DNA-Polymer Conjugates by Photoinduced RAFT Polymerization

Article abstract of DOI:10.1021/acs.biomac.8b01328

Conventional grafting-to approaches to DNA-polymer conjugates are often limited by low reaction yields due to the sterically hindered coupling of a presynthesized polymer to DNA. The grafting-from strategy, in contrast, allows one to directly graft polymers from an initiator that is covalently attached to DNA. Herein, we report blue-light-mediated reversible addition-fragmentation chain-transfer (Photo-RAFT) polymerization from two different RAFT agent-terminated DNA sequences using Eosin Y as the photocatalyst in combination with ascorbic acid. Three monomer families (methacrylates, acrylates and acrylamides) were successfully polymerized from DNA employing Photo-RAFT polymerization. We demonstrate that the length of the grown polymer chain can be varied by altering the monomer to DNA-initiator ratio, while the self-assembly features of the DNA strands were maintained. In summary, we describe a convenient, light-mediated approach toward DNA-polymer conjugates via the grafting-from approach.

Full text of DOI:10.1021/acs.biomac.8b01328

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Article
DNA-Polymer Conjugates by Photo-induced RAFT Polymerization
Thorsten Lückerath, Tina Strauch, Kaloian Koynov, Christopher Barner-Kowollik, David Ng, and Tanja Weil
Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b01328 • Publication Date (Web): 08 Nov 2018
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DNA-Polymer Conjugates by Photo-induced RAFT
Polymerization
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Thorsten Lueckerath,1 Tina Strauch,1 Kaloian Koynov,1 Christopher Barner-Kowollik,2,3* David
Ng,1* Tanja Weil1*
1 Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz (Germany)
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School of Chemistry, Physics and Mechanical Engineering, Queensland University of
Technology (QUT), George Street, QLD 4000, Brisbane (Australia)
3
Macromolecular Architectures, Institut für Technische Chemie und Polymerchemie, Karlsruhe
Institute of Technology (KIT), Engesserstraße 18, 76131 Karlsruhe (Germany)
KEYWORDS: Photo-RAFT, DNA-polymer conjugate, radical polymerization, DNA
Abstract: Conventional grafting-to approaches to DNA-polymer conjugates are often limited by
low reaction yields due to the sterically hindered coupling of a pre-synthesized polymer to DNA.
The grafting-from strategy, in contrast, allows to directly graft polymers from an initiator that is
covalently attached to DNA. Herein, we report blue light-mediated reversible addition-
fragmentation chain-transfer (Photo-RAFT) polymerization from two different RAFT agent-
terminated DNA sequences using Eosin Y as the photocatalyst in combination with ascorbic acid.
Three monomer families (methacrylates, acrylates and acrylamides) were successfully
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polymerized from DNA employing Photo-RAFT polymerization. We demonstrate that the length
of the grown polymer chain can be varied by altering the monomer to DNA-initiator ratio, while
the self-assembly features of the DNA strands were maintained. In summary, we describe a
convenient, light-mediated approach towards DNA-polymer conjugates via the grafting-from
approach.
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Introduction
The combination of synthetic polymers with the unique specificity and recognition properties of
DNA gave rise to an all-new class of DNA-polymer hybrid materials.1–5 In recent years, DNA-
polymer conjugates have emerged as versatile building blocks that provided enhanced stability to
complex DNA nanostructures6,7 or facilitated, e.g., the precise organization of polymeric structures
by DNA.8–12 In particular, the combination of DNA with a hydrophobic polymer chain resulted in
amphiphilic species that formed supramolecular architectures such as micelles or vehicles.13–15
These assemblies were intensively investigated in the context of drug delivery,16–19 as scaffolds
for directing organic reactions20 or as virus-like particles.21 Inspired by recent advances in
designing more sophisticated 2D and 3D DNA architectures equipped with various
functionalities,22,23 polymer chains were organized within complex DNA scaffolds or grown from
DNA origami templates in distinct nanostructures.24,25
Until recently, grafting-to strategies of pre-synthesized polymers to DNA sequences were
mainly accomplished following solid phase synthesis13,20,26,27 or bioconjugation strategies, such as
copper-mediated azide-alkyne,28,29 thiol-ene,30–32 amide33–35 or recently also tetrazine-norbornene
coupling.36 While the major advantage related to both approaches is that the polymer can be
thoroughly characterized prior to the coupling reaction, the nature of the polymer has to be
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carefully selected, e. g., it must remain stable under the strongly basic conditions of DNA-cleavage
from the solid support. Critically, variation of polymer lengths to, i.e., improve stability and
bioactivity is more tedious via the grafting-to strategy and low reaction yields were reported.32,37
Most likely, steric hindrance of the pre-formed polymer chain or the necessity to use organic
solvents during the conjugation step caused problems, particularly when hydrophobic, high
molecular weight polymers were employed.36
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Therefore, grafting-from polymerization from the DNA sequence containing the covalently
attached initiator would be highly desirable for achieving DNA-polymer conjugates. Compared to
the grafting-to strategy, grafting-from polymerization offers more convenient purification and
higher coupling yields. Here, reversible-deactivation radical polymerizations38 such as atom
transfer radical polymerization (ATRP)39,40 or RAFT polymerization41,42 are often favoured due to
their exceptional characteristics in terms of non-demanding reaction conditions, high control over
molecular weight, molecular weight distribution (MWD) and end-group functionality.
