Detoxifying polyhalogenated catechols through a copper-chelating agent by forming stable and redox-inactive hydrogen-bonded complexes with an unusual perpendicular structure

Article abstract of DOI:10.1002/chem.201402833

The use of selective metal chelating agents with preference for binding of a specific metal ion to investigate its biological role is becoming increasingly common. We found recently that a well-known copper-specific chelator 2,9-dimethyl-1,10-phenanthroline (2,9-Me₂OP) could completely inhibit the synergistic toxicity induced by tetrachlorocatechol (TCC) and sodium azide (NaN₃). However, its underlying molecular mechanism is still not clear. Here, we show that the protection by 2,9-Me₂OP is not due to its classic copper-chelating property, but rather due to formation of a multiple hydrogen-bonded complex between 2,9-Me₂OP and TCC, featuring an unusual perpendicular arrangement of the two binding partners. The two methyl groups at the 2,9 positions in 2,9-Me₂OP were found to be critical to stabilize the 2,9-Me₂OP/TCC complex due to steric hindrance, and therefore completely prevents the generation of the reactive and toxic semiquinone radicals by TCC/NaN₃. This represents the first report showing that an unexpected new protective mode of action for the copper "specific" chelating agent 2,9-Me₂OP by using its steric hindrance effect of the two CH₃ groups not only to chelate copper, but also to "chelate" a catechol through multiple H-bonding. These findings may have broad biological implications for future research of this widely used copper-chelating agent and the ubiquitous catecholic compounds.

Full text of DOI:10.1002/chem.201402833

DOI: 10.1002/chem.201402833
Full Paper
&
Hydrogen Bonds
Detoxifying Polyhalogenated Catechols through a Copper-
Chelating Agent by Forming Stable and Redox-Inactive Hydrogen-
Bonded Complexes with an Unusual Perpendicular Structure
Yan Li,[a] Chun-Hua Huang,[a] Yu-Xiang Liu,[a] Li Mao,[a] and Ben-Zhan Zhu*[a, b]
Abstract: The use of selective metal chelating agents with
preference for binding of a specific metal ion to investigate
its biological role is becoming increasingly common. We
found recently that a well-known copper-specific chelator
2,9-dimethyl-1,10-phenanthroline (2,9-Me2OP) could com-
pletely inhibit the synergistic toxicity induced by tetrachloro-
catechol (TCC) and sodium azide (NaN3). However, its under-
lying molecular mechanism is still not clear. Here, we show
that the protection by 2,9-Me2OP is not due to its classic
copper-chelating property, but rather due to formation of
a multiple hydrogen-bonded complex between 2,9-Me2OP
and TCC, featuring an unusual perpendicular arrangement of
the two binding partners. The two methyl groups at the 2,9
positions in 2,9-Me2OP were found to be critical to stabilize
the 2,9-Me2OP/TCC complex due to steric hindrance, and
therefore completely prevents the generation of the reactive
and toxic semiquinone radicals by TCC/NaN3. This represents
the first report showing that an unexpected new protective
mode of action for the copper “specific” chelating agent 2,9-
Me2OP by using its steric hindrance effect of the two CH3
groups not only to chelate copper, but also to “chelate”
a catechol through multiple H-bonding. These findings may
have broad biological implications for future research of this
widely used copper-chelating agent and the ubiquitous
catecholic compounds.
Introduction
We found recently that the combination of tetrachlorocate-
chol (TCC) and sodium azide (NaN3) caused a pronounced syn-
ergistic cytotoxicity in a bacterial model.[14, 15] During our study
on the role of transition-metal ions, especially Cu and Fe, we
found, unexpectedly, that only 2,9-Me2OP, but not any other
well-known metal chelating agents, markedly inhibited the
synergistic toxicity induced by TCC/NaN3. Further studies indi-
cate that the protection by 2,9-Me2OP may not be due to its
chelation of copper, but possibly due to the formation of an
unknown supramolecular complex with TCC and/or NaN3.
