A Robust Fungal Allomelanin Mimic: An Antioxidant and Potent π-Electron Donor with Free-Radical Properties that can be Tuned by Ionic Liquids

Article abstract of DOI:10.1002/cplu.201900195

Developing effective strategies to increase the chemical stability and to fine-tune the physico-chemical properties of melanin biopolymers by rational control of π-electron conjugation is an important goal in materials science for biomedical and technological applications. Herein we report that poly-1,8-dihydroxynaphthalene (pDHN), a non-nitrogenous, catechol-free fungal melanin mimic, displays a high degree of structural integrity (from MALDI-MS and CP/MAS ¹³C NMR analysis), a strong radical scavenging capacity (DPPH and FRAP assays), and an unusually intense EPR signal (g=2.0030). Morphological and spectral characterization of pDHN, along with deassembly experiments in ionic liquids, indicated amorphous aggregates of small globular structures with an estimated stacking distance of 3.9 ? and broadband absorption throughout the visible range. These results indicate that DHN-based melanins exhibit a high structural integrity and enhanced antioxidant and free-radical properties of potentially greater biomedical and technological relevance than for typical indole-based eumelanins.

Full text of DOI:10.1002/cplu.201900195

Accepted Article
Title: A Robust Fungal Allomelanin Mimic: An Antioxidant and Potent
π-Electron Donor with Free-Radical Properties that can be Tuned
by Ionic Liquids
Authors: Paola Manini, Valeria Lino, Paola Franchi, Gennaro Gentile,
Teresa Sibillano, Cinzia Giannini, Emanuela Picardi,
Alessandra Napolitano, Luca Valgimigli, Cinzia Chiappe, and
Marco d'Ischia
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ChemPlusChem
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A Robust Fungal Allomelanin Mimic: An Antioxidant and Potent
π-Electron Donor with Free-Radical Properties that can be Tuned
by Ionic Liquids
[
a]*
[a]
[b]
[c]
[d]
Paola Manini, Valeria Lino, Paola Franchi, Gennaro Gentile, Teresa Sibillano, Cinzia
[
d]
[a]
[a]
[b]
[e]
Giannini, Emanuela Picardi, Alessandra Napolitano, Luca Valgimigli, Cinzia Chiappe, Marco
[
a]
d’Ischia
Abstract: Developing effective strategies to increase the chemical
stability and to fine tune the physico-chemical properties of melanin
biopolymers by rational control of π-electron conjugation is an
important goal in materials science for biomedical and technological
applications. Herein we report that poly-1,8-dihydroxynaphthalene
damaging radiations. Melanins are usually defined on the basis
[
1,2]
of their biogenetic origin.
Animal pigments consist mainly of
black nitrogenous eumelanins, which arise from the oxidative
polymerization of tyrosine via 5,6-dihydroxyindole (DHI) and 5,6-
[
1]
dihydroxyindole-2-carboxylic acid (DHICA).
In plants and
(pDHN), a non-nitrogenous, catechol-free fungal melanin mimic,
microorganisms, different kinds of melanin-type pigments are
displays a high degree of structural integrity on MALDI-MS and
produced, originally termed allomelanins, which arise from non-
13
[3,4]
CP/MAS C NMR analysis, a strong radical scavenging capacity
DPPH and FRAP assays) and an unusually intense EPR signal (g =
.0030). Morphological and spectral characterization of pDHN, along
nitrogenous phenolic substrates.
All melanins exhibit almost
(
complete insolubility in all solvents, a high degree of structural
and supramolecular disorder and an antioxidant capacity
2
[
5]
with de-assembly experiments in ionic liquids, indicated amorphous
aggregates of small globular structures with an estimated stacking
distance of 3.9 Å and broadband absorption throughout the visible
range. These results indicate that DHN-based melanins exhibit a
high structural integrity and enhanced antioxidant and free radical
properties of potentially greater biomedical and technological
relevance compared to typical indole-based eumelanins.
associated with efficient H-atom donor behavior. All these
properties are associated with characteristic set of
a
physicochemical properties, including broad band visible light
absorption, accounting for their dark colors, an intrinsic free
radical character associated with
a
permanent electron
paramagnetic resonance (EPR) signal and water-dependent
[
6,7]
hybrid ionic-electronic conductor behavior.
