Carbazole modified oligonucleotides: Synthesis, hybridization studies and fluorescence properties

Article abstract of DOI:10.1039/d0ob01553a

Synthesis of the novel thiophenyl carbazole phosphoramidite DNA building block 5 was accomplished in four steps using a Suzuki-Miyaura cross-coupling reaction from the core carbazole and it was seamlessly accommodated into a 9-mer DNA-based oligonucleotide by incorporation at the flanking 5′-end in combination with a central insertion of an LNA-T nucleotide. The carbazole-containing oligonucleotide was combined in different duplex hybrids, which were characterized by thermal denaturation, circular dichroism and fluorescence studies. The carbazole monomer modulates the duplex stability in various ways. Thus, monomer Z increased the thermal stability of the 9-mer towards the complementary 9-mer/15-mer DNA duplex by 4.2 °C. Furthermore, indications of its intercalation into the duplex were obtained by modeling studies and robust decreases in fluorescence emission intensities upon duplex formation. In contrast, no clear intercalating tendency was corroborated for monomer Z within the DNA/RNA hybrid duplex as indicated by moderate quenching of the fluorescence and similar duplex thermal stabilities relative to the corresponding control duplex. The recognition efficiencies of the carbazole modified oligonucleotide toward single nucleotide mismatches were studied with two 15-mer model targets (DNA and RNA). For both systems, mismatches positioned at the juxtaposition of the carbazole monomer showed pronounced deceases in thermal denaturation temperature. Steady-state fluorescence emission studies of all mismatched duplexes with incorporation of Z monomer typically displayed efficient fluorescence quenching.

Full text of DOI:10.1039/d0ob01553a

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21 December 2017
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DOI: 10.1039/D0OB01553A
Carbazole modified oligonucleotides: Synthesis, hybridization studies and fluorescence
properties†
Alaa S. Gouda,a,c,‡ Łukasz Przypis,b,‡ Krzysztof Walczak,b Per T. Jørgensena and Jesper Wengel*,a
aBiomolecular Nanoscale Engineering Center, Department of Physics, Chemistry and
Pharmacy, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark. Tel +45
65502510; *E-mail: jwe@sdu.dk
bDepartment of Organic Chemistry, Biochemistry and Biotechnology, Silesian University of
Technology, Poland.
cDepartment of Chemistry, Faculty of Science, Benha University, Benha13518, Egypt.
‡These authors contributed equally to this work
Abstract. Synthesis of the novel thiophenyl carbazole phosphoramidite DNA building block 5
was accomplished in four steps using a Suzuki-Miyaura cross-coupling reaction from the core
carbazole and was seamlessly accommodated into a 9-mer DNA-based oligonucleotide by
incorporation at the flanking 5´-end in combination with a central insertion of an LNA-T
nucleotide. The carbazole-containing oligonucleotide was combined in different duplex hybrids,
which were characterized by thermal denaturation, circular dichroism and fluorescence studies.
The carbazole monomer modulates duplex stability in various ways. Thus, monomer Z increased
the thermal stability of the 9-mer towards complementary 9-mer/15-mer DNA duplex by 4.2 °C.
Further, indications of its intercalation into the duplex were obtained by modeling studies and
robust decreases in fluorescence emission intensities upon duplex formation. On the contrary, no
clear intercalating tendency was corroborated for monomer Z within the DNA/RNA hybrid duplex
as indicated by moderate quenching of the fluorescence and similar duplex thermal stabilities
relative to the corresponding control duplex. The recognition efficiencies of carbazole modified
oligonucleotide toward single nucleotide mismatches were studied with two 15-mer model targets
(DNA and RNA). For both systems, mismatches positioned at juxtaposition of the carbazole
monomer showed pronounced deceases in thermal denaturation temperature. Steady-state
fluorescence emission studies of all mismatched duplexes with incorporation of Z monomer
typically displayed efficient fluorescence quenching.
1

Organic & Biomolecular Chemistry
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DOI: 10.1039/D0OB01553A
Introduction
Over the last two decades, research in the field of synthetic nucleic acids has sparked a plethora of
biologically relevant applications.1-5 Previous studies reported that oligonucleotide (ON)
analogues inducing high duplex stability and sequence specificity towards target DNA or RNA
single strands are interesting as therapeutics or diagnostics.6 To date, a diverse assortment of
modified ONs have been prepared by solid-phase DNA synthesis using chemically modified
nucleosides and non-nucleosidic building blocks with the aims of optimizing e.g. binding affinity,
mismatch discrimination, spectroscopic properties and in vivo stability.7-11
One nucleotide modification also used in this study is locked nucleic acids (LNA) with a
conformationally locked C3´-endo (North-type) furanose ring thus mimicking RNA
nucleotides.12,13 Incorporation of one or more LNA monomers into ONs has been shown to induce
excellent hybridization properties, especially towards complementary single-stranded RNA.14
A variety of fluorescent probes have been inserted to ONs with the purpose of specific and
sensitive detection of hybridization towards complementary DNA/RNA. This can be achieved by
observing changes of fluorescence, through enhanced or quenched emission upon duplex
formation.15-20
Fluorescent probes comprising polyaromatic compounds have been intensively studied to label
DNA for biochemical and biological applications. Typical labels of this class include pyrene,21
perylene,22 and phenanthroline23 known to possibly forming noncovalent π-stacking interactions
and/or intercalation with nucleic acids24-27 or with other polyaromatic compounds.28-34
Carbazole and its derivatives which in general have interesting spectral and electronic
properties were found to integrate well in DNA without compromising hybrid stability. Thus, it
has been reported that carbazole monomers are known to modulate G-quadruplexes and
duplexes.35-38 In addition, carbazole nucleotide monomers have been reported to induce elevated
duplex stability as a result of increased stacking interactions involving the extended aromatic
structure.38 Furthermore, 3-cyanovinylcarbazole-modified ONs with threoninol linker showed
promising DNA photo-cross-linking reactivity,39 and specific and reversible photochemical
labeling of single- and double-stranded plasmid DNA.40
Taking into account the general sensitivity of carbazole-functionalized probes toward
hybridization,36-38 and the high-affinity hybridization of LNA-T,14,41-45 we present here a new ON
design that combine the interesting fluorescence properties of the thiophenyl-substituted carbazole
(absorbs around 310 nm and emits around 400 nm) together with the known stabilization properties
of the LNA-T monomer. We choose to incorporate one LNA-T monomer into the middle of the 9-
mer DNA ON due to the known positive effect of LNA on duplex thermostability, favorable
2

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Organic & Biomolecular Chemistry
mismatch discrimination and improved resistance toward nucleolytic digestiDoOnI:.4105.10W39/eD0OaBls0o1553A
investigate the impact on the intercalating properties when placing a bulky 3-thiophenyl-
substituted carbazole probe at the 5’-end of a 9-mer DNA-type ON. Furthermore, we describe the
synthesis of the thiophenyl carbazole phosphoramidite derivative 5, its 5’-end incorporation into
a 9-mer DNA-type ON, and its effects on binding affinity, mismatch discrimination and
fluorescence emission properties (Fig. 1).
View Article Online
O
O
NH
N
O
O
O
Carbazole dsDNA/dsRNA
O
P
Compl. ssDNA/ssRNA
O
O
TL
9-mer
Z
TL
Z
Carbazole ssDNA (9-mer)
15-mer
S
Z
N
O
P
O
TL
O
Fig. 1. Structures of the carbazole and LNA-T monomers employed, and the duplexes formed between
the modified 9-mer ON and complementary DNA/RNA target strands.
Results and discussion
Synthesis of carbazole phosphoramidite building block
A synthetic route toward the thiophenyl carbazole phosphoramidite monomer 5 from the
commercially available carbazole 1 was developed in four high yielding steps as shown in
Scheme 1. The direct iodination reaction was performed on the carbazole core using mild
conditions previously described by Przypis and Walczak,46 i.e. using Barluenga’s reagent
(IPy2BF4) as an iodinating agent and copper sulphate in anhydrous acetonitrile under an
atmosphere of nitrogen to afford 3-iodo-9H-carbazole (2a) as the major product in 79% yield.
