Protein-resistant surfaces through mild dopamine surface functionalization

Article abstract of DOI:10.1002/chem.200801134

The synthesis and evaluation of new dopamine-based catechol anchors coupled to poly(ethylene glycol) (PEG) for surface modification of TiO₂ are reported. Dopamine is modified by dimethylamine-methylene (7) or trimethylammonium-methylene (8) groups, and the preparation of mPEG-Glu di-dopamine polymer 11 is presented. All these PEG polymers allow stable adlayers on TiO₂ to be generated through mild dip-and-rinse procedures, as evaluated both by variable angle spectroscopic ellipsometry and X-ray photoelectron spectroscopy. The resulting surfaces substantially reduced protein adsorption upon exposure to full human serum.

Full text of DOI:10.1002/chem.200801134

FULL PAPER
DOI: 10.1002/chem.200801134
Protein-Resistant Surfaces through Mild Dopamine Surface
Functionalization
Jean-Yves Wach,[a] Barbora Malisova,[b] Simone Bonazzi,[a] Samuele Tosatti,[b]
Marcus Textor,[b] Stefan Zꢀrcher,[b] and Karl Gademann*[a]
Abstract: The synthesis and evaluation
of new dopamine-based catechol an-
chors coupled to poly(ethylene glycol)
(PEG) for surface modification of TiO2
are reported. Dopamine is modified by
dimethylamine–methylene (7) or trime-
thylammonium–methylene (8) groups,
and the preparation of mPEG-Glu di-
dopamine polymer 11 is presented. All
these PEG polymers allow stable
adlayers on TiO2 to be generated
through mild dip-and-rinse procedures,
as evaluated both by variable angle
spectroscopic ellipsometry and X-ray
photoelectron spectroscopy. The result-
ing surfaces substantially reduced pro-
tein adsorption upon exposure to full
human serum.
Keywords: biofouling · biomimetic
strategies
·
natural products
·
surface chemistry · synthesis design
Introduction
poly-2-methyl-2-oxazoline.[11] The remaining general chal-
lenge consists of attaching these polymers to surfaces in a
mild, rapid and efficient way. Several approaches have been
developed based on polyelectrolytes,[12] silanes,[13] or thiols[14]
on gold. All of these approaches suffer from limitations re-
garding the scope, stability or reactivity of the anchoring
groups. Recently, catechols have been presented as promis-
ing alternative adhesives in surface modifications.[15] These
anchors are found in mussel-adhesive proteins (MAPs),[16]
which are responsible for the very strong wet adhesion of
mussels to surfaces. MAPs have been shown to contain up
to 27% of the catechol dihydroxyphenylalanine (DOPA),
which is the key constituent for adhesion. Based on early
work by Waite and co-workers,[15a] and then Grꢀtzel and co-
workers,[15b] Messersmith and co-workers[15c–h] demonstrated
the usefulness of catechols for surface functionalization. In
these systems, however, oligomers of DOPA, such as 2, are
required to achieve sufficient adhesion stability and protein
resistance. We were able to overcome this limitation by in-
troducing anchor 3[17a] for effective single-site attachment
based on the iron chelator anachelin.[18] This biomimetic
strategy allowed PEG-based, protein-resistant,[17a] and cell-
resistant[17b] surfaces to be generated, with the drawback
that the anchor required multistep organic synthesis, involv-
ing heavy-metal-based[18a,b] or enzymatic[18c,d] conversions.
We sought to structurally simplify this anchor and, while re-
taining its benefits, make its preparation more straightfor-
ward. Herein, we present structurally simple, easy-to-pro-
duce anchors that allow protein-resistant TiO2 surfaces to be
generated through straightforward dip-and-rinse procedures.
The nonspecific attachment of macromolecules of biological
nature, such as proteins, or of microorganisms, such as bac-
teria or fungi, to surfaces (so-called biofouling) is a funda-
mental problem in areas ranging from medical devices and
biosensors[1] to protein production, handling, and storage.[2]
Surfaces suffer from biofouling as soon as they are exposed
to biological fluids or protein solutions. This process[3] is
often considered to be the first step in nosocomial infec-
tions,[4] which are a serious problem in hospital settings
nowadays.