The peculiar properties of natural eumelanins have prompted
[8]
intense research efforts aimed at exploiting synthetic mimics
as tailorable biomaterials for nanosized functional systems,
polymer stabilization, surface functionalization and coating,
signal transducing biointerfaces for organic (bio)electronics,
Introduction
[9,10]
The production of black, insoluble and highly π-conjugated
melanin pigments is part of the adaptive response of man,
mammals, birds, fish, cephalopods, plants and fungi, to various
cues, such as UV irradiation, oxidative stress and highly
drug delivery and other biomedical and technological
applications.
Despite considerable progress in the understanding of melanin
properties and the definition of basic structure-property-function
[
5,11]
relationships for polymer design,
there are still a number of
gaps that hinders the definitive maturation of a eumelanin-based
technology.
[
a]
Prof. P. Manini, Dr E. Picardi, Prof. A. Napolitano, Prof. M. d’Ischia
Department of Chemical Sciences
University of Napoli Federico II
Via Cintia 4, I-80126 Napoli, Italy
E-mail: paola.manini@unina.it
Prof. P. Franchi, Prof. L. Valgimigli
Department of Chemistry “G. Ciamician”
University of Bologna
Via S. Giacomo 11, I-40126 Bologna, Italy
Dr G. Gentile
Institute for Polymers Composites and Biomaterials
National Research Council of Italy
Via Campi Flegrei 34, I-80078 Pozzuoli, Italy
Dr T. Sibillano, Dr C. Giannini
Istituto di Cristallografia (IC) CNR
via Amendola 122/O, I-70126 Bari, Italy
Prof. C. Chiappe
Rational selection of monomer building blocks has recently
emerged a sa valuable strategy for controlling and tailoring π-
electron properties of melanins for antioxidant, optoelectronic
[
[
b]
c]
[
12]
and biomedical applications.
In support of this view, in a
recent study we showed that replacing nitrogen by sulfur
markedly affects the mode of polymerization of DHI leading to
novel melanin-type materials with
regioregularity.
a
high degree of
[
13]
Fungal pathways of melanin synthesis have so far been little
considered as a source of bio-inspiration in the field of functional
[
[
d]
e]
[14]
materials. In Ascomyces a black melanin pigment is produced
by the oxidative polymerization of 1,8-dihydroxynaphthalene
[
15]
Department of Pharmacy
University of Pisa
via Bonanno Pisano 6, I-56126 Pisa, Italy
(DHN).
An interesting property of Ascomyces melanin is its
[16]
reported ability to shield organisms from ionizing radiation.
In
addition, fungal melanin synthesis has been related to energy
transduction mechanisms and biological properties such as
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
[17]
virulence.
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Intrigued by these and other properties of Ascomyces melanins,
Structural and morphological characterization of pDHN.
we have recently investigated the black melanin polymer
pDHN was synthesized using the enzymatic system horseradish
[
18]
produced by the oxidative polymerization of DHN
in
peroxidase/H
2
O
2
in phosphate buffer, pH 7.4, as previously
[
18]
comparison with DHI- and DHICA-melanins as reference
nitrogen-containing biomimetic polymers. MS analysis indicated
the presence of regular patterns of oligomers with intact
structural units (see SI section). This feature revealed a marked
difference from DHI melanins in which extensive breakdown of
reported.
Under these conditions, the reaction led to the
transient formation of a greenish blue species which rapidly
evolved toward the formation of a black insoluble precipitate.
This latter was isolated by centrifugation and washed for
structural and spectroscopic characterization.
Preliminary solubility tests were performed by suspending pDHN
in different solvents (isopropyl alcohol, N-methylpyrrolidone,
[
19]
quinone units during polymerization is usually observed.
