Photo-cross-linking probes for trapping G-quadruplex DNA

Article abstract of DOI:10.1002/anie.201307413

We have developed a straightforward synthetic pathway to a set of six photoactivatable G-quadruplex ligands with a validated G4-binding motif (the bisquinolinium pyridodicarboxamide PDC-360A) tethered through various spacers to two different photo-cross-linking groups: benzophenone and an aryl azide. The high quadruplex-versus-duplex selectivity of the PDC core was retained in the new derivatives and resulted in selective alkylation of two well-known G-quadruplexes (human telomeric G4 and oncogene promoter c-myc G4) under conditions of harsh competition. The presence of two structurally different photoactivatable functions allowed the selective alkylation of G-quadruplex structures at specific nucleobases and irreversible G4 binding. The topology and sequence of the quadruplex matrix appear to influence strongly the alkylation profile, which differs for the telomeric and c-myc quadruplexes. The new compounds are photoactive in cells and thus provide new tools for studying G4 biology. Catch me if you can: A readily accessible set of photoactivatable G-quadruplex (G4) ligands with a bisquinolinium core showed high G4-versus-duplex selectivity. Alkylation under UV/Vis irradiation occurred at G4 nucleobases located in either the loops or the external G-quartets (see picture), depending on the cross-linker and the topology of the quadruplex. These probes might be used to irreversibly trap G4 structures for the study of G4 biology. Copyright

Full text of DOI:10.1002/anie.201307413

.
Angewandte
Communications
DOI: 10.1002/anie.201307413
G-Quadruplex Ligands
Photo-Cross-Linking Probes for Trapping G-Quadruplex DNA**
Daniela Verga, Florian Hamon, Florent Poyer, Sophie Bombard,* and Marie-Paule Teulade-
Fichou*
Abstract: We have developed a straightforward synthetic
pathway to a set of six photoactivatable G-quadruplex ligands
with a validated G4-binding motif (the bisquinolinium pyr-
idodicarboxamide PDC-360A) tethered through various
spacers to two different photo-cross-linking groups: benzo-
phenone and an aryl azide. The high quadruplex-versus-
duplex selectivity of the PDC core was retained in the new
derivatives and resulted in selective alkylation of two well-
known G-quadruplexes (human telomeric G4 and oncogene
promoter c-myc G4) under conditions of harsh competition.
The presence of two structurally different photoactivatable
functions allowed the selective alkylation of G-quadruplex
structures at specific nucleobases and irreversible G4 binding.
The topology and sequence of the quadruplex matrix appear to
influence strongly the alkylation profile, which differs for the
telomeric and c-myc quadruplexes. The new compounds are
photoactive in cells and thus provide new tools for studying G4
biology.
lish a stronger interaction by coordinating to nucleic bases of
quadruplexes were developed.[5] These metal complexes form
highly stable adducts with the human telomeric quadruplex,
but the intrinsic reactivity of platinum may also induce the
formation of nonspecific adducts. Recently, hybrid G4 binders
bearing a classical alkylating agent (e.g. an oxirane)[6] and
a chemically activatable alkylating moiety (e.g. a quinone
methide) have been reported.[7] However, the formation of
the resulting DNA adducts appeared to be reversible, which
caused low cross-linking efficiency, below 15%. So far, the
possibility of using light to trigger the alkylation of G4
structures has not been reported, although the photoalkyla-
tion of duplex DNA is rather well documented.[8]
In this context, we were keen to develop hybrid agents
containing a validated G4-binding motif tethered to a photo-
reactive group. With this aim, we chose to link the bisquino-
linum pyridodicarboxamide scaffold (abbreviated PDC and
also known as 360A), one of the best G4 ligands reported,[9] to
two motifs XL currently used for alkylating biomolecules,
that is, benzophenone (Bp) and 4-azido-2,3,5,6-tetrafluoro-
benzoic acid (N3; Scheme 1). Both of these groups can be
Oligonucleotides containing runs of three or four guanine
residues may arrange into four-stranded DNA supramolec-
ular structures called G-quadruplexes (G4s).[1] The discovery
of these noncanonical structures in functional genomic
regions suggests possible consequences for key processes,
such as telomere maintenance, replication, and transcrip-
tion.[2] Therefore, G4s have been the object of intense study
with the aim of defining their potential as regulatory elements
and/or therapeutic targets.[3] G4 DNA can be stabilized by
small synthetic molecules, but so far most of these compounds
bind in a reversible process through noncovalent interactions.
