Dopamine polymerization promoted by a catecholase biomimetic CuII(μ-OH)CuII complex containing a triazine-based ligand

Article abstract of DOI:10.1039/c6dt02032a

We describe herein the catecholase-like catalytic activity and dopamine polymerization by using a dinuclear [LCu^II(μ-OH)₂Cu^II](ClO₄)₂ (1) complex where L is the dinucleating triazine-based ligand 6-chloro-N²,N²,N⁴,N⁴-tetrakis(pyridin-2-ylmethyl)-1,3,5-triazine-2,4-diamine. The kinetic parameters (k_cat = 0.318 s^-1, K_M = 1.6 × 10^-3 mol L^-1, and k_cat/K_M = 198.8 L s^-1 mol^-1), mechanistic insights into the oxidation of 3,5-di-tert-butyl catechol and early characterization of poly(dopamine) are presented.

Full text of DOI:10.1039/c6dt02032a

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COMMUNICATION
Dopamine polymerization promoted by a
II
II
catecholase biomimetic Cu (μ-OH)Cu complex
Cite this: DOI: 10.1039/c6dt02032a
containing a triazine-based ligand†
Received 20th May 2016,
Accepted 9th September 2016
a
a
a
a
DOI: 10.1039/c6dt02032a
José A. F. de Oliveira, Marcos P. da Silva, Bernardo de Souza, Tiago P. Camargo,
b
a
a
Bruno Szpoganicz, Ademir Neves* and Adailton J. Bortoluzzi*
www.rsc.org/dalton
We describe herein the catecholase-like catalytic activity and hours or even days to reach the desirable coating of the sur-
II
II
6
dopamine polymerization by using a dinuclear [LCu (μ-OH)
2
Cu ] faces, but enzymes or biomimetic catalysts can offer a smart
7
(
ClO
4
)
2
(1) complex where L is the dinucleating triazine-based solution to this problem.
2
2
4
4
ligand
6-chloro-N ,N ,N ,N -tetrakis(pyridin-2-ylmethyl)-1,3,5-
triazine-2,4-diamine. The kinetic parameters (kcat = 0.318 s− , K
Catechol Oxidase from Ipomoea batatas (ibCO) can catalyze
the oxidation reaction of a wide range of catechols to the
= 198.8 L s−1 mol−1), mechanistic corresponding o-quinones in the presence of O . In
1
M
=
.6 × 10− mol L−1, and kcat/K
3
8
1
M
2
insights into the oxidation of 3,5-di-tert-butyl catechol and early mammals, tyrosinase is involved in the two-step oxidation of
characterization of poly(dopamine) are presented.
L-tyrosine to melanin pigment biosynthesis and widely used in
the production of eumelanin and pheomelanin. It is very well
9
In the last decade, catalytically relevant biomimetic complexes
of catechol oxidase have been reported, especially after the elu-
known that model dinuclear copper(II) complexes are also able
to promote such reactions and that the ability of the catalyst to
show catecholase activity depends upon various factors such
as Cu⋯Cu distances, Cu(II)/Cu(I) redox potentials, the ligand
structure, the nature of the solvent, pH of the medium and the
cidation of the structure of the dinuclear active site of ibCO
1
(
Ipomoea batatas catechol oxidase). However, there is still
great discrepancy on the catalytic efficiency observed for model
complexes in comparison to the native enzyme. ibCO oxidizes
10
nature and number of the bridging ligands. In fact, it is well
2
−1
o-catechol with k = 2.293 s (KM = 2.5 mM) while many bio-
cat
established that dinuclear Cu(II) complexes with a Cu⋯Cu dis-
tance of 2.9–3.2 Å show the highest catecholase activity
because of the requirement of a steric match between the sub-
mimetic complexes are only able to oxidize the activated
model substrate 3,5-di-tert-butyl catechol with a much lower
3
catalytic efficiency. On the other hand, the study of structural
10
strate and the catalyst. Since dinuclear copper(II) complexes
and catalytic properties has extended the performance of
biomimetic complexes to promote more than one chemical
transformation, like cleavage of phosphodiester bonds, protein
are also known to promote such reactions, we investigated the
possibility of applying them for the same purpose. Here we
report the solution studies, catecholase-like activity, and cata-
lytic dopamine polymerization promoted by a biomimetic
model for the met form of the active site of ibCO, which shows
the solid state structure with a Cu⋯Cu distance of 2.97 Å
4
cleavage and the oxidation of catechols.
