) oxidase (ACCO) is a
key enzyme that catalyzes the final step in the biosynthesis of the
plant hormone ethylene [1–10]. ACCO has been classified as a mem-
ber of a family of non-heme iron proteins, and suggested that the
substrate oxidation proceeds through two successive monoelec-
tronic oxidation steps including the formation of peroxoiron(III)
and oxoiron(IV) intermediates [11–17]. However, there are only
few reported functional models of ACCO. These are mainly using
iron and copper ions. As heme and nonheme iron-containing func-
tional model of the ACCO we have found, that complexes
Cu-based models for the ACCO analogously to the Cu(I)–a-ketog-
lutarate complex reported by Tolman – ‘‘to emphasize the parallel
that might be established between copper and iron systems’’ [26].
In addition, in Cu-containing enzymes such as the peptidylglycine
a
-hydroxylating monooxygenases (PHMs, responsible for the
production of amidated peptide hormones), the initial -C–H
activation of the C-terminal glycine substrate is promoted by
the so-called Cu cofactor [27–30]. An analogy to the ACCO is
a
M
the use of ascorbate cosubstrate. Therefore mechanistic studies
on Cu-based models that oxidize AAs can be useful in relevance
to PHMs, too.
III
0
[
[
[
Fe (Salen)Cl] (Salen = N,N -bis(salicylidene)-ethylenediaminato)
III
18,19], [Fe (TPP)Cl] (TPP = meso-tetraphenylporphyrin) and
II
Fe (CH
3
CN)(N
4
Py)](ClO
4
)
2
(N
4
Py = N,N-bis(2-pyridylmethyl)-N-
We will present here the synthesis, crystal structure and prop-
II
bis(2-pyridyl)methylamine) [20] highly selectively and efficiently
catalyze the oxidation of ACCH to ethylene, and the alternate
-aminoisobutyric acid (AIBH) substrate to acetone. In the catalytic
erties of two copper(II)–AIB complexes [Cu
2
(AIB)
2
(PBI)
2 3
(CH OH)
II
(ClO
4
)]ClO
4
(2) and [Cu
2
(AIB)
2
(PBT) (H
2 2
O)](ClO
)
4 2
ꢀCH
3
OH (3) with
a
a various hetero-bidentate ligands (PBT = 2-pyridin-2-yl-benzothia-
cycles of these systems above high-valent oxoiron(IV) species,
zole and PBI = 2-pyridin-2-yl-1H-benzoimidazole) that exhibit
IV
Å+
IV
IV
2+
namely [Fe O(Salen)] [18,19], [Fe O(TPP)] and [Fe O(N
20], have been identified as key intermediates, but no evidence
has been found for the presence of the proposed peroxo species.
4
Py)]
efficient ACCO activity under ambient conditions using H
oxidant. To get insight into the structure–activity relationships, reac-
tivity of these complexes toward H was systematically studied
and will be discussed with respect to the stereochemistry and redox
2 2
O as an
[
2 2
O
II
⇑
Corresponding author. Tel.: +36 88 624 720; fax: +36 88 624 469.
E-mail address: kaizer@almos.vein.hu (J. Kaizer).
behavior compare to the recently published [Cu (AIB)(bpy)(H
(ClO (1) system (Schemes 1 and 2).
2
O)]
4 2
)
http://dx.doi.org/10.1016/j.poly.2015.05.043
277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
0

D. Lakk-Bogáth et al. / Polyhedron 98 (2015) 12–17
13
2
. Experimental
2.1. Materials
Solvents used for the reactions were purified by literature
0
methods and stored under argon. The 2,2 -bipyridine (bpy),
Cu(ClO
4
)
2
ꢀ6H
2
O and the AIBH were purchased from commercial
II
sources. [Cu (AIB)(bpy)]ClO
4 2
ꢀH O (1) [22], PBT [31] and PBI [31]
were synthesized following a previously described procedures.
CAUTION! Perchlorate salts of metal complexes with organic ligands
are potentially explosive. Only a small amount of material should be
prepared and handled with caution.
Scheme 2. Structures of the complexes (ligands) used in this study.
Table 1
Crystal structure details for 2 and 3.