Thus, these reversible binders have to exhibit high specificity
for the G4 target to minimize off-target effects in a cellular
context.[4] More recently, platinum complexes able to estab-
[*] Dr. D. Verga, Dr. F. Hamon, Dr. M.-P. Teulade-Fichou
Institut Curie, Section Recherche, CNRS, UMR 176
Universitꢀ Paris-Sud
Scheme 1. Detailed chemical structure of the six PDC–XL derivatives
synthesized in this study.
Bat. 110–112, 91405 Orsay (France)
E-mail: mp.teulade-fichou@curie.fr
Homepage: http://umr176.curie.fr/en/MPTF
excited by UVA irradiation (see Figures S10 and S11 in the
Supporting Information) at wavelengths close to the visible
(330–365 nm) to generate highly reactive intermediates
evolving through diverse photochemical pathways. For exam-
ple, the excited triplet state of benzophenone undergoes
[2+2] cycloaddition to alkenes to produce oxetanes by
a Paternꢀ–Buchi reaction[10] and may also abstract a hydrogen
atom from accessible H donors to form a covalent bond
between the resulting germinal alkyl and ketyl radicals.[11] On
the other hand, photoexcited aryl azides form a nitrene, both
in the singlet and triplet state, and this nitrene can initiate
Dr. S. Bombard
UMR-S INSERM 1007, Universitꢀ Paris Descartes
45 rue des Saint-Pꢁres, 75006 Paris Cedex 06 (France)
E-mail: Sophie.Bombard@parisdescartes.fr
Dr. F. Poyer
Institut Curie, Section Recherche, U759 INSERM
Universitꢀ Paris-Sud
Bat. 110-112, 91405 Orsay (France)
[**] We thank Prof. Dr. Elmar Weinhold, IOCRWTH Aachen for a fruitful
discussion on photo-cross-linking.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307413.
À
addition reactions with double bonds, insertion into C H and
994
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 994 –998

Angewandte
Chemie
À
N H sites, or subsequent ring expansion and reaction with
We next investigated the ability of PDC–XL derivatives to
a nucleophile.[12] Therefore, both motifs have been exploited
for photoaffinity labeling of protein–protein,[13] protein–
ligand,[14] and DNA–ligand interactions.[15]
Six PDC–XL derivatives combining the two XL motifs
and three different linkers were prepared and evaluated for
their ability to form photoadducts with two well-known G4s,
that is, the human telomeric sequence (22AG) and the
oncogene promoter c-myc sequence (myc 22/G4T-G23T).
Remarkably, most PDC–XL derivatives retained the high
quadruplex-versus-duplex selectivity of the PDC core. As
a result, selective alkylation of G4 was observed under
competitive conditions. Furthermore, the alkylation occurred
at nucleobases localized either in the loops or in the G-
quartets, depending on the alkylating agent and on the
topology of the quadruplex.