Recently, catechol-containing polymers like poly(dopamine)
(PDA) have attracted much attention due to their potential
coating of various materials such as nanoparticles and nano-
tubes, with promising applications in energy and environ-
(
Fig. 1).
5
mental technology. In general, surface polymerization
involves simple immersion of the substrate in a dilute aqueous
solution of dopamine, buffered to an alkaline pH. It can take
a
Laboratório de Bioinorgânica e Cristalografia, Universidade Federal de Santa
Catarina, Trindade Campus, Florianópolis, SC, 88040-900, Brazil.
E-mail: ademir.neves@ufsc.br, adailton.bortoluzzi@ufsc.br;
Tel: +55 (48) 3721-6844 R219, +55 (48) 3721-6847 R221
b
Departamento de Química, Universidade Federal de Santa Catarina,
Trindade Campus, Florianópolis, SC, 88040-900, Brazil
†
Electronic supplementary information (ESI) available: General experimental
procedures, mass spectrometry, potentiometric titration, kinetic data, and
poly(dopamine) characterization. See DOI: 10.1039/c6dt02032a
Fig. 1 Active site of ibCO in the met state (left) and structure of the
II
II 2+
2
cation complex [LCu (μ-OH) Cu ] (1) (right).
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II
II
The synthesis and spectroscopic characterization of 1 were [Cu (μ-OH)Cu ]. The dependence of the initial rates at pH 6.0
1
1
first described by Massoud and co-workers. By using a on the concentration of the substrate reveals saturation kine-
different reaction path, we obtained crystals of 1 with the same tics with Michaelis–Menten-like behavior (Fig. S5†).
−
1
structure as revealed by X-ray diffractometry.
The kinetic parameters (kcat = 0.318 s , K
M
= 1.6 × 10−3
The dinuclear complex was synthesized from a hexadentate- mol L− , and kcat/KM = 198.8 L s mol ) were obtained from
triazine ligand and its two copper centres are in a square nonlinear fits. Compared to the uncatalyzed reaction,
pyramidal arrangement with two pyridines and a tertiary complex 1 accelerates the oxidation of 3,5-dtbc by 1.90 × 10
amine in a facial coordination, and with two μ-OH bridges times. This promising oxidation rate of 1 is just similar to one
between the copper centres (see Fig. 1). Indeed the bridging complex described by Casella et al. as being the most active
1
−1
−1
1
1
15
7
1
6
hydroxo groups show important features in this complex: a biomimetic system reported in the literature.
II
II
very short Cu ⋯Cu distance of 2.97 Å, and a strong antiferro-
Another important aspect is the need for excess oxygen
These features are very during the reaction, since molecular oxygen is essential for
similar to those determined for the met form of ibCO (−2J > recovering the binuclear copper center, from Cu Cu to
−
1 11
magnetic coupling J = −311.2 cm
.
I
I
00 cm− with a silent EPR and dCu–Cu = 2.87 Å).
1
1,12
Cu Cu during the catalytic cycle. The oxidation reaction
II
II
3a
6
Full potentiometric titration data of 1 (Fig. S2†) in MeOH/ was monitored under optimal kinetic conditions with an
H
2
O (1 : 1, v/v) provided the formation constants for the first excess of substrate ([3,5-dtbc]/[complex] = 100 times) in the
4
+
[Cu L(H O) ] species at lower pH values (log K = 10.77), a presence of oxygen for 14 hours. Complex 1 converted 93% of
2 2 n
relatively low stability without the μ-OH bridges (Table 1 – the initial substrate concentration (∼93 turnovers). The for-
ESI†). Short pyridine-containing arms and a pronounced mation of H as a side product was assayed by reacting it
Jahn–Teller effect may contribute to the relatively low for- with iodide to give I (78% of the expected amount was detec-
2 2
O
−
3
3
a
mation constant. Titration data also reveal the two catalytically ted). Thus the oxidation of 3,5-dtbc occurs following the stoi-
important deprotonation constants: pKa1 = 5.03 and pKa2 chiometry 3,5-dtbc + O → 3,5-dtbq + H , which is distinct
.03. These pK ’s are, respectively, attributed here to the for- when compared to the mechanism proposed for the native
=
2
2 2
O
6
a
II
II
II
II
17
mation of Cu (μ-OH)Cu and Cu (μ-OH)
E – Fig. S2†). Some strong antiferromagnetically coupled
dinuclear copper(II) complexes also show pKa values in the anion binds as a bridging ligand to both copper(II) ions. After
2
Cu species (D and enzyme ibCO in which O
2 2
is reduced to H O.