Chemical studies indicated that DHN tends to polymerize via
,2’-, 2,4’- and 4,4’-bondings to give regular patterns of
oligomers that may exhibit extended quinone-methide
2
N,N-dimethylformamide (DMF) and dimethylsulfoxide at a
nominal concentration of 1 mg/mL, then exposing suspensions
to ultrasounds for 30 minutes and measuring the final
absorbance value following filtration. The most homogeneous
suspension was obtained in DMF (Figure 2, inset). The UV-
visible spectrum of the DMF suspension showed broadband
absorption throughout the whole spectrum, typical of eumelanin
polymers, with an intense absorption in the visible region,
[
20]
moieties.
OH OH
DHN
denoting
a high degree of polymerization. Moreover, the
[
O]
presence of shoulders at 390, 460 and 580 nm suggested the
presence of extended quinonoid moieties, as reported in a
previous work, indicating a high π-electron conjugation.
OH OH
[20]
OH OH
OH OH
+
2
,2-dimer
4,4'-dimer
+
HO
HO
OH OH
OH OH
1
2
,4'-dimer
pDHN
0
300
400
500
600
700
800
Figure 1. First stages of the oxidative polymerization of DHN.
Wavelength (nm)
The symmetry and the meta-like peri dioxygenation pattern of
DHN, precluding labile o-quinone formation in the first step of
the process, could drive the generation of relatively more regular
melanin-type polymers and their efficient assembly into
aggregates. This reactivity can account for the different
properties exhibited by the DHN melanin compared to DHI-
based eumelanins.
Figure 2. UV-vis spectrum of a pDHN suspension in DMF.
A typical melanin-type spectrum was observed by ATR analysis
(see SI section). MALDI-MS analysis confirmed the previously
[
18]
reported regular polymeric nature of pDHN.
C
1
3
cross-polarization/magic-angle-spinning (CP/MAS) NMR
analysis (see SI section) revealed the presence of a wide
overlap of bands in the range of δ 100-150, attributable to the
aromatic carbons, with a small peak around 160 ppm, attributed
to OH-bearing carbons, and a broader band at 190 ppm
consistent with carbonyl-type carbons. These resonances
overall were compatible with extended quinone-methide
moieties framed within more or less large oligomeric scaffolds.
No detectable aliphatic resonances, denoting oxidative
breakdown of quinonoid moieties, were observed, unlike with
To verify this prediction, we undertook a detailed spectral and
morphological characterization of pDHN with
a view to
elucidating the impact of extended quinone-methide formation
within catechol- and o-quinone-free scaffolds on the degree of
π-conjugation and stacking in the polymer aggregates. Electron
paramagnetic resonance (EPR) data were coupled with de-
stacking experiments in ionic liquids (IL) for probing aggregation
dependent effects on the EPR signal. Finally the radical
scavenging properties of pDHN were preliminarily assessed in
comparison with those of reference eumelanins.
[21]
most DHI melanins.
Transmission electron microscopy (TEM) analysis(Figure 3)
revealed that pDHN is constituted by porous, quasi-spherical
primary particles, similar to those observed in the case of
[5]
melanins from DHI, connected in loose aggregates with
approximate size in the range 200 nm - 2 mm. The statistical
Results and Discussion
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analysis of the lateral dimensions of the primary particles
revealed a narrow unimodal size distribution, well fitted with a
lognormal function(Figure 4c, adj R-square = 0.9726, shape
EPR investigations and effect of pDHN de-assembly in ionic
liquids. Electron paramagnetic spectra (EPR) recorded on solid
pDHN revealed an unusually intense melanin-type signal.
parameter σ = 0.33 ± 0.02), with median value ρ = 21.1 ± 0.5 nm. The g-factor value determined for pDHN (g = 2.0030) was
smaller than that measured for nitrogenous melanins from DHI
[
5]
and DHICA (2.0034), but in line with that measured for
[
24]
polyphenolic polymers; this suggested a more pronounced C-
centered character of unpaired spins in the fungal melanin mimic
(
Table 1, Figure 5).
g = 2.0032
ΔB = 5.0
Spin/g = 8.088 x 10
g = 2.0030
1
8
W = 4.8 G
Figure 5. EPR spectrum of pDHN in the solid state.