form covalent adducts within the G4. Dose-dependent assays
showed that a 2.5:1 ratio gave the highest photochemical
yields, and these conditions were therefore applied through-
out the study. The derivatives were treated with 22AG, both in
NaCl and KCl buffers, and irradiated with a flash lamp (l =
360 nm).[14a] Analysis of the mixture by denaturing gel
electrophoresis showed the formation of retarded bands
attributed to alkylated 22AG, as the photoadducts provide
additional mass and positive charges (Figure 1). The alkyla-
tion of the six compounds appeared significantly higher in
K+ buffer than in Na+ buffer and was found to be strongly
dependent on the nature of the spacer (Figure 1). Indeed, the
highest yields were observed for the C4 alkyl and PEG
derivatives (26–36% in K+, 13–29% in Na+); however, the
latter required stronger irradiation conditions (90 versus 150
flashes; see Table S1 in the Supporting Information). In stark
contrast, alkylation was low or substantially absent for the C8
derivatives (0–12%; Figure 1), irrespective of the irradiation
conditions. Finally, the number of alkylated bands varied from
one for C4 derivatives to three for PEG chains (Figure 1).
Altogether, these data indicate that C4 and PEG are the most
appropriate linkers for alkylation of the quadruplex. The low
efficiency of the C8 alkyl chain might be related to its strong
lipophilic character: It is less prone to stay close to the highly
hydrated DNA structure.
The photoalkylating motifs were introduced on the
central pyridine ring, since the derivatization of this posi-
tion[16] does not modify significantly the quadruplex-binding
behavior.[17] To evaluate their influence, we chose three
different connecting chains: two diaminoalkyl chains with
four and eight carbon atoms (C4 and C8, respectively) and
a diaminopoly(ethylene glycol) chain (PEG; Scheme 1; see
also Schemes S1–S3 in the Supporting Information). The
affinity of the new derivatives for G4 and duplex DNA was
first investigated by the G-quadruplex fluorescent intercala-
tor displacement (G4-FID)
assay, which is based on the
competitive displacement
of thiazole orange (TO).[18]
All PDC–XL derivatives
were found to displace TO
with an efficacy compara-
ble to that of PDC when c-
myc was used as the G4
target (see Figure S1). In
contrast, the displacement
activity was more affected
in the case of 22AG: It
varied from a moderate to
a more significant decrease
in displacement (20–40%),
depending on the spacer
(see Figure S1). The influ-
ence of the XL moiety
appeared
except in the case of the
PDC-C4-Bp derivative,
negligible,
which showed lower affin-
ity than its N3 analogues.
Nonetheless,
all
com-
pounds showed quite low
activity towards the duplex
control (ds lac), thus indi-
cating poor binding (see
Figure S1). This result con-
firms the structural prefer-
ence for quadruplex con-
formations.
Figure 1. Denaturing gel electrophoresis (15% acrylamide) of the alkylation products of 22AG (10 mm) in the
presence of Na+ and K+ buffer (100 mm) with A) PDC-C4-Bp, PDC-C8-Bp, and PDC-PEG-Bp, and B) PDC-C4-
N3, PDC-C8-N3, and PDC-PEG-N3 (25 mm). Lanes 1, 2, 3, 4, 6, 8, 10, 12, and 14 are control experiments that
show the effect of no irradiation (lane 1, 4, 6, 8, 10, 12, and 14) and irradiation for 90 flashes (lane 2) and
150 flashes (lane 3). Lanes 5 and 7 show the results of irradiation at 360 nm for 90 flashes; lanes 9, 11, 13,
and 15 show the results of irradiation at 360 nm for 150 flashes. Bands have been numbered with respect to
their migration as compared to a control (band 1.1: no migration; bands 1.2, 1.3, 1.4: retarded bands,
numbered in the order of the extent of migration. ln: À absence of irradiation, + 90 flashes, + + 150 flashes.
Angew. Chem. Int. Ed. 2014, 53, 994 –998
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
995

.
