Enantioselective cyclization of photochromic dithienylethenes bound to DNA

Article abstract of DOI:10.1002/anie.201209773

Guiding light: Enantioselectivity is obtained for the photocyclization of a photochromic dithienylethene when isomerization is carried out in the presence of DNA (see scheme). Copyright

Full text of DOI:10.1002/anie.201209773

Angewandte
Chemie
DOI: 10.1002/anie.201209773
Supramolecular Chemistry
Enantioselective Cyclization of Photochromic Dithienylethenes Bound
to DNA**
Tamara C. S. Pace, Vilhelm Mꢀller, Shiming Li, Per Lincoln, and Joakim Andrꢁasson*
Since the majority of the DNA-mediated processes occurring
in our cells can be inhibited by molecules binding to the
double helix, photoactivation of the DNA binding process
would provide a dramatically improved spatiotemporal con-
trol over the action of many biochemical processes. With this
idea as the inspiration, the DNA binding properties of
a number of photochromic systems have recently been
studied.[1] A spiropyran,[1a,b] a chromene,[1c] and a benzothia-
zoloquinolinium[1d] have all been shown to have only one form
of the photochromic molecule which interacts with DNA, and
DNA binding can therefore be modulated by irradiation.
Conversely, both the isomers of a dithienylethene (DTE)
derivative were shown to bind to DNA with similar binding
affinities.[1e] The typical derivative from the DTE photo-
chromic family can be reversibly switched between the open
and closed forms with high fatigue resistance and no thermal
isomerization.[2] Photocyclization to the closed form of DTE
produces two enantiomers, which are usually found as
a racemic mixture in solution. Enrichment of one enantiomer
for a symmetrically substituted DTE has previously only been
reported in single crystals, wherein two types of crystals
formed, each consisting of only one conformer of the open
DTE, which could be subsequently isomerized to the
corresponding closed form with high enantioselectivity.[3]
Diastereoselectivity, however, has been more readily
obtained for DTEs using chiral, and often bulky, substituents
in solution,[4] in gels,[5] or in single crystals.[6]
photoisomerization of cyclooctene when complexed with
DNA.[11] Herein we report the binding of a methylpyridinium-
and a methylquinolium-appended DTE to DNA and the
achievement of enantioselectivity for the closing reaction of
the methylquinolium DTE derivative in the presence of
DNA. This is a rare example of direct transfer of chirality
from DNA to a photoproduct and it is also the first reported
example of enantioselectivity for a DTE derivative in
solution.
DTE symmetrically substituted with methylpyridinium
chloride (1) or methylquinolium chloride (2) groups
(Scheme 1) were synthesized as described in the Supporting
Scheme 1. Dithienylethenes 1 and 2 in the open form and in the
closed form with enantiomers of S,S or R,R configuration.
In solution, inclusion of achiral photoreactants in chiral
supramolecular systems has been used to influence the
chirality of the products for photocyclizations, photoisomeri-
zations, photodimerizations, or photoadditions in systems
including cyclodextrins,[7] modified zeolites,[8] and proteins.[9]
Though stereoselective binding to DNA is commonly
observed, and DNA has been used to indirectly transfer
chirality through an asymmetric catalyst in ground-state
reactions,[10] there is only one reported example using DNA
to directly influence the chiral outcome of a photoreaction.
Wada et al. observed modest enantiomeric excesses for the
Information. The permanent positive charges which result
from methylation of the heterocyclic nitrogen atoms lead to
good water solubility of both 1 and 2. The open forms 1open
and 2open absorb exclusively in the UV region (Figure 1).
Isomerization to the closed form is triggered by UV light.
After isomerization with l = 366 nm UV light the photosta-
1
tionary states were determined from H NMR spectra to be
greater than 98% 1closed for 1 and approximately 92:8 2closed
/
2open for 2. The closed forms 1closed and 2closed have significant
absorption in the visible region (Figure 1), and reverse
isomerization using broadband visible light (l > 530 nm)
converts both 1 and 2 into 100% of the open form. No
thermal interconversion was observed after weeks at ambient
temperature.
Changes in the absorption spectra of both the open and
closed forms of 1 and 2 on addition of calf thymus DNA (ct-
DNA) indicated that both isomers of these DTE derivatives
bind to DNA. Similarly an amine-terminated DTE has been
previously shown to bind to DNA in both forms when
protonated.[1e] The changes in the absorption spectra for both
isomeric forms of 1 and 2 upon addition of ct-DNA are
similar. An initial decrease in the absorption was seen with no
change in the wavelength of the absorption maxima at high
[*] Dr. T. C. S. Pace, V. Mꢀller, Dr. S. Li, Prof. Dr. P. Lincoln,
Prof. Dr. J. Andrꢁasson
Department of Chemical and Biological Engineering
Physical Chemistry, Chalmers University of Technology
41296 Gçteborg (Sweden)
E-mail: a-son@chalmers.se
[**] This work was supported by the Swedish Research Council (grant
622-2010-280) and the European Research Council (ERC FP7/2007-
2013 grant No. 203952).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209773.