The direct approach towards preventing biofouling is the
generation of antifouling surfaces,[5] that is, coatings that
repel the adhesion of proteins and cells. Over recent years,
effective antifouling polymers have been developed, includ-
ing poly(ethylene glycol)[6] polyglycerol,[7] poly(ethylene
oxide)–poly(propylene oxide)/pluronics,[8,9] peptoids,[10] and
[a] J.-Y. Wach, S. Bonazzi, Prof. Dr. K. Gademann
Chemical Synthesis Laboratory (SB-ISIC-LSYNC)
Swiss Federal Institute of Technology (EPFL)
1015 Lausanne (Switzerland)
Fax : (+41)216931500
E-mail: karl.gademann@epfl.ch
[b] B. Malisova, Dr. S. Tosatti, Prof. Dr. M. Textor, Dr. S. Zꢁrcher
Laboratory for Surface Science and Technology
Swiss Federal Institute of Technology (ETH)
8093 Zꢁrich (Switzerland)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801134.
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Results and Discussion
The goal of this investigation was the design of an effective
catechol surface anchor that could generate protein-resistant
surfaces and that was readily available by a short synthetic
route. We initially proposed that the key to successful sur-
face immobilization through single-site attachment is the
combination of both catechol and a positive ammonium
group, such as that in 3.[17a] These functional units should
synergistically act on surface binding with the catechol as
the anchoring group in a similar fashion to MAPs and the
positive charge inducing favorable interactions with the neg-
atively charged surface of the metal oxide. Therefore, we
turned our attention to compound 4, which is readily avail-
able from Boc-l-DOPA.[18a,b] This monoprotected catechol
diamine exists in a charged state under physiological condi-
tions, thus displaying the desired properties of catechol and
a positive charge. A permanent, pH-independent positive
charge was introduced by N-methylation of 4 in one step
with excellent yield (97%; Scheme 1). The resulting com-
pound 5 can therefore be obtained through a short synthetic
sequence from DOPA. In addition to the monocatechol an-
chors, such as 4 and 5, we were interested in dicatechol an-
chors because of their increased adhesion stability on surfa-
ces.[15g] Thus, dicatechol–diamine 6 was prepared in two
steps from 4, simply through deprotection of 4 and coupling
to Boc-l-DOPA (Boc=tert-butoxycarbonyl). The three an-
chors (4–6) were deprotected and coupled to poly(ethylene
glycol) (PEG-5000) through the corresponding PEG-succi-
nidyl esters and purified by size exclusion chromatography.
The resulting polymers (7–9) were thus readily available for
surface functionalization.
Scheme 1. Preparation of catechol-PEG conjugates for surface modifica-
tion: a) Boc2O, NaOH (aq), dioxane, RT, 16 h; b) 1) iBuOCOCl, THF,
À358C, 0.5 h; 2) HNMe2, THF, À358C to RT, 16 h, 68% (over three
steps); c) Cs2CO3, BnBr, acetone, reflux, 4 h, 83%; d) 1) trifluoroacetic
acid (TFA), CH2Cl2, 08C, 1 h; 2) BH3·THF, THF, 08C to RT, 16 h;
3) Boc2O, NaOH (aq), dioxane, RT, 16 h, 53% (over 3 steps); e) H2, Pd/
C, AcOH, MeOH, 14 h, RT, 99%; f) CH3I, CH2Cl2/MeOH 2:1, 6 h, RT;
g) TFA, CH2Cl2, 08C, 1 h; h) Boc-l-DOPA, NEt3, 1-hydroxybenzotriazole
(HOBt), N-(3-dimethylaminopropyl)-N-ethylcarbodiimide (EDC), 14 h,
RT, 54% (over two steps); i) HCl (4m) in dioxane, dioxane, 08C, 2 h;
j) methoxy(polyethylene glycol) N-hydroxysuccinidylpropionate (mPEG-
SPA), N-methylmorpholine (NMM), CH2Cl2/DMF 1:1, RT, 64% (for 7),
44% (for 8), 60% (for 9).
point of view, this compound could also allow the role of
the amino group in 9 to be investigated with respect to the
two catechol units. Compound 11 should also display in-
creased binding properties when compared with dopamine
PEG polymer 12, as noted previously for DOPA-based poly-
mers.[10,15g]
The generation of protein-resistant surfaces was then car-
ried out by means of spontaneous adsorption (self-assembly)
by using a dip-and-rinse protocol. TiO2 was chosen as the
surface in view of its relevance in the field of biomedical im-
plants and biosensors.[19] Thus, clean TiO2 surfaces were
In addition, we prepared didopamine polymer 11 in three
steps from Boc-l-Glu, through coupling to dopamine, depro-
tection and PEGylation with mPEG succinidyl ester
(Scheme 2). Compound 11 was designed in view of benefi-
cial properties through bidentate anchoring of the catechols
on the surface, while also being readily available through a
short three-step synthetic sequence. From a mechanistic
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Protein-Resistant Surfaces
FULL PAPER
lymer 3 and dopamine-derived 12. Tertiary amine polymer 7
showed a significantly lower thickness, with a measured
value of 9.8 ꢃ.