First grafting-from approaches from DNA sequences immobilized on different surfaces
following either ATRP or RAFT polymerization have been reported.43–47 However, no particular
insights in understanding the polymerization process from DNA were given and only limited
characterization of the resulting DNA-polymer conjugates was provided as they mostly remained
at the surface. Matyjaszewski and co-workers significantly contributed by providing full analytics
on the composition of DNA-polymer conjugates synthesized via conventional ATRP as well as
photoATRP directly in solution.48,49
ATRP methods that rely on toxic transition metal catalysts have several limitations as catalysts
are challenging to remove from the polyanionic DNA-polymer conjugates. However, important
progress in metal-free ATRP has been made recently,50–52 which offers key future potential for
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achieving DNA-polymer conjugates. RAFT polymerization does typically not require metal
catalysts and, in addition, offers similar flexibility in terms of monomer scope and end-group
functionalization.53 In particular, recent developments in photo-induced RAFT polymerizations –
particularly the introduction of photo-induced electron transfer (PET)-RAFT polymerization by
Boyer and co-workers54–56 – allow initiating the RAFT process solely by irradiating a
biocompatible dye with visible light.57 Therefore, the RAFT approach provides an elegant
polymerization platform particularly for biological systems such as cells or proteins.58,59
We believe that photo-induced RAFT (Photo-RAFT) polymerizations will substantially enlarge
the polymer chemist’s toolset for the synthesis of DNA-polymer conjugates, yet it has, to the best
of our knowledge, not been exploited for the grafting-from preparation of DNA-polymer
conjugates freely in solution. Herein, we report the solution-based Photo-RAFT polymerization
from two RAFT agent-terminated single-stranded DNA (RAFT-ssDNA) sequences, which were
derived from the established RAFT agents 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid
(CPADB) and 2-(butylthiocarbonothioyl)propionic acid (BTPA). Methacrylates, acrylates and
acrylamides were applied as monomers for blue light-mediated RAFT polymerization. A series of
new ssDNA-polymer conjugates of varying monomers and chain lengths were achieved while the
DNA terminus remained functionally intact throughout the course of polymerization. In summary,
we provide a powerful and robust grafting-from method in solution by light activation that offers
many opportunities for achieving tailored DNA-polymer conjugates.
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Experimental
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Materials. 2-Bromopropionic acid (Alfa Aesar, 98 %), 1-butanethiol (Fluka, >97 %), carbon
disulphide (Acros Organics, 99,9 %), 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid
(CPADB, 99,9 %), N,N’-diisopropylcarbodiimide (DIC, Sigma, 99 %), N,N-
diisopropylethylamine
(DIPEA,
Roth,
>99,5
%),
N-(3-dimethylaminopropyl)-N’-
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ethylcarbodiimide hydrochloride (EDC*HCl, Sigma, 98 %), 4-dimethylaminopyridine (DMAP,
Sigma, >99 %), 3-hydroxypicolinic acid (Sigma, >99 %), N-hydroxysuccinimide (NHS, Sigma,
98 %), pentafluorophenol (PFP, Merck, >99 %), organic solvents (VWR), dry solvents (Sigma,
99,8 %), deuterated solvents (Sigma or Deutero GmbH, 99,9 %) and silica gel 60 (Macherey-Nagel
GmbH) were all used as received. The nucleotides for DNA synthesis were purchased from Link
Technologies Ltd. Amine-terminated ssDNA was either self-synthesized by using a commerically
available C6 amino modifier from Glen Research or purchased from Sigma or biomers.net. All
other functional DNAs used within this study were purchased from biomers.net. Poly(ethylene
glycol) methyl ether methacrylate (Sigma, Mn = 300 g/mol, stabilized with 100 ppm MeHQ and
300 ppm BHT), poly(ethylene glycol) methyl ether acrylate (Sigma, Mn = 480 g/mol, stabilized
with 100 ppm MeHQ and 100 ppm BHT) and N,N-dimethylacrylamide (Acros Organics, stabilized
with 500 ppm MeHQ) were passed through a short column of alumina prior to use. N-
Isopropylacrylamide (NIPAM, Sigma, 97 %) was recrystallized twice from a 1 to 1 mixture of
toluene and pentane. The blue LED array lamp (item number: LIU470a) was purchased from
Thorlabs, Inc.