Purification of the 3-iodocarbazole 2a was challenging when using chromatography. This is
ascribed to the formation of a by-product tentatively assigned as the corresponding 3,6-diiodo-9H-
carbazole according to TLC. However, it turned out to be possible to purify the product by
recrystallization. N-Alkylation of 3-iodocarbazole 2a was accomplished by treatment with
3

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2-bromoethanol in the presence of potassium hydroxide in anhydrous dimethylformamide to
DOI: 10.1039/D0OB01553A
furnish the corresponding N-hydroxyethylated 3-iodocarbazole 3 in 77% yield. An alternative
synthetic methodology has been applied for synthesis 3 from the carbazole precursor 1 by
performing the above-mentioned two steps in a reversed order.47 A thiophenyl moiety was
introduced into the 3-position of the iodocarbazole alcohol 3 by means of a Suzuki-Miyaura cross-
coupling reaction with 2-thienylboronic acid in the presence of Na2CO3 as a base in anhydrous
1,4-dioxane under nitrogen atmosphere via palladium catalysis to attain the corresponding N-
hydroxyethylated 3-thiophenyl carbazole 4 in 64% yield. The obtained thiophenyl carbazole
primary alcohol 4 was subsequently phosphitylated by treatment with 2-cyanoethyl N,N-
diisopropylchlorophosphoramidite in the presence of anhydrous ethyldiisopropylamine under an
atmosphere of argon to afford the corresponding carbazole phosphoramidite 5 in 78% yield.
Due to its high sensitivity to humidity and light, the obtained thiophenyl carbazole
phosphoramidite 5 was immediately used on an automated DNA synthesizer in order to synthesize
the oligonucleotides ON3 with the inserted carbazole monomer Z using the standard
phosphoramidite approach (Table 1). The correct molecular weights of the synthesized ONs were
verified by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass
spectrometry, and the purity was verified as >90% according to ion exchange HPLC analysis.
I
ia
iia
I
N
H
N
H
2a
N
1
iib
OH
ib
N
3
OH
2b
iii
S
S
S
v
iv
N
N
N
O
O
O
OH
P
N
CN
P
O
O
Monomer Z
5
4
4

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DOI: 10.1039/D0OB01553A
Scheme 1: Synthesis of carbazole intermediates and phosphoramidite monomer 5. Reagents and
conditions: (ia) IPy2BF4, CuSO4, 65 °C, anhydrous CH3CN, 10 min, 79%; (ib) KOH, anhydrous DMF, 30
min, RT, Br(CH2)2OH, 80 °C, 20 h, 83%; (iia) KOH, anhydrous DMF, 30 min, RT, Br(CH2)2OH, 80 °C,
20 h, 77%; (iib) IPy2BF4, CuSO4, 65 °C, anhydrous CH3CN, 10 min, 85%; (iii) 2-thienylboronic acid,
anhydrous 1,4-dioxane, Na2CO3, Pd(dppf)Cl2∙DCM, 64%; (iv) NC(CH2)2OP(NPri2)Cl, dry EtN(Pri)2, dry
DCM, RT, Ar, 2 h, 78%; (v) ON synthesis.
Hybridization studies of duplexes: Binding affinity and mismatch sensitivity
The influence of the non-nucleosidic thiophenyl carbazole monomer Z on the stability of various
DNA/DNA and DNA/RNA duplexes was investigated by thermal denaturation experiments in
medium salt buffer (2.5 μM concentration of each strand) at pH 7.0 using the UV melting method
at 260 nm with thermal denaturation temperatures (Tm, °C) determined as the maximum of the first
derivative plots. In this frame, the fully characterized nonamer 5´-GTGATATGC-3´ DNA
sequence ON1 was used in this study, in which the carbazole monomer Z was terminally
incorporated into the 5´-end together with substitution of the central DNA-T nucleotide by an
LNA-T monomer to give the sequence ON3 (Table 1).
The binding affinity and specificity of the carbazole-modified oligonucleotide ON3 was
assessed towards both complementary 9-mer DNA and RNA strands (D0 and R0, respectively) as
well as towards 15-mer DNA and RNA strands (D1 and R1, respectively) each having a three-
nucleotide overhang at each end relative to the 9-mer central region designed to partake in duplex
formation. These longer targets were included as models for longer more biologically relevant
targets. More specifically we wished to evaluate the capability of the carbazole intercalator Z to
undergo π-π stacking with surrounded nucleobases at the overhang regions in addition to explore
the intercalating effectiveness of monomer Z as a staking lid stabilizing the 9-mer DNA/DNA and
DNA/RNA duplexes. The Tm values obtained for the unmodified all-DNA 9-mer strand ON1 and
the central LNA-T modified 9-mer strand ON2, both hybridized with complementary 9-mer/15-
mer single-stranded DNAs or RNAs, are considered as the control measurements referred for
comparison. The Tm values of the carbazole-modified duplexes were recorded and compared with
the Tm of the unmodified duplexes to determine the differences in melting temperatures (∆Tm
values, Table 1). The resulting denaturation duplex curves all displayed the expected monophasic
sigmoidal transitions (Fig. S17, ESI).
For possible perfectly matched 9-mer unmodified duplexes, all DNA reference ON1 revealed
a thermal denaturation temperature of 32.2 °C with the complementary DNA strand D0 and of
30.2 °C with the RNA counterpart R0, whereas the thermal meltings for the control LNA-T ON2
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was 38.0 °C and 39.4 °C with the corresponding DNA and RNA complements, respectively. These
DOI: 10.1039/D0OB01553A
values correspond to other measurements made earlier.48 As can be seen from Table 1, the
carbazole-modified DNA/DNA duplex ON3/D0 forms a significantly more thermally stable
duplex with ∆Tm 8.0 °C than for the wild type DNA-T ON1/D0. In fact, this duplex is even more
thermostable by + 2.2 °C relative to the corresponding LNA-T control ON2/D0 duplex indicating
that the thiophenyl carbazole moiety is well accommodated at the 5’-end of the duplex. This
stabilizing influence of the carbazole monomer Z is most likely attributable to favorable
positioning of the aromatic polycyclic system for inter-strand stacking including the underlying
nucleobases (known as the lid-effect). It should be noted that our aromatic carbazole lid has a
duplex stabilizing effect relative to pyrene twisted intercalating nucleic acid (TINA)49 and for the
phenanthroimidazole intercalator (Amany),50 but similar to intercalating nucleic acid (INA)51 and
to 2´-deoxyribose phenanthrene nucleotide at the 5´-ends.52
The thermal stability of the carbazole-modified DNA/RNA duplex ON3/R0 was approximately
9.0 °C higher than the Tm value of the wild type duplex ON1/R0. However, the Tm decreased
weakly by 0.4 °C compared to the corresponding duplex containing LNA-T only ON2/R0.
Table 1 Tma (°C), ∆Tmb (°C) data for thermal denaturation temperatures of unmodified and modified DNA/DNA and
DNA/RNA duplexes at 260 nm.
5´-GTG ATA TGC-3´
5´-GTG ATLA TGC-3´
5´-ZGTG ATLA TGC-3´
Complementary sequence
Code
(ON1)
(ON2)
(ON3)
D0
R0
D1
R1
3´-CAC TAT ACG-5´
3´-CAC UAU ACG-5´
3´-TTC CAC TAT ACG CTC-5´
32.2
30.2
36.2
33.6
38.0 (+5.8)
39.4 (+9.2)
42.0 (+5.8)
42.2 (+8.6)
40.2 (+8.0)
39.0 (+8.8)
46.2 (+10.0)
42.0 (+8.4)
3´-UUC CAC UAU ACG CUC-5´
a
Conditions: 2.5 µM of each strand in a medium salt buffer 5.8 mM NaH2PO4/ Na2HPO4 buffer (pH 7.0), containing 100 mM
NaCl and 0.10 mM EDTA. b The Tm values reflect the average of two measurements. Numbers in parentheses are differences in
Tm (ΔTm) = Tm (modified) - Tm (unmodified). Z = carbazole monomer, TL = LNA-T monomer.
Hereafter, we investigated the hybridization of the carbazole-modified 9-mer sequence ON3 to
two target 15-mer ONs, i.e. DNA (D1) and RNA (R1) strands encompassing three unpaired
nucleotides as overhangs at both ends of the parental 9-mer DNA and RNA complements. As
evident for the DNA/DNA duplexes (Table 1), again monomer Z and the LNA-T monomers
induced similar increases as described above for the all 9-mer duplexes. However, relative to the
LNA-T only modified duplex ON2/D1, the increased thermal stability even increased by 4.2 °C
(Table 1), suggesting favorable stacking interaction from the large aromatic surface of the
carbazole intercalator with nucleobases anchored as overhangs, and not only acting as lids, into
the 15-mer DNA complement. Again, similar trends were detected with RNA strands as targets
where the lack of any stabilizing effect originating from the carbazole moiety is hypothesized to
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be due to inefficient intercalation of monomer Z into the DNA/RNA duplex ON3/R1, as has been
DOI: 10.1039/D0OB01553A
earlier observed with other intercalating units.51,53
Based on the above data, we conclude that the carbazole-based monomer Z is well fitted as 5’-
end modification to engage in stabilizing interactions within the B-type helix geometry of a
DNA/DNA duplex, whereas neither stabilizing nor destabilizing interactions are observed for a
corresponding DNA/RNA duplex likely adopting a A/B-type helical structure.