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Figure 3. CD spectra of 2closed (14.3 mm) in 5 mm NaCl in the presence
of increasing concentrations of ct-DNA (0–270 mm b.p.). Inset: binding
isotherm at l=675 nm for the binding of 2closed to ct-DNA fit to
Equation S1 in the Supporting Information.
such as groove binding or intercalation will give rise to
a strong CD signal. Unlike the UV titrations above, the CD
titrations showed no obvious indication of multiple binding
modes between the DTEs and ct-DNA. We therefore assign
the behavior observed at high DTE/DNA ratios in the
absorption titrations to electrostatic binding of the positively
charged 1 or 2 with the negative phosphate backbone of
DNA, as such nonspecific interactions are likely to contribute
little to the CD spectra. The second binding mode observed at
low DTE/DNA ratios in the absorption titrations is the
binding observed in the CD spectra, which we assign to
a specific binding interaction between the DTE and DNA.
Nonspecific binding is likely to have little effect on the chiral
outcome of the DTE-closing reaction and we expect any
observed enantioselectivity to arise from intercalative or
groove binding interactions, especially at the relatively high
DNA concentration used where the specific binding mode
predominates.
Figure 1. UV/Vis absorption spectra in 5 mm NaCl of a) 1open (red) and
1closed (black) and b) 2open (red) and 2closed (black).
DTE/DNA ratios, whereas upon further addition of DNA
a red shift in the absorption maximum and an increase in the
absorption was observed (see Figure S1 in the Supporting
Information). This behavior is indicative of more than one
type of binding interaction between each DTE and ct-DNA
(see assignment below).
To show circular dichroism (CD) molecules must be
inherently chiral or be perturbed by a chiral environment. In
solution the achiral open form of symmetric DTEs or the
racemic mixture (S,S + R,R) of the chiral closed form show no
CD signal, however when bound to DNA induced CD signals
are observed. CD titrations showed an increase in signal on
addition of ct-DNA to DTE for both isomers of 1 and 2
(Figures 2 and 3 for 2open and 2closed, respectively; see
Figures S2 and S3 in the Supporting Information for 1open
and 1closed, respectively). Only specific binding interactions
Apparent binding constants for the specific binding mode
were estimated from the CD titrations assuming a 1:1 binding
stoichiometry per DNA base pair (see the Supporting
Information for details). Binding constants of (5.0 ꢀ 0.1) ꢀ
105 mꢁ1 and (2.3 ꢀ 0.1) ꢀ 105 mꢁ1 were obtained for 1open and
1
closed, respectively, and binding constants of (2.8 ꢀ 0.5) ꢀ
105 mꢁ1 and (7.6 ꢀ 1.1) ꢀ 105 mꢁ1 were obtained for 2open and
2closed, respectively.
In flow-oriented linear dichroism (LD) experiments DNA
is oriented in a shear flow and any LD signal from the DTE is
due to specific binding to DNA. The sign of the LD signal
contains valuable information about the orientation of the
DTE relative to DNA. Molecules which interact with DNA
through intercalation between the base pairs exhibit only
negative bands in the LD spectra, whereas molecules which
bind in the DNA groove tend to have both negative and
positive LD bands.[12] Inspection of the LD spectra shows only
negative bands for 1open, while both negative and positive
bands were seen for 1closed (Figure 4a). This observation shows
that the interaction of 1open with ct-DNA is intercalative, while
1closed interacts with ct-DNA through groove binding. Only
negative bands were observed in the LD spectra of 2open and
2closed (Figure 4b), thus demonstrating that both isomers of 2
Figure 2. CD spectra of 2open (15.5 mm) in 5 mm NaCl in the presence
of increasing concentrations of ct-DNA (0–290 mm b.p.). Inset: binding
isotherm at l=405 nm for the binding of 2open to ct-DNA fit to
Equation S1 in the Supporting Information.
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Figure 5. a) CD spectra of 2 (50 mm) closed with l=366 nm light in
Figure 4. a) LD spectra for 1open (red) and 1closed (black) (18 mm) in
5 mm NaCl with 200 mm ct-DNA. b) LD spectra for 2open (red) (14 mm)
and 2closed (black) (13 mm) in 5 mm NaCl with 175 mm ct-DNA.
bulk solution then added to 480 mm ct-DNA (red) and closed with
l=366 nm light in the presence of 480 mm ct-DNA (black). b) CD
spectra of above solutions on addition of 50 mm SDS.
interact with ct-DNA through intercalation. The binding
mode of the closed forms is also supported by a comparison of
the intensity of the bands in the LD spectra with those in the
corresponding isotropic absorption spectra (see the Support-
ing Information).