Synthesis of 2-(((butylthio)carbonothioyl)thio)propanoic acid (BTPA). BTPA was synthesized
according to a literature procedure with slight changes.[1] Briefly, 1-butanethiole (1.67 mL, 1.40 g,
15.5 mmol, 1.00 eq.) and carbon disulphide (1.02 mL, 1.30 g, 17.1 mmol, 1.10 eq.) were stirred in
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1 M NaOH (17 mL) for 1 hour. The reaction mixture was cooled to 0 °C and a solution of 1-
bromopropionic acid (2.61 g, 17.1 mmol, 1.10 eq.) in 1 M NaOH (17 mL) was slowly added. After
stirring overnight, the aqueous phase was overlaid with cyclohexane and acidified to pH = 1 with
concentrated hydrochloric acid solution. The organic phase was collected, dried over magnesium
sulphate and concentrated in vacuo. Recrystallization from cyclohexane gave the clean product as
a yellow solid after storage in the fridge (1.80 g, 7.55 mmol, 49 %). 1H NMR (300 MHz, DMSO-
d6): δ(ppm) = 13.17 (brs, 1H), 4.68 (q, J = 7.3 Hz, 1H), 3.45-3.28 (m, 2H), 1.70-1.56 (m, 2H), 1.51
(d, J = 7.3 Hz, 3H), 1.44-1.27 (m, 2H), 0.89 (t, J = 7.3 Hz, 3H). 13C NMR (125 MHz, DMSO-d6):
δ(ppm) = 222.9, 172.0, 48.6, 36.7, 30.1, 21.9, 17.2, 13.9. ESI-MS: Calc.: [M+H]+ = 239.38, found:
[M+H]+ = 239.
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Synthesis of BTPA-NHS. BTPA-NHS was synthesized according to a literature procedure with
slight changes.[2] BTPA (250 mg, 1.05 mmol, 1.00 eq.), NHS (117 mg, 1.15 mmol, 1.10 eq.) and
DMAP (12.8 mg, 105 µmol, 0.10 eq.) were added to dry DMF (10 mL). The mixture was cooled
to 0 °C and EDC*HCl (261 mg, 1.36 mmol, 1.30 eq.) was slowly added as a solid. After stirring
overnight, the reaction mixture was diluted with DCM (30 mL), washed with brine (3x) and dried
over sodium sulphate. Concentration in vacuo gave a yellow liquid as the crude product, which
had to be further purified using column chromatography (CH/EA = 2:1). The final product was
1
obtained as a yellow viscous oil (156 mg, 0.46 mmol, 46 %). H NMR (300 MHz, DMSO-d6):
δ(ppm) = 5.13 (q, J = 7.4 Hz, 1H), 3.41 (t, J = 7.3 Hz, 2H), 2.80 (s, 4H), 1.72-1.57 (m, 5H), 1.45-
1.30 (m, 2H), 0.89 (t, J = 7.3 Hz, 3H). 13C NMR (125 MHz, DMSO-d6): δ(ppm) = 221.0, 170.3,
167.7, 45.4, 37.0, 30.0, 26.0, 21.9, 16.6, 13.9. ESI-MS: Calc.: [M+Na]+ = 358.44, found: [M+Na]+
= 358.
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Synthesis of BTPA-PhF5. BTPA-PhF5 was synthesized according to a literature procedure with
slight changes.[2] Briefly, BTPA (200 mg, 0.84 mmol, 1.00 eq.), PFP (263 mg, 1.43 mmol, 1.70
eq.) and DMAP (10.3 mg, 84.0 μmol, 0.10 eq.) were added to dry DCM (10 mL.) The mixture was
cooled to 0 °C and a solution of DIC (260 µL, 212 mg, 1.68 mmol, 2.00 eq.) in dry DCM (10 mL)
was added dropwise. After stirring overnight, the precipitate was filtered off and the reaction
mixture was concentrated in vacuo. Column chromatography (CH/EA = 9:1) gave the final product
as a yellow liquid (227 mg, 0.56 mmol, 67 %). 1H NMR (300 MHz, CD2Cl2): δ(ppm) = 5.02 (q, J
= 7.4 Hz, 1H), 3.32 (t, J = 7.3 Hz, 2H), 1.72-1.56 (m, 5H), 1.43-1.28 (m, 2H), 0.89 (t, J = 7.3 Hz,
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3H). F NMR (470 MHz, DMSO-d6): δ(ppm) = -152.2 (d, J = 20.8 Hz, 2F), -157.4 (t, J = 21.6
Hz, 1F), -162.0 (t, 21.1 Hz, 2F) ESI-MS: Calc.: [M+H]+ = 405.43, found: [M+H]+ = 405.
Synthesis of CPADB-NHS. CPADB (300 mg, 1.07 mmol, 1.00 eq.), NHS (136 mg, 1.18 mmol,
1.10 eq.) and DMAP (13.1 mg, 107 μmol, 0.10 eq.) were dissolved in dry DCM (10 mL). The
mixture was cooled to 0 °C and EDC*HCl (309 mg, 1.61 mmol, 1.50 eq.) was slowly added as a
solid. After stirring overnight, the reaction mixture was diluted with DCM (30 mL) and washed
with brine (1x), 1 M NaOH (2x) and 1 M HCl (2x). The organic phase was dried over magnesium
sulphate and was concentrated in vacuo to afford a pink solid as the crude product. Further
purification via column chromatography (CH/EA = 2:1) gave the final product as a pink powder
(145 mg, 0.39 mmol, 36 %). 1H NMR (300 MHz, DMSO-d6): δ(ppm) = 7.98-7.92 (m, 2H), 7.74-
7.66 (m, 1H), 7.56-7.48 (m, 2H), 3.14-2.90 (m, 2H), 2.83 (s, 4H), 2.75-2.56 (m, 2H), 1.96 (s, 3H).