2
.2. Analytical and physical measurements
Compound reference
Chemical formula
Formula mass
Crystal system
a (Å)
2
C
3
C
33
H
38Cl
2
Cu
N
2 8
O
13
32.5 2 2 6 13.5 2
H36Cl Cu N O S
Infrared spectra were recorded on an Avatar 330 FT-IR Thermo
952.69
494.39
monoclinic
10.6516(3)
15.6635(5)
24.1701(8)
90.341(1)
4021.0(2)
293(2)
monoclinic
9.9792(3)
17.9434(7)
22.5983(8)
99.095(1)
3995.6(2)
293(2)
Nicolet instrument using samples mulled in KBr pellets. UV–Vis
spectra were recorded on a Cary 60 spectrophotometer equipped
with a fiber-optic probe with 1 cm pathlength. Microanalyses were
done by the Microanalytical Service of the University of Pannonia.
Cyclic voltammograms (CV) were taken on a VoltaLab 10 poten-
tiostat with VoltaMaster 4 software for data process. The elec-
trodes were as follows: glassy carbon (working), Pt (auxiliary),
and Ag/AgCl in 3 M KCl (reference). The potentials were referenced
versus the ferrocenium ferrocene redox couple (+416 mV in metha-
nol in our setup). The crystal evaluations and intensity data collec-
tions were performed on a Bruker–Nonius Kappa CCD single-
b (Å)
c (Å)
b (°)
3
Unit cell volume (Å )
T (K)
Space group
Number of formula units per
unit cell (Z)
Radiation type
P2
4
1
/c
P2
4
1
/c
Mo K
1.263
a
Mo Ka
1.375
Absorption coefficient
ꢂ1
(l
mm
)
No. of reflections measured
No. of independent
reflections
9140
6167
10302
4894
crystal diffractometer (2, 3) using Mo
K
a
radiation
(k = 0.71073 Å) at 293 K. Data collection was performed with
R
int
0.0515
0.0658
0.1830
0.0984
1.025
0.065
COLLECT [32], cell refinement and data reduction with
DENZO/SCALEPACK [33]. The structure were solved with SIR92
Final R1 values (I > 2
r
(I))
0.0685
0.1662
0.1679
1.041
Final wR values (I > 2
Final R1 values (all data)
Goodness of fit
CCDC number
r
(I))
[
34] and SHELXL-2013 [35] was used for full matrix least squares
refinement. Crystallographic data and details of the structure
determination are given in Table 1, whereas selected bond lengths
and angles are listed in Tables 2 and 3. CIF files are available in the
CCDC database: CCDC 1036340–1036341 (2, 3). GC measurements
were performed on a HP 5890 gas chromatograph equipped with a
1036340
1036341
Table 2
Selected bond lengths (Å) for 2 and 3.
3
0 m Supelcowax column.
2
II
O1–Cu1
N1–Cu1
N2–Cu1
N4–Cu1
O16–Cu1
O7–Cu1
C5–N1
1.917(3)
2.015(3)
1.989(3)
1.982(3)
2.788(4)
2.744(6)
1.354(5)
1.463(6)
O2–Cu2
O3–Cu2
N5–Cu2
N6–Cu2
N8–Cu2
C13–O1
2.252(3)
1.948(3)
2.085(4)
1.962(4)
1.974(4)
1.272(5)
1.523(6)
1.499(5)
2
.3. Synthesis of [Cu
2
(AIB)
2
(PBI)
2
(CH
3
OH)(ClO
4 4
)]ClO (2)
II
To a stirred solution of Cu (ClO
4
)
2
ꢀ6H O (0.372 g, 1 mmol) of in
2
methanol (20 mL) PBI (0.195 g, 1 mmol) and AIBH (0.11 g,
mmol) were added. Dropwise addition of triethylamine
140 L, 1 mmol, freshly distilled and stored under argon)
1
(
C13–C14
C14–N4
l
C5–C6
resulted in a dark blue solution. After stirring for 1 h at room
temperature the solution was filtered and left to evaporate slowly
to ꢁ3 mL. A dark blue crystalline product was formed that was
filtered, briefly washed with a minimal amount of cold methanol
3
O1–Cu1
O5–Cu1
N1–Cu1
N2–Cu1
N3–Cu1
C14–N3
C13–O1
C13–C14
1.926(4)
2.314(4)
2.035(5)
2.024(4)
1.998(4)
1.498(6)
1.276(6)
1.518(7)
O2–Cu2
O3–Cu2
N4–Cu2
N5–Cu2
N6–Cu2
C6–N1
2.252(4)
1.931(4)
2.016(4)
2.036(4)
1.992(4)
1.313(7)
1.439(8)
1.339(7)
2 2 8 13
and dried. Yield: 0.64 g (66%). Anal. Calc. for C34H42Cl Cu N O :
C, 42.15; H, 4.37; N, 11.57. Found: C, 41.98; H, 4.37; N, 11.39%.