In order to assert the formation of all 9-mer/15-mer carbazole hybrids and unmodified duplexes,
circular dichroism (CD) spectra were recorded. For all carbazole-modified DNA/DNA duplexes
(ON3/DNA) as well as LNA-T modified counterparts (ON2/DNA), CD spectra displayed intense
positive and negative amplitudes at ~280 nm and ~250 nm, respectively, with no major variations
relative to those of the unmodified duplexes ON1/DNA. Such data are consistent with the
characteristic features of a DNA B-conformation (Fig. S18, ESI). In parallel, CD spectra of
carbazole-modified DNA/RNA variants (ON3/RNA) revealed the characteristic profile of an
intermediate A/B-type helical environment with ellipticities from both geometries. Thus, positive
bands of strong intensity at ~265 nm (A-type) with a shoulder at ~280 nm (B-type) were recorded
along with negative bands of rather low intensities at ~240 nm and ~210 nm (A-type). The
intensity of the bands is not significantly changed relative to those of the unmodified DNA/RNA
duplexes ON1/RNA suggesting that carbazole monomer do not induce any changes in the overall
A/B-type duplex structure (Fig. S18, ESI).
As a next step, we investigated the binding specificity of the carbazole-modified
oligonucleotide ON3 using 15-mer D1 and R1 as targets with overhangs and two types of
mismatched nucleotides. The capability for mismatch discrimination was first examined by
comparing melting temperatures of DNA/DNA duplexes ON1-ON3/D2-D7 and DNA/RNA
duplexes ON1-ON3/R2-R7, focusing on a mismatch (either in juxtaposition or in a directly
opposite position to the carbazole-functionalized monomer Z) with melting temperatures of the
corresponding fully matched duplexes ON1-ON3/D1 and ON1-ON3/R1 (∆Tm values).
The first type of mismatches was specifically introduced at position 13 of D1 and R1
complementary strands directly opposite to the monomer Z in ON3, i.e. in the first overhang
position relative to the nine-mer fully matched duplex segment. As outlined in Table 2, when the
cytosine nucleobase was replaced by the purines adenine or guanine, a small increase in Tm ranging
from 0.8 °C to 2.4 °C (for ON1-ON3/D2,D3 duplexes) and from 2.2 °C to 4.4 °C (for ON1-
ON3/R2, R3 duplexes) was detected relative to their perfectly matched duplexes. However, these
increases were less pronounced than obtained with the non-carbazole containing duplexes used as
controls (ON1 and ON2). Negligible mismatch discriminations were observed when the cytosine
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nucleobase was replaced by the pyrimidine bases thymine (ON1-ON3/D4) or uracil (ON1-
DOI: 10.1039/D0OB01553A
ON3/R4). These data show that monomer Z positioned as in these systems is unable to induce
stabilizing stacking effects (Fig. S19, S20, ESI).
Table 2 Tma (°C), ∆Tmb (°C) data for thermal denaturation temperatures at 260 nm of matched and mismatched
duplexes involving complementary DNA and RNA strands with mismatches directly opposite to modification
(positions 12 and 13).
Complementary sequence
B (DNA)
B (RNA)
Code
c
DNA/RNA
3´-TTB CAC TAT ACG CTC-5´
C
G
A
T
C
G
A
U
D1
D2
D3
D4
R1
R2
R3
R4
36.2
42.0
46.2
+2.4
+1.8
+0.8
+1.8
+2.2
+0.8
+0.2
±0.0
-0.4
33.6
42.2
42.0
+3.4
+3.6
+2.2
+4.4
+4.2
+2.2
+0.6
+0.8
±0.0
ON1
ON2
ON3
5´-GTG ATA TGC-3´
5´-GTG ATLA TGC-3´
5´-ZGTG ATLA TGC-3´
Complementary sequence
B (DNA)
B (RNA)
Code
c
DNA/RNA
3´-TTC BAC TAT ACG CTC-5´
C
G
A
T
C
G
A
U
D1
D5
D6
-9.8
-9.0
D7
R1
R5
R6
R7
36.2 -11.4
42.0 -9.4
46.2 -10.0 -10.2
-8.0
-7.6
-8.2
33.6 -20.4 -10.0
42.2 -11.8
42.0 -10.6
-9.6
-8.2
-8.8
ON1
ON2
ON3
5´-GTG ATA TGC-3´
5´-GTG ATLA TGC-3´
5´-ZGTG ATLA TGC-3´
-8.4
-9.2
a
Conditions: 2.5 µM of each strand in a medium salt buffer 5.8 mM NaH2PO4/ Na2HPO4 buffer (pH 7.0), containing 100 mM
NaCl and 0.10 mM EDTA. b The Tm values reflect the average of two measurements. ΔTm values for mismatches were calculated
as the difference in Tm values between mismatched and fully matched duplexes. In RNA targets, the uracil bases are present
c
instead of the thymine bases. Z = carbazole monomer, TL = LNA-T monomer and B are bases mismatched to duplexes.
Next, mismatches were also presented into position 12 of the target strands substituting the
relevant cytosine base (D5-D7 for DNA and R5-R7 for RNA). All possible mismatches in the
D1/R1 targets at position 12 resulted in significant mismatch recognition (∆Tm = 7.6-11.4 °C for
DNA/DNA duplexes and ∆Tm = 8.2-20.4 °C for DNA/RNA duplexes) as depicted in Table 2. In
the case of the mismatched DNA/DNA duplexes, the highest ∆Tm values of discrimination
displayed by the ON1/D5 duplex was 11.4 °C for replacement of C by G, and 10.2-10.0 °C
displayed by carbazole-functionalized duplexes ON3/D5,D6 upon substituting C with G or A (Fig.
S21, ESI). While for mismatched DNA/RNA duplexes, the highest value of discrimination was
observed for the ON1/R5 duplex (20.4 °C upon replacement of C by G; Fig. S22, ESI).
Surprisingly, the mismatched DNA/RNA duplex ON1/R5 showed an S-shaped melting curve
using a medium salt buffer. To unveil this unclear transition, thermal denaturation experiments
were performed for ON1/(R1, R5, R6 and R7) in a high salt buffer concentration containing 1 M
(instead of 100 mM) NaCl. The resulting melting curves displayed clear sigmoidal transitions with
similar (∆Tm = 9.0-9.4 °C, ON1/R6, R7) or slightly lower (∆Tm = 16.8 °C, ON1/R5) mismatch
discrimination when compared to the values measured at a medium salt concentration (Table 3
and Fig. 2, Fig. S23 ESI).
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DOI: 10.1039/D0OB01553A
Table 3. Tma (°C), ∆Tmb (°C) data for thermal denaturation
temperatures at 260 nm of matched (R1) and mismatched
DNA/RNA duplexes (R5-R7) at high salt concentration.
ON1/R1
ON1/R5
ON1/R6
ON1/R7
0,76
0,72
0,68
0,64
5´-GTG ATA TGC-3´
Complementary sequence
Code
(ON1)
R1
R5
R6
R7
3´-UUC CAC UAU ACG CUC-5´
3´-UUC GAC UAU ACG CUC-5´
3´-UUC AAC UAU ACG CUC-5´
3´-UUC UAC UAU ACG CUC-5´
43.8
27.0 (-16.8)
34.4 (-9.4)
34.8 (-9.0)
a
Conditions: 2.5 µM of each strand in a high salt buffer 5.1 mM NaH2PO4/
Na2HPO4 buffer (pH 7.0), containing 1 M NaCl and 0.10 mM EDTA. b The Tm
values reflect the average of two measurements. Numbers in parentheses are
differences in Tm (ΔTm) = Tm (mismatched) - Tm (matched). The underlined
bases are mismatched to duplexes.
8
20
32
44
56
68
Temperature (°C)
Fig. 2 UV melting curves of matched and mismatched DNA/RNA duplexes with mismatches at position
12. Conditions are described in Table 3. R1 is fully matched DNA/RNA duplex, while R5, R6 and R7 are
mismatched DNA/RNA duplexes.