Figure 3. TEM images and size distribution of the nanostructures from pDHN.
Table 1. EPR spectral parameters (W = line width in Gauss; g-factor) recorded
The X-ray powder diffraction (XPD) spectrum of pDHN, shown in
Figure 4, presents the typical pattern of an amorphous material
for pDHN and other synthetic melanins and polyphenols in the solid state.
-1
with a major peak at q = 1.6 Å , corresponding to a stacking
distance d = 2π/q = 3.9 Å. This spectrum is compared in the
same figure with the XRD pattern collected on DHI-melanin, with
Sample
W (G)
g-factor
pDHN
DHI melanin
DHICA melanin
Caffeic acid-polymer
Gallic acid-polymer
4.8
5.6
4.4
3.7
3.3
2.0030
2.0035
2.0034
2.0033
2.0030
[
5]
-1
the major diffraction peak at q = 1.8 Å (i.e. d = 3.5 Å). Fungal
melanins have recently been reported to produce diffraction
[5]
[24]
24]
-1
[
spectra with a peak at approximately q = 1.4 Å (corresponding
[
22]
to a stacking distance d = 2π/q = 4.5 Å),
which is at an
angular position smaller than that measured for polymerized L-
DOPA and other eumelanins reported previously.
A reasonable explanation may be based on the lack of o-
[
23,8]
semiquinone free radicals, which typically account for g-factor
[
10]
On this basis, stacking distances in the 4.1–4.5 Å range were
deduced for fungal melanins, which are similar to the distance
determined for our pDHN, and quite larger from those of DHI-
values in eumelanins higher than 2.0040.
This observation
would be compatible with extended quinone-methide moieties
ensuring larger spin delocalization across planar segments of
polymeric carbon platforms. The line width (W) of the signal, 4.8
G, proved to be intermediate between the DHI melanin value
[
8]
melanin (3.5 Å).
(
5.6 G) and the DHICA melanin value (4.4 G). This data
confirmed that the pDHN structure allowed a tridimensional spin
delocalization not only along the polymer chain in the conjugated
quinonoid planar regions, but also through π-stacking
interactions between the aromatic units, as suggested also for
DHI melanin.
Recently, we discovered that ionic liquids (ILs) can cause
extensive de-assembly of polydopamine aggregates up to the
nanosize level, allowing unprecedented insights into the role of
[
25]
aggregation in melanin paramagnetic properties. Accordingly,
in further experiments the effect of ILs on pDHN EPR properties
was investigated to inquire into the origin of the intense signal
and for probing possible structure-property relationships. To this
aim, two sets of ILs (based on cation structure) were
synthesized (Figure 6). IL1-10 were characterized by an
imidazolium moiety and differed only in the anion counterpart,
Figure 4. X-ray powder diffraction spectra of pDHN (green trace) and DHI-
melanin (black trace).
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while IL11-12 featured a tetraalkylammonium moiety. IL1, IL4,
IL10 and IL11 were all characterized by a dimethylphosphate
anion component and different cation counterparts. Special
interest was focused on the effect of N-methyl-N,N,N-
trioctylammoniumoleate (IL12), which proved highly effective in
de-assembling polydopamine by means of the long alkyl chains
on both the cationic and anionic portions.
The same effect can be observed also by examining the band
width (W) of the signals of the pDHN in ILs and in the solid state.
In particular, all ILs induced a marked signal broadening (high
W) suggesting a higher degree of spin delocalization in the
melanin backbone due to the disassembly of the polymer. This
effect can be rationalized considering the Kamlet-Taft solvent
[
26-28]
parameters reported for some of the ILs tested (Table 3).