However, the exact chemical structure and the composition of
such a complex remained unclear because we were unable to
detect and identify this complex by the typical techniques
applied in the characterization of organic compounds in
solutions.
The use of selective transition-metal chelating agents with
a preference for the binding of a specific metal ion to investi-
gate its role in oxidative stress reactions is becoming increas-
ingly common.[1–5] 2,9-Dimethyl-1,10-phenanthroline (2,9-
Me2OP; also called neocuproine) and its water-soluble ana-
logue bathocuproine disulfonate (BCS) are two such chelating
agents. The cuprous complexes with these ligands possess
greater stability than other biologically relevant complexes
with other transition metals due to the steric hindrance provid-
ed by the two CH3 groups at the 2,9 positions of the 1,10-phe-
nanthroline ring structure.[3,4] Reductions in toxicity and de-
creased molecular damage, which are caused by addition of
these chelators, have been used to support the involvement of
copper in deleterious reactions in vivo and in vitro.[5–13] In one
of these studies, 2,9-Me2OP protected isolated rat hearts
against ischemia/reperfusion-induced injury.[13] In another
study, BCS reversed copper-mediated growth inhibition of
L1210 cells.[4]
In this study, we plan to address the following questions by
the complementary application of single-crystal X-ray diffrac-
tion, IR, and solid state NMR spectroscopic methods: 1) What is
the exact chemical structure and composition of this complex?
2) What is the nature of the binding forces for this complex?
3) Could 2,9-Me2OP form similar complexes with other poly-
halogenated catechols? 4) Why are the two CH3 substituents at
the 2,9 positions in the 2,9-Me2OP critical? 5) What is the un-
derlying molecular mechanism of the protection of 2,9-Me2OP
against TCC/NaN3 induced synergistic toxicity?
[a] Dr. Y. Li, Dr. C.-H. Huang, Dr. Y.-X. Liu, Dr. L. Mao, Prof. Dr. B.-Z. Zhu
State Key Laboratory of Environmental Chemistry
and Ecotoxicology, Research Center for Eco-Environmental Sciences
The Chinese Academy of Sciences, Beijing 100085 (P.R. China)
E-mail: bzhu@rcees.ac.cn
[b] Prof. Dr. B.-Z. Zhu
Linus Pauling Institute, Oregon State University, Corvallis, OR 97331 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402833.
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Experimental Section
Instrumentation
All the complexes were determined by single-crystal X-
ray diffraction. Data collection was performed on an Agi-
lent Gemini A Ultra diffractometer, using graphite-mono-
chromated MoKa radiation (l=0.71073 ꢁ) for 2,9-Me2OP/
TCC and 2,9-Me2OP/4,5-DCC or CuKa radiation (l=
1.5418 ꢁ) for other complexes. The determination of
crystal class and unit cell parameters was carried out by
CrysAlis (Agilent 2011) program package, raw frame data
was processed using CrysAlis (Agilent 2011), and the
structures were solved by use of SHELX97 program or
new versions and refined by full-matrix least-squares on
F
values. Crystallographic data are summarized in
Tables S1 and S2 (in the Supporting Information).
The infrared spectra of the obtained complexes were re-
corded using KBr discs on Perkin–Elmer 1430 Infrared
Spectrophotometer, in the range 4000–400 cmÀ1. The
solid-state NMR experiments were performed on
a Bruker AVANCE III 400 MHz solid-state NMR spectrom-
eter with Total Sideband Suppression (TOSS) pulse se-
quence at 5 KHz magic angle spinning (MAS) rate. The
Figure 1. A multiply H-bonded complex with an unusual perpendicular conformation
was formed between 2,9-Me2OP and TCC. A) IR spectra; B) and C) Crystal structures of
the 2,9-Me2OP/TCC complex by ORTEP drawing with 30% thermal ellipsoids, N···HÀO H-
bond is indicated by the dashed line, the secondary attractive N···HÀO interactions and
C-H···O H-bond are indicated by the dotted line (all other structures were drawn in the
same form); B) View face-on to the aromatic rings of 2,9-Me2OP and C) View along the
dissolved oxygen was measured by Thermo Scientific edge of both TCC and 2,9-Me2OP; D) Solid-state 13C NMR spectra.