Generally, the mismatched carbazole duplexes demonstrated similar (DNA/DNA duplex;
ON3/D5-D7) or substantially lower (DNA/RNA duplex; ON3/R5-R7) mismatch discrimination
relative to the corresponding mismatched reference duplexes containing ON1 and ON2.
To study structural features of all mismatched reference as well as carbazole modified duplexes
in detail, CD spectroscopy was employed. All the mismatched dsDNA duplexes are of the B-form
with positive maxima around 280 nm and negative minima around 250 nm. As expected for all
mismatched DNA/RNA duplexes, CD spectra imply no major structural variations in the overall
A/B-form duplex with positive peaks around 265 nm and two negative ones around 240 nm and
210 nm with similar amplitudes (except for D5 and D7) (Fig. S24-S27 ESI).
Fluorescence studies for carbazole containing ONs
Steady-state fluorescence emission spectra of the carbazole-modified single-strand ON3, and the
full spectrum of the matched and mismatched carbazole DNA/DNA and DNA/RNA duplexes were
recorded using excitation of the carbazole moiety at 310 nm. In all cases, the fluorescence spectra
were scanned from 350 to 600 nm in the same buffer solutions as used for UV thermal melting
studies. ONs containing carbazole monomer Z displayed characteristic emission maxima around
400 nm (representative fluorescence spectra are shown in Fig. 3). For all possible matched
carbazole duplexes, the fluorescence emission intensity was typically quenched upon
hybridization with DNA/RNA targets with more pronounced decreases typically being observed
upon DNA binding (Fig. 3A and 3B). Interestingly, the 9-mer matched carbazole duplexes without
overhangs (ON3/D0 and ON3/R0) are noticeably more fluorescent than the matched carbazole
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duplexes (ON3/D1 and ON3/R1) containing 15-mer complementary sequences.DOTI:h1i0s.10i3n9d/Di0cOaBte01s553A
that the carbazole moiety Z, when positioned at the 5’-end of the 9-mer ON, interacts strongly
upon hybridization, most likely via intercalation leading to quenched fluorescence.
A)
B)
120
100
80
60
40
20
0
Buffer
60
50
40
30
20
10
0
ss-ON3
ON3/D0
ON3/R0
ON3/D1
ON3/R1
ON3
ON3/D0 ON3/R0 ON3/D1 ON3/R1
350
400
450
500
550
600
Wavelength (nm)
A (λ=400 nm)
C)
60
D)
Buffer
Buffer
60
50
40
30
20
10
0
ss-ON3
ON3/R1
ON3/R2
ON3/R3
ON3/R4
ON3/R5
ON3/R6
ON3/R7
ss-ON3
ON3/D1
ON3/D2
ON3/D3
ON3/D4
ON3/D5
ON3/D6
ON3/D7
50
40
30
20
10
0
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 3 A): Steady-state fluorescence emission spectra of carbazole single-strand (ss-ON3), and its
matched duplexes with complementary DNA and RNA. B): Fluorescence intensities for matched
carbazole modified duplexes and their remaining percentages at 400 nm relative to ON3. C): Steady-state
fluorescence emission spectra of carbazole single-strand (ss-ON3), fully matched DNA/DNA carbazole
duplex ON3/D1, and mismatched DNA/DNA carbazole duplexes (ON3/D2-D7). D): Steady-state
fluorescence emission spectra of carbazole single-strand (ss-ON3), fully matched DNA/RNA carbazole
duplex ON3/R1, and mismatched DNA/RNA carbazole duplexes (ON3/R2-R7). All fluorescence
measurements were obtained using 2.5 µM concentrations of the two strands in a medium salt buffer (5.8
mM NaH2PO4/ Na2HPO4 buffer (pH 7.0), containing 100 mM NaCl and 0.10 mM EDTA) at 20 °C; λex =
310 nm.
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Thereafter, fluorescence properties of carbazole duplexes formed by ON3 and mismatched
DOI: 10.1039/D0OB01553A
DNA/RNA targets were investigated (Fig. 3C and 3D). Generally, probe ON3 was seen to be
highly sensitive to the presence of mismatched nucleotides in DNA than RNA targets for both
types of mismatches (at positions 12 or 13), showing 2.2-5.0 fold decreases in fluorescence
intensities for mismatched DNA/DNA duplexes and between 1.3- to 3.5 fold for DNA/RNA
mismatches relative to fluorescence obtained for the single-stranded probe ON3. Specifically,
when a mismatch was due to G (D2 and R2) or A (D3) opposite to the insertion of monomer Z,
significant hybridization-induced decreases in carbazole monomer fluorescence intensities were
observed upon mixing with ON3. Only the carbazole mismatched ON3/R3 displayed the same
fluorescence intensities in reference with the fully matched ON3/R1. It is noteworthy that the
decreases in fluorescence emission intensities of mismatches in juxtaposition to the carbazole
monomer upon mixing with ON3 are generally less distinct than the decreases observed for
mismatches directly opposite to monomer Z. The fluorescent properties for carbazole single-strand
ON3 suggest this to be an interesting new probe scaffold for selective detection of DNA and RNA
target strands.
Molecular modeling of probe-target duplexes
To gain more insight in the efficacy of thiophenyl carbazole monomer Z to stabilize the duplex
via intercalation, and implicitly to further interpret the variations in Tm values, modeling
calculations were carried out on the fully matched duplexes formed between 9-mer ON3 sequence
and complementary 15-mer D1/R1 strands. This would also give some information about the
capability of the thiophenyl carbazole moiety to undergo π-π stacking interactions with surrounded
nucleobases. A modified AMBER* force field in Macro Model 9.2 molecular modeling was
subjected to dynamics simulation and minimization in order to generate representative low-
energy structures of the modified duplexes. For the fully matched DNA duplex ON3/D1, a
standard B-type DNA/DNA duplex conformation was built with subsequent incorporations of Z
and LNA-T monomers into ON3, and trimer overhangs nucleotides into D1 (for details, see
experimental section). While for the DNA/RNA ON3/R1 hybrid, the starting NMR duplex
solution structure was initially downloaded from the Protein Data Bank, code 1HHW54 accession
number and corrected, followed by terminal insertions of Z monomer into ON3 and overhangs
into R1 (for details, see experimental procedures). In agreement with the modeling simulation
presented below for Z-modified DNA duplex structure ON3/D1, the carbazole moiety in
monomer Z (cyan) is positioned nicely on the top of the underlying guanosine of the Watson-
Crick 9-mer duplex via π-π stacking interaction, whereas the thiophene ring in Z (magenta)
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interacts with the two adjacent up and down cytosines in a manner that facilitate more favourable
DOI: 10.1039/D0OB01553A
π-π interactions, and thus remains stably intercalated in the duplex core as depicted in Fig. 4. In
addition, the capability of twisting the aromatic moieties of the Z intercalator around the single
bond allows them to adjust their positions to the local secondary structure of the duplex. This
clearly means that thiophenyl carbazole intercalator Z, which behaves as a further base pair, is
well-accommodated into the DNA duplex. Based on these data, we speculate that the proper
intercalation modes together with the stacking properties contribute, together with the LNA
nucleotide, to the 10.0 °C increase in Tm for ON3/D1 when compared to the native duplex. Along
the same line the 4.2 °C increase in Tm for ON3/D1 when compared to the LNA-containing duplex
ON2/D1 underlines the affinity enhancing effect of monomers Z.
A)
B)
Fig. 4: Representative low-energy conformations of the fully
matched carbazole-modified DNA/DNA duplex ON3/D1
adopted between the 9-mer sequence: 5´-ZGTGATLATGC-3´
(ON3) and its complementary 15-mer strand: 3´-TTCCAC
TATACGCTC-5´ (D1) produced by an Amber*-minimized
model. A) Side view displaying hybridization and stacking
mode interactions between thiophenyl carbazole monomer Z
and the surrounding nucleobases of dsDNA. B) Overall view
displaying stacking interactions and overlapping for possible
conformation of ON3/D1. Carbazole moiety in monomer Z is
shown in cyan, thiophene ring in Z is shown in magenta, and
LNA-T centrally inserted in ON3 is marked in cyan. Hydrogen
bonds between the canonical nucleobases are displayed in red.
Guanosines, cytosines, adenines, and thymidines displayed in
their standard colors.