In
detail, the signal broadening proved to be more pronounced for
ILs with low polarity/polarizability (π*) and high hydrogen-bond
acceptor basicity (β).
+
+
+
N
N
N
N
N
N
O
O
O
P
P
P
-O
IL1
-O
IL2
O
-O
IL3
O
H
Table 2. EPR spectral parameters (I = signal intensity in arbitrary units; W =
line width in Gauss; g-factor; N×spins/L) recorded for pDHN suspended in
different ionic liquids.
O
CH3
O
Ionic
liquid
[a]
+
+
+
I (A.U.)
W (G) g-factor
N×spins/L
N
N
N
N
N
N
O
IL1
IL2
IL3
IL4
IL5
IL6
IL7
IL8
1.05 E+05
1.40 E+05
1.14 E+05
4.60 E+04
1.10 E+04
1.03 E+04
3.80 E+04
5.05 E+04
2.20 E+05
1.80 E+04
1.20 E+04
4.88 E+04
3.01 E+06
5.1
5.3
5.3
7.4
5.4
6.2
5.4
5.7
7.5
5.2
6.4
6.3
4.8
2.0038
2.0038
2.0037
2.0032
2.0047
2.0039
2.0030
2.0035
2.0040
2.0031
2.0036
2.0043
2.0030
1.09 E-05
1.52 E-05
1.24 E-05
1.05 E-05
1.34 E-06
1.60 E-06
4.63 E-06
6.78 E-06
5.17 E-05
2.03 E-06
2.05 E-06
8.09 E-06
-
O
NH2
O
-O S
P
-O
O
-
O
O
IL4
O
IL5
IL6
+
+
+
N
N
N
N
N
N
OH
O-
IL9
-
-
O
S
O
IL10
IL11
IL12
None
N
N
S
O
N
N
N
F3C
CF3
O O
IL7
IL8
IL9
[
a] N×spins/L= concentration of radicals (unpaired electrons) in the IL
C H
8
17
+
OH
suspension (corresponding to molar concentration in solution).
+
N
+N
H C
C H
8 17
N
3
C H
17
Table 3.Kamlet-Taft solvent parameters(π*=
polarity/polarizability, β =
O
P
O
8
O
hydrogen-bond acceptor basicity) measured for ILs.
P
-O
-O
O
O
-O
Ionic Liquid
π*
β
Reference
[26]
[26]
[27]
[26,28]
[26,28]
(CH2)
7
(CH ) CH
2 7 3
O
O
IL4
IL6
IL2
IL8
IL7
0.98
1.04
1.05
1.09
1.13
1.13
1.09
1.10
0.24
0.62
IL10
IL11
IL12
Figure 6. Formulas of the ILs synthesized in this work
pDHN was added to ILs at 1 mg/mL concentration and subjected
to ultrasound treatment for 30 min in an ice bath. The
suspensions thus obtained were centrifuged and the
supernatants subjected to EPR spectroscopy. As reported in
Tables 2, pDHN-containing IL samples gave signals with
variable g-factor values and line widths. This behavior would
suggest aggregation-dependent inhibition (“freezing” effect) to
spin delocalization in the solid state. Upon de-aggregation in ILs,
an increased proportion of the spin population becomes
exposed to the IL, with consequent removal of aggregation
dependent constraints to delocalization. Data reported in Table 2
indicated that g-factors varied in most of the tested ILs
compared to the solid state, with values mainly ranging 2.0037-
From these data it was concluded that ILs can give rise to
variable, structure-specific interactions with free radical centers
in pDHN aggregates, at variance with the case of
[
25]
polydopamine,
for which constant shifts in g-factor were
observed with destacking with all ILs. It appears, moreover, that
the hydrophobic and basic character of the IL plays an important
role in promoting the disassembly of the polymer with
consequent tuning of spin delocalization.
Antioxidant properties. DHI and related diffusible eumelanin
[
29]
[5]
precursors as well as their polymers display marked
antioxidant properties of potential relevance for applications as
functional materials. Recent studies indicated a correlation
between the paramagnetic properties of synthetic eumelanins
2
.0047, typical of phenoxyl O-centered radicals.