Eutech DO2700 dissolved oxygen meter. ESR spectra
were obtained using Bruker ER 200 D-SRC spectrometers
and calibrated with DPPH (g=2.0037).
its poor solubility in both water and organic solvents. Further
Synthesis
IR studies suggest that the original weak hydrogen-bonds (H-
bonds) in the individual TCC or 2,9-Me2OP (the wide peak at
3451 cmÀ1 for 2,9-Me2OP was attributed to the N···H and OÀH
H-bonds because of the presence of trace amount of water)
were no longer detected when they combined together, possi-
bly forming new N···HÀO H-bonds between 2,9-Me2OP and
TCC (Figure 1A; for detailed descriptions, see the Supporting
Information). To obtain the unequivocal evidence for the for-
mation of such H-bonds and to further characterize the chemi-
cal structure of the complex, single crystals of the complex
were grown (for details, see “Synthesis” in the Experimental
Section). The solid-state structure was determined by using
single-crystal X-ray diffraction. The solution of the diffracted
data clearly showed the formation of a H-bonded complex be-
tween 2,9-Me2OP/TCC with a 1:1 stoichiometry (Figure 1B and
Table 1; for detailed crystal data, see Table S1 in the Supporting
Information).
2,9-Me2OP (50 mL, 100 mm in methanol) were mixed in the phos-
phate buffer (pH 7.4). After 10 min of vigorous stirring, the resul-
tant white precipitate was filtered, then washed with water and
methanol and dried in vacuum. The clean precipitate was then re-
dissolved in methanol/dichloromethane. After 3 days, single crys-
tals suitable for X-ray analysis were grown at room temperature.
All other complexes mentioned in this study were also synthesized
and crystallized by the procedure as described above.
Crystallographic data
CCDC-980526 (2,9-Me2OP/quercetin), -980527 (2,9-Me2OP/TCC),
-980528 (4,7-Me2OP/TCC), -980529 (5,6-Me2OP/TCC), -980530 (2,9-
Me2OP/TBrC), -980531 (2,9-Me2OP/TFC), -980532 (2,9-Me2OP/3,5-
DCC), -980533 (2,9-Me2OP/4,5-DCC), -980534 (2,9-Me2OP/4-CC), and
-980535 (2,9-Me2OP/Cat) contain the supplementary crystallograph-
ic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Table 1. Selected angles (a, [8]) and distances (d, [ꢁ]) of the complexes formed
between Me2OP and TCC.