13C NMR (125 MHz, DMSO-d6): δ(ppm) = 224.1, 170.6, 168.2, 144.4, 134.2, 129.5, 127.0, 118.9,
46.2, 31.8, 26.7, 25.9, 23.4. ESI-MS: Calc.: [M+H]+ = 377.45, found: [M+H]+ = 377.
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Synthesis of CPADB-PhF5. CPADB (250 mg, 0.89 mmol, 1.00 eq.), PFP (181 mg, 0.98 mmol,
1.10 eq.) and DMAP (10.9 mg, 89.2 μmol, 0.10 eq.) were added to dry DCM (10 mL.) The mixture
was cooled to 0 °C and a solution of DIC (152 µL, 124 mg, 0.98 mmol, 1.10 eq.) in dry DCM (10
mL) was added dropwise. After stirring overnight, the precipitate was filtered off and the reaction
mixture was concentrated in vacuo. Column chromatography (CH/EA = 10:1) gave the final
product as a pink oil (119 mg, 0.27 mmol, 30 %). 1H NMR (300 MHz, CD2Cl2): δ(ppm) = 7.89-
7.82 (m, 2H), 7.57-7.48 (m, 1H), 7.39-7.32 (m, 2H), 3.02-2.95 (m, 2H), 2.74-2.44 (m, 2H), 1.90
(s, 3H). 19F NMR (470 MHz, DMSO-d6): δ(ppm) = -153.1 (d, J = 20.3 Hz, 2F), -158.5 (t, J = 21.8
Hz, 1F), -162.9 (t, J = 21.1 Hz, 2F). ESI-MS: Calc.: [M+H]+ = 446.42, found: [M+H]+ = 446.
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Synthesis of NH2-DNA. After DNA synthesis, the oligonucleotides were cleaved from the solid
support by incubation with concentrated ammonia at ambient temperature for 16 hours. The MMT
group was manually removed by following a standard protocol for MMT-deprotection of
oligonucleotides.[3]
Synthesis of CPADB-DNA via NHS or PFP Coupling. NH2-DNA (1.00 mg, 172 nmol, 1.00 eq.)
at a concentration of 250 µM relative to the total amount of solvent, CPADB-NHS (3.23 mg, 8.58
µmol, 50.0 eq.) or CPADB-PhF5 (3.82 mg, 8.58 nmol, 50.0 eq.), respectively, and DIPEA (0.60
µL, 0.44 mg, 3.43 µmol, 20.0 eq.) were shaken in a 1 to 1 mixture of DMF and nuclease-free water
for 30 minutes. Residual CPADB-NHS was removed via centrifugation (5 min, 12500 rpm, 20
°C). The supernatant was evaporated and the residue was picked up in a 1 to 1 mixture of
acetonitrile and water. The sample was then subjected to HPLC purification and the desired peak
was collected between 11.74 min and 12.77 min. The fractions were combined, the solvent was
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removed and the residue was picked up in nuclease-free water to determine the concentration
(682 µg, 112 nmol, 65 %). For MALDI ToF mass spectra, the DNA was once precipitated from
isopropanol to desalt the sample from the buffer salts of HPLC purification.
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Synthesis of BTPA-DNA via NHS or PFP Coupling. NH2-DNA (1.00 mg, 172 nmol, 1.00 eq.)
at a concentration of 250 µM relative to the total amount of solvent, BTPA-NHS (2.88 mg, 8.58
µmol, 50.0 eq.) or BTPA-PhF5 (3.47 mg, 8.58 µmol, 50.0 eq.), respectively, and DIPEA (0.60 µL,
0.44 mg, 3.43 µmol, 20.0 eq.) were shaken in a 1 to 1 mixture of DMF and nuclease-free water for
2 hours. The supernatant was evaporated and the residue was picked up in a 1 to 1 mixture of
acetonitrile and water. The solution was then subjected to HPLC purification and the desired peak
was collected between 12.30 min and 13.21 min. The fractions were combined, the solvent was
removed and the residue was picked up in nuclease-free water to determine the concentration
(835 µg, 138 nmol, 80 %). For MALDI ToF mass spectra, the DNA was once precipitated from
isopropanol to desalt the sample from the buffer salts of HPLC purification.
Conventional Photo-RAFT Polymerization of Acrylamides and Acrylates. A typical
experiment was conducted by charging a custom-built schlenk tube with DMA (2.06 µL, 1.98 mg,
20.0 µmol, 200 eq.), BTPA (23.9 µg, 0.10 µmol, 1.00 eq.), Eosin Y (6.48 µg, 0.01 µmol, 0.10 eq.),
ascorbic acid (18.0 µg, 0.10 µmol, 1.00 eq.) and water (17.9 µL) as the solvent. The schlenk flask
was properly sealed and degassed by freeze-pump-thaw (3 x). The tube was then placed in the
photoreactor as shown above and irradiated by a blue LED (λmax. = 470 nm, 4 mW/cm2) at room
temperature. The samples were analyzed by GPC with DMF as the eluent in order to determine
apparent molecular weights and dispersities.
