Control of aggregation-induced emission by DNA hybridization

Article abstract of DOI:10.1039/c3cc42706d

Aggregation-induced emission (AIE) was studied by hybridization of dialkynyl-tetraphenylethylene (DATPE) modified DNA strands. Molecular aggregation and fluorescence of DATPEs are controlled by duplex formation.

Full text of DOI:10.1039/c3cc42706d

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Control of aggregation-induced emission by DNA
hybridization†
Cite this: Chem. Commun., 2013,
49, 5835
Shaoguang Li, Simon M. Langenegger and Robert Haner*
¨
Received 12th April 2013,
Accepted 9th May 2013
DOI: 10.1039/c3cc42706d
www.rsc.org/chemcomm
Aggregation-induced emission (AIE) was studied by hybridization
of dialkynyl-tetraphenylethylene (DATPE) modified DNA strands.
Molecular aggregation and fluorescence of DATPEs are controlled
by duplex formation.
DNA is a useful and versatile scaffold for the assembly of
functional groups with important applications in the fields of
diagnostics, electronics, and materials science.1–13 In particular,
the introduction of a wide range of chromophores into DNA has
drawn considerable attention.14–23 Distinct photophysical effects
are observed in such DNA-organized architectures including
the formation of exciplexes,24–28 H- and/or J-aggregates29–31 or
distinct helical arrangements with unique optical or chiral
properties.32–39 In recent years, tetraphenylethylene (TPE) and
related molecules have attracted interest because of their
Scheme 1 Preparation of phosphoramidites 6 and 7; (a) CuI, Pd(PPh3)2Cl2, NEt3,
THF; 38%; (b) DMTCl, THF, pyridine; 29% for 4; 23% for 5; (c) CEPCl, Hunig’s base,
¨
DCM; DMT = 4,40-dimethoxytrityl; CEP = 2-cyanoethyl-N,N-diisopropyl-phosphor-
amidite; 61% for 6; 71% for 7.
unusual fluorescence properties. These chromophores are DNA strands using an enzymatic method.47 Herein, we describe
weakly or non-emissive as unassociated monomers but they the synthesis and incorporation of the two stereoisomers of a
become strongly fluorescent upon aggregation. Aggregation- dialkynyl-tetraphenylethylene (DATPE) into DNA and the AIE
induced emission (AIE) is a result of restricted intramolecular properties of the obtained conjugates (Fig. 1b).
rotation in the aggregated state (Fig. 1a).40,41 AIE features are of
The synthetic strategy of the two building blocks (ME and
interest in diverse areas including fluorescence sensors42 and MZ) is shown in Scheme 1. Compound 1 was obtained accord-
OLEDs.40 However, reports describing the interaction of AIE ing to the reported procedure as a mixture of E-/Z-isomers.43
molecules (AIEs) with DNA,43 proteins44,45 or other bio- The butynyl chains were introduced through Sonogashira
structures46 are limited and only one recent publication has coupling using 3-butyn-1-ol. The E- and Z-isomers (2 and 3) were
reported the incorporation of an AIE-active silole derivative into separated by chromatography and obtained in nearly equal
amounts. The products were recrystallized and their configura-
tions established by X-ray crystallography (ESI†). Compounds 2
and 3 exhibited typical AIE characteristics. They are non-emissive
in well-solubilizing solvents, such as THF. In THF–water mix-
tures, the fluorescence intensity strongly increases once the
water fraction rises above 60%. In a 95/5 (v/v) water–THF
solution the quantum yields (FF, ESI†) are 0.40 and 0.39 for 2
and 3, which compare well with similar compounds.40
The two diols were transformed into the corresponding mono-
DMT protected alcohols 4 and 5, which were subsequently con-
verted into phosphoramidites 6 and 7. These building blocks were
used in the automated synthesis of oligomers ON1-8 (Table 1)
containing one or two DATPEs. All oligomers were purified by
Fig. 1 AIE properties of tetraphenylethylene (a) as a monomer and (b) as a
building block in single and double stranded DNA.
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3,
CH-3012 Bern, Switzerland. E-mail: robert.haener@ioc.unibe.ch; Tel: +41 31 631 4382
† Electronic supplementary information (ESI) available: Synthetic and analytical
details; additional spectroscopic data; X-ray structures. See DOI: 10.1039/c3cc42706d reversed-phase HPLC and characterized by ESI-MS (see ESI†).
c
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Table
1
Oligonucleotides (ONs) containing E- or Z-DATPE building blocks
(ME and MZ) and Tm values of DNA hybrids
a
b
Sequence
Tm (1C) DTm (1C)
DR R1 50-AGC TCG GTC ATC GAG AGT GCA
R2 30-TCG AGC CAG TAG CTC TCA CGT
45.0
—
D1 ON1 50-AGC TCG GTC AMEC GAG AGT GCA 47.0
2.0
2.0
4.5
5.0
ON2 30-TCG AGC CAG TMEG CTC TCA CGT
D2 ON3 50-AGC TCG GTC AMZC GAG AGT GCA 47.0
ON4 30-TCG AGC CAG TMZG CTC TCA CGT
D3 ON5 50-AGC TCG GTC MEMEC GAG AGT GCA 49.5