ꢂ1
FT-IR bands (KBr pellet, cm ) 3441, 3288, 3157, 2971, 2926,
C5–C6
C5–N2
1
1
637, 1609, 1593, 1483, 1458, 1430, 1390, 1370, 1317, 1236,
199, 1118, 1073, 1009, 923, 733, 616, 567. UV–Vis (CH OH)
3
Table 3
Selected bond angles (°) for 2 and 3.
2
N1–Cu1–N4
O1–Cu1–N2
–
174.84(14)
172.54(15)
O3–Cu2–N5
N6–Cu2–N8
168.65(15)
178.83(16)
0.166
s(Cu2)
3
N1–Cu1–O1
N3–Cu1–N2
173.33(17)
162.72(18)
0.176
N4–Cu2–N6
O3–Cu2–N5
s(Cu2)
170.56(18)
166.49(17)
0.067
s(Cu1)
Scheme 1. Reactions catalyzed by ACCO.

1
4
D. Lakk-Bogáth et al. / Polyhedron 98 (2015) 12–17
II
2 2 2 3 4 4
Fig. 1. ORTEP drawings of the [Cu (AIB) (PBI) (CH OH)(ClO )]ClO (2) (A) including the two copper centers (B and C). Ellipsoids are plotted at 30% probability level, hydrogen
atoms are omitted.
[
E
k
max
,
nm (log
pc = ꢂ37 mV. Crystals suitable for X-ray structural determination
were obtained from CH OH.
e
)] 326 (4.40), 339 (sh) (3.60), 623 (2.01).
3. Results and discussion
3
3.1. Synthesis and characterization of the complexes
Complexes 3 was synthesized analogously to 2.
The complexes were synthesized in methanol. Crystalline prod-
ucts were obtained by slow evaporation of the methanol from the
reaction mixture. Selected crystallographic data are reported in
Table 1, the single crystal structures are shown in Figs. 1 and 2.
Selected bond lengths and angles are listed in Tables 2 and 3.
Similar complexes to 2 and 3 have been structurally characterized
II
2
.4. Synthesis of [Cu
2
(AIB)
2
(PBT)
2
(H
2
O)](ClO
4
)
2
ꢀCH
3
OH (3)
Yield: 0.59 g (60%). Anal. Calc. for C33
9.45; H, 3.81; N, 8.36; S, 6.38. Found: C, 39.39; H, 3.87; N, 8.29;
S, 6.27%. FT-IR bands (KBr pellet, cm ) 3307, 3162, 2971, 2923,
343, 2014, 1618, 1605, 1498, 1397, 1363, 1326, 1228, 1111,
083, 1013, 761, 622, 571, 545, 477. UV–Vis (CH OH) [kmax, nm
)] 315 (4.16), 328 (sh) (2.97), 706 (1.88). Epc = ꢂ167 mV.
Crystals suitable for X-ray structural determination were obtained
from CH OH.
H38Cl
2 2 6 14 2
Cu N O S : C,
3
ꢂ1
earlier. These were the [Cu(ACC)(bpy)(OH
2
)](ClO
), [Cu(ACPC)(bpy)(OH
and [Cu(ACC)(2-picolylamine)]
) [22]. Our newly characterized complexes show structural
4
), [Cu(AIB)(bpy)
2
1
(
(
(
OH
ClO
ClO
2
)](ClO
4
), [Cu(ACBC)(bpy)(OH
2
)](ClO
4
2
)]
3
4
), [Cu(ACHC)(phen)](ClO
4
)
(
loge
4
analogy to the latter two examples, where polymer chain with car-
boxylate bridging ligands instead of axial water coordination was
observed. The solid structure of 2 reveals the presence of two cop-
per ions bridged by one of the AIB group. The Cu1 cation (Fig. 1B) is
in pseudo-octahedral geometry. The AIB and the PBI ligands form
the basal plane and a methanol molecule is coordinated to the
Cu(II) on elongated axial position (at distance of ca. 2.783 Å).