We next investigated the adlayer thickness of divalent di-
dopamine derivatives 9 and 11 (Figure 1). Interestingly, de-
rivative 9, featuring additional amino groups, displayed a
similar low initial adlayer thickness when compared to the
parent monovalent polymer 7 and after 24 h exposure to the
buffer an adlayer thickness of 8.8 ꢃ was determined. Diva-
lent dopamine derivative 11, however, gave the highest
adlayer thickness, both after adsorption (ca. 25 ꢃ) and after
24 h incubation in buffer (ca. 18 ꢃ).
Scheme 2. Preparation of mPEG didopamine polymer 11: a) dopamine
hydrochloride, NEt3, HOBt, EDC, 14 h, RT, CHCl3; b) HCl (4m) in diox-
ane, dioxane, 08C, 2 h; c) mPEG-SPA, NMM, CH2Cl2/DMF 1:1, RT,
58%.
From these data, the following conclusions can be drawn:
1) A permanent positive charge, present, for example, in 8,
results in higher values for the adlayer thickness (on nega-
tively charged surfaces), and therefore, a higher adsorbed
mass for the monovalent compounds. Thus, polymers such
as 8 and control 3 have superior adsorption properties when
compared with nonpermanently charged aminodopamines
such as 7. 2) Didopamine polymer 11 resulted in the highest
adlayer thickness, as determined by VASE. Clearly, an opti-
mal arrangement of both catechols is beneficial for surface
attachment. In contrast, diamino dopamine polymer 9 per-
formed poorly, with layer thicknesses similar to its monova-
lent counterpart 7. Increased steric hindrance expected for
molecule 9, when compared with 11, is possibly the reason
for the observed lower adlayer thickness. Molecular model-
ing results supports this view, in which the dimethylamino
group in 9 appears to separate the catechol units spatially
and thus prevent multivalent attachment (data not shown).
From the adsorption data after incubation for 24 h under
physiological conditions, it can be concluded that the perfor-
mance regarding adlayer thickness of the group of polymers
3, 8, 11, and 12 are superior to 7 and 9.
dipped in dilute solutions of 7–9 and 11–12 (polymers
(0.1 mgmLÀ1) in high salt buffer (0.1m MOPS/0.6m NaCl/
0.6m K2SO4)) for 4 h at 508C. These cloud-point conditions
were chosen to achieve dense packing of PEG and thus
maximize PEG surface density.[5a] After incubation, the re-
sulting surfaces were briefly rinsed with water and the thick-
ness of the adlayer was measured by variable angle spectro-
scopic ellipsometry (VASE). This measured thickness is pro-
portional to the amount of polymer adsorbed and thus di-
rectly reflects the mass of PEG on the surface. From these
*
values (Figure 1, ), it is evident that polymer 8, featuring
the quaternary ammonium anchor, as well as didopamine
polymer 11 displayed the highest mass adsorbed. Among
the monocatechol polymers, the performance of 8 is compa-
rable to anachelin chromophore polymer 3 and to that mea-
sured for dopamine-derived 12. Clearly, the nonpermanently
charged polymer 7 displayed a lower adlayer thickness. As a
next step, the polymers were equilibrated by exposing the
surfaces to physiological buffer (HEPES 2; pH 7.4) for 24 h.
At the same time, the PEG chains become rehydrated in
this process. The adlayer thickness of all polymers was again
measured by VASE (Figure 1, &). Quaternary ammonium
polymer 8 again displayed a high value, with a measured
thickness of roughly 17 ꢃ. This is comparable to control po-
Next, we investigated protein adsorption to the TiO2 sur-
faces coated with the different polymers prepared as de-
scribed above. The resulting surfaces were subsequently ex-
posed to human serum for 20 min, rinsed with buffer and
ultra-pure water, and the resulting increase in layer thick-
ness was measured by VASE (Figure 2). These results dem-
Figure 1. Adlayer thickness of polymers 3 (8), 7 (10), 8 (9), 9 (10), 11 (6),
and 12 (8) as measured by VASE (numbers in parentheses refer to the
number of independent experiments, error bars denote the standard devi-
Figure 2. Measured increase in layer thickness due to serum adsorption
on polymer adlayers of 3, 7, 8, 9, 11, and 12 plotted against polymer
adlayer thickness after equilibration for 24 h in physiological buffer. Con-
trol refers to the bare TiO2 surface.