Conversely, in case of the Z-modified DNA/RNA ON3/R1 hybrid, the structural
conformation of the thiophenyl carbazole building block Z is considerably changed. Molecular
modeling depicts that the carbazole moiety (cyan) in Z is positioned substantially outward the
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duplex core and consequently no clear π-π stacking interactions to the undeDrlOyI:i1n0g.103g9u/Da0nOiBn0e1553A
nucleobase are rendered possible. Whereas the thiophene ring (magenta) is twisted to a large
extent, in comparison with the carbazole moiety, stacking interactions with the surrounding
cytosines appear weak at best which weaken the intercalating efficiency of Z towards the DNA
/RNA hybrid (Fig. 5). Another noteworthy observation is that the large extent of twisting of the
thiophene ring as well as its inefficient intercalation into the ON3/R1 duplex is ascribed to the
presence of the negatively charged hydroxyl group of the opposite cytidine ribosugar (C12) located
in very close proximity to the negatively charged sulfur of the thiophene ring in Z. Such type of
electrostatic repulsion precludes the interior accommodation of the thiophene moiety into the
helical DNA/RNA structure, and hence, reflects the inability of Z monomer to intercalate between
the up and down cytosines. Another reason perturbing the intercalating properties of Z is the
unfavorable steric clashes and/or strain within the DNA/RNA duplex construct which impairs the
interactions with the neighboring nucleobases. This entails the equivalency of thermostability
observed for carbazole-modified duplex ON3/R1 and the corresponding LNA-T control duplex
ON2/R1 where LNA-T (cyan) is oriented towards the major groove of the double helix, adding to
the stability of the DNA/RNA duplex.
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Fig. 5 Representative low-energy conformations of the fully matched carbazole-modified DNA/RNA
DOI: 10.1039/D0OB01553A
hybrid duplex ON3/R1 formed between the 9-mer sequence: 5´-ZGTGATLATGC-3´ (ON3) and its
complementary 15-mer strand: 3´-UUCCAC UAUACGCUC-5´ (R1) produced by an Amber*-minimized
model. Carbazole moiety in monomer Z is shown in cyan, thiophene ring in Z is shown in magenta, and
LNA-T centrally inserted in ON3 is marked in cyan. Hydrogen bonds between the canonical nucleobases
are displayed in yellow. Guanosines, cytosines, adenines, thymidines and uridines displayed in their
standard colors.
Conclusions
Synthesis of a novel thiophenyl carbazole intercalating building block suitable for solid-supported
DNA synthesis was developed via Suzuki-Miyaura cross-coupling. This building block was used
on an automated nucleic acid synthesizer for 5’-end incorporation of monomer Z into a 9-mer
sequence. This 9-mer displayed high-affinity hybridization towards complementary 9-mer/15-mer
DNA and comparable affinity toward the corresponding RNA complements. With matched and
singly mismatched DNA and RNA 15-mer targets, the carbazole containing 9-mer probe was able
to signal full target complementarity using fluorescence spectroscopy, although the quenching was
more profound than in cases of DNA/RNA hybrids. Evident increases of fluorescence intensities
were revealed upon hybridization of carbazole single-strand probe with all 9-mer complements
rather than all 15-mer complements encompassing overhangs. Based on modeling studies, the
increased thermal stabilities and the fluorescence quenching observed for Z-modified DNA/DNA
duplexes is suggested to be a result of efficient π-π stacking interactions. The interesting
fluorescence profiles reported herein, taken together with high-affinity binding and mismatch
sensitivity to complementary targets, make ONs containing carbazole monomer Z as dangling 5’-
end modification as a new design for exploitation within nucleic acid diagnostic and in nucleic
acid targeting.
Experimental section
All reactions were carried out under a N2 or Ar atmosphere using anhydrous solvents and glassware
that had been dried at 120 °C. Column chromatography was carried out under pressure using
Merck Millipore silica gel 60 (0.040-0.063 mm). Analytical silica gel thin layer chromatography
(TLC) was performed using Merck Kieselgel 60 F254 (0.22 mm thickness, precoated aluminum
plates). The silica was pretreated with a solvent containing 1% Et3N, dried over activated
molecular sieves (3 Å, 2-3 mm), for compounds sensitive to acid. All reagents used were purchased
from Sigma-Aldrich, Fluka and used without purification. Dichloromethane (DCM), N,N-
diisopropylethylamine (DIPEA), N,N-dimethylformamide (DMF), ethyl acetate (AcOEt) were
dried over activated molecular sieves (4 Å, 2-3 mm) and measured on a Karl Fischer titrator (<12
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ppm). Petroleum ether: bp. 60-80 °C, triethylamine (Et3N) and acetonitrile (CH3CN) were dried
DOI: 10.1039/D0OB01553A
over activated molecular sieves (3 Å, 2-3 mm). Acetone and ethanol were used as received.
Barluenga’s reagent (IPy2BF4) was synthesized in a safe and scalable procedure from iodine,
pyridine and silver salt according to the procedure reported by Davis et al.55
NMR spectra were recorded for compounds 5 at room temperature on a Bruker AVANCE III
400 spectrometer at 400 MHz for 1H, 101 MHz for 13C, and at 162 MHz for 31P with TMS as an
internal standard for 1H NMR. For compounds 2-4, NMR spectra were recorded at 298 K on an
Agilent NMR Magnet-400 MHz (399.89 MHz for 1H and at 100.56 MHz for 13C). Chemical shifts
are reported in parts per million (δ), relative to the residual non-deuterated solvents peaks (CDCl3:
7.26 ppm for 1H and 77.16 ppm for 13C; DMSO-d6: 2.50 ppm for 1H and 39.5 ppm for 13C; 85 %
aq. H3PO4 as an external standard with 0.00 ppm for 31P NMR). Multiplicities are abbreviated as
follows: s = singlet, d = doublet, t = triplet, m = multiplet. For the final phosphoramidite 5, the
spectral assignment for 1H NMR and 13C NMR has been confirmed by H-H correlation (COSY),
H-C correlation (HSQC) and long-range H-C correlation (HMBC). High-resolution electrospray
ionization mass spectroscopy (ESI-MS) experiments were performed for compounds 2a, 2b and 3
using a Waters Xevo G2 QTOF instrument equipped with an injection system (cone voltage 50 V;
source 120 °C). For compounds 3 and 4, electrospray ionization high-resolution mass spectra (ESI-
HRMS) were performed on a Bruker APEX III FT-ICR mass spectrometer using chloroform or
acetonitrile as solvents. For accurate ion mass determinations, the (M+H+) or (M+Na+) ions were
peaks matched by calibration with NaI. Melting points were determined with a Boetius apparatus
and are not corrected. Elemental analysis was determined with the PerkinElmer 2400 Series II
CHNS/O Elemental Analyzer. The progress of the reactions were monitored by thin-layer
chromatography (TLC) using silica gel coated aluminum plates with a fluorescence indicator
(Merck, SiO2 60, F254) and visualized by UV light (254 and 365 nm) or dipping into a solution of
vanillin (400 mg of vanillin in 200 mL of EtOH, 4 mL of H2SO4).
Commercially available DNA phosphoramidite monomers, solid supports, additional reagents
and the wild types DNA, as well as RNA oligonucleotides, were purchased from Sigma-Aldrich,
Glen Research or GE Healthcare. Acetonitrile and 5-[3,5-bis(trifluoromethyl)phenyl]-1H-
tetrazole, required for hand coupling step, were dried over activated molecular sieves (3 Å, 2-3
mm) and their dryness was measured on Karl Fischer titrator (<10 ppm). The synthesized
oligonucleotides were purified by IE-HPLC with HPLC grade acetonitrile or methanol as the
solvent. The composition of the synthesized oligonucleotide was verified by MALDI-TOF
analysis on a Bruker Daltonics Microflex LT (MALDI-LIFT system) MS instrument in ES+ mode
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with HPA-matrix (10 mg 3-hydroxypicolinic acid in 50 mM ammoniumcitrate/70% acetonitrile)
DOI: 10.1039/D0OB01553A
matrix.