Moreover, the slightly distorted perchlorate anion is weakly
bonded to the metal center at distance of ca. 2.737 Å. The second
oxygen of the AIB group is ligated on the apical position of a second
copper ion (Cu2). The Cu2 ion is in a distorted square pyramidal
3
2
.5. Determination of products and kinetic measurements
II
II
A corresponding complex ([Cu (AIB)(bpy)]ClO
4
ꢀH
2
O, [Cu
OH)(ClO
2
(AIB)
)]ClO
2
II
(
PBT)
was dissolved in 10 mL of DMF/H
a sealable tube of 20 mL in the presence of appropriate amount of
NH OH with MeCN (10 l) as inner standard. Hydrogen peroxide
(H O)](ClO )
2 2 4 2
ꢀCH
3
OH or [Cu
2
(AIB)
2
(PBI)
2
(CH
3
4
4
2
O mixture (3/1 V/V) at 35 °C, in
4
l
was added through the septum with a syringe and the evolved
acetone was measured by removing 0.25 mL of the headspace
with a gastight syringe and the sample was injected into a gas
chromatograph. The concentration of acetone in the head space is
linearly proportional to the concentration of that in the reaction
mixture.
geometry (s = 0.166) with the basal plane completed by the PBI
moiety and one of the oxygen of the bridging amino acid ligand.
The PBI ligand is coordinated to the Cu1 and Cu2 ions at distances
that range from 1.95 to 2.09 Å and the Cu–AIB distances, Cu–O
are ranging from 1.92 to 2.25 Å, depending on the geometry.

D. Lakk-Bogáth et al. / Polyhedron 98 (2015) 12–17
15
II
Fig. 2. ORTEP drawings of the [Cu
2
(AIB)
2
(PBT)
(H
2 2
O)](ClO
4
)
2 3
ꢀCH OH (3) (A) including the two copper centers (B and C).
from 1.925 to 1.928 Å and the Cu–Oax from 2.250 to 2.313 Å
regardless whether the donor is from water or carboxylate in this
position.
The IR spectral analysis of the solid complexes can be useful,
2 2 s
since the characteristic m(CO )as and m(CO ) should be sensitive
to coordination modes. The marked shift of the carboxylate stretch
from those of the ligand clearly shows that the ligand is bonded to
the metal center through the carboxylate oxygen. The IR spectra of
the compounds 2 and 3 show a characteristic
frequencies for the carboxylate groups at 1637, and 1618 cm
m
(CO
2
)
as, stretching
ꢂ1
,
respectively. However, without isotope labeling the assignment
of the (CO stretching for complexes 2 and 3 can be ambiguous
due to the presence of two bands in the concerned 1400–
m
2 s
)
ꢂ1
ꢂ1
ꢂ1
1
300 cm region (1390, 1370 cm for 2 and 1396, 1363 cm
for 3). The multiple frequencies of symmetric stretching modes
may originate from the different binding positions for the carboxy-
lates. The electrochemical properties of the complexes 2, 3 were
studied by cyclic voltammetry (CV) to the cathodic direction, in
methanol, at room temperature under argon. The CVs all show
one irreversible cathodic reduction peak that varies with the
ligands 1–3; ꢂ366, ꢂ37 and ꢂ167 mV, respectively. The cathodic
current peaks can be attributed to the Cu(II)–Cu(I) reduction.
Very similar CV curve was recorded for the ACC complex [22].
The electronic spectrum of 2 and 3 shows a band due to d–d tran-
sitions at 623 and 706 nm, respectively. Apart from the d–d transi-
tion, compounds 2 and 3 show absorption bands at 326, 339 (sh),
and 315, 328 (sh) nm, which values shifted by ꢁ24 and ꢁ13 nm,
respectively, compared to those found for compound 1 (302,
Fig. 3. Correlation between the Cu(II)/Cu(I) reduction potential and the LMCT
transition of the complexes 1–3.
