A Biomimetic principle for the chemical modification of metal surfaces: Synthesis of tripodal catecholates as analogues of siderophores and mussel adhesion proteins

Article abstract of DOI:10.1002/chem.201100715

By following a biomimetic design principle, tetravalent scaffolds based on an adamantyl and trisalkylmethyl core structure have been synthesized. These scaffolds have been coupled to three catecholamines, thus resembling the characteristic tripodal recognition motif of many natural metal binders, such as mussel adhesion proteins and siderophores, for example, enterobactin. Besides this tripodal recognition element, our scaffolds provide a fourth position for the conjugation of effector molecules. These effectors can be conjugated through biocompatible conjugation techniques to the scaffold and can be used to tailor the properties of different metal surfaces for a range of applications, for example, in implant engineering. Herein, we describe the synthesis of several tripodal metal binders and their immobilization on TiO₂ surfaces by using a simple dip-coating procedure. Furthermore, we demonstrate the conjugation of our surface binders to the dye eosin Y as an effector molecule by peptide coupling. The resulting surfaces have been analyzed by using ellipsometry, time-of-flight secondary ion mass spectrometry, IR spectroscopy, and contact-angle measurements to confirm the specific loading on TiO ₂ films and nanoparticles with our trivalent surface binders. As a proof of concept, we have demonstrated the functionalization of TiO₂ nanoparticles with the eosin Y dye.

Full text of DOI:10.1002/chem.201100715

DOI: 10.1002/chem.201100715
A Biomimetic Principle for the Chemical Modification of Metal Surfaces:
Synthesis of Tripodal Catecholates as Analogues of Siderophores and Mussel
Adhesion Proteins
Elisa Franzmann,[a] Faiza Khalil,[a] Christoph Weidmann,[b] Michael Schrçder,[b]
Marcus Rohnke,[b] Jꢀrgen Janek,[b] Bernd M. Smarsly,[b] and Wolfgang Maison*[a]
Abstract: By following a biomimetic
design principle, tetravalent scaffolds
based on an adamantyl and trisalkyl-
methyl core structure have been syn-
thesized. These scaffolds have been
coupled to three catecholamines, thus
resembling the characteristic tripodal
recognition motif of many natural
metal binders, such as mussel adhesion
proteins and siderophores, for example,
enterobactin. Besides this tripodal rec-
ognition element, our scaffolds provide
a fourth position for the conjugation of
effector molecules. These effectors can
be conjugated through biocompatible
conjugation techniques to the scaffold
and can be used to tailor the properties
of different metal surfaces for a range
of applications, for example, in implant
engineering. Herein, we describe the
synthesis of several tripodal metal
binders and their immobilization on
TiO2 surfaces by using a simple dip-
coating procedure. Furthermore, we
demonstrate the conjugation of our
surface binders to the dye eosin Y as
an effector molecule by peptide cou-
pling. The resulting surfaces have been
analyzed by using ellipsometry, time-
of-flight secondary ion mass spectrom-
etry, IR spectroscopy, and contact-
angle measurements to confirm the
specific loading on TiO2 films and
nanoparticles with our trivalent surface
binders. As a proof of concept, we
have demonstrated the functionaliza-
tion of TiO2 nanoparticles with the
eosin Y dye.
Keywords: biomimetic synthesis
·
catecholates · siderophores · surface
chemistry · synthetic methods
Introduction
on the use of appropriately functionalized anchor molecules,
including thioles for gold and silver surfaces,[5] silanols for
glass surfaces,[6] and phosphates or phosphonates for various
metal surfaces.[7] All of these immobilization techniques
have drawbacks, such as limited stability or a limited range
of substrate materials. Recently, catecholates have received
considerable interest as reagents for the mild modification
of metal and metal oxide surfaces by using convenient dip-
and-rinse protocols.[8] This approach may be considered bio-
mimetic because catecholate moieties are key components
of natural metal binders such as mussel adhesion pro-
teins.[8c,9] In the Mytilus edulis foot protein 5 (Mefp-5), de-
rived from marine mussels, the catecholate derivative 3,4-di-
hydrophenylalanine (DOPA) is the key constituent for ad-
hesion and makes up about 27% of all amino acids.[10] In ad-
dition, catecholates are abundant motifs in many sidero-
phores, such as enterobactin (Figure 1).[11]
The chemical modification of material surfaces by the mo-
lecular self-assembly of functionalized anchor molecules is
an attractive concept for various applications in fields such
as marine technology,[1] medicine,[2] hygiene technology,[3]
and microelectronics.[4] The major advantages of this princi-
ple for the generation of functional surfaces are 1) the avail-
ability of efficient and scalable coating techniques (compati-
ble with large surface areas and 3D objects), 2) the genera-
tion of chemically well-defined surfaces, and 3) the need for
relatively low amounts of anchor molecules. Methods for
the covalent immobilization of molecular monolayers rely
[a] Dipl.-Chem. E. Franzmann, Dipl.-Chem. F. Khalil,
Prof. Dr. W. Maison
Justus-Liebig-University Giessen
The use of mussel adhesion proteins and catecholates de-
rived thereof for surface immobilization was pioneered by
Waite[9] and Grꢀtzel and co-workers[12] and later established
by Messersmith and co-workers.[8a–c,e,13] Many different cate-
cholates, which range from synthetic derivatives to natural
products in monomeric and also polymeric forms, have been
used for the modification of various metal and metal oxide
surfaces.[10d,14] Particularly interesting for surface engineering
is the use of small bifunctional catecholate derivatives, such
as dopamine as an anchor group, and a small selection of
Institute of Organic Chemistry
Heinrich-Buff-Ring 58, 35392 Giessen (Germany)
Tel : (+49)641-99-34330
E-mail: wolfgang.maison@org.chemie.uni-giessen.de
[b] Dipl.-Chem. C. Weidmann, Dipl.-Chem. M. Schrçder, M. Rohnke,
Prof. Dr. J. Janek, Prof. Dr. B. M. Smarsly
Justus-Liebig-University Giessen
Institute of Physical Chemistry
Heinrich-Buff-Ring 58, 35392 Giessen (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100715.