Results and Discussion
A multiple hydrogen-bonded complex with an
unusual perpendicular conformation was formed
between 2,9-Me2OP and TCC
Complex
Inter-planar
angle[a]
N···HÀO
Secondary electro-
static interactions
CÀH···O
H bonds
H bonds
d(N O) a(NHO) d(N···O) a(NHO) d(C···O) a(CHO)
2,9-Me2OP/TCC 78.55
4,7-Me2OP/TCC 59.83
5,6-Me2OP/TCC 52.66
2.709 163.72 3.092
2.709 163.72 3.092
2.640 156.54 3.452
2.655 151.86 3.281
2.656 149.57 3.450
2.656 149.57 3.450
117.40
117.40
127.17
123.06
132.78
132.78
3.384 128.18
3.384 128.18
Our preliminary studies demonstrated that a white
precipitate was formed when a solution of 2,9-Me2OP
was mixed with TCC, but not with NaN3. We also
found that the typical analytical methods used for
solutions were not suitable for structure determina-
tion of the unknown precipitate mainly because of
–
–
–
–
–
–
–
–
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According to the H-bond distance and angle, the strength of
the two primary H-bonds was moderate. Interestingly, 2,9-
Me2OP and TCC bound in the complex adopt an unusual per-
pendicular relative arrangement, which was C2 symmetrical
(further supported by data from solid-state 13C NMR spectros-
copy, Figure 1D; for detailed descriptions, see the Supporting
Information), probably to minimize the steric hindrance caused
by the two CH3 substituents of 2,9-Me2OP (Figure 1C). The ge-
ometry of the complex allowed the hydroxyl H atom to also
form a H-bond with N atom on the other side and established
two secondary attractive H-bonding interactions between the
two adjacent primary H-bonds. Furthermore, the distance be-
tween the H atoms of the two CH3 substituents and the two
adjacent oxygen atoms in the TCC was found to be as short as
gioisomers of 2,9-Me2OP, 4,7- and 5,6-dimethyl-1,10-phenan-
throline (4,7-Me2OP and 5,6-Me2OP). The only difference be-
tween the three regioisomers is the substitution positions of
the two CH3 groups on the 1,10-phenanthroline (OP) structure
(the Supporting Information, Figure S1). The unsubstituted OP
was selected as a reference model system.
Similar to our previous observations with 2,9-Me2OP, we
found that both 4,7-Me2OP and 5,6-Me2OP, but not the unsub-
stituted OP, could also form H-bonded complexes with TCC,
and the two primary N···HÀO H-bonds of the three Me2OP/TCC
complexes were similar to each other (Table 1 and Figure 2).
However, for geometrical reasons the TCC complexes formed
with Me2OP without the CH3 substituents at the 2,9 positions
cannot establish CÀH···O interactions between the CH3 groups
2.690 ꢁ, and this may lead to the formation of two weak CÀ and the O atoms of the TCC.
H···O H-bonds, which should further stabilize the 2,9-Me2OP/
TCC complex. The blueshift observed for the CÀH stretching vi-
bration of the two CH3 groups in the IR spectrum of the com-
plex provides further support to this hypothesis (from 2954 to
2965 cmÀ1; Figure 1A).
Furthermore, the solid-state binding geometries of the com-
plexes of 4,7-Me2OP/TCC and 5,6-Me2OP/TCC showed that the
binding partners are arranged in a less perpendicular orienta-
tion. The observed change in the binding geometry of these
complexes is a consequence of a more parallel orientation
adopted by the two N···HÀO intermolecular H-bonds. The H-
bonding interactions are energetically more favored when the
atoms involved are linearly oriented. The absence of substitu-
ents at the 2,9 positions on the OP made the complex structur-
ally flexible, which most likely made the complex less stable
and more prone to dissociation with competing agents like
NaN3 or solvent molecules (see below). We also observed that
the secondary H-bonding interactions were significantly re-
duced in these complexes because the less perpendicular ori-
entation made the hydroxyl group far away from the N atom
on the other side.
Therefore, the combination of two primary N···HÀO H-bonds
with two additional attractive secondary H-bonding interac-
tions and two weak CÀH···O H-bonds, should provide a high
thermodynamic stability to the 2,9-Me2OP/TCC complex.
Similar H-bonded complexes were formed between
2,9-Me2OP and other polyhalogenated catechols
We found that similar H-bonded complexes were formed
when TCC was substituted by other tetrahalogenated cate-
chols such as tetrabromocatechol (TBrC) and tetrafluorocate-
chol (TFC; Supporting Information, Figure S2). The two 2,9-
Me2OP/TBrC and 2,9-Me2OP/TFC complexes also adopted a per-
pendicular arrangement of the binding partners and featured
similar intermolecular distances to those described for 2,9-
Me2OP/TCC complex (the Supporting Information, Figure S2
and Table S2). Interestingly, we found that 2,9-Me2OP is also
able to form analogous H-bonded complexes with catechols
featuring a reduced number of chlorine substituents as 3,5-di-
clorocatechol (3,5-DCC), 4,5-diclorocatechol (4,5-DCC), and 4-
chlorocatechol (4-CC). Even the un-substituted catechol (Cat)
formed a H-bonded complex with 2,9-Me2OP. However, the
lengths measured for the primary H-bonding interactions in
the solid-state structures of these complexes increase as the
substitution level of the catechol is reduced. This finding sug-
gests that the thermodynamic stability of the complexes is
proportional to the acidity of the phenolic groups of the cate-
chol (the Supporting Information, Figure S2 and Tables S2 and
S3).