The Cu–NAIB distances in these chelate rings (j
2N,O-coordination
mode) are almost the same, approximately 1.98 Å. X-ray diffrac-
tion analysis was also performed on complexes 3. The copper(II)
ions are in square-pyramidal geometries. For the two centers,
s
are 0.07 and 0.18, indicating small deviation from perfectively
square-pyramidal geometry. In general, the bond lengths are in
good agreement with the expected values, the Cu–NAIB range from
3
13 nm). These bands can be assigned to N?Cu(II), ligand to metal
1
.995 to 2.001 Å, the Cu–NPBT from 2.018 to 2.035 Å, the Cu–Oeq
charge-transfer transitions (LMCT). More importantly, a correlation

1
6
D. Lakk-Bogáth et al. / Polyhedron 98 (2015) 12–17
Table 4
Kinetic data and oxidation yields for the H
2
O
2
oxidation of complexes 1–3.
[H (M)
0
ꢂ2
0
a
ꢂ4 ꢂ1
0
ꢂ2
ꢂ1 ꢂ1
Complex
[Cu]
0
(10 M)
2
O
]
2 0
E
pc (mV)
k
obs (10
s
)
k
ox (10
M
s
)
Yield (%)
1
3
2
2
2
2
1.20
1.20
1.20
1.20
1.20
1.20
0.036
0.036
0.036
0.24
0.60
1.20
ꢂ366
ꢂ167
ꢂ37
47.7
6.23
0.47
3.36
8.4
13.25
1.73
0.13
0.14
0.14
0.13
80b
0.82
8.8
20
45
92
ꢂ37
ꢂ37
ꢂ37
15.6
Reactions were performed in DMF/water (3:1 V/V) at 35 °C.
a
The value of kobs was calculated from the ln[product] vs t plots.
Ref. [19].
b
Fig. 4. Hydrogen peroxide dependence in the oxidation reaction of
0
Fig. 5. The logk ox values as a function of Epc of the 1–3 complexes. Inset: cyclic
II
[
Cu
2
(AIB)
2
(PBI)
2
(CH
3
OH)(ClO
4
)]ClO
4
(2) in DMF/water (3/1) at 35 °C. Inset: Progress
voltammograms of the complexes (DMF, 1 mM complex, 0.1 M TBAP electrolyte, at
ꢂ2
of the oxidation reaction of 2 in DMF/water (3/1) at 35 °C. [2]
0
= 1.2 ꢃ 10 M,
ꢂ2
ꢂ2
100 mV/s
scan
rate)
[complex]
0
= 1.2 ꢃ 10 M,
[H
2
O
]
2 0
= 3.6 ꢃ 10 M,
ꢂ2
[
NH
4
OH]
0
= 6 ꢃ 10 M, DMF/water (3/1) mixture T = 35 °C.
ꢂ2
[
4
NH OH]
0
= 6 ꢃ 10 M, DMF/water (3/1) mixture T = 35 °C.
was found between the LMCT band and the reduction potential, Epc
of the copper center (Fig. 3). This indicates that the observed shift
in the LMCT transitions can be attributed indirectly to the elec-
tronic effect of the bidentate ligands used in our model systems.
0
reduction (E° pc) shifted in the order 2 > 3 > 1 that corresponds
the opposite order in the LMCT transition and the reactivity toward
H O , suggesting that the metal-bound amino acid is sensitive to
2 2
the nature of the bidentate ligand in the 1–3 series. Fig. 5 shows
0
that the logk ox is inversely related to the reduction potential for
complexes 1–3. This underlines that reactivity of the Cu(I) interme-
diate is an important factor in the reaction rate.
2 2
3.2. Reactivity of the Cu(II)–AIB complexes toward H O
AIBH as a chelating ligand forms stable complex with copper. At
elevated temperature (35 °C) the stoichiometric H oxidation of
complexes 1, 2 and 3 proceeds reasonably fast in DMF/H O mix-
ture. The reactions were performed by adding 6 equivalents of
ammonium hydroxide as base and 3–100 equivalents of H as
2 2
O
2
2 2
O
oxidant versus the complex (Table 4). Product formation from the
stoichiometric reactions of the bpy-based complexes was followed
by GC. The product, acetone was analyzed as reported earlier [19].