&
ation). : values measured directly after adsorption in cloud point buffer,
&: values measured after equilibration in physiological buffer at pH 7.4.
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K. Gademann et al.
onstrate that coatings of 7 and 8 substantially reduce non-
specific serum adsorption when compared with bare TiO2
(Figure 2, control). The measured protein resistance values
for 7 and 8 are slightly lower than that of anachelin chromo-
phore 3, which reflects their lower polymer adlayer thick-
nesses (Figure 1). In addition, whereas the introduction of a
permanent charge, such as in 8, leads to increased PEG sur-
face coverage, as determined by VASE (Figure 1), it has no
influence on protein resistance (Figure 2) because both poly-
mers 7 and 8 are equally efficient towards this goal.
Boc-l-Glu and dopamine in a straightforward and conven-
ient one-step process. Benefits of the new anchors included
in 8 and 11 are 1) ease of synthesis compared with previous-
ly reported systems such as 3, 2) operationally simple dip-
and-rinse protocols for the generation of protein-resistant
surfaces, and 3) mild surface functionalization through cate-
chols, which are compatible with many functional groups.
Applications of the novel surface-active molecules for the
generation of functional surfaces are currently underway in
our laboratories.
Surprisingly, the diamino catechol polymer 9 coating was
inefficient at reducing the protein-adsorbed mass (Figure 2),
in contrast to the expectation given the adlayer thickness of
10 ꢃ after buffer incubation (Figure 1). The reason for this
unexpected result is unclear at present. Didopamine poly-
mer 11 showed a significantly better performance in this re-
spect, with over 95% reduction of protein adsorption when
compared with the control. Thus, polymer 11 as well as the
charged monovalent 8 generally display the best values with
regard to performance in adlayer thickness and protein re-
sistance.
The data obtained from VASE experiments were further
corroborated by X-ray photoelectron spectroscopy (XPS)
measurements. For protein-resistant surfaces, the chemical
composition of the adsorbate layer is expected to remain
unchanged, whereas surfaces that adsorb proteins should
show a substantial increase in the surface nitrogen concen-
tration as well as changes in the composition of the carbon
components of the C1s signal. The XPS data confirmed that
surfaces identified by VASE as protein resistant, for exam-
ple, 3, 11, and 12, also show the smallest change in their
chemical composition (shown in the Supporting Informa-
tion). The combined XPS and VASE findings for these
three polymers clearly demonstrate that the measured layer
thickness after the serum test mostly reflects the polymer
film and that adsorption or exchange of polymer molecules
by proteins has not taken place to a significant degree. In
contrast, the films obtained from 7 and 9 show a substantial
increase in the nitrogen and carbon surface concentrations.
The XPS data also indicate that thicker polymer layers (cor-
responding to a higher C/Ti atomic concentration ratio) with
correspondingly higher PEG-chain surface density result in
more favorable protein resistance, which is in agreement
with the ellipsometry data (Figure 2).
Experimental Section
Materials and methods: Chemicals were purchased from Fluka, ABCR,
or Acros and used without further purification. Analytical thin-layer
chromatography (TLC) was performed on Merck silica gel 60 F254
plates (0.25 mm thickness) precoated with a fluorescent indicator. The
developed plates were examined under UV light and stained with ceric
1
ammonium molybdate followed by heating. All H and 13C NMR spectra
were recorded by using a Bruker DPX 400 MHz (1H) or 100 MHz (13C)
FT spectrometer at RT; chemical shifts (d) are given in ppm and coupling
constants (J) in Hz. IR spectra were recorded by using a Varian 800 FT-
IR ATR spectrometer. The absorptions are reported in cmÀ1 and the IR
bands were assigned as strong (s), medium (m), or weak (w). All mass
spectra were recorded by the Mass Spectroscopy service of EPF Lau-
sanne on a MICROMASS (ESI) Q-TOF Ultima API instrument. Melting
points were determined by using a Bꢁchi B-545 apparatus in open capil-
laries and are uncorrected. Compound 4 was prepared according to the
literature,[18] and procedures and data are reported in the Supporting In-
formation.