Taken together, the results described above give a significant
thermodynamic and kinetic advantage to the 2,9-Me2OP/TCC
H-bonded complex.
The two CH3 substituents at the 2,9 positions in 2,9-Me2OP
are critical to stabilize the H-bonded complex with TCC due
to steric hindrance
Figure 2. Less stable H-bonded complexes with TCC could be formed by
4,7-Me2OP and 5,6-Me2OP, the two regioisomers of 2,9-Me2OP. A) IR spectra;
B) and C) Crystal structure of the 4,7-Me2OP/TCC and 5,6-Me2OP/TCC
complex; D) Solid-state 13C NMR spectra.
To investigate the effect of the two CH3 substituents in 2,9-
Me2OP on the thermodynamic stabilization of the complex 2,9-
Me2OP/TCC, we evaluated the binding properties of two re-
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Mechanism of protection by 2,9-Me2OP against the synergis-
tic toxicity induced by TCC/NaN3: Inhibition of the reactive
and toxic semiquinone radical generation by forming
a stable and redox-inactive H-bonded complex
It has been shown that both the chemical structure and reac-
tivity of different functional groups within a molecule could be
modulated by forming H-bonded complexes.[16,17] Since 2,9-
Me2OP was found to form a rather thermodynamically stable
H-bonded complex with TCC, we wondered whether it could
be effective in reducing the known redox-activity of TCC. We
expected that the trapping of TCC in the H-bonded complex
may inhibit its reaction with NaN3. Our previous work showed
that the reaction between TCC and NaN3 was accompanied
with enhanced oxygen consumption (from 100 to 60%, Fig-
ure 3A: Control) and the formation of O-tetrachlorosemiqui-
CÀ
none anion radical (O-TCSQ ) and other semiquinone radicals
CÀ
(SQ ; Figure 3C), which were considered to be the reactive
and toxic species responsible for cell death.[15] We found that
both the oxygen consumption and semiquinone radicals gen-
eration (as monitored by direct ESR) was inhibited by 2,9-
Me2OP in a dose-dependent manner. When the molecular ratio
of [2,9-Me2OP]/[TCC]=1:1, no oxygen consumption was ob-
served and the semiquinone radicals relative value was re-
duced to the base line, indicating that the reaction between
TCC and NaN3 was completely inhibited by 2,9-Me2OP
(Figure 3A and D). Even when 2,9-Me2OP was added 3 min
after the reaction was initiated, a sharp decrease of semiqui-
none radical signal close to the base line values was observed
(Figure 3E).
Figure 3. Inhibition of the reaction between TCC and NaN3 by 2,9-Me2OP
and its isomers. The 40% oxygen consumption caused by 1 mm TCC and
2 mm NaN3 (as a control) was completely inhibited by 1 mm 2,9-Me2OP (A),
but only partly inhibited by its two regioisomers 4,7-Me2OP, 5,6-Me2OP and
CÀ
OP under the same condition (B); The ESR signal of the O-TCSQ and other
CÀ
In contrast, 4,7-Me2OP, 5,6-Me2OP, and OP only partially in-
hibited the oxygen consumption and semiquinone radical gen-
eration under the same experimental conditions used with 2,9-
Me2OP (Figure 3B and F). These results are in good agreement
with our previous finding in the bacterial system.[15]
SQ radicals (C); The semiquinone radical generation was completely inhib-
ited by 1 mm 2,9-Me2OP (D), but only partly inhibited by its two isomers 4,7-
Me2OP, 5,6-Me2OP and OP under the same condition (F); The semiquinone
radical generation could be inhibited by 2,9-Me2OP even after the reaction
was initiated for 3 min (E). D–F were drawn according to the relative value
of the highest peak of C with the time interval of 1.66 min. All the reaction
mixtures contain 1 mm TCC and 2 mm NaN3 (also as control), and the
reaction was conducted in phosphate buffer (100 mm, pH 7.4) at room
temperature.