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Conventional Photo-RAFT Polymerization of Methacrylates. A typical experiment was
conducted by charging a custom-built schlenk tube with OEGMA (5.71 µL, 6.00 mg, 20.0 µmol,
200 eq.), CPADB (27.9 µg, 0.10 µmol, 1.00 eq.), Eosin Y (6.48 µg, 0.01 µmol, 0.10 eq.), ascorbic
acid (18.0 µg, 0.10 µmol, 1.00 eq.) and water (14.3 µL) as the solvent. The schlenk flask was
properly sealed and degassed by freeze-pump-thaw (3 x). The tube was then placed in the
photoreactor as shown above and irradiated by a blue LED (λmax. = 470 nm, 4 mW/cm2) at room
temperature. The samples were analyzed by GPC with DMF as the eluent in order to determine
apparent molecular weights and dispersities.
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Photo-RAFT Polymerization of Acrylamides and Acrylates from DNA. A typical experiment
was conducted by charging a custom-built schlenk tube with DMA (2.06 µL, 1.98 mg, 20.0 µmol,
200 eq.), BTPA-DNA (605 µg, 0.10 µmol, 1.00 eq.), Eosin Y (6.48 µg, 0.01 µmol, 0.10 eq.),
ascorbic acid (18.0 µg, 0.10 µmol, 1.00 eq.) and water (17.9 µL) as the solvent. The schlenk flask
was properly sealed and degassed by freeze-pump-thaw (3 x). The tube was then placed in the
photoreactor as shown above and irradiated by a blue LED (λmax. = 470 nm, 4 mW/cm2) at room
temperature. After a predetermined time interval, the polymerization was stopped by removing the
light source and exposing to oxygen. The DNA-polymer conjugate was purified by membrane
filtration (3 x, MWCO = 10 kDA) and was subsequently analyzed by GPC, native PAGE and FCS.
Photo-RAFT Polymerization of Methacrylates from DNA. A typical experiment was
conducted by charging a custom-built schlenk tube with OEGMA (5.71 µL, 6.00 mg, 20.0 µmol,
200 eq.), CPADB-DNA (609 µg, 0.10 µmol, 1.00 eq.), Eosin Y (6.48 µg, 0.01 µmol, 0.10 eq.),
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ascorbic acid (18.0 µg, 0.10 µmol, 1.00 eq.) and water (14.3 µL) as the solvent. The schlenk flask
was properly sealed and degassed by freeze-pump-thaw (3 x). The tube was then placed in the
photoreactor as shown above and irradiated by a blue LED (λmax. = 470 nm, 4 mW/cm2) at room
temperature. After a predetermined time interval, the polymerization was stopped by removing the
light source and exposing to oxygen. The DNA-polymer conjugate was purified by membrane
filtration (3 x, MWCO = 10 kDA) and was subsequently analyzed by GPC, native PAGE and FCS.
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Results and Discussion
In automated DNA synthesis, the terminal functionality is generally introduced to ssDNA
sequences by using phoshoramidite coupling chemistry.60 This process requires incubation with
concentrated ammonia for several hours, e.g., for DNA cleavage from the solid support, and the
newly incorporated functionality has to remain stable under these harsh conditions. In contrast to
ATRP initiators that have been attached to ssDNA sequences directly on the solid support,48,49
RAFT agents are typically not stable in very basic media or in the presence of primary amines and
they would thus decompose during cleavage from the solid support.61–63 Therefore, two RAFT-
ssDNA sequences (BTPA-DNA and CPADB-DNA) were achieved by conjugating the RAFT
agent to the amine-terminated ssDNA sequence (NH2-ssDNA) in solution and not via the
phosphoramidite approach as depicted in Figure 1.
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DNA sequence
3'-ATCATCCACCATCTCTTTT-5'
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6
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9
R
O
P
O
O
O
NH2
CPADB-NHS/PFP
DIPEA
BTPA-NHS/PFP
DIPEA
3'
5'
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H2O/DMF (1:1)
65 %
H2O/DMF (1:1)
80 %
NH2-DNA
CN
S
S
H
H
O
P
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O
P
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O
R
N
O
O
R
N
S
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3'
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5'
5'
O
O
O
CPADB-DNA
BTPA-DNA
Acrylic monomers
Eosin Y
Ascorbic acid
Methacrylic monomers
Eosin Y
Ascorbic acid
H2O, hv (470 nm)
H2O, hv (470 nm)
H
O
P
O
S
S
O
O
O
O
R
R
N
3'
3'
5'
5'
O
O
S
or
CN
H
N
O
P
O
S
S
Figure 1. Schematic Overview on the Synthesis towards DNA-Polymer Conjugates by Photo-
RAFT Polymerization.
Synthesis of the DNA-Photo-RAFT initiators
The conjugation of the RAFT agent to DNA was accomplished by amide conjugation as reported
for RAFT polymerization from surface-anchored DNA before.46,47 CPADB and BTPA were
selected for controlling RAFT polymerizations of either methacrylic or acrylic monomers. The
ligation was conducted by reacting 19-mer NH2-ssDNA (3’-ATCATCCACCATCTCTTTT-5’-
AminoC6) with the activated N-hydroxysuccinimide (NHS) or pentafluorophenyl (PFP) esters of
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the two chosen RAFT agents in a mixture of water and DMF, which was necessary to ensure
sufficient solubility of all reagents. Due to the competitive hydrolysis reaction in partly aqueous
medium, the activated esters were added in large excess (≥ 50 eq.) and in conjunction with N,N-
diisopropylethylamine (DIPEA) as an auxiliary base in order to increase the reactivity of the amine
moiety. Reaction control by HPLC indicated that less equivalents of the PFP ester were required
to achieve higher conversions, presumably due to its higher hydrolytic stability when compared to
the respective NHS ester.64 Full conversion of the NH2-DNA was achieved after 30 minutes for
activated CPADB and after 2 hours for activated BTPA, respectively. In both reactions, a clear
shift in the corresponding HPLC spectra (Figure 2) was observed during product formation.