Angewandte
Communications
To assess whether the formation of adducts with 22AG
was indeed selective for the G4 conformation, we reproduced
the experiment with two control oligonucleotides, that is, the
17 bp duplex (ds lac) used in the FID assay and the single-
stranded scrambled sequence 22Agmut. No adduct was
observed in the presence of ds lac (see Figure S3), whereas
low reactivity (8%) was observed with 22Agmut. This result
is not surprising, as single-stranded DNA is known to trap
cationic compounds through nonspecific electrostatic inter-
actions. Subsequently, the photolysis experiments were repro-
duced in the presence of a large excess (8 molequiv) of
nonlabeled 22Agmut or ds lac. Under these conditions, G4
alkylation was retained with only a slight to moderate
decrease in yield for the short-chain compounds (0–20%;
Figure 2), whereas the two PEG derivatives were more
size was determined by comparison with digested non-
alkylated 22AG. The following alkylation sites were found
for PDC-PEG-Bp: T6, T11, T12, T17, and T18, in both K+
and Na+ buffer (Figure 3A,B; see also Figure S4C). Interest-
Figure 2. Quantitative analysis of the alkylation of the telomeric
sequence 22AG (10 mm, 100 mm K+-rich buffer) by the PDC–XL
derivatives (25 mm) in the absence (black bars) or in the presence of
a competitor (80 mm): ds lac (dark-gray bars), 22Agmut (light-gray
bars).
Figure 3. Denaturing gel electrophoresis of alkylated products of 22AG
in K+-rich buffer. A) 3’-Exonuclease digestion of bands 1.1–1.4 obtained
by alkylation with PDC-PEG-Bp (see Figure 1A): enzyme units/ml:
lane a, 0; lane b, 0.04; lane c, 0.08. Band 1.1 corresponds to non-
alkylated 22AG. B) Treatment of the digested fragments with piper-
idine. The numbered pyrimidine sites correspond to the digestion
arrests.
affected by the presence of the competitors (ca. 30% decrease
to 45% with 22Agmut). This result indicates that a long linker
is deleterious to selectivity by favoring multiple positioning of
the XL moiety, whereas a short linker that restricts dynamic
motion[19] promotes alkylation with better G4 selectivity,
thereby demonstrating the significance of proximity effects.
Nonetheless, we cannot exclude the occurrence of static
quenching of the excited triplet state of the cross-linker by
electron-transfer and/or energy-transfer processes involving
nucleic acid competitors.[20]
With the aim of identifying the alkylation sites, we isolated
the adducts from the gel and subjected them to two different
sequencing procedures. For the PDC–N3 adducts, classical
alkaline treatment, which induces cleavage on the 5’ side of
alkylated bases, could be applied. Conversely, the PDC–Bp
adducts reverted to the starting material at high temper-
ature,[19b] thus making this sequencing method useless. In this
case, the oligonucleotides were digested by 3’-exonuclease,
which stops at alkylated sites.[21] The digested fragments were
then isolated from the gel and treated with piperidine to
remove the adducts, which affect their migration, and their
ingly, only two alkylation sites (T6 and T11) were identified
for PDC-C4-Bp (see Figure S5C). In conclusion, the benzo-
phenone conjugates react exclusively with the loop thymine
residues of 22AG: PDC-C4-Bp alkylates the first and second
loop (starting from the 5’ end), whereas PDC-PEG-Bp
alkylates the three loops (Figure 5A; see also Figure S2).
In contrast, the direct treatment of PDC-PEG-N3 adducts
with piperidine revealed exclusive alkylation at guanine
residues G10 and G14, regardless of the buffer cation
(Figure 4; see also Figure S6). The same sites were observed
for PDC-C4-N3, with one additional alkylation at G16 in K+-
rich buffer (see Figure S7B). This result demonstrates that
G10 and G14 are the only accessible and reactive residues for
the electrophilic species produced by the photolysis of azide
conjugates. As shown in Figure 5A, G10 and G14 belong to
996
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 994 –998

Angewandte
Chemie
Figure 4. Denaturing gel electrophoresis of alkylated products of 22AG
in K+-rich buffer. Treatment with piperidine of bands 1.1 and 1.2
obtained by alkylation with PDC-PEG-N3 (see Figure 1B): lane a: non-
treated sample; lane b: sample (band 1.1, which corresponds to non-
alkylated 22AG) treated with DMS (dimethyl sulfate) and piperidine to
give a reference scale for fragment migration; lane c: piperidine-treated
sample.