3-Iodo-9H-carbazole (2a)46
Carbazole (5.01g, 30.0 mmol) was added to a solution of IPy2BF4 (12.27 g, 33.0 mmol) and CuSO4
(270 mg, 1.65 mmol) under a N2 atmosphere (Schlenk line) in anhydrous acetonitrile (50 mL) and
the solution turned a deep-blue color. The reaction was stirred at 65 °C for 10 min and then diluted
with 90 mL of a sat. sodium thiosulfate solution. The precipitation formed was filtered off and
rinsed with distilled water. The solid was dried in a vacuum desiccator over P2O5. The crude
product had traces amounts of 3,6-diiodo-9H-carbazole byproduct. The product was recrystallized
from hot water. The 3-iodo-9H-carbazole was obtained as a white-cream solid (6.95 g, 23.7 mmol,
1
79% yield); mp 193 °C (lit.56 mp 192-194 °C). H NMR (DMSO-d6) δ (ppm): 7.17 (ddd, J = 8.0,
7.1, 1.0 Hz, 1H), 7.34 (d, J = 8.5 Hz, 1H), 7.42 (d, J = 7.1 Hz, 1H), 7.49 (dd, J = 8.0, 1.0 Hz, 1H),
7.63 (dd, J = 8.5, 1.8 Hz, 1H), 8.15 (dd, J = 8.0, 1.0 Hz, 1H), 8.50 (d, J = 1.8 Hz, 1H), 11.38 (s,
13
1H, NH). C NMR (DMSO-d6) δ (ppm): 81.3(Caro.-I), 111.0, 113.4, 118.9, 120.6, 121.1, 125.1,
126.2, 128.6, 133.3, 138.7, 139.7 (Caro.). HRMS (ESI) m/z [M + H]+ calcd for C12H9IN: 293.9779,
found 293.9780. Anal. Calcd for C12H8IN: C, 49.17; H, 2.75; N, 4.78. Found: C, 49.04; H, 2.87;
N, 4.63.
2-(9H-Carbazol-9-yl)ethan-1-ol (2b)47
The synthetic method for compound 2b is optimized and fully characterized in this report.
Powdered potassium hydroxide (14.0 g) was stirred with DMF (80 mL) at room temperature for
10 min. The mixture was then stirred with carbazole (6.6 g, 0.040 mol) at room temperature for 45
min. 2-Bromoethanol (3.5 mL, 0.05 mol) was added slowly, and the resultant mixture was allowed
to stir at room temperature for 10 h. The mixture was poured into water (1.2 L), and the white solid
was filtered, washed with water, and air-dried. The white solid was dissolved in 70% ethanol, and
the insoluble residue was filtered out. Water was added to the filtrate until the precipitation was
completed. Precipitated was filtered and dried under vacuum. Yield: 83% (6.95 g) as a white solid;
1
mp 80 oC. H NMR (CDCl3) δ (ppm): 1.63 (bs, 1H, NCH2CH2OH), 3.94 (d, J = 5.3 Hz, 2H,
NCH2CH2OH), 4.34-4.47 (m, 2H, NCH2CH2OH), 7.19-7.26 (m, 2H), 7.38-7.49 (m, 4H), 8.06 (d,
13
J = 7.8 Hz, 2H). C NMR (CDCl3) δ (ppm): 45.6 (NCH2CH2OH), 61.6 (NCH2CH2OH), 108.9,
119.3, 120.5, 123.1, 125.9, 140.8 (Caro.). MS (ESI) m/z [M + H]+ calcd for C14H13NO: 212.11,
found 212.11. Anal. Calcd for C14H13NO: C, 79.59; H, 6.20; N, 6.63. Found: C, 79.69; H, 6.07; N,
6.44.
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DOI: 10.1039/D0OB01553A
2-(3-Iodo-9H-carbazol-9-yl)ethan-1-ol (3)
Method A. To the solution of 3-iodocarbazole (2a,1.47 g, 5.02 mmol) in anhydrous DMF (25
mL), potassium hydroxide (2.25 g, 40.16 mmol) was added while stirring. The reaction mixture
was stirred for 30 min at room temperature and 2-bromoethanol (540 µL, 7.55 mmol) was slowly
added (approx. 45 min). The temperature was increased to 80 °C and stirring was continued for 20
h. The reaction mixture was cooled down to room temperature and poured into water (300 mL).
The precipitation was filtered off and recrystallized from ethanol. Yield: 77% (1.31 g) as a pall
beige solid.
Method B. 2-(9H-Carbazol-9-yl)ethan-1-ol (2b, 3.17 g, 15.0 mmol) was added to a solution of
IPy2BF4 (6.14 g, 16.5 mmol) and CuSO4 (135 mg, 0.83 mmol) under a N2 atmosphere (Schlenk
line) in anhydrous acetonitrile (25 mL), and the solution turned a deep-blue color. The reaction
was stirred at 65 °C for 10 min and then diluted with 42 mL of a sat. sodium thiosulfate solution.
The precipitation formed was filtered off and rinsed with distilled water. The solid was dried in a
1
vacuum desiccator over P2O5. Yield: 85% (4.31 g) as a white-cream solid; mp 105-108 oC. H
NMR (CDCl3) δ (ppm): 1.57 (bs, 1H, NCH2CH2OH), 3.99 (m, 2H, NCH2CH2OH), 4.39 (t, J = 5.3
Hz, 2H, NCH2CH2OH), 7.22 (d, J = 8.2 Hz, 1H), 7.24 (d, J = 6.7 Hz, 1H), 7.42 (d, J = 8.2 Hz,
1H), 7.46 (d, J = 7.2 Hz, 1H), 7.68 (d, J = 7.2 Hz, 1H), 8.00 (d, J = 7.8 Hz, 1H), 8.37 (s, 1H). 13C
NMR (CDCl3) δ (ppm): 45.6 (NCH2CH2OH), 61.5 (NCH2CH2OH), 81.8(Caro.-I), 109.1, 111.1,
119.8, 120.7, 121.8, 125.6, 126.7, 129.0, 134.8, 140.0, 140.7 (Caro.). HRMS (ESI) m/z [M + H]+
calcd for C14H13INO: 338.0041, found 338.0043. Anal. Calcd for C14H12INO: C, 49.87; H, 3.59;
N, 4.15. Found: C, 50.03; H, 3.58; N, 4.11.
2-(3-(Thiophen-2-yl)-9H-carbazol-9-yl)ethan-1-ol (4)
To the suspension of 2-(3-iodo-9H-carbazol-9-yl)ethan-1-ol (3, 1.02 g, 3.0 mmol) in
anhydrous 1,4-dioxane (10 mL), 2-thienylboronic acid (0.576 g, 4.5 mmol) was added
while stirring, followed by 2 M Na2CO3 (3 mL) and Pd(dppf)Cl2∙DCM [1,1′-
bis(diphenylphosphino)ferrocene]dichloropal-ladium(II) complex with dichloromethane (0.196 g,
0.24 mmol) were added. The reaction mixture was placed in Schlenk line and after degassing
refluxed under nitrogen for 18 h. After that time the reaction mixture was concentrated under
reduced pressure and purified by silica gel packed column using petroleum ether:ethyl acetate (3:1
to 7:3 v/v). Yield: 64% (0.806 g) as a white powder; mp 120-122 oC. 1H NMR (CDCl3) δ (ppm):
1.59 (bs, 1H, NCH2CH2OH), 3.99 (t, 2H, NCH2CH2OH), 4.41 (t, J = 5.6 Hz, 2H, NCH2CH2OH),
7.09 (dd, J = 4.8 Hz, J = 3.6 Hz, 1H, Hthio.), 7.22-7.26 (m, 2H), 7.32 (dd, J = 3.6, 1.1 Hz, 1H),
7.39-7.47 (m, 3H), 7.69 (dd, J = 8.4 Hz, J = 1.6 Hz, 1H, Hthio.), 8.10 (d, J = 7.6 Hz, 1H), 8.29 (d,
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DOI: 10.1039/D0OB01553A
J = 1.6 Hz, 1H). C NMR (CDCl3) δ (ppm): 45.7 (NCH2CH2OH), 61.6 (NCH2CH2OH), 109.1,
109.3, 118.0, 119.6, 120.7, 122.3, 123.1, 123.5, 123.9, 124.6, 126.3, 126.3, 128.1, 140.4, 141.3,
145.7 (Caro.). HRMS (ESI) m/z [M+H]+ calcd for C18H16NOS: 294.0952, found 294.0953. Anal.
Calcd for C18H15NOS: C, 73.69; H, 5.15; N, 4.77. Found: C, 73.50; H, 4.93; N, 4.49.
2-Cyanoethyl (2-(3-(thiophen-2-yl)-9H-carbazol-9-yl)ethyl) diisopropylphosphoramidite (5)
The alcohol 4 (0.293 g, 1.0 mmol) was coevaporated with anhydrous acetonitrile (3 × 20 mL) and
dissolved together with anhydrous N,N-diisopropylethylamine (0.33 g, 2.55 mmol) under an argon
atmosphere
in
anhydrous
dichloromethane
(10
mL).