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FULL PAPER
which many different anchor molecules and effectors might
be conjugated. With an exchange of a catecholate for anoth-
er adhesive, for example, it would be easy to tune the sur-
face specificity of our devices.
Herein, we describe the synthesis of triscatecholates 3, 4,
6, 8, 10, and 12–14 and their application to the functionaliza-
tion of TiO2 films and nanoparticles. The resulting surfaces
were analyzed by using IR spectroscopy, contact-angle
measurements, time-of-flight secondary ion mass spectrome-
try (TOF-SIMS), and ellipsometry.
Figure 1. Natural catecholate derivatives. A) A fragment of the mussel
adhesion protein Mefp-5 of Mytilus edulis with its characteristic trimeric
repeats of l-DOPA and B) the siderophore enterobactin (enterochelin).
Results and Discussion
The compounds central to our concept are modular scaf-
folds 3, 4, 6, 8, 10, and 12–14. These scaffolds have several
desirable properties: they permit the assembly of three cate-
cholates to a tripodal motif, they can be easily coupled to ef-
fector molecules either before or after surface immobiliza-
tion, and they are relatively easy to synthesize.[19]
suitable compounds is presented in Figure 2. These anchors
may be conjugated to effector molecules such as poly(ethy-
lene glycol), biopolymers, drugs, and dyes for the specific
Synthesis of triscatecholates: The preparation of trisalkyl-
methyl scaffold 1 is scalable and has been reported by New-
kome et al. previously.[19a] Therefore, scaffold 1 was an ideal
starting material for the synthesis of a first set of flexible
triscatecholates. The free amine was protected with CbzCl
to give 2, and the obtained tert-butyl esters were cleaved
under acidic conditions to give the corresponding tricarbox-
ylic acid, which was subsequently coupled to three equiva-
lents of dopamine with EDC/HOBt to give triscatecholate 3
(Scheme 1). Final deprotection of the Cbz group gave trisca-
techolate 4 with a free primary amine group for effector
conjugation. A second derivative 6 was obtained following a
similar reaction sequence. In this case, amine 1 was acylated
with propiolic acid to give tert-butyl ester 5. Acidic cleavage
of the ester and subsequent conjugation to dopamine gave
triscatecholate 6. The propiolate moiety in 6 may be used
for the conjugation of effectors through the Huisgen cyclo-
addition. A third triscatecholate 8 was synthesized from
nitro derivative 7. In a first step, the tert-butyl esters were
cleaved under acidic conditions and the resulting tricarbox-
ylic acid was coupled to three equivalents of dopamine
using EDC/HOBt to give triscatecholate 8.
The corresponding adamantyl derivatives 10 and 12–14
were prepared according to Scheme 2 from the known pre-
cursor 9, which is available in two high-yielding steps from
commercial precursors.[19b] The coupling of tricarboxylic acid
9 to dopamine gave triscatecholate 10 without an additional
functional group at the remaining fourth bridgehead posi-
tion of the adamantane unit. Alternatively, amino derivative
11 was synthesized as a hydrochloride salt by following a
two step-procedure reported previously by us.[18d] The result-
ing amine 11 was protected with a Cbz group and coupled
to dopamine to give the Cbz-protected triscatecholate 12.
Subsequent deprotection gave free amine 13. Alternatively,
amino tricarboxylic acid 11 was first acylated with the N-hy-
droxysuccinimide (NHS) ester of 4-pentynoic acid and then
Figure 2. Surface modification by the self-assembly of bifunctional cate-
cholate derivatives with dip-and-rinse protocols.
modulation of surface properties.[8a,b,11b,15] Inspired by the in-
triguing molecular symmetry of siderophores, such as enter-
obactin,[16] we have designed triscatecholates 3, 4, 6, 8, 10,
and 12–14 with a tripodal binding motif based on adaman-
tane and trisalkylmethyl scaffolds. Many enterobactin mim-
etics are known.[17] However, to the best of our knowledge,
none of these mimetics have been applied to surface modifi-
cation. By using trimeric catecholates 3, 4, 6, 8, 10, and 12–
14 instead of monomeric derivatives, such as dopamine, we
assume to achieve binding to metal surfaces characterized
by high thermal, mechanical and pH stability. In addition,
our scaffolds permit the conjugation of effector molecules
by using standard peptide chemistry or other bioorthogonal
techniques. We have used these scaffolds before as modular
components for targeted chemotherapy and molecular imag-
ing.[18] Thus, they are a valuable molecular platform to
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W. Maison et al.
Scheme 2. Synthesis of triscatecholates 10 and 12–14 with adamantyl scaf-
folds and different conjugation sites for effector moieties. DMSO=di-
methyl sulfoxide.