Figure 5. A) Schematic structure of the human telomeric G4 in Na+-
and in K+-rich buffer, and sites alkylated by PDC–Bp and PDC–N3
derivatives. B) Schematic structure of the c-myc 22G4T-G23T G4 and
sites alkylated by PDC–N3 derivatives. Guanine residues are repre-
sented by blue squares (G-quartets, dashed squares are syn and plain
squares are anti) or blue spheres (G-loops); yellow and green spheres
stand for T and A in the loops, respectively. Alkylated guanine residues
are labeled in black; alkylated nucleobases in the loops are shown with
red arrows.
the 5’ external quartet in almost all quadruplex conformations
identified in solution (by NMR spectroscopy) for the
telomeric sequence (see Figure S2).[22] Altogether, the results
suggest that the quartet containing G10 and G14 may open
transiently to expose the nucleophilic sites of the guanine
residues (the ring N7 atoms), as previously observed with
platinum complexes.[5b,23] Altogether, the alkylation profile
for the Bp derivatives is consistent with a p-stacking of the
ligands on the two external quartets to enable alkylation of
the surrounding loops. The increased number of alkylation
sites observed with the PEG derivative is in line with the
longer length and higher flexibility of the spacer. In contrast,
the two azide derivatives alkylate selectively the guanine
residues located on the 5’ side with a high preference for G10
and G14, irrespective of the linker size. This result is again
consistent with predominant p-stacking on the 5’ side but also
suggests that the alkylation pattern is essentially determined
by the nature of the reactive intermediate.
The four compounds were then evaluated with the c-myc
sequence. Unexpectedly, the two Bp derivatives showed
sluggish reactivity, which prevented analysis of the alkylation
(see Figure S8). The near absence of reactivity of the Bp
derivatives could reflect unfavorable conditions for photo-
adduct formation, possibly as a result of the high compactness
of the c-myc G4 structure, which displays short loops that are
arranged laterally and thus more difficult to access. The two
azide derivatives induced the formation of a mixture of both
retarded and accelerated bands (see Figure S8),[5b] which
displayed alkylation of either the external G-quartet or the
loops (Figure 5B; see also Figure S9). However, the absence
of reactivity of the Bp derivatives and the multiple alkylation
pattern of the N3 derivatives could also result from the ligand-
induced formation of dimeric G4 forms (see Figure S8), the
presence of which may strongly impact the photoreactivity.[24]
Finally, the compounds were evaluated for their cytotox-
icity with regard to two cancer cell lines (breast tumor MCF7
and non-small-cell lung carcinoma A549). All compounds
showed no or low toxicity in the dark, whereas toxicity was
triggered by UV/Vis irradiation (320–420 nm), with IC50
values ranging from 0.13 to 14 mm (see Table S1). Most
importantly, the IC50 values are clearly dependent on the size
of the linker (C4 alkyl > C8 alkyl > PEG) and on the nature
of the alkylating agent Bp @ N3, irrespective of the cell-line
sensitivity. As a result, PDC-C4-Bp appears as the most active
compound. Further studies devoted to analysis of the cellular
targets (telomeres, transcription of proteins as controlled by
G4-prone promoters) as well as cell-cycle analysis could
indicate whether the activity of the compounds is mediated by
G4 formation.
In conclusion, the use of two structurally different photo-
activatable moieties linked to an efficient quadruplex-binding
motif allowed us to selectively alkylate G4 structures in vitro,
Angew. Chem. Int. Ed. 2014, 53, 994 –998
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
997

.
Angewandte
Communications
Richter, C. Bertipaglia, M. Mella, C. Sissi, M. Palumbo, M.
Freccero, J. Am. Chem. Soc. 2009, 131, 13132 – 13141.