The oxidation reaction in all cases were selective, no other prod-
ucts were obtained.
The reaction was performed either under argon or under air
with 1, but no significant difference between the conversions and
0
k values could be observed, supporting the negligible role of air
O
2
in the oxidation process [19]. In order to determine the rate-de-
pendence on the H , stoichiometric reactions were performed at
different H concentrations (Fig. 4). A straight line was obtained
in a plot of the reaction rate versus the initial concentration of
2 2
O
2 2
O
H
2
O
2
to establish a second-order rate expressions d[acetone]/dt =
0
ꢂd[2]/dt = k ox[2][H
2 2
O ].
Electrochemical studies allowed us to draw some conclusions
0
from comparison of the E° pc and the pseudo-first-order rate coef-
ficient, k ox. We have found that the potential of the irreversible
Fig. 6. Formation of Brown-intermediates in the reaction of 1–3 and ascorbate in
0
ꢂ3
ꢂ2
MeOH at ꢂ10 °C. [complex]
0
= 0.5 ꢃ 10 M, [AscH]
0
= 0.5 ꢃ 10 M.

D. Lakk-Bogáth et al. / Polyhedron 98 (2015) 12–17
17
2 2
Scheme 3. Possible mechanism and proposed intermediates for the H O oxidation of 1–3.
Earlier we were able to generate the proposed Cu(I) species
Brown intermediate) for a series of amino acids with the same
spectral features in each case in methanol by using ascorbate or
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434.
7
(
[
[
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H
2
O
2
as reductant at ꢂ10 °C [22,24]. Similar spectral behavior
[
[
7] Z. Zhang, J.-S. Ren, I.J. Clifton, C. Schofield, J. Chem. Biol. 11 (2004) 1383.
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(Fig. 6). This suggests that the Cu(II)/Cu(I) redox cycling due to
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[
[
[
the presence of H
2 2
O plays important role in the peroxide/copper
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Two copper(II)–AIB complexes with hetero-bidentate NN-donor
ligands have been prepared and structurally characterized to
model the metal site of ACCO. Furthermore, as a functional model
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[
[
of ACCO we have investigated their reactivity toward H
2
O
2
and
17] L.M. Mirica, J.P. Klinman, Proc. Natl. Acad. Sci. U.S.A. 105 (2008) 1814.
18] G. Baráth, J. Kaizer, J.S. Pap, G. Speier, N. El Bakkali-Taheri, A.J. Simaan, J. Chem.
Commun. 46 (2010) 7391.
0
found that the potential of the irreversible reduction (E° pc) shifted
in the order 2 > 3 > 1 that corresponds the opposite order in the
LMCT transition of the used NN-donor ligands and the reactivity
[
[
[
[
[
[
[
[
19] S. Góger, D. Bogáth, G. Baráth, A.J. Simaan, G. Speier, J. Kaizer, J. Inorg. Biochem.
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2 2
toward H O .
20] D. Lakk-Bogáth, G. Speier, M. Surducan, R. Silaghi-Dumitrescu, A.J. Simaan, B.
Faure, J. Kaizer, RSC Adv. 5 (2015) 2075.
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24] J.S. Pap, N. El Bakkali-Tahéri, A. Fadel, S. Góger, D. Bogáth, M. Molnár, M. Giorgi,
G. Speier, A.J. Simaan, J. Kaizer, Eur. J. Inorg. Chem. 17 (2014) 2829.
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Polyhedron 73 (2014) 37.
Acknowledgments
Financial support of the Hungarian National Research Fund
OTKA K108489), TÁMOP-4.2.2.A-11/1/KONV-2012-0071, and
COST CM1003 are gratefully acknowledged.
(
Appendix A.
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1
29 (2007) 14190.
CCDC 1036340–1036341 contains the supplementary crystallo-
graphic data for 2 and 3. These data can be obtained free of charge
via http://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 e-mail:
deposit@ccdc.cam.ac.uk.
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