2-Cyanoethyl-N,N-
diisopropylchlorophosphoramidite (0.284 g, 1.2 mmol) dissolved in anhydrous dichloromethane (5
mL) was added dropwise, and the reaction mixture was stirred at room temperature for 2 h. The
resulting mixture was transferred with a dry syringe and directly applied onto the top of a silica gel
column for purification under super dry conditions (AcOEt:TEA 99:1 v/v). The phosphoramidite is
sensitive to moisture and light and it was immediately used for DNA synthesis. Yield: 78% (0.385
1
g) as a yellow semi-solid. Rf = 0.32 (99: 1 AcOEt/TEA v/v). H NMR (DMSO-d6) δ (ppm): 0.83
and 0.99 (2d, J = 6.8 and 6.8 Hz, 12H, 4 × CH3), 2.57 (t, J = 5.9 Hz, 2H, CH2CN), 3.35-3.40 (m,
2H, 2 × NCH), 3.45-3.51 (m, 2H, NCCH2CH2OP), 3.90-4.01 (m, 2H, CH2CH2OP), 4.62 (t, J = 5.7
Hz, 2H, CH2CH2OP), 7.15 (dd, J = 5.1 Hz, J = 3.6 Hz, 1H, Hthio.), 7.19-7.24 (m, 1H, Haro.), 7.43-
7.53 (m, 3H, 1Hthio. and 2Haro.), 7.62-7.68 (m, 2H, 1Hthio. and 1Haro.), 7.74 (dd, J = 8.6 Hz, J = 1.8
Hz, 1H, Haro.), 8.23 (d, J = 7.5 Hz, 1H, Haro.), 8.44 (d, J = 1.6 Hz, 1H, Haro.). 13C NMR (DMSO-d6)
δ (ppm): 19.6 (CH2CN), 24.2 (4 × CH3), 42.2 (2 × NCH), 43.6 (CH2CH2OP), 58.0, 61.2 (CH2CH2OP
and NCCH2CH2OP), 109.8 (Caro.), 110.1 (Caro.), 117.0 (CH2CN), 118.7, 118.9, 120.4, 122.1, 122.2,
122.6, 123.6, 124.1, 125.0, 125.8, 128.3, 139.8, 140.7, 144.8 (Caro.). 31P NMR (DMSO-d6) δ (ppm):
146.67. HRMS (ESI) m/z [M+Na]+ calcd for C27H32N3NaO2PS: 516.1850, found: 516.1868.
Oligonucleotide synthesis, purification and analysis
Oligonucleotide synthesis was performed on a PerSeptive Biosystems expedite 8909 automated
DNA/RNA synthesizer in 1.0 μmol scale on polystyrene supports 40s (from Amersham
Biosciences) for oligonucleotides ON1-ON3. The manufacturer´s standard cycle protocol was
carried out using 4,5-dicyanoimidazole as an activator for the commercial phosphoramidites. A
solution of the synthesized carbazole phosphoramidite 5 was prepared in a 1 mL dry plastic
syringes by addition of anhydrous 5-[3,5-bis(trifluoromethyl)phenyl]-1H-tetrazole (0.25 M, in
anhydrous acetonitrile) as an activator, followed by incorporation via hand-coupling57 into the
growing oligonucleotides chain during extended coupling time (25 min). Due to its high sensitivity
to humidity and light, the carbazole phosphoramidite 5 was immediately used for DNA synthesis.
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The coupling time for standard as well as LNA-T monomers was 144 s and steDpOwI:i1s0e.10c3o9/uDp0OliBn0g1553A
efficiencies, determined by the absorbance of the liberated trityl cation at 495 nm on a UV-VIS
spectrophotometer and were >98% for standard DNA monomers and >99% for LNA-T monomer.
Removal of nucleobase protecting groups and cleavage from the solid polystyrene supports for
ON1-ON3 was performed under standard conditions (1 ml of 32% aqueous ammonia, 12 h at
55 °C). Purification of the resulting oligonucleotides ON1-ON3 were accomplished by DMT-ON
reverse-phase HPLC (RP-HPLC) using the Waters system 600 equipped with a Waters XBridge
OBD C18-column (19 × 1000 mm, 5 µm + precolumn: XBridge 10 × 10 mm, 5 µm, temperature
column oven: 50 °C). Elution was performed starting with an isocratic hold of buffer A for 2 min
followed by a linear gradient to 70% buffer B over 17 min at a flow rate of 5 mL/min (Buffer A:
0.05 M triethylammonium acetate in Milli-Q water, pH 7.4; Buffer B: 75% CH3CN/25% Buffer
A). Corresponding fractions with oligonucleotides were evaporated under a flow of nitrogen, and
oligonucleotides ON1 and ON2 were detritylated using an 80% aqueous solution of acetic acid
(100 μL) for 30 min. Desalting was performed by addition of an aqueous solution of sodium acetate
(3 M, 15 µL) and sodium perchlorate (5 M, 15 µL) and precipitation from cold acetone (1 mL).
ON3 was treated with 100 µL doubly filtered water followed by the addition of sodium perchlorate
(5 M, 15 µL) before precipitation from cold acetone (1 mL). The resulting suspension for all
oligonucleotides ON1-ON3 was stored at -20 °C for 1 h. After centrifugation (13000 rpm,10 min,
4 °C), the supernatant was removed and the pellet further washed with cold acetone (2 × 1 mL),
dried for 30 min under a flow of nitrogen, and dissolved in Milli-Q water (1 ml).
The composition of the oligonucleotides (Table 4) was confirmed by MALDI-TOF analysis on
a Bruker Daltonics Microflex LT (MALDI-LIFT system) MS instrument in the ES+ mode.
The purity for all final oligonucleotides ON1-ON3 was found to be more than 93% when
recorded by analytical IE-HPLC traces using Merck Hitachi La-Chrom system equipped with a
DNAPac PA100, analytical column (13 μm, 250 mm × 4 mm) heated to 60 °C. Elution was
performed with an isocratic hold of buffer (10%), starting from 2 min hold on 2% Eluent in Milli-
Q water (solvent A), followed by a linear gradient to 30% eluent in 23 min at a flow rate of 1.1
mL/min (Eluent: 0.6 M sodium perchlorate; buffer: 0.25 M Tris-Cl, pH 8.0; solvent A: Milli-Q
water).
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DOI: 10.1039/D0OB01553A
Table 4. MALDI-MS and HPLC purity of synthesized ONs
ON#
ON1
ON2
ON3
Sequence
Calc. m/z
2753.84
2781.84
3138.19
Found m/z HPLC purity
5´-GTG ATA TGC-3´
5´-GTG ATLA TGC-3´
5´-ZGTG ATLA TGC-3´
2749.76
2779.23
3136.01
94%
96%
93%
UV thermal melting studies
In order to determine duplex melting temperatures (Tm), UV melting measurements were
performed on a Perkin-Elmer Lambda 35 UV-Vis spectrometer fitted with a PTP-6 Peltier
temperature programmer using Hellma SUPRASIL synthetic quartz optical cuvettes with a path
length of 10 mm. Concentrations of purified oligonucleotides were determined by UV at 260 nm,
assuming identical molar absorptivities for unmodified DNA, RNA and LNA nucleoside
constituents (ε260: dG = 10.8, dT = 8.4, dA = 13.7, dC = 7.3, rG = 10.8, rU = 9.8, rA = 13.7, rC =
7.1, TL = 8.4 OD260/µmol). Extinction coefficient58 was determined for the thiophenyl carbazole
moiety (ε260 = 19.71 OD260/µmol) after measuring the absorbance average of three measurements
for Z linker. The synthesized oligonucleotides ON1-ON3 (2.5 µM) were mixed with a
complementary DNA/RNA strand concentration of 2.5 μM and a volume of 1.0 mL before
monitoring at 260 nm. Samples were prepared as follows: The two strands were mixed in a 1:1
ratio in 2 mL Eppendorf tubes before medium salt buffer (2 times, 11.7 mM sodium phosphate,
200 mM NaCl, 0.20 mM EDTA, pH 7.0, 500 μL) was added, which was completed in 1.0 mL
using Milli-Q water. Thus, all oligonucleotide samples were dissolved in 1× buffer condition (5.8
mM sodium phosphate, 100 mM NaCl and 0.10 mM EDTA) to furnish duplex-forming
oligonucleotides. The samples were denatured by heating to 90 °C (10 min) in a water bath
followed by gradually cooling to the starting temperature of the experiment 8 °C and were then
kept at this temperature for 120 min before they were transferred into the cuvettes. The thermal
melting temperatures (Tm, °C) were determined as the maximum of the first derivative plots of the
smoothed melting curves obtained by UV absorbance at 260 nm (as a function of time) against
increasing temperature from 8 °C to 70 °C (gradient 0.5 °C/min) programmed by a Peltier
temperature controller. All melting temperatures are within the uncertainty ± 0.5 °C as determined
by repetitive experiments and Tm values were calculated using UV-WinLab software, taking an
average of the two separate melting curves. ΔTm values were calculated as the difference in Tm
values between unmodified and modified duplexes. For mismatched duplexes, ΔTm values were
calculated as the difference in Tm values between mismatched and matched duplexes. Due to
unclear transition for the mismatched duplex melting curve for ON1/R5 using a medium salt
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buffer, UV melting measurements were repetitively recorded at pH 7.0 using a high salt buffer
DOI: 10.1039/D0OB01553A
condition (5.1 mM sodium phosphate, containing 1 M NaCl and 0.10 mM EDTA) for ON1/(R1,
R5, R6, R7).