The observed rate of the closing reaction is not signifi-
cantly changed in the presence of DNA. Thus, although
binding is a dynamic process, isomerization of 1 and 2 can
easily occur while bound to DNA without dissociation from
the double helix. However, a remarkable difference was
observed between the CD spectra obtained for a sample of 2
where racemic 2closed was formed in bulk solution then added
from DNA (black line in Figure 5b). This inherent CD signal
confirms that this solution has an excess of one of the
enantiomers of 2closed. Further irradiation of this extracted
solution with visible light to form 2open and subsequent UV
irradiation to reform 2closed led to the disappearance of the CD
signal, that is, the enantioselectivity achieved when 2 is bound
to DNA is lost following isomerization in bulk solution.
Unfortunately, to date, attempts to separate the enantiomers
and quantify the enantiomeric excess achieved have not been
successful.
For 1 no difference was observed for the CD spectra of
1closed bound to DNA when the closing reaction was carried
out in the absence or presence of DNA (see Figure S4 in the
Supporting Information), thereby indicating that no signifi-
cant enantioselectivity is achieved for the 1open!1closed reac-
tion in the presence of DNA. Previously, a similar experiment
with a DTE that underwent photoisomerization to the closed
form while included in a cyclodextrin cavity showed no
indication of enantiomer enrichment.[13]
to DNA, and for a sample where the closing reaction 2open
!
2closed was carried out in the presence of DNA (Figure 5a).
The sample that was isomerized in the absence of DNA
displays a positive CD band centered at l = 668 nm when
DNA is added. This CD signature is assigned to the induced
CD from the racemic solution of 2closed when bound to DNA.
In sharp contrast, the sample that was isomerized in the
presence of DNA displays only a very weak CD signal. This
observation implies that closing 2open in the presence of DNA
does not lead to the formation of racemic 2closed, but instead to
The conformers of the DTE open form undergo fast
exchange in solution, thus resulting in the formation of
a racemic mixture of the closed form on photocyclization. The
observed enantioselectivity of the photocyclization of 2 in the
presence of DNA indicates either that one conformer of 2open
is preferentially bound to DNA or that when bound one
conformer is more easily able to undergo isomerization to
the preferential formation of one of the enantiomers of 2closed
.
The inherent CD signal of the favored enantiomer must be of
opposite sign to that of the CD signal induced by DNA
binding, thus resulting in the apparent disappearance of the
long-wavelength band in the CD spectra for the latter
solution.
SDS micelles were added to the above solutions to extract
2 from DNA. After addition of SDS the solution containing
racemic 2closed had no CD activity (red line in Figure 5b), as
expected for complete extraction of 2 from DNA. In contrast,
the solution where 2closed was formed in the presence of DNA
showed a significant negative CD signal following extraction
2closed. The binding affinities for both forms of 1 and 2 with ct-
DNA are of a similar magnitude and the main difference in
the interaction of 1 and 2 with DNA is that the binding modes
of 2open and 2closed are the same (intercalation), while 1open and
1closed have different binding modes (intercalation and groove
binding, respectively). The fact that enantioselectivity is
achieved for the closing reaction of 2 but not for 1 could be
explained by this difference in binding modes. Since the
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[2] a) M. Irie, Chem. Rev. 2000, 100, 1685 – 1716; b) H. Tian, S. Yang,
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likely that this would lead to a disruption of the interaction of
1 with the chiral environment of DNAwhen moving along the
reaction coordinate from 1open to 1closed. This would not be the
case for 2 where isomerization may occur freely within the
chiral environment of the double helix. Consequently, chir-
ality transfer from DNA occurs in the case of 2, but not in the
case of 1.
In summary, we report two water-soluble, positively
charged DTEs, which bind to ct-DNA in both the open and
closed forms. Significant enantioselectivity was observed for
the DTE closing reaction for the DTE that interacts with
DNA through intercalation in both forms. The observed
enantiomeric enhancement in this system is a rare realization
of direct chirality transfer from DNA. Moreover, using this
approach to affect the chiral outcome of the DTE photo-
cyclization allowed us to obtain enantioselectivity for sym-
metric DTE derivatives in solution for the first time.
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Received: December 6, 2012
Revised: February 1, 2013
Published online: && &&, &&&&
Keywords: DNA · enantioselectivity · heterocycles ·
.
photochromism · supramolecular chemistry
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Angew. Chem. Int. Ed. 2013, 52, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Supramolecular Chemistry
Guiding light: Enantioselectivity is
obtained for the photocyclization of
a photochromic dithienylethene when
isomerization is carried out in the pres-
ence of DNA (see scheme).
T. C. S. Pace, V. Mꢁller, S. Li, P. Lincoln,
J. Andrꢂasson*
&&&& — &&&&
Enantioselective Cyclization of
Photochromic Dithienylethenes Bound to
DNA
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!