Scheme 1. Synthesis of triscatecholates 3, 4, 6, and 8 with trisalkylmethyl
scaffolds and different conjugation sites for effector moieties. CbzCl=
benzyl chloroformate, EDC·Cl=1-ethyl-3-(3-dimethylaminopropyl)car-
dodiimide hydrochloride, HOBt=hydroxybenzotriazole TFA=trifluoro-
acetic acid.
Figure 3. Schematic illustration of the dip-coating procedure of catecho-
lates and TiO2 in aqueous solution of MOPS buffer.
coupled to dopamine by using the standard procedure to
give triscatecholate 14.
Overall, four scaffolds 10 and 12–14 with either amino or
alkynyl groups for effector conjugation were synthesized in
gram quantities.
dipped into solutions of triscatecholates 6, 8, 10, 12, and 13
(1.25 mg/15 mL) in concentrated salt buffer (0.1m MOPS/
0.6m NaCl/0.6mK2SO4) for 13 hours at room temperature.
After this incubation, the coated surfaces were carefully and
repeatedly rinsed with purified water and methanol to wash
away residual buffer and unbound catecholate derivatives.
The resulting modified surfaces were submitted to differ-
ent analytical techniques to verify the binding of our trisca-
techolates to the TiO2 films.
Evaluation of surface-binding properties: We focused on
TiO2 surfaces for our first binding studies because many sur-
face-modified titanium-based materials are used as nanoe-
lectronics, sensors, energy-storage devices, photovoltaics,
and biomaterials, such as implants.[15c20] In addition, TiO2
surfaces are readily available in the form of nanoparticles
and films.
The immobilization of triscatecholates 6, 8, 10, 12, and 13
by self-assembly on TiO2 films (prepared on silicon wafer)
was performed by aqueous dip-coating in MOPS buffer
under standard conditions (Figure 3).[15a,b]
Surface analysis: First, we examined the polarity of the sur-
faces by measuring the water contact angles. In each case,
the initial water contact angle for the uncoated TiO2 surface
was between 25 and 308 (Table 1). After storage of the TiO2
films in the pure MOPS buffer solution for 13 hours, the
water contact angle decreased to about 58, thus reflecting a
change of surface polarity as a result of the alkaline buffer
TiO2 films (layer thickness of 100 nm on a silicon wafer of
2.5ꢂ2.5 cm) were cleaned with water and methanol and
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Biomimetic Triscatecholate-Functionalized Surfaces
FULL PAPER
Table 1. Results of the water contact-angle (V) measurements of cate-
cholate derivatives 6, 8, 10, 12, and 13.
Catecholate
V (TiO2)
[8][a]
V (TiO2 +MOPS)
[8][a]
V (TiO2 +MOPS+
catecholate) [8][a]
6
8
10
12
13
27.9
32.3
28.0
25.6
29.1
30.9
8.2
5.8
5.0
8.2
5.0
7.6
18.5
22.9
38.5
22.9
11.7
10.7
dopamine
[a] Average values for 3–4 measurements are given.
solution. After incubation with the triscatecholate deriva-
tives, a substantial increase in the water contact angle V was
observed, thus reflecting a decrease in surface polarity upon
immobilization of the organic molecules. As expected, the
largest effect was observed for triscatecholate 10 with an un-
substituted adamantyl scaffold (V=38.58; Figure 4).
Figure 5. TOF-SI mass spectrum of 8 immobilized on TiO2. The negative-
mode spectrum of the functionalized surfaces shows the characteristic
À
mass signal at m/z 45.99 u for NO2
.
dopamine to form polydopa-
mine on TiO2.[21] In contrast,
triscatecholate 13 showed no
tendency to polymerize and
the adlayer thickness stayed
constant.
Figure 4. A) Pure TiO2 surface: contact angle V=248; B) TiO2 surface treated with MOPS buffer: contact
angle V=58; C) TiO2 surface treated with 10 in MOPS buffer: contact angle V=38.58.
In a last set of experiments,
we immobilized triscatecho-
Analysis by TOF-SIMS was used to verify the nature of
the bound organic material on the TiO2 films.
late 12 and the eosin Y-loaded derivative 15 on TiO2 nano-
particles (P25). The conjugation of eosin Y to triscatecho-
late 13 was done under standard peptide-coupling conditions
to give the triscatecholate/dye conjugate 15 as a red powder,
which was characterized by means of MS (ESI) and used
without further purification for surface immobilization. The
loading of TiO2 nanoparticles (P25) was performed with tris-
catecholates 12 and 15 under sonication conditions in
MOPS buffer for 12 hours. The binding of triscatecholates
For the detection of the bound organic molecules, the sur-
+
face was analyzed with Bi3 primary ions. Each sample was
measured in the positive and negative modes of operation,
that is, by detecting positive and negative secondary ions.
For the elimination of the sample background in each case,
a reference area without immobilized molecules was mea-
sured and afterward the difference spectrum was calculated.
Secondary molecular ions were detectable for monomeric
+
dopamine only and the characteristic signals for C8H9NO2
,
TiC8H9NO2+, and Ti2C8H9NO2 were clearly identified in
the positive-mode mass spectra. As a result of the high frag-
mentation rate of the triscatecholate derivatives, it was im-
possible to detect the molecular ions of these larger com-
pounds in most cases. However, characteristic fragments
were observed in each case. Compound 8, for example, was
+
À
identified unambiguously by a characteristic NO2 signal in
the negative mode (Figure 5).