Circular dichroism studies
CD measurements were performed on a Jasco J-815 spectropolarimeter using 1 mL quartz cuvettes
with 5-mm path length. Oligonucleotides (2.5 μM) were mixed in a medium salt buffer (5.8 mM
NaH2PO4/Na2HPO4, containing 100 mM NaCl and 0.10 mM EDTA) at pH 7.0 using the same
solution used for Tm measurements. All samples were annealed for 2 min at 90 °C and slowly
cooled to room temperature before data collection. The measurements were performed at 20 °C in
the 200-400 nm wavelength range with a continuous scanning mode, 50 nm/min as a scanning
speed, 4 s for a response, 2.0 nm for bandwidth. In order to confirm the very low thermal melting
value for the mismatched duplex ON1/R5, the CD spectra were recorded at pH 7.0 using a high
salt buffer condition (5.1 mM sodium phosphate, containing 1 M NaCl and 0.10 mM EDTA) for
the mismatched duplexes ON1/(R1, R5, R6, R7) in addition to measurements using a medium salt
buffer. All CD profiles were attained by taking the average of five accumulations from which the
spectrum of the background buffer was subtracted. The spectra were smoothened in Microcal
Origin 6.0 using a Savitzky–Golay filter.
UV absorption spectra
UV-vis absorption spectra (range: 200-600 nm) were recorded at 20 °C using the same samples
and instrumentation as in the thermal denaturation experiments.
Steady-state fluorescence emission spectra
Fluorescence spectra (350–600 nm) of ONs modified with carbazole-functionalized monomer Z
and the corresponding matched and mismatched duplexes with complementary DNA/DNA and
DNA/RNA targets, were recorded with a Varian-Cary Eclipse spectrophotometer using quartz
Suprasil cuvettes with a path length of 10.00 mm. The fluorescence experiments were determined
at 20 °C in non-deoxygenated thermal denaturation buffer (5.8 mM NaH2PO4/Na2HPO4,
containing 100 mM NaCl and 0.10 mM EDTA at pH 7.0, each strand at 2.5 μM concentration)
and obtained as an average of five scans with the use of an excitation wavelength of 310 nm, an
excitation slit of 5.0 nm, an emission slit of 5.0 nm and a scan speed of 600 nm min-1 with a
medium scanning mode using medium voltage. The samples from the thermal denaturation
experiments were reused for the fluorescence measurements. All samples were annealed at 90 °C
for 10 min and allowed to cool slowly to room temperature prior to measurements. Background
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spectra of buffer solution were recorded with an excitation wavelength of 310 nm and were
DOI: 10.1039/D0OB01553A
subtracted from the relevant spectra.
Molecular Modelling
Molecular modelling was performed with Maestro v9.2 from Schrödinger. All calculations were
conducted with AMBER* force field59 and the implicit GB/SA water model60 as implemented in
MacroModel v9.2. Extended cut-offs were used for non-bonded interactions (van der Waals 8 Å
and electrostatics 20 Å). The molecular dynamic simulations were performed with stochastic
dynamics, a SHAKE algorithm to constrain bonds to H-atoms, time step of 1.5 fs, and simulation
temperature of 300 K. Simulation for 0.5 ns with an equilibration time of 150 ps generated 250
individual structures, which were minimized using the Polak-Ribiere Conjugate Gradient (PRCG)
method61 with maximum iterations 5000 and convergence threshold of 0.05 kJ/mol. The resulting
structures were compared in order to determine the lowest-energy conformers. The global
minimum was used for analysis. For the Z-modified DNA/DNA duplex ON3/D1, the starting
structure was generated by building a standard B-type DNA duplex conformation using
MacroModel v9.2, followed by: (i) modifying with LNA-T monomer together with terminal
incorporation of the desired carbazole monomer Z at the DNA 9-mer strand ON3, (ii) terminal
insertions of two 3-mer overhangs (CTT, CTC) at the 3´- and 5´-opposing ends of the
complementary DNA stand D1, respectively.
For the Z-modified DNA/RNA duplex ON3/R1, the seed structure was generated by taking
previously reported NMR solution structure of the LNA:RNA hybrid duplex obtained from Protein
Data Bank (PDB ID: 1HHW).54 For wrong number of bonds and missing hydrogens observed in
the best representative conformer of the NMR ensemble, manual fixing was carried out using Build
panel for bonds and Add hydrogens preprocessing step for missing hydrogens, while observed
hydrogen overlaps were fixed via optimizing H-bond assignment. For precise positioning and
accurate model calculations, the cytosine attached at the 5´-end of ON3 was modified by guanine,
whereas the guanine at the 3´-opposing end of R1 was adjusted by cytosine in order to build a
corrected base pair which fully matched with the tested DNA/RNA duplex construct. Model of the
modified ON3/R1 duplex was performed by incorporation of the carbazole monomer Z at the 5´-
end of ON3, followed by incorporations of the two 3-mer overhangs (CUU, CUC) at the 3´- and
5´-opposing ends of the complementary RNA stand R1, respectively.
Initially, all atoms except those in the carbazole monomer Z and LNA-T were frozen to perform
a relaxation of the modified nucleotide, followed by a complete minimization of the full duplexes
(ON3/D1, ON3/R1) were carried out to attain a relaxed low-energy duplex before applying
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dynamic simulations described above. Hundreds of conformations were foundDOfoI:r10.c10a3r9b/Da0zOoBl0e1-553A
modified DNA/DNA (ON3/D1) as well as DNA/RNA (ON3/R1) duplex structures within the
energy window. Resulting structures were further processed in VMD (v1.9.3a6, 2015.
http://www.ks.uiuc.edu) or in PyMOL (v1.7.4.5, 2010. http://pymol.org) Molecular Graphics
Systems.
Acknowledgements
This work was financially supported by Biomolecular Nanoscale Engineering Center of
Excellence (BioNEC) funded by the VILLUM FONDEN for studies on nucleic acid chemical
biology, Grant Number VKR18333, from the European Union’s Horizon 2020 research and
innovation program under grant agreement No 810685, and from the Silesian University of
Technology BKM-RCh-2020.
†Electronic supplementary information (ESI) available. See DOI:
Supplementary data (1H-,13C-NMR, 31P-NMR, HSQC, HMBC, H-HCOSY and HRMS of
synthesized compounds 2-5 (Fig. S1-S16). IE-HPLC trace and MALDI-TOF mass spectrometry
data of oligonucleotides (ON1-ON3). Representative UV melting absorption curves, first
derivatives curves and CD spectral analysis for all DNA/DNA and DNA/RNA duplexes are shown
in Fig. S17-S27.
Conflict of interest: The authors declare no conflict of interest.
Keywords: Carbazole duplexes. Oligonucleotides, Mismatch discrimination. Fluorescent probes.
Molecular modeling.
ORCID
Alaa S. Gouda
Łukasz Przypis
Krzysztof Walczak
Per T. Jørgensen
Jesper Wengel
https://orcid.org/0000-0002-8862-4859
https://orcid.org/0000-0001-5195-8751
https://orcid.org/0000-0001-8511-2009
https://orcid.org/0000-0003-3932-5921
https://orcid.org/0000-0001-9835-1009
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DOI: 10.1039/D0OB01553A
Graphical abstract
Investigation of modified carbazole, as a nucleoside mimetic, binding affinity towards
complementary DNA/RNA models exhibit improved thermal stability and fluorescence
emission of carbazole probe-target hybrid duplexes.