Slightly higher yields were obtained for activated BTPA (~ 80%) than for activated CPADB (~ 65
%), probably again due to the higher hydrolytic stability of the trithiocarbonate group compared
to the dithiobenzoic acid moiety of CPADB. In contrast to the often low coupling yields of
sterically demanding polymers to DNA, high yields and clean products were achieved for
conjugation of the RAFT agent as depicted in Figure 2.
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The BTPA-DNA and CPADB-DNA conjugates were further analyzed by UV-VIS, revealing
pronounced shoulders at the maximum UV absorbance of the RAFT agents compared to the
precursor DNA (Figure S4), and by MALDI ToF-MS (Figure 2c). As reported in the literature,65,66
RAFT agents can undergo fragmentation via a six-membered transition state during the MALDI
ToF-MS measurement process. As depicted in Figure 2, CPADB revealed a significantly higher
tendency to fragment due to its less stable dithiobenzoic acid moiety. Monofunctionalization of
the ssDNA sequences with each RAFT agent was clearly confirmed and after HPLC, purified
products were isolated (Figure S5).
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Figure 2. a) HPLC chromatograms of N-hydroxysuccinimide (NHSOH, line 1) and NH2-ssDNA
before (line 2) and after the reaction with BTPA-NHS (line 3) or CPADB-NHS (line 4),
respectively. b) Schematic overview on the fragmentation reactions of BTPA-DNA (left) and
CPADB-DNA (right) occurring during MALDI ToF measurements in accordance with the
literature.65,66 c) MALDI ToF mass spectra of BTPA-DNA (left) and CPADB-DNA (right). M =
3-hydroxypicolinic acid.
Photo-RAFT Model Polymerizations
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Oligonucleotides are still relatively expensive and challenging to access in large milligram
scales. For DNA conjugation, high ionic strength buffers are typically required and tolerance of
organic solvents is limited. Therefore, in view of later applications, it is important to assess the
limits of the Photo-RAFT polymerization process from ssDNA sequences in terms of reaction
volume and DNA concentration in aqueous media, while maintaining a functioning RAFT
equilibrium. Based on previous reports on RAFT polymerization in biological environments,58,59
the RAFT agent concentration was set to 5 mM, which is typically considered as dilute, and
polymerizations were conducted in ultralow volumes of 20 μL with water as the only solvent.
One major limitation when scaling to such small volumes is the increased sensitivity of the
reaction mixture to oxygen due to the higher surface area exposed to air. Conventional
deoxygenation techniques such as freeze-pump-thaw or inert gas purging are extremely
challenging at these volumes. Boyer and co-workers have recently introduced an aqueous Photo-
RAFT polymerization process for ultralow volumes that employs Eosin Y (EY) in the combination
with ascorbic acid (AscA) as the reducing agent without the need for deoxygenation.67 In such
EY/AscA photopolymerizations, oxygen is converted into hydrogen peroxide, which subsequently
reacts with ascorbic acid to form radical species capable of initiating polymerization. However,
high monomer concentrations were typically employed (50 wt% monomer),67 whereas very low
concentrations are necessary for polymerization from DNA. Therefore, optimal reaction
conditions for DNA-Photo-RAFT polymerization had to be identified.
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Table 1. Summary of Polymers prepared by Photo-RAFT Polymerization
No.
P1
P2
P3
P4
P5
P6
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Monomer
ratio xa
200
100
BTPA
DMA
2
400
100
80
200
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9
RAFT agent BTPA
Monomer DMA
BTPA
DMA
2
BTPA
NIPAM
1.5
BTPA
OEGA
1
CPADB
OEGMA
2.5
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Irr. Time [h] 1.5
Mn,app. [kDa] 12.4
6.3
18.1
1.14
11.6
11.2
1.36
13.5
Đ
1.15
1.21
1.22
1.40
a) Polymerizations were conducted at [Monomer]:[RAFT agent]:[EY]:[AscA] ratios of x:1:0.1:1 in
water under blue light irradiation (λmax. = 470 nm, 4 mW/cm2) at room temperature using [RAFT agent]
= 5 mM. b) Apparent molecular weights and dispersities were determined by GPC with DMF as the
eluent using PMMA calibration standards.
We initially assessed whether the EY/AscA system can initiate the aqueous RAFT
polymerization of DMA under blue light irradiation (λmax. = 470 nm, 4 mW/cm2, Figure S3)
without prior deoxygenation using BTPA as the RAFT agent and a RAFT agent concentration of
5 mM. Employing a [DMA]:[BTPA]:[EY]:[AscA] ratio of 200:1:0.1:1 and custom-made Schlenk
flasks (Figure S2), the model polymer (Table S1-P2) was obtained after 2 h irradiation time.