Another good marker is the adamantyl cation C10H11
+
,
which was clearly detected for triscatecholates 10, 12, and
13. A small section of the mass spectrum in the positive
mode for 10 is depicted in Figure 6.
The adlayer thickness of the functionalized TiO2 surfaces
was measured by ellipsometry for immobilized triscatecho-
late 13 and dopamine (Figure 7). A clear trend was observed
for dopamine: an increase in the concentration of dopamine
in the buffer solutions led to a significant increase in the
adlayer thickness. This outcome reflects the tendency of
Figure 6. TOF-SI mass spectrum of 10. The positive-mode spectrum of
the functionalized surfaces shows the characteristic mass signal at m/
z 131 u for the adamantyl cation.
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W. Maison et al.
Figure 9. 1) Eosin Y-labeled triscatecholate 15 immobilized on TiO2
nanoparticles after thorough washing with water and methanol; 2) TiO2
nanoparticles after incubation with pure eosin Y and thorough washing
with water and methanol; 3) pure TiO2 (P25).
Figure 7. Changes of the adlayer thickness of different concentrations of
triscatecholate 13 and dopamine.
to the nanoparticles after thorough washing was clearly con-
firmed by characteristic signals in the IR spectra (as depict-
ed for triscatecholate 12 on TiO2 in Figure 8).
In addition, the successful immobilization of the eosin Y-
labeled triscatecholate 15 on TiO2 nanoparticles (P25) was
clearly visible by the deep-red color after washing (Figure 9
and Scheme 3). Eosin Y itself is bright red and does not
bind to the TiO2 surface (Figure 9, tube 2).
Conclusion
Bifunctional catecholate derivatives are versatile tools to
tailor surface properties by using reliable and convenient
dip-and-rinse procedures. This approach is biomimetic and
has been derived from mussel adhesion proteins and sidero-
phores containing catecholates. With the synthesis of bifunc-
tional triscatecholates 3, 4, 6, 8, 12–15, we have extended
this biomimetic principle that imitates tripodal catecholate
assemblies in nature. These triscatecholates bind to TiO2
Scheme 3. Synthesis of the eosin Y-labeled triscatecholate 15 for immobi-
lization on TiO2 nanoparticles.
films and nanoparticles, which was confirmed by using con-
tact-angle measurements, ellipsometry, IR spectroscopy, and
TOF-SIMS, and allow surface functionalization with effector
molecules. These effectors may be introduced before surface
immobilization as demonstrat-
ed with the dye conjugate 15.
Alternatively, alkyne-substitut-
ed catecholates, such as 6 and
14, allow the generation of
ꢃclickableꢄ materials that may
be modified by effector cou-
pling to the surface.
By using multivalent cate-
cholate derivatives, we expect
high thermal, mechanical, and
pH stability of triscatecholate-
functionalized surfaces while
conserving a molecularly well-
defined immobilization pat-
tern. It should be noted that
our surface binders have
a
modular character and their
surface specificity may there-
fore be tailored by the use of
other adhesives than catecho-
lates.
Figure 8. The characteristic FTIR spectrum for a) pure TiO2 nanoparticles (P25), b) pure triscatecholate 12,
and c) triscatecholate 12 immobilized on TiO2 nanoparticles.
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Biomimetic Triscatecholate-Functionalized Surfaces
FULL PAPER
Preparation of triscatecholate 4: Triscatecholate 3 (35 mg, 0.044 mmol)
was dissolved in anhydrous MeOH (10 mL) and a catalytic amount of
Pd/C was added. The resulting suspension was stirred at room tempera-
ture for 12 h under H2 (balloon). Filtration over celite and concentration
of the filtrate gave compound 4 (287 mg, 0.044 mmol, quant.). 1H NMR
(400 MHz, [D6]DMSO): d=6.76 (d, 3H, 3J=8.0 Hz), 6.70 (s, 3H), 6.56
(d, 3H, 3J=8.0 Hz), 3.50–3.48 (m, 12H), 2.21–2.19 (m, 6H), 1.67–
1.65 ppm (m, 6H); 13C NMR (100 MHz, [D6]DMSO): d=172.1, 145.0,
143.5, 130.2, 119.1, 115.9, 115.5, 48.6, 40.6, 34.6, 29.7 ppm; HRMS (ESI):
m/z: calcd for C34H44N4O9: 653.3187 [MÀH+]; found: 653.3186.
Preparation of triester 5: Compound 1 (3.6 g, 8.7 mmol) was dissolved in
CH2Cl2 (100 mL) and cooled to 08C. Propiolic acid (0.73 g, 10.4 mmol)
and N,N-dicyclohexylcarbodiimide (DCC; 2.2 g, 10.4 mmol) were added
and the solution was stirred at room temperature for 12 h. CH2Cl2 was
removed in vacuo and the residue was dissolved in EtOAc, washed three
times with 1m HCl, three times with an aqueous saturated solution of
NaHCO3, dried over Na2SO4, filtered, and concentrated. The resulting
crude product was purified by flash chromatography on silica gel (petrol
ether/EtOAc) to give triester 5 as a colorless solid (2.5 g 5.3 mmol,
61%). Rf =0.47 (petroleum ether/EtOAc, 2:1, molybdophosphoric acid);
m.p. 1398C; 1H NMR (400 MHz, CDCl3): d=6.38 (s, 1H), 2.69 (s, 1H),
2.23 (t, 6H, 3J=7.6, 3J=8.1 Hz), 1.98 (t, 6H, 3J=8.1, 3J=7.6 Hz),
1.43 ppm (s, 27H); 13C NMR (100 MHz, CDCl3): d=172.7, 151.3, 81.0,
78.0, 71.8, 59.0, 30.0, 29.8, 28.2 ppm; IR (KBr): n˜ =3260, 1724, 1539,
1154 cmÀ1; HRMS (ESI): m/z: calcd for C25H41NO7: 490.2781 [MÀNa+];
found: 490.2781; elemental analysis calcd (%) for C 64.22, H 8.84, N
3.00; found: C 64.22, H 8.84, N 3.01.