Worthy of note is the case of IL-5 and IL-12 inducing the most
significant shift of the g-factors. This suggested the determinant
role played by hydrophobic (IL-12) and H-bond accepting (IL-5)
properties of ILs in tuning the spin delocalization via
disassembly of the polymer.
[5,24]
and (poly)phenolic polymers and their antioxidant activity.
Accordingly, in a final group of experiments the potential
antioxidant behavior of pDHN were preliminarily explored. To
this aim, two commonly used chemical tests were used, namely
the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay and the ferric
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[
31,32]
reducing antioxidant power (FRAP) test.
The first one
systems, and its presence and fate can be efficiently probed by
EPR.
accounts for the H-donor ability of a given species, whereas the
second one measures the iron(III) reducing power. pDHN was
assayed as fine suspension obtained by homogenization in a
glass/glass potter.
Data obtained from DPPH and FRAP assays on pDHN are
reported in Figure 7 and Table 4 and compared with those
reported in the literature from other melanin and
polyphenolicpolymers.
Non-nitrogenous fungal melanin mimics devoid of catechol
functions are thus proposed as innovative prototypes of melanin-
based functional materials with exceptionally robust π-electron
[
35,36]
properties and radical scavenging ability
suitable for
applications in biomedicine and materials science. In addition,
these results may provide an improved background to inquire
into the fate of PAH-derived carbon-containing species in
[
37]
potential astrochemical and prebiotic environments.
Table 4. EC50 values from DPPH assay and ferric reducing power expressed
as Trolox equivalents of pDHN and other melanin and polyphenol samples.
Sample
DPPH-EC50
µg/mL)
FRAP
Experimental Section
(
(Trolox eqs)
Synthetic procedures for ILs. IL1-IL4 and IL10 were prepared by
addition of the alkylating agent (trimethyl phosphate or
dimethyl−methylphosphonate or dimethyl phosphite, 1.1 molar
pDHN
6.5+0.8
14.4+ 0.7
21.5 + 0.7
10.5 + 0.7
1.82+0.3
0.90+ 0.06
0.65 + 0.05
0.49 + 0.05
[
30]
DHICA melanin
equivalents) to a molar equivalent of the proper alkylimidazole (methyl or
[
24]
[25]
Caffeic acid-polymer
butylimidazole), according to the previously reported procedure.
N-
[
24]
Methyl-N,N,N-trioctylammonium oleate (IL12) was prepared by addition
of a molar equivalent of oleic acid to a methanolic solution of N-methyl-
N,N,N-trioctylammonium methylcarbonate (43%), using the same
Gallic acid-polymer
Data are expressed as mean ± SD of three independent experiments.
[
38]
experimental conditions described in the literature. Analogously, ionic
liquids IL5-IL9 were prepared by addition of an equimolar amount of the
desired acid (2-aminoethanesulfonic acid, acetic acid, dicyanamide,
bis(trifluoromethane)sulfonimide, mandelic acid) to the commercial 1-
butyl-3-methylimidazolium methylcarbonate methanol solution, at room
temperature. The mixture was gently heated to 50 °C. After 2 h, the
mixture was concentrated in vacuo at 50 °C to remove the methanol and
then was dried under high vacuum and continuous stirring at 50 °C for 8
h. In the case of IL5, due to the low solubility of this reagent in methanol,
a water solution was used. Finally, choline dimethylphosphate (IL11) was
obtained by reaction of choline chloride with a small excess of trimethyl
phosphate (10% mol) at 80 °C for 12h. Any volatile compound was
Both assays showed that pDHN has a strong reducing capacity,
higher than that exhibited by DHICA melanin and by the
polymers obtained from catechols (e.g. caffeic acid) and
triphenols (e.g. gallic acid). Indeed, in DPPH-EC50 test lower
values indicate higher reducing capacity, while in FRAP test this
is indicated by higher values. It should be noted, however, that
the two tests are not simply inversely proportional, as they are
based on different reaction mechanisms: the first investigates
the formal H-atom transfer ability of the potential antioxidant,
[33]
while FRAP assesses its ability to give electron-transfer. Both
tests converge promising higher antioxidant activity for pDHN,
which could be based on both mechanisms. This behavior can
be in part explained by considering the efficient antioxidant
properties exhibited by the monomer precursor, 1,8-DHN. This is
due to the stabilizing effect exerted on the aryloxyl radical both
by intramolecular hydrogen bonding and by the naphthalene ring
removed by vacuum evaporation. The pure product was subject to
[39]
AgNO
3
analysis to confirm the absence of a starting material.