TiO2 surfaces and surface preparation: Silicon wafers (Si-Mat Silicon Ma-
terials Landsberg/Deutschland) were coated with TiO2 (15/20 nm) by
magnetron sputtering (PSI Villingen, Switzerland). Metal oxide coated
wafers were subsequently sawn into 1 cmꢄ1 cm pieces. Prior to polymer
modification, TiO2-coated silicon wafers were sonicated in toluene twice
for 10 min, followed by sonication in 2-propanol twice for 7 min, dried
under a stream of nitrogen, and finally exposed to O2 plasma (Harrick
Scientific Corporation, Ossining, NY) for 3 min to remove adventitious
contamination from the surface.
Ellipsometry (VASE): The adlayer thickness on the TiO2 wafers was
measured by M-2000F Variable angle spectroscopic ellipsometry (J. A.
Woollam Co.) at 65, 70, and 758C by using wavelengths from 370 to
1000 nm. VASE spectra were fitted with multilayer modes by using a
custom analysis software (WVASE 32).
X-ray photoelectron spectroscopy (XPS): XPS data were acquired on a
SIGMA probe thermo XPS system spectrophotometer. The instrument
was equipped with a multichannel detector and an Alpha 110 hemispher-
ical analyzer. All spectra were acquired at 300 W with an AlKa X-ray
source (1486.6 eV) with a large area spot size. High-resolution spectra (C
1s, N 1s) were recorded with 0.1 eV step size and 25 eV pass energy. Re-
corded spectra were referenced to the aliphatic hydrocarbon C1s signal
at 285.0 eV. Data were analyzed by using the program CasaXPS (Ver-
sion 2.3.5 http://www.casaxps.com). The signals were fitted by using Gaus-
sian–Lorentzian functions and an Marquardt–Levenberg optimization al-
gorithm following Shirley iterative background subtraction.
Conclusion
We have presented the synthesis and evaluation of a series
of new dopamine-based anchors for surface functionaliza-
tion. Two anchors with excellent properties were identified.
Dopamine 8 with a permanent positive charge displayed a
comparable adlayer thickness as reference 3. Didopamine
polymer 11 was identified as the anchor of choice because
this compound showed the highest adlayer thickness while
displaying a large reduction in protein attachment. In addi-
tion, the didopamine anchor of 11 can be prepared from
Preparation of trimethylammonium dopamine 5: Compound 4 (44 mg,
0.14 mmol, 1.0 equiv) was dissolved in CH2Cl2/MeOH 2:1 (1 mL). MeI
(44 mL, 0.71 mmol, 5.0 equiv) was added and the solution was stirred at
RT for 6 h under N2. The solvent was removed under reduced pressure
to give compound
5
(62 mg, 0.14 mmol, 97%). 1H NMR (CD3OD,
400 MHz): d=1.36–1.39 (s, 9H; Boc rotamers), 2.61–2.74 (m, 2H), 3.17
(s, 9H), 3.49 (m, 2H), 4.20–4.23 (m, 1H), 6.59–6.61 (m, 1H), 6.72–
6.74 ppm (m, 2H); 13C NMR (CD3OD, 100 MHz): d=27.5, 27.6 (Boc ro-
tamers), 40.7, 48.4, 54.8, 73.8, 79.5, 115.6, 116.8, 123.2, 132.0, 144.6, 145.8,
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Protein-Resistant Surfaces
FULL PAPER
155.6 ppm; HRMS: m/z: calcd for C17H29N2O4 [M+H]+: 325.2122; found:
325.2127.
purified by chromatography on SiO2 (CH2Cl2/MeOH 9:1) to give (S)-tert-
butyl-1,5-bis(3,4-dihydroxyphenethylamino)-1,5-dioxopentan-2-ylcarba-
mate (477 mg, 0.92 mmol, 76%). 1H NMR (400 MHz, CDCl3): d=1.41–
1.44 (s, 9H; Boc rotamers), 1.76–1.87 (m, 2H), 2.19 (t, J=7.10 Hz, 2H),
2.62–2.68 (m, 4H), 3.31–3.37 (m, 4H), 3.64–3.77 (m, 2H), 3.94–4.00 (m,
1H), 6.56–6.61 (m, 2H), 6.63 (d, J=0.96 Hz, 2H), 6.79 ppm (d, J=
8.32 Hz, 2H); 13C NMR (CDCl3, 100 MHz): d=27.2, 28.2, 29.6, 34.6, 34.8,
40.4, 41.1, 51.8, 80.4, 115.3, 115.5, 115.7, 115.8, 120.5, 120.6, 130.5, 130.7,
143.0, 143.1, 144.0, 144.1, 155.9, 171.9, 173.9 ppm; IR 3402 (m), 3010 (m),
1780 (s), 1580 (s), 1273 (s), 1151 (s).