Experimental Section
Synthetic precursors: The following educts were prepared according to
literature procedures: 1,[19a] 2,[22] 7,[19a] 9,[19b] and 11.[18d]
Immobilization on TiO2 surfaces: Silicon wafers ((100)-oriented Si single
crystal (surface area: 2.5ꢂ2.5 cm)) were coated with TiO2 films of
100 nm thickness by hydrolysis of TiCl4 in water and ethanol. The surfa-
ces were cleaned twice with water and methanol and were dried in a
compressed air steam.
The appropriate catecholate (5 mg) was dissolved in MeOH (5 mL) and
H2O (15 mL). An aliquot of this stock solution (5 mL) was added to 3-
(N-morpholino)propanesulfonic acid (MOPS) buffer (10 mL; 0.1m
MOPS/0.6m NaCl/0.6mK2SO4). TiO2 surfaces were dipped into the solu-
tion of catecholate in buffer and left for 13 h. The plates were rinsed ex-
cessively with water and methanol.
Immobilization of catecholates on TiO2 nanoparticles: The TiO2 nanopar-
ticles are commercially available from Degussa (TiO2, P25). These nano-
particles have a specific surface area of 50Æ15 m2 gÀ1, an average particle
size of 21 nm, and a purity of 99.5%.
TiO2 nanoparticles (1 equiv by mass) and the appropriate catecholate
(4 equiv) were stirred for 12 h in MOPS buffer with sonication, separated
by centrifugation, washed with excessively H2O and MeOH, and dried in
vacuo.
Preparation of triester 2: Compound 1 (3.0 g, 7.2 mmol) was dissolved in
dioxane/water (100 mL; 1:1) and cooled to 08C. Na2CO3 (0.7 g,
8.3 mmol) and CBzCl (1.4 g, 1.2.0 mL, 8.3 mmol) were added, and the so-
lution was stirred at 08C for 2 h and at room temperature for 2 h. The re-
action mixture was extracted three times with CH2Cl2, the combined or-
ganic layers were washed three times with 1m HCl, dried over Na2SO4,
filtered, and concentrated. The resulting crude product was purified by
flash chromatography on silica gel (petrol ether/EtOAc) to give triester 2
as a colorless solid (2.5 g, 4.6 mmol, 64%). Rf =0.42 (petroleum ether/
EtOAc 8.5:1.5); cerium ammonium nitrate; m.p. 1008C; 1H NMR
(400 MHz, CDCl3): d=7.35–7.32 (m, 5H), 5.04 (s, 2H), 4.83 (s, 1H), 2.20
Preparation of triscatecholate 6:
A solution of triester 5 (500 mg,
1.67 mmol) in CH2Cl2 and TFA (45 mL, 1:1) was stirred at room temper-
ature for 3 h. The mixture was concentrated in vacuo and the resulting
oil was coevaporated with CH2Cl2 three times to give the corresponding
triacid (410 mg) as an intermediate. This crude product (320 mg,
1.10 mmol) and Et3N (1.44 mL; 10.4 mmol) were dissolved in anhydrous
DMF (10 mL) and cooled to 08C. EDC·Cl (600 mg, 4.3 mmol), HOBt
(580 mg, 4.3 mmol), dopamine hydrochloride (650 mg, 4.3 mmol), and
Et3N (0.91 mL, 6.6 mmol) were added, and the resulting solution was
stirred at room temperature for 72 h. The reaction mixture was diluted
with EtOAc (20 mL) and washed three times with 1m HCl and brine.
The organic layer was dried over Na2SO4, filtered, and concentrated in
vacuo. This crude product was suspended in Et2O (100 mL) and stirred
for 2 h. Filtration gave compound 6 (600 mg, 0.85 mmol, 51%). 1H NMR
(400 MHz, [D4]CH3OD): d=6.68 (d, 3H, 3J=7.9 Hz), 6.64 (s, 3H), 6.52
(d, 3H, 3J=7.9 Hz), 3.51 (s, 1H), 3.39–3.34 (m, 6H), 2.63 (t, 6H, 3J=
7.1 Hz), 2.12 (s, 6H), 2.13–2.09 (m, 6H), 1.94–1.92 ppm (m, 6H);
13C NMR (100 MHz, [D4]CH3OD): d=175.5, 146.2, 144.8, 132.0, 121.1,
117.0, 116.4, 74.6, 60.3, 42.4, 35.8, 31.5, 31.2 ppm; HRMS (ESI): m/z:
calcd for C37H44N4O10: 727.2955 [M+Na+]; found: 727.2961.