Based on these results, we propose that the catecholate
I
I
5
.0–6.5 pH range, attributed to the formation of bridging μ-OH the electron transfer, the two copper centres (now Cu Cu ) are
3
b,13
between the metal centers.
reoxidized by molecular oxygen in the reaction medium,
In fact, this proposal is supported by the ESI-MS data of forming H O and completing the catalytic cycle (Fig. 2).
2
2
complex 1 under kinetic experimental conditions (Fig. S3 and
Considering these results, we were motivated to test the
S4†). An analysis of the fragment ion pattern reveals a group of complex in the oxidation of dopamine (3,4-dihydroxyphen-
peaks centered at a mass-to-charge (m/z) ratio of 335.50. This ethylamine) to produce a melanin-like polymer under the
II
2+
group corresponds to the [(C27
H
24
N
9
Cl)Cu (OH)
2
]
species, in same kinetic conditions as those employed for the 3,5-dtbc
2
full agreement with the potentiometric titration and X-ray assays. Complex 1 was able to promote PDA polymerization
−
structure as well. Another two species containing MeO anions efficiently, with the oxidation of dopamine monitored by
were observed in mass spectrometry under kinetic conditions, UV-Vis spectroscopy. The initial colourless solution turned
since methoxy groups may arise from the fragmentation of deep blue in 5 minutes and after 20 minutes, a dark powder
methanol in the ion spray, occupying the labile positions of precipitated from the solution.
the complex (Fig. S4†).
The spectrum of the PDA solution (Fig. S6†) shows an
The catecholase-like activity of 1 was determined using the intense band at λ = 609 nm (with a shoulder at λ = 568 nm),
model substrate 3,5-di-tert-butylcatechol (3,5-dtbc) at 25.0 °C which may be assigned to dopasemiquinone intraligand tran-
II
−1
in a MeOH/H
2
O (1 : 1, v/v) mixture saturated with O
2
. The reac- sition. A similar band was reported in [Zn (SQ)
3
]
(SQ = dopa-
tion was monitored by means of UV-vis spectroscopy under the
conditions of excess substrate and performed through the
stopped-flow technique. The 3,5-dtbc model substrate is
widely used because its oxidation potential is lower than the
catechol which facilitates its oxidation and its bulky substitu-
3
,14
ents prevent further oxidative processes.
The ability of 1 to
catalyze the oxidation of 3,5-dtbc is strongly influenced by the
pH of the reaction mixture with a bell-shaped profile and an
optimum pH of 6.0, which is very close to that found for ibCO
2
(
pH 6.5) and correlates with the titration results.
Sigmoidal fits resulted in kinetic pKa values of 5.21 and
6
.43, in good agreement with pKa1 = 5.03 and pKa2 = 6.03
obtained from the potentiometric titration experiments, indi-
cating that the catalytically active species should be of the type Fig. 2 Mechanistic proposal for the oxidation of 3,5-dtbc promoted by 1.
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model for the ibCO enzyme. The complex was also able to
convert dopamine to its respective polymer. These results
bring new opportunities on the use of dopamine to coat a
wide range of surfaces, faster than traditional methods. We are
currently studying the application of this poly(dopamine)
coating on nanostructures such as carbon nanotubes and
silica and investigating the resulting properties.
We are grateful to FINEP, INCT-Catálise, INCT-Biologia
Molecular Estrutural e Bioimagem, CNPq, Deonildo Faggion
Jr. and LCME-Lab for SEM analysis.
Fig. 3 SEM micrographs at two different magnifications of poly
(
dopamine).
1
8
semiquinone) and related complexes. It is likely that dopa-
mine binds complex 1 in the same way as proposed for 3,5- Notes and references
dtbc, generates the SQ-radical and starts polymerization.
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−
1
a large band between 3440 and 3381 cm , which can be
assigned to the axial stretching of the O–H bond, confirming
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−
1
between 2950 and 2841 cm , corresponding to ν(C–H) stretch-
−
1
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to the bending vibrations of CvC and N–H, respectively, are
2
0
consistent with the indole or indoline structures.
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2
1,22
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7
1
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