However, the observed MWD was relatively broad (Đ = 1.92) and exhibited tailing in the low
molecular weight region while five times more concentrated conditions ([BTPA] = 25 mM)
resulted in a unimodal and narrow MWD (Table S1-P1, Đ = 1.21). These results suggested that at
sufficiently high EY/AscA concentrations, the influence of molecular oxygen can be minimized.
Interestingly, at low EY/AscA concentrations, the polymerization did not fail completely but rather
proceeded with a significant loss in control. As such, we attempted to explore the limits by
conducting the polymerization in air with substantially increased EY or AscA ratios. Nonetheless,
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with a RAFT agent concentration of 5 mM, we found no notable improvement and that the MWDs
still remained rather broad (Table S2-P4-P7). These observations imply that while the presence
of AscA facilitated the polymerization to take place under ambient conditions, it is still insufficient
to ensure good polymerization control.
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Therefore, in order to improve the MWDs at low concentrations, polymerizations were
additionally degassed via the freeze-pump-thaw technique while maintaining AscA as a reducing
agent, thereby eliminating oxygen interfering with the polymerization process. Under these
conditions, the Photo-RAFT polymerization of DMA afforded a polymer with narrow MWD and
low dispersity (P1). The length of the polymer chain varied in a convenient fashion by altering the
monomer to RAFT agent ratio (P2, P3). These polymerization conditions were successfully
transferred to other monomers, including NIPAM (P4), OEGA (P5) and OEGMA (P6, SI Table
1-P3), and sufficient polymerization control was maintained. Noteworthy, in case the
polymerization reaction was degassed via freeze-pump-that and EY was used without AscA, the
polymerization results were not well reproducible. Most likely, varying amounts of oxygen still
remained in solution after deoxygenation, which then interfered with the Photo-RAFT process. In
addition, it has been reported previously that an optimal [AscA]:[RAFT agent] ratio exists for
peroxide/AscA redox pairs.68,69 For the EY/AscA system, an optimal polymerization rate was
found at [AscA]:[RAFT agent] = 0.5-1 and above this ratio, the polymerization rates were
significantly compromised due to undesired side reactions.67 Therefore, in all further experiments,
the [AscA]:[RAFT agent] ratio was kept constant at 1 to minimize side reactions.
DNA-Photo-RAFT Polymerization
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With the optimized conditions in hand, Photo-RAFT polymerization was performed from either
BTPA- or CPADB-DNA. The resulting DNA-polymer conjugates were purified by membrane
filtration (MWCO = 10 kDa) in order to remove potentially unreacted precursor ssDNA sequences.
GPC analysis indicated successful polymer growth from both RAFT-ssDNA sequences for every
monomer (DP1-DP6). According to the GPC data, the polymer lengths were successfully varied
by adjusting the monomer to RAFT-ssDNA ratio (DP1, DP2). However, compared to the model
polymerizations with dispersities between 1.14 and 1.40 (Table 1), increased values between 1.6
and 2.96 were obtained when polymerizing directly from the respective RAFT-ssDNA sequence
(Table 2). The observed tailing in the low molecular weight region could be due to chain transfer
reactions to the ssDNA sequence, which could result in the occurrence of minor low molecular
weight DNA-polymer conjugates. In addition, small shoulders of high molecular weight were
observed in the case of PEGylated monomers presumably due to undesired side reactions, such as
bimolecular termination events, in situ transesterification or chain transfer reactions to the
polymers, which has been observed previously in aqueous media.70
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Native PAGE revealed successful polymer growth from both RAFT-ssDNA sequences as new
bands emerged after conducting the polymerization, indicating the formation of higher molecular
weight compounds (Figure S8). These bands were relatively broad and exhibited significantly
reduced mobility compared to the sharp bands of the precursor ssDNA sequence. Such diffuse
bands in PAGE are relatively common, e.g., for polymer-protein conjugates71 and reflect the
polydisperse nature of the grown polymers. Hybridization of a complementary 19-mer ssDNA
sequence (3’-GAGATGGTGGATGATTTTT-5’) to the DNA-polymer conjugates before gel
electrophoresis provided improved staining efficiency of the employed staining agent SYBR gold.
The DNA-polymer conjugates of higher molecular weight exhibited slower gel movement than
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the lower molecular weight conjugates and these results correspond well with the obtained GPC
data in Figure 3.
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Figure 3. GPC traces of the polymers measured in DMF with PMMA calibration standards. a)
Variation of the polymer lengths by varying monomer to RAFT agent ratio: P1, P2, P3 according
to Table 1; b) Photo-RAFT polymerization of different monomers (NIPAM, OEGA, OEGMA):
P4, P5, P6 according to Table 1; c, d) DNA-polymer conjugates of different monomers (DMA,
NIPAM, OEGA, OEGMA): DP1-DP6 according to Table 2.
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Table 2. Summary of DNA-Polymer Conjugates prepared by Photo-RAFT Polymerization
5
6
7
8
9
No.