3
3
3
3
(t, J=7.5, J=8.3 Hz, 6H), 1.91 (t, J=8.2, J=7.4 Hz, 6H), 1.43 ppm (s,
27H); 13C NMR (100 MHz, CDCl3): d=172.7, 154.3, 136.7, 128.6, 128.2,
128.1, 80.7, 66.3, 56.7, 30.2, 29.8, 28.2 ppm; IR (KBr): n˜ =3348, 1726,
757 cmÀ1; HRMS (ESI): m/z: calcd for C30H47NO8: 572.3199 [M+Na+];
found: 572.3195; elemental analysis calcd (%) for C 65.63, H 8.65, N
2.55; found: C 65.55, H 8.62, N 2.55.
Preparation of triscatecholate 3: A solution of triester 2 (2.0 g, 3.6 mmol)
in CH2Cl2 and TFA (45 mL, 1:1) was stirred at room temperature for 3 h.
The reaction mixture was concentrated and coevaporated twice with
CH2Cl2 to give the corresponding triacid (1.3 g, 3.3 mmol, 92%).
1H NMR (400 MHz, [D6]DMSO): d=7.34–7.33 (m, 5H), 4.98 (s, 2H),
2.12 (t, 6H, 3J=7.8, 3J=7.8 Hz), 1.79 ppm (t, 6H, 3J=8.4, 3J=7.8 Hz);
13C NMR (100 MHz, [D6]DMSO): d=174.5, 154.3, 137.5, 128.4, 127.7,
Preparation of triscatecholate 8: Compound 7 (100 mg, 0.36 mmol) and
Et3N (0.47 mL; 3.40 mmol) were dissolved in absolute DMF (10 mL) and
cooled to 08C. EDC·Cl (0.25 g, 1.62 mmol), HOBt (0.22 g, 1.62 mmol),
dopamine hydrochloride (0.25 g, 1.62 mmol), and Et3N (0.47 mL,
3.40 mmol) were added sequentially and stirred at room temperature for
72 h. The reaction mixture was diluted with EtOAc (20 mL) and washed
three times with 1m HCl and brine. The organic layer was dried over
Na2SO4, filtered and concentrated. This crude product was suspended in
Et2O (100 mL) and stirred for 30 min in a water bath at 308C. The result-
ing slurry was filtered and the procedure was repeated five times to give
127.5, 64.7, 55.8, 29.2, 28.1 ppm; IR (KBr): n˜ =3433, 1712, 697 cmÀ1
HRMS (ESI): m/z: calcd for C18H23NO8: 404.1321
404.1326; elemental analysis calcd (%) for C 52.46, H 6.17, N 3.34;
found: 52.69, 6.08, 3.37. The tricarboxylic acid (200 mg,
;
AHCTUNGTRENNUNG
C
H
N
0.52 mmol) and Et3N (1.44 mL; 10.4 mmol) were dissolved in anhydrous
DMF (10 mL) and cooled to 08C. EDC·Cl (330 mg, 1.72 mmol), HOBt
(230 mg, 1.72 mmol), and dopamine hydrochloride (260 mg, 1.72 mmol)
were added, and the resulting solution was stirred at room temperature
for 72 h. The reaction mixture was diluted with EtOAc (20 mL) and
washed three times with 1m HCl and brine. The organic layer was dried
over Na2SO4, filtered, and concentrated. This crude product was suspend-
ed in Et2O (100 mL) and stirred for 2 h. Filtration gave triscatecholate 3
(140 mg, 0.18 mmol, 34%). 1H NMR (400 MHz, [D6]DMSO): d=7.35–
7.30 (m, 5H), 6.62 (d, 3H, 3J=7.87 Hz), 6.57 (s, 3H), 6.42 (d, 3H, 3J=
7.87 Hz), 5.00 (s, 2H), 3.41–3.36 (m, 6H), 3.15–3.14 (m, 6H), 2.00 (s,
6H), 1.78 ppm (s, 6H); 13C NMR (100 MHz, [D6]DMSO): d=172.0,
145.1, 143.5, 137.4, 130.3, 128.4, 127.7, 127.5, 64.9, 56.2, 40.7, 34.8, 30.4,
29.7 ppm; HRMS (ESI): m/z: calcd for C42H50N4O11: 809.3374 [MÀNa+];
found: 809.3374.
triscatecholate
8
(160 mg, 0.23 mmol, 65%).1H NMR (400 MHz,
[D4]CH3OH): d=6.68 (d, 3H, 3J=8.0 Hz), 6.63 (d, 3H, 4J=2.0 Hz), 6.50
3
4
3
(dd, 3H, J=8.1, J=2.0 Hz), 3.34–3.30 (m, 6H), 2.63 (t, 6H, J=7.4 Hz),
2.16–2.09 ppm (m, 12H); 13C NMR (100 MHz, [D4]CH3OH): d=174.1,
146.2, 144.7, 132.0, 121.1, 117.0, 116.4, 94.1, 42.2, 35.8, 32.3, 31.2 ppm;
HRMS (ESI): m/z: calcd for C34H42N4O11: 681.2798 [MÀH+]; found:
681.2797.