The
structure and purity of the recovered liquids (yields 95−97%) were
checked by NMR analysis.
Synthesis of poly-1,8-dihydroxynaphthalene (pDHN) and treatment
with ILs. The oxidation of 1,8-DHN was carried out according to a
[
18]
[34]
previously reported procedure.
In brief, a solution of 1,8-DHN (500
system.
mg,3.12 mmol) in acetonitrile (20 mL) was added to a 0.1 M solution of
phosphate buffer, pH 7.4 (300 mL) and treated under vigorous stirring
with HRP (15 U/mL) and hydrogen peroxide (375 μL, 1.2 molar
equivalents). After 24 h the reaction was acidified with a solution of HCl
Conclusions
(
2 M) until pH 2 was reached. The reaction mixture was centrifuged at
pDHN, a synthetic mimic of fungal melanin from Aspergillus, has
been shown to display typical features of a eumelanin-type
polymer but a greater structural stability, stronger antioxidant
properties and a marked paramagnetic behavior. When viewed
in terms of structure-property relationships, these results
demonstrate that most of the characteristic properties of
eumelanins, i.e. a broadband absorption spectrum, an intrinsic
EPR signal and H-atom/electron donating capacity, are not
specifically due to catechol-containing building blocks, nor to
nitrogenous centers. Actually, DHN-derived structures can
provide stronger stabilization to C-centered radicals rather than
to O-centered ones, providing useful probe systems for studies
of the origin of free radical properties in melanin type polymers.
pDHN can be incorporated into various materials polymers and
4 °C (7000 rpm for 15 minutes) to collect the dark precipitate that was
washed twice with water and then dried under reduced pressure. In
separate experiments, pDHN samples have been suspended in the
proper ionic liquid at a concentration of 1 mg/mL and subjected to
[
25]
ultrasound treatment for 30 min in an ice bath (amplitude 30%).
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exhibiting dark colorations have been used for EPR measurements.
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RigakuD/teX Ultra detector. An asymmetric Johansson Ge(111) crystal is
used to select the monochromatic Cu Kα1 radiation (λ = 1.54056 Å).
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10.1002/cplu.201900195
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Measurements were executed in transmission mode, by introducing the
powders in Lindemann capillaries.
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Entry for the Table of Contents
FULL PAPER
Extended
π electron
conjugation
Antioxidant power
FRAP
Paola Manini,* Valeria Lino, Paola
Franchi, Gennaro Gentile, Teresa
Sibillano, Cinzia Giannini, Emanuela
Picardi, Alessandra Napolitano, Luca
Valgimigli, Cinzia Chiappe, Marco
d’Ischia
1
OH OH
W = 4.8 G
1
.82 trolox eqs
OH OH
Tunable
EPR signal
0
300
400
500
600
700
pDHN
Wavelength (nm)
Biomimetic oxidation of 1,8-dihydroxynaphthalene (1,8-DHN), the monomer
precursor of Aspergillus allomelanin, leads to a black and insoluble melanin-like
polymer with potent antioxidant activities and an unusually intense EPR signal
subject to modulation by de-assembly in ionic liquids.
Page No. – Page No.
A Robust Fungal Allomelanin Mimic:
An Antioxidant and Potent π-Electron
Donor with Free-Radical Properties
that can be Tuned by Ionic Liquids
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