Preparation of compound 6: Compound 4 (250 mg, 0.81 mmol, 1.0 equiv)
was suspended in CH2Cl2 (4 mL) and cooled to 08C under N2. TFA
(2 mL) was added dropwise and the solution stirred at 08C for 1 h and at
RT for 1 h. The reaction mixture was then concentrated, redissolved in
toluene (1 mL), concentrated, and dried under high pressure. The residue
was dissolved in CH2Cl2 (10 mL) before Et3N (450 mL, 4.0 equiv) and
HOBt (130 mg, 1.2 equiv) were added and the solution was stirred for
5 min at RT. EDC·HCl (165 mg, 1.2 equiv) and a solution of BOC-l-
DOPA (259 mg, 1.1 equiv) in CH2Cl2 (3 mL) were added. The solution
was stirred at 08C for 1 h and at RT for 18 h under N2 in the dark. The
reaction mixture was concentrated, diluted with Et2O and washed with
1m HCl (2ꢄ10 mL), saturated NaHCO3 (2ꢄ10 mL) and brine (10 mL).
The organic layer was dried (MgSO4), concentrated, and the residue puri-
fied by chromatography on SiO2 (CH2Cl2/MeOH 8:1) to give 6 (287 mg,
0.63 mmol, 54%).
(S)-tert-Butyl 1,5-bis(3,4-dihydroxyphenethylamino)-1,5-dioxopentan-2-
ylcarbamate (15 mg, 30 mmol, 3.0 equiv) was dissolved in CH2Cl2 (1 mL),
cooled at 08C under N2, and CF3CO2H (1 mL) was added dropwise. The
solution stirred for 1 h at 08C and for 1 h at RT. The solvent was re-
moved and the residue was dissolved in toluene and concentrated. The
residue was dissolved in CH2Cl2/DMF 1:1 (1 mL) under N2. NMM
(50 mL) was added dropwise and the solution was stirred at RT for
15 min. mPEG-SPA (50 mg, 10 mmol, 1.0 equiv) was added and the solu-
tion was stirred for 14 h at RT under N2. The solution was filtered, dilut-
ed with Et2O (30 mL), and stored at 48C for 4 h. Filtration and purifica-
tion by chromatography on Sephadex LH-20 (MeOH) gave 11 (30 mg
5 mmol, 56%).
Preparation of mPEG-SPA: mPEG-propionic acid (150 mg, 30 mmol,
1.0 equiv) was dissolved in CHCl3 (3 mL) at 08C under N2. Dicyclohexyl-
carbodiimide (DCC; 7 mg, 33 mmol, 1.1 equiv) was added and the solu-
tion stirred for 15 min at 08C. N-Hydroxysuccinimide (NHS; 4 mg,
33 mmol, 1.1 equiv) was added and the solution was stirred for 1 h at 08C
and for 14 h at RT under N2. The reaction mixture was kept at 48C for
4 h and filtered. The solvent was removed and the residue was dissolved
in Et2O (30 mL) and stored at 48C for 1 h. Filtration gave mPEG-SPA
(146 mg, 29 mmol, 97%), which was used without further purification.
Acknowledgements
Preparation of polymer 7: Compound 4 (14 mg, 30 mmol, 3.0 equiv) was
dissolved in CH2Cl2 (1 mL), cooled at 08C under N2, and TFA (1 mL)
was added dropwise. The solution was stirred for 1 h at 08C and for 1 h
at RT. The solvent was removed and the residue was dissolved in toluene
and concentrated. The residue was dissolved in CH2Cl2/DMF 1:1 (1 mL)
under N2. NMM (50 mL) was added dropwise and the solution was stirred
at RT for 15 min. mPEG-SPA (50 mg, 10 mmol, 1.0 equiv) was added and
the solution was stirred for 14 h at RT under N2. The solution was fil-
tered, diluted with Et2O (30 mL), and stored at 48C for 4 h. Filtration
and purification by chromatography on Sephadex LH-20 (MeOH) gave 7
(22 mg, 4 mmol, 44%).
K.G. is a European Young Investigator (EURYI). We thank the SNF for
support of this work (200021-115918/1).
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Received: June 10, 2008
Published online: October 16, 2008
10584
www.chemeurj.org
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 10579 – 10584