Preparation triscatecholate 10: Tris(2-carboxyethyl)adamantane (9;
500 mg, 1.42 mmol) was dissolved in anhydrous DMF (60 mL) and
cooled to 08C (ice bath). Et3N (2.54 mL, 18.32 mmol) was added to the
reaction mixture, which was stirred for 5 min. EDC·Cl (900 mg,
Chem. Eur. J. 2011, 17, 8596 – 8603
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8601

W. Maison et al.
4.69 mmol) and HOAt (9.38 mg, 4.69 mmol) were added to the reaction
mixture, which was stirred for 30 min. Dopamine hydrochloride (890 mg,
4.69 mmol) was added to the reaction mixture, which was stirred for 72 h
at room temperature. The reaction mixture was concentrated and the res-
idue was dissolved in EtOAc (30 mL) and 1m HCl (5 mL) and then
washed with saturated aqueous KHSO4 solution three times. The organic
layer was dried over Na2SO4, filtered, and concentrated. Freeze drying
gave a light-brown solid, which was suspended in Et2O (100 mL) and
stirred for 30 min in a water bath at 308C. The resulting slurry was fil-
tered and the procedure was repeated five times to give triscatecholate
Preparation of triscatecholate 14: Compound 11 (250 mg, 0.62 mmol)
was dissolved in dimethyl sulfoxide (DMSO; 20 mL) and Et3N (0.35 mL,
2.50 mmol) and succinimidyl-4-pentiolate (181 mg, 0.93 mmol) were
added. The reaction mixture was stirred for 12 h at room temperature
and concentrated. The residue was dissolved in EtOAc (20 mL) and
washed with saturated aqueous KHSO4 solution. The organic layer was
concentrated and the residue dissolved in 1m NaOH (20 mL), washed
with EtOAc, and concentrated to give the crude product. This crude
product was dissolved in DMF (60 mL) and the solution was cooled to
08C (ice bath). Et3N (0.90 mL, 8.002 mmol) was added and the solution
was stirred for 5 min. EDC·Cl (405 mg, 2.05 mmol) and HOBt (277 mg,
2.05 mmol) were added and the solution was stirred for 5 min. Dopamine
hydrochloride (389 mg, 2.05 mmol) was added and the solution was
stirred for 72 h at room temperature. The reaction mixture was concen-
trated, dissolved in EtOAc (30 mL) and 1m HCl (5 mL), and washed
with saturated aqueous KHSO4 solution three times. The organic layer
was dried over Na2SO4, filtered, and concentrated. Freeze drying gave a
colorless solid, which was suspended in Et2O (100 mL) and stirred for
30 min in a water bath at 308C. The resulting slurry was filtered and the
procedure was repeated five times to give triscatecholate 14 (282 mg,
1
10 (687 mg, 0.906 mmol, 64%). H NMR (400 MHz, [D4]MeOH): d=6.62
(d, 3H, 2J(H,H)=8.0 Hz), 6.49–6.47 (m, 3H), 6.47 (d, 3H, 2J
ACHTUNGTRENNUNG AHCTUNGTRNEN(UGN H,H)=
3
3
8.0 Hz), 3.33–3.30 (m, 6H), 2.55 (t, 6H, J
ACHTUNGTRENNUNG
ACHTUNGTNER(NUNG H,H)=8.0 Hz), 2.10 (t, 6H, J-
(H,H)=8.0 Hz), 1.42–1.35 (m, 12H), 1.13–1.05 ppm (m, 6H); 13C NMR
(100 MHz, [D4]MeOH): d=177.2, 164.9, 146.3, 144.8, 142.7, 132.0, 129.6,
121.1, 116.9, 116.4, 47.3, 42.3, 42.0, 40.9, 37.0, 34.6, 31.1 ppm; HRMS
(ESI): m/z: calcd for C43H55N3O9: 756.3866 [MÀH+]; found: 756.3862.
Preparation of triscatecholate 12: Tris(2-carboxyethyl)aminoadamantane
(11; 500 mg, 1.20 mmol) was dissolved in dioxane (30 mL) and added to
NaHCO3 (300 mg, 4.09 mmol) dissolved in water (30 mL). NaOH (2m)
was added to the reaction mixture to achieve a pH of 9, and the solution
was cooled to 08C (ice bath). Benzyl chloroformate (CbzCl; 0.24 mL,
1.74 mmol) was added to the reaction mixture, which was stirred for 12 h
at room temperature. The reaction mixture was washed three times with
CH2Cl2 (30 mL) and three times with EtOAc (30 mL), acidified to pH 1,
and extracted six times with EtOAc. The combined organic layers were
dried over Na2SO4, filtered, and concentrated to give the intermediate
Cbz-protected triacid (524 mg, 1.05 mmol, 84%). M.p. 2218C; 1H NMR
(400 MHz, [D6]DMSO): d=12.06 (br, 3H), 7.43–7.39 (m, 5H), 7.07 (s,
0.331 mmol, 53%,
(400 MHz, [D4]MeOH): d=6.65 (d, 3H, 2J
3H), 6.50 (d, 3H, 2J(H,H)=8.0 Hz), 3.32–3.27 (m, 6H), 2.60 (t, 6H, 2J-
(H,H)=8.0 Hz), 2.39–2.35 (m, 2H), 2.29–2.23 (m, 2H), 2.11–2.07 (m,
7H), 1.59–1.57 (m, 6H), 1.43–1.39 (m, 6H), 1.04 (dd, 3H, 2J
(H,H)=
12.0 Hz), 1.03 ppm (dd, 3H, 2J(H,H)=12.0 Hz); 13C NMR (100 MHz,
a
small impurity of residual HOBt). 1H NMR
(H,H)=8.0 Hz), 6.61–6.60 (m,
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
[D4]MeOD): d=176.8, 173.3, 146.3, 144.8, 132.0, 121.1, 116.9, 116.4, 79.3,
67.2, 49.9, 46.3, 45.6, 42.3, 40.2, 36.8, 36.1, 35.8, 31.2, 19.7, 15.8 ppm;
HRMS (ESI): m/z: calcd for C48H60N4O10: 851.4237 [MÀH+]; found:
851.4232.