DP1
200
DP2
500
DP3
200
DP4
80
DP5
80
DP6
200
Monomer
ratio xa
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BTPA-
DNA
BTPA-
DNA
BTPA-
DNA
BTPA-
DNA
BTPA-
DNA
CPADB-
DNA
RAFT agent
Monomer
DMA
2
DMA
2.5
NIPAM
2
OEGA
1
OEGA
2.33
OEGMA
1.5
Irr. Time [h]
Mn,app. [kDa] 13.8
1.60
31.2
1.68
14.6
1.80
10.1
1.77
22.9
12.7
Đ
2.19
2.96
a) Polymerizations were conducted at [Monomer]:[RAFT-ssDNA]:[EY]:[AscA] ratios of x:1:0.1:1 in
water under blue light irradiation (λmax. = 470 nm, 4 mW/cm2) at room temperature using [RAFT-
ssDNA] = 5 mM. b) Apparent molecular weights and dispersities were determined by GPC with DMF
as the eluent using PMMA calibration standards.
It is a special feature of ssDNA sequences that they can be equipped with various functionalities
by simple hybridization with the complementary ssDNA strand carrying the desired functionality.
Rhodamine 6G-terminated complementary ssDNA was applied to the polymer solution, resulting
in immediate labeling of the DNA-polymer conjugates. Fluorescence Correlation Spectroscopy71
was then used to measure the diffusion coefficients and hydrodynamic radii of free Rhodamine
6G-terminated ssDNA and the DNA-polymer conjugates hybridized with the Rhodamine 6G-
terminated ssDNA sequence. Typical FCS autocorrelation curves recorded in aqueous solution (c
 40 nM) are shown in Figure 4a. The curves could be well represented by monomodal component
fits (eq. S1, Supporting Information) that yielded the diffusion coefficients and consecutively the
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hydrodynamic radii of the studied fluorescent species. The hydrodynamic radii substantially
increased upon hybridization, evidencing the successful growth of polymer chains from RAFT-
ssDNA. DP1 and DP4 revealed similar hydrodynamic radii as expected for polymers of similar
molecular weight as determined by GPC. Importantly, DP2 showed a significant increase in the
hydrodynamic radius compared to DP1 due to the higher molecular weight of the attached
polymer, again standing in agreement with the corresponding GPC data (Figure 3).
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Figure 4. a) Normalized FCS autocorrelation curves (symbols) measured in aqueous solutions of
Rhodamine 6G-terminated ssDNA and DNA-polymer conjugates hybridized with it. The solid
lines represent the corresponding fits with eq. S1 that yielded the hydrodynamic radii of the studied
fluorescent species. b) Schematic representation of the labelling of ssDNA-polymer conjugates
with the complementary rhodamine 6G-terminated ssDNA sequence.
Conclusions
We introduce the first Photo-RAFT polymerization from ssDNA based on an optimized
polymerization protocol. The DNA-Photo-RAFT polymerization was accomplished with four
monomers yielding new DNA-polymer conjugates with varying molecular weights and acceptable
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dispersities. Noteworthy, also high molecular weight conjugates above 30000 g mol-1 were
achieved. Photo-RAFT polymerization does not require organic solvents or metal catalysts and is
particularly suitable for achieving DNA-polymer conjugates in biological environments, thus
complementing other synthetic and biopolymerization techniques such as ATRP and PCR.
Emerging applications of this technology, e.g., for constructing more sophisticated DNA-polymer
architectures controlled by light, will yield unique DNA origami-polymer nanostructures. With the
current achievements in DNA upscaling, we envision many promising opportunities based on the
unique characteristics of ssDNA and polymers to achieve precisely defined polymeric objects, e.g.,
by single chain folding where polymer shapes and functions could be controlled precisely.
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ASSOCIATED CONTENT
Supporting Information. Instrumentation details and experimental setup can be found in the
supporting information. NMR spectra and further characterizations are as well provided. The
files are available free of charge.
AUTHOR INFORMATION
Corresponding Authors
christopher.barnerkowollik@qut.edu.au, christopher.barner-kowollik@kit.edu, david.ng@mpip-
mainz.mpg.de, weil@mpip-mainz.mpg.de
Author Contributions
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The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
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Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
T.W. thanks the support of the European Union for a Synergy Grant (319130-BioQ). T.W. and
D.Y.W.N. gratefully acknowledge funding support from the SFB Transregio (TRR) 234 – Project
B01. C.B.-K. is grateful for funding in the context of an Australian Research Council (ARC)
Laureate Fellowship underpinning his photochemical research program, as well as continued key
support from the Queensland University of Technology (QUT). C.B.-K. additionally
acknowledges continued support by the BIFTM program of the Helmholtz association.
ABBREVIATIONS
DMA – N,N-dimethylacrylamide; NIPAM – N-isopropylacrylamide; OEGA – oligo(ethylene
glycol) methyl ether acrylate; OEGMA – oligo(ethylene glycol) methyl ether methacrylate.
REFERENCES
(1) Alemdaroglu, F. E.; Herrmann, A. DNA meets synthetic polymers--highly versatile
hybrid materials. Org. Biomol. Chem. 2007, 5, 1311–1320.
(2) Kwak, M.; Herrmann, A. Nucleic acid amphiphiles: synthesis and self-assembled
nanostructures. Chem. Soc. Rev. 2011, 40, 5745–5755.
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