Fig. 2 Left: absorption spectra of single strands ON1 and D1; right: fluores-
cence spectra of ON1, ON2, calculated sum of spectra of ON1 and ON2, and D1
(20 1C; lex: 335 nm, ex. slit: 5 nm; em. slit: 5 nm; detector: 600 V; other conditions
as in Table 1; for more details see ESI†).
ON6 30-TCG AGC CAG MEMEG CTC TCA CGT
D4 ON7 50-AGC TCG GTC MZMZC GAG AGT GCA 50.0
ON8 30-TCG AGC CAG MZMZG CTC TCA CGT
a
Conditions: 1.0 mM oligonucleotide (each strand), 10 mM sodium
phosphate buffer (pH 7.4) and 100 mM NaCl, recorded in 2,2,2-
b
trifluorethanol–water (70/30 v/v). Compared to unmodified duplex DR.
A mixed solvent system composed of a buffered system with
2,2,2-trifluoroethanol (TFE, 70%, v/v) and water (30%, v/v, ESI†)48
was used to analyze the spectroscopic properties of the
DATPE-modified oligonucleotides.49 Measurements performed
in buffered water alone or smaller fractions of TFE provided
qualitatively similar results but the effects were less pronounced.
Generally, single strands showed considerably weaker fluores-
cence in TFE–water mixtures than in water alone. We ascribe this
to reduced molecular interactions between DATPE and the
nucleotides and, consequently, to increased internal rotation
in DATPEs42 in the presence of the less polar TFE.
Fig. 3 Change of fluorescence by addition of ON4 (individual steps = 0.1 mM;
conditions as in Fig 2) to ON3; arrow: increasing ON4 conc.
The melting temperature (Tm) values of the different hybrids
obtained by thermal denaturation are summarized in Table 1.
The DATPE units have a positive effect on the stability of the Formation of the duplex results in a gradual shift in the emission
duplex with DTm values ranging from +2 to +5 1C, for hybrids maximum from 476 nm (ON3) to 490 nm for D2. Fluorescence
containing two or four DATPE units, respectively. This stabili- intensity is significantly increased by annealing of the two strands
zation can be attributed to favorable stacking interactions (an increase by a factor of 5.6 compared to ON3, Fig. 3), indicating
between the propeller-twisted DATPE molecules in the duplex. a restriction of internal rotation by aggregation of the two
The UV-vis and fluorescence spectra recorded for hybrid D1 are DATPEs.41,42 Thus, the AIE character of the building blocks is
shown in Fig. 2 along with the spectra of the single strands. For controlled by the hybridization process. Emission maxima and
the single strand ON1, the absorption band of the E-DATPE quantum yields are summarized in Table 2.
molecule locates around 327 nm. The maximum of this band is
Fig. 4 illustrates the effect of molecular aggregation of
shifted to 335 nm during formation of the duplex D1. Aggregation DATPEs by DNA hybridization. The comparison between ON7
of the two DATPE molecules by hybridization of the two single and D2 shows that the emission by two DATPE molecules is
strands results in a considerable enhancement of the fluorescence considerably higher in the duplex (FF = 0.19) than in the single
intensity (Fig. 2, right). The fluorescence intensity of D1 at 490 nm strand (FF = 0.07). This is expected since the molecular aggre-
is about 9 times higher than that of ON1 and roughly 3 times gation should be positively influenced by the well-organized
higher than that of ON2. These differences were also reflected in duplex structure compared to the single stranded random coil.
the fluorescence quantum yields (FF) given in Table 2. FF rises to A further extension of the DATPE stack, as in D4, results in a
0.22 upon hybridization compared to 0.05 and 0.10 for ON1 and further significant increase in the fluorescence intensity (FF =
ON2, respectively. Similar effects are observed for D2 containing 0.32). The FF values of D3 and D4 are three times higher than
two Z-isomers. Fig. 3 displays the titration of ON3 with ON4, those of the corresponding single strands and close to the
in which FF gradually increases to 0.19 in the duplex D2. monomeric building blocks 2 and 3 in their aggregated states
Table 2 Emission maxima and quantum yields (FF) of E- and Z-DATPE molecules (2 and 3) and modified single strands and hybrids (quinine sulfate as standard, for
conditions see Table 1 and ESI)
E-DATPE (2)a
Z-DATPE (3)a
ON1
ON2
D1
ON3
ON4
D2
ON5
ON6
D3
ON7
ON8
D4
lmax (nm)
FF (%)
490
39.9
490
39.2
480
5.1
480
10.5
489
22.4
476
6.8
478
3.7
490
19.3
488
9.4
488
6.1
496
30.6
486
7.1
486
9.9
491
32.1
a
Values determined in a 95/5 H2O–THF (v/v) mixture.
c
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In conclusion, we have demonstrated that AIE can be con-
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This work was supported by the Swiss National Foundation
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 5835--5837 5837