1H), 5.01 (s, 2H), 2.20 (t, 6H, 3J
6H, 3J(H,H)=8.2 Hz), 1.08 (d, 3H, 2J
2J(H,H)=13.2 Hz); 13C NMR (100 MHz, [D6]DMSO): d=175.0, 154.1,
ACHTUNGTRENNUNG
Preparation of triscatecholate 15: Triscatecholate 12 (6.2 mg,
0.008 mmol) was dissolved in anhydrous DMF (10 mL) and the solution
was cooled to 08C (ice bath). Et3N (0.004 mL, 0.032 mmol) was added
and the solution was stirred for 5 min. EDC·Cl (1.9 mg, 0.010 mmol) and
HOBt (1.4 mg, 0.010 mmol) were added and the solution was stirred for
5 min. Eosin Y disodium salt (7.2 mg, 0.010 mmol) was added and the so-
lution was stirred for 24 h at room temperature. The reaction mixture
was concentrated, characterized by using MS (ESI), and used without fur-
ther purification.
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
137.3, 128.3, 127.7, 127.7, 64.5, 51.9, 44.5, 44.5, 37.4, 34.3, 27.9 ppm;
HRMS (ESI): m/z: calcd for C27H35N1O8: 524.2255 [M+Na+]; found:
524.2260. The Cbz-protected triacid (50 mg, 0.099 mmol) was dissolved in
anhydrous DMF (30 mL) and the solution was cooled to 08C (ice bath).
N,N-diisopropylethylamine (DIPEA; 0.22 mL, 0.127 mmol) was added to
the reaction mixture, which was stirred for 10 min. EDC·Cl (63 mg,
0.327 mmol) and HOBt (44 mg, 0.327 mmol) were added to the reaction
mixture, which was stirred for 20 min. Dopamine hydrochloride (62 mg,
0.327 mmol) was then added, and the solution was stirred for 72 h at
room temperature. The reaction mixture was concentrated and the resi-
due dissolved in EtOAc (30 mL) and 1m HCl (5 mL). This solution was
washed with saturated aqueous KHSO4 solution three times. The organic
layer was dried over Na2SO4, filtered, and concentrated. Freeze drying
gave a light-white solid, which was suspended in Et2O (10 mL) and
stirred for 30 min in a water bath at 308C. The resulting slurry was fil-
tered and the procedure was repeated five times to give triscatecholate
Acknowledgements
We acknowledge analytical support from Prof. Dr. Bernd Wçstmann,
Dipl.-Ing. Michael Kçhl, and helpful discussions with Gero Winkler and
Dipl.-Chem. Alexander Rein.
1
12 (43 mg, 0.047 mmol, 48%). H NMR (400 MHz, [D4]MeOH): d=7.17–
7.11 (m, 5H), 6.54 (d, 3H, 2J
ACHTUNGTRENNUNG
3H, 2J
ACHTUNGTRENNUNG
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ACHTUNGTRENNUNG
2
2
3J=7.6 Hz), 0.96 (d, 3H, J
ACTHUNTGRENNG(U H,H)=12.0 Hz), 0.88 ppm (d, 3H, J=(H,H)
12.0 Hz); 13C NMR (100 MHz, [D4]MeOH): d=176.8, 156.8, 146.2, 144.7,
138.5, 132.0, 128.9, 128.8, 129.4, 121.1, 116.9, 116.4, 66.7, 53.7, 46.3, 45.9,
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:
Preparation of triscatecholate 13: Triscatecholate 12 (43 mg, 0.047 mmol)
was dissolved in anhydrous MeOH and a catalytic amount of Pd/C was
added. The resulting mixture was stirred under N2 for 72 h, filtered, and
concentrated to give free amine 13 (30 mg, 0.039 mmol, 82%). 1H NMR
(400 MHz, [D4]MeOD): d=6.64 (d, 3H, 2J=8.0 Hz), 6.62–6.60 (m, 3H),
6.48 (d, 3H, 2J
ACHTUNGTRENNUNG ACHTUNGTNER(NUGN H,H)=7.0 Hz), 2.60 (t,
(H,H)=8.0 Hz), 3.27 (t, 6H, 3J
6H, 3J(H,H)=7.7 Hz), 2.08 (t, 6H, 3J=7.7 Hz), 1.44–1.42 (m, 12H),
ACHTUNGTRENNUNG
1.11–1.08 ppm (m, 6H); 13C NMR (100 MHz, [D4]MeOD): d=176.4,
146.2, 144.8, 132.0, 121.1, 117.1, 116.4, 54.9, 45.3, 45.0, 42.2, 39.6, 36.4,
35.8, 31.1 ppm; HRMS (ESI): m/z: calcd for C43H56N4O9: 773.4120
[MÀH+]; found: 773.4120.
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Biomimetic Triscatecholate-Functionalized Surfaces
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Received: March 8, 2011
Published online: June 15, 2011
Chem. Eur. J. 2011, 17, 8596 – 8603
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8603