[8] a) D. Verga, M. Nadai, F. Doria, C. Percivalle, M. Di Antonio,
M. Palumbo, S. N. Richter, M. Freccero, J. Am. Chem. Soc. 2010,
132, 14625 – 14637; b) F. Doria, S. N. Richter, M. Nadai, S.
Colloredo-Mels, M. Mella, M. Palumbo, M. Freccero, J. Med.
Chem. 2007, 50, 6570 – 6579.
[9] G. Pennarun, C. Granotier, L. R. Gauthier, D. Gomez, F.
Hoffschir, E. Mandine, J. F. Riou, J. L. Mergny, P. Mailliet,
F. D. Boussin, Oncogene 2005, 24, 2917 – 2928.
[10] T. Bach, Synthesis 1998, 683 – 703.
[11] P. J. Wagner, Acc. Chem. Res. 1971, 4, 168 – 177.
[12] a) W. Sander, D. Grote, S. Kossmann, F. Neese, J. Am. Chem.
Soc. 2008, 130, 4396 – 4403; b) R. Poe, K. Schnapp, M. J. T.
Young, J. Grayzar, M. S. Platz, J. Am. Chem. Soc. 1992, 114,
5054 – 5067.
[13] a) G. D. Prestwich, G. Dormꢃn, J. T. Elliott, D. M. Marecak, A.
Chaudhary, Photochem. Photobiol. 1997, 65, 222 – 234; b) G.
Dorman, G. D. Prestwich, Biochemistry 1994, 33, 5661 – 5673.
[14] a) C. Dalhoff, M. Hꢂben, T. Lenz, P. Poot, E. Nordhoff, H.
Kçster, E. Weinhold, ChemBioChem 2010, 11, 256 – 265;
b) L. R. Odell, N. Chau, A. Mariana, M. E. Graham, P. J.
Robinson, A. McCluskey, ChemMedChem 2009, 4, 1182 – 1188.
[15] a) M. W. Berns, Z. Wang, A. Dunn, V. Wallace, V. Venugopalan,
Proc. Natl. Acad. Sci. USA 2000, 97, 9504 – 9507; b) P. L. Gilbert,
D. E. Graves, J. B. Chaires, Biochemistry 1991, 30, 10925 – 10931.
[16] P. Yang, A. De Cian, M.-P. Teulade-Fichou, J.-L. Mergny, D.
Monchaud, Angew. Chem. 2009, 121, 2222 – 2225; Angew. Chem.
Int. Ed. 2009, 48, 2188 – 2191.
which led to their irreversible trapping. Overall, the observed
alkylation patterns reflect a subtle balance between the
nature of the alkylating agent, the influence of the quadruplex
topology, and the ligand spacer length. The new compounds,
which display G4-binding selectivity combined with G4-
topology-dependent photochemical behavior, are unique
probes suitable for further investigations of quadruplex
effects in various functional biological systems. Finally, the
cellular results are promising, since only irradiated com-
pounds were active, and in particular the benzophenone
derivatives, which also reacted exclusively with the telomeric
quadruplex in vitro. Whether the cellular target is a G4
structure remains to be evaluated. Nonetheless, our study
already constitutes a step towards the discovery of specific G4
ligands that can be used to target preferentially certain G4-
forming domains or subclasses of quadruplexes.
Received: August 22, 2013
Revised: November 14, 2013
Published online: December 11, 2013
Keywords: alkylation · DNA · G-quadruplexes ·
.
photoactivatable probes · photo-cross-linking
[1] a) D. J. Patel, A. T. Phan, V. Kuryavyi, Nucleic Acids Res. 2007,
35, 7429 – 7455; b) J. B. Chaires, FEBS J. 2010, 277, 1098 – 1106.
[2] a) H. J. Lipps, D. Rhodes, Trends Cell Biol. 2009, 19, 414 – 422;
b) J. L. Huppert, S. Balasubramanian, Nucleic Acids Res. 2007,
35, 406 – 413.
