Toward singlet oxygen delivery at a measured rate: A self-reporting photosensitizer

Article abstract of DOI:10.1021/ol501084n

A Bodipy-based energy transfer cassette with a singlet oxygen reactive linker between the donor and acceptor modules has an interesting emergent property, if the acceptor module is also a photosensitizer. Singlet oxygen produced by the photosensitizer rea

Full text of DOI:10.1021/ol501084n

Letter
pubs.acs.org/OrgLett
Toward Singlet Oxygen Delivery at a Measured Rate: A Self-
Reporting Photosensitizer
Sundus Erbas-Cakmak† and Engin U. Akkaya*,†,‡
‡
†UNAM-National Nanotechnology Research Center and Department of Chemistry, Bilkent University, Ankara TR-06800, Turkey
S
* Supporting Information
ABSTRACT: A Bodipy-based energy transfer cassette with a
singlet oxygen reactive linker between the donor and acceptor
modules has an interesting emergent property, if the acceptor
module is also a photosensitizer. Singlet oxygen produced by
the photosensitizer reacts rapidly with the molecule itself to
liberate the energy donor, resulting in an enhanced
fluorescence emission. The result is a self-reporting photo-
sensitizer providing an assessment of the singlet oxygen
production rate under the operational conditions.
ensing analytes in solution is highly important both for the
elucidation of the processes they are involved in and/or for
S
determining therapeutic/diagnostic approaches in diseases
where corresponding analytes are markers for various biological
anomalies.1,2 Apart from using analyte responsive molecules as
reporters, activatable therapeutic systems have been developed
as well, where the disease related parameter is used to initiate a
chemical/physical transformation in the therapeutic agent to
alter its activity. Activatable photosensitizers are one of such
therapeutics with a modulated activatability property.3−8
Exchange of ideas between the fields of therapeutic design
and molecular sensors is expected to yield great improvements
in personalized treatment and diagnostics. However, one of the
main problems not addressed so far in this overlap area, is the
inadequacy of measuring the effect of any therapeutics directly.
Rather, the biological effect is analyzed on a cellular level. For
the photosensitizers, the effect of light, oxygen concentration,
cell penetration, and dark-toxicity determines the overall effect
of the photosensitizer on the cell;9,10 hence, one has to be
cautious in speaking about the efficiency of singlet oxygen
production of a photosensitizer in a cell-culture experiment
since the observed outcome is a cumulative result, namely cell
death.
In this work, we address the issue of direct monitoring of
photosensitizer activity with the use of a singlet oxygen (1O2)
labile linker between the fluorophore and a photosensitizer,
where the former is an electronic energy transfer (EET) donor
and the latter being an EET acceptor (Figure 1, BOD 2). The
rationale behind the design is such that EET from the
fluorophore (FL, blue module in Figure 1) to the photo-
sensitizer (PS, green module on BOD 2) quenches the
emission of the FL to a great extent. PS and FL are attached to
one another with (Z)- 1,2-bis(alkylthio)ethene bridge due to
Figure 1. Structure of the model compound (BOD 1), energy-transfer
cassette with self-reporting property (BOD 2), and the individual
energy-transfer modules.
cleave the linker, liberating the fluorophore as a consequence.
Fluorescence of the free FL is reinstated which is the reporter
1
the fact that O2 susceptibility of this electron-rich olefinic
linker is extensively exploited by us and others.11−16
1
According to the design, O2 generated by the PS upon
Received: April 15, 2014
Published: May 21, 2014
irradiation with red light is to act on the molecule itself and
© 2014 American Chemical Society
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Letter
1
for the O2 generation activity of the PS. A model compound
Table 1. Photophysical Characterization of BOD 1 