Based on previous studies,[15] we proposed that for TCC to
be oxidized to generate the reactive and toxic semiquinone
radicals, it is necessary that the hydroxyl group of TCC be first
deprotonated to form its anionic form TCCÀ(Scheme 1).
As mentioned above, the steric hindrance effect caused by
the two CH3 substituents at the 2,9 positions in 2,9-Me2OP
plays a critical role in stabilizing the H-bonded 2,9-Me2OP/TCC
complex. The combination of two primary N···HÀO H-bonds
with two additional attractive secondary H-bonding interac-
tions and two weak CÀH···O H-bonds, should provide a high
thermodynamic stability to the 2,9-Me2OP/TCC complex. These
special features may completely inhibit the deprotonating pro-
cess of TCC, and therefore inhibiting further oxidation of TCC
(with or without NaN3) to produce toxic semiquinone
radical species.
cause the less perpendicular orientation makes the hydroxyl
group far away from the N atom on the other side (Figures 1
and 2, and Table 1). Thus the two 4,7-Me2OP/TCC and 5,6-
Me2OP/TCC complexes are less stable as compared with 2,9-
Me2OP/TCC, and therefore, TCC may be partly deprotonated
and more prone to be attacked by N3Àto generate the reactive
and toxic semiquinone radicals.
In summary, we have discovered that the steric hindrance
provided by the two CH3 substituents in 2,9-Me2OP assists in
In contrast, due to the absence of the two CH3
groups at the 2,9 positions, although 4,7-Me2OP and
5,6-Me2OP can form similar two primary N···HÀO H-
bonds with TCC, they cannot form the two weak CÀ
H···O H-bonds with TCC; and the secondary attractive
interactions in the two 4,7-Me2OP/TCC and 5,6-
Scheme 1. Proposed possible reaction pathway for TCC/NaN3 to produce a toxic
Me2OP/TCC complexes are also markedly reduced be- semiquinone radical species.
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ordination structure, but could also “chelate” the or-
ganic catecholic compounds by forming perpendicu-
lar conformation through multiple H-bonding. Due to
its unique steric hindrance effect of the two CH3
groups at 2,9 positions, both of the two complexes
were restricted, and the catechol group and/or CuI
were also protected against the attack from other
competing agents.
To our knowledge, this represents the first report
showing that an unexpected new protective mode of
action for the copper “specific” chelating agent 2,9-
Me2OP by using its steric hindrance effect of the two
CH3 groups not only to chelate copper, but also to
“chelate” a catechol through multiple H-bonding
(Scheme 1).
Scheme 2. Proposed mechanism of protection by the copper-chelating agent 2,9-Me2OP
against the synergistic toxicity induced by TCC/NaN3: Inhibition of the reactive and toxic
semiquinone radicals generation by forming a stable and redox-inactive multiple
H-bonded complex with TCC.
Potential biological implications
We found that the formation of the unusual H-
the formation of a multiple H-bonded complex with TCC. The
2,9-Me2OP/TCC complex features an unusual perpendicular ar-
rangement of binding units. The complex is thermodynamical-
ly highly stable and is capable of inhibiting the reaction be-
tween TCC and NaN3, and the subsequent generation of the
reactive and toxic semiquinone radicals. In short, 2,9-Me2OP
completely eliminates the synergistic toxic effect induced by
TCC/NaN3 through multiple H-bonding with TCC (Scheme 2).