[17] a) E. Largy, F. Hamon, M.-P. Teulade-Fichou, Methods 2012, 57,
129 – 137; b) A. Renaud de La Faverie, F. Hamon, C. Di Primo,
E. Largy, E. Dausse, L. Delauriꢄre, C. Landras-Guetta, J.-J.
Toulmꢁ, M.-P. Teulade-Fichou, J.-L. Mergny, Biochimie 2011, 93,
1357 – 1367.
[3] a) D. Monchaud, M.-P. Teulade-Fichou, Org. Biomol. Chem.
2008, 6, 627 – 636; b) S. Balasubramanian, L. H. Hurley, S.
Neidle, Nat. Rev. Drug Discovery 2011, 10, 261 – 275.
[18] E. Largy, F. Hamon, M.-P. Teulade-Fichou, Anal. Bioanal. Chem.
2011, 400, 3419 – 3427.
[19] a) M. C. Cuquerella, V. Lhiaubet-Vallet, J. Cadet, M. A. Mir-
anda, Acc. Chem. Res. 2012, 45, 1558 – 1570; b) K. Nakatani, T.
Yoshida, I. Saito, J. Am. Chem. Soc. 2002, 124, 2118 – 2119.
[20] C. J. Burrows, J. G. Muller, Chem. Rev. 1998, 98, 1109 – 1152.
[21] S. Redon, S. Bombard, M.-A. Elizondo-Riojas, J.-C. Chottard,
Biochemistry 2001, 40, 8463 – 8470.
[22] a) Y. Wang, D. J. Patel, Structure 1993, 1, 263 – 282; b) K. N. Luu,
A. T. Phan, V. Kuryavyi, L. Lacroix, D. J. Patel, J. Am. Chem.
Soc. 2006, 128, 9963 – 9970; c) A. T. Phan, K. N. Luu, D. J. Patel,
Nucleic Acids Res. 2006, 34, 5715 – 5719.
[4] S. Neidle, FEBS J. 2010, 277, 1118 – 1125.
[5] a) E. Largy, F. Hamon, F. Rosu, V. Gabelica, E. De Pauw, A.
Guꢁdin, J.-L. Mergny, M.-P. Teulade-Fichou, Chem. Eur. J. 2011,
17, 13274 – 13283; b) H. Bertrand, S. Bombard, D. Monchaud, E.
Talbot, A. Guꢁdin, J.-L. Mergny, R. Grꢂnert, P. J. Bednarski, M.-
P. Teulade-Fichou, Org. Biomol. Chem. 2009, 7, 2864 – 2871.
[6] F. Doria, M. Nadai, M. Folini, M. Scalabrin, L. Germani, G.
Sattin, M. Mella, M. Palumbo, N. Zaffaroni, D. Fabris, M.
Freccero, S. N. Richter, Chem. Eur. J. 2013, 19, 78 – 81.
[7] a) M. Nadai, F. Doria, M. Di Antonio, G. Sattin, L. Germani, C.
Percivalle, M. Palumbo, S. N. Richter, M. Freccero, Biochimie
2011, 93, 1328 – 1340; b) F. Doria, M. Nadai, M. Folini, M.
Di Antonio, L. Germani, C. Percivalle, C. Sissi, N. Zaffaroni, S.
Alcaro, A. Artese, S. N. Richter, M. Freccero, Org. Biomol.
Chem. 2012, 10, 2798 – 2806; c) M. Di Antonio, F. Doria, S. N.
[23] I. Ourliac-Garnier, M.-A. Elizondo-Riojas, S. Redon, N. P.
Farrell, S. Bombard, Biochemistry 2005, 44, 10620 – 10634.
[24] H. T. Le, M. C. Miller, R. Buscaglia, W. L. Dean, P. A. Holt, J. B.
Chaires, J. O. Trent, Org. Biomol. Chem. 2012, 10, 9393 – 9404.
998
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 994 –998