H-bonding is a contemporary research interest because of
its fundamental importance in many branches of science.[18–21]
Usually, the formation of H-bond would enhance chemical and
enzymatic reactions by stabilizing the transition states or reac-
tion intermediates.[22–24] However, in this study, we found an
unique example where the H-bonding between 2,9-Me2OP and
TCC is so tight that it will make the reactive catechol group
too stable to go further redox reactions.
bonded 2,9-Me2OP complex is not only limited to TCC, but it is
also a general mechanism for all polyhalogenated catecholic
compounds. Therefore, our findings may have interesting bio-
logical and environmental implications because these poly-
halogenated catecholic compounds are the reactive and toxic
metabolites, or degradation products for many widely used
polyhalogenated aromatic compounds (such as pentachloro-
phenol, Agent Orange, and hexachlorobenzene), which are
considered probable human carcinogens and have also been
detected in discharges from pulp and paper mills.[26–28]
Polyphenolic compounds, which are found in large amounts
in fruits and vegetables, have been reported to exhibit benefi-
cial antioxidant and anticancer activities.[29,30] However, they
could also exert deleterious effects by generating reactive phe-
noxyl or semiquinone radicals.[31] Interestingly, 2,9-Me2OP was
also found to efficiently inhibit cytotoxic effects induced by
33]
some of these polyphenolic compounds.[32,
Since many of
them contain the characteristic catecholic structure, we specu-
late that they probably underwent similar reaction with 2,9-
Me2OP to form H-bonded complexes. Indeed, we found that
2,9-Me2OP could combine with quercetin, a typical polyphenol-
ic compound, to form 2:1 2,9-Me2OP/quercetin complex (the
Supporting Information, Figure S3), which was stable enough
to inhibit radical generation from the oxidation of quercetin
(the Supporting Information, Figure S4 and Table S4). Therefore
the formation of H-bonded complexes with catecholic com-
pounds may serve as a general, but previously unrecognized
copper-independent new detoxication mechanism for the
widely used 2,9-Me2OP. We suggest that special care should be
paid when 2,9-Me2OP was used to study the role of copper in
the toxicity induced by polyphenolic compounds, especially
when they possess a catecholic structure.
Comparison between 2,9-Me2OP/TCC and (2,9-Me2OP)2CuI
complexes
Compared with OP, 2,9-Me2OP has two more CH3 groups at po-
sitions 2 and 9. The steric hindrance of these two CH3 groups
leads it to bind favorably with cuprous ion to form the pre-
ferred tetrahedral structure, which is typical of many CuI com-
plexes. Due to its high redox potential (+510.59 mV), as well
as its high stability constant [logb2 =19.1],[25] once (2,9-
Me2OP)2CuI is formed, copper will be stabilized at the CuI state,
thus will be unable to allow redox-cycling between CuI and
CuII.
Interestingly, these two CH3 groups also lead 2,9-Me2OP to
bind favorably with TCC to form the preferred perpendicular
structure. Due to its high stability through multiple H-bonding,
once 2,9-Me2OP/TCC was formed, TCC would be stabilized at
the catechol state, thus will be unable to allow redox-cycling
between catechol, its corresponding O-semiquinone radical
and O-quinone (see above). In other words, 2,9-Me2OP could
not only chelate the inorganic CuI by forming a tetrahedral co-
Therefore, our findings may have broad chemical, biological,
and environmental significance for future research on both
polyhalogenated aromatic pollutants and natural polyphenolic
compounds, which are two important classes of catecolic com-
pounds of major environmental and biomedical concern that
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The work in this paper was supported by the Strategic Priority
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Keywords: chelates · hydrogen bonds · radical reactions ·
reaction mechanisms · steric hindrance
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Published online on August 14, 2014
Chem. Eur. J. 2014, 20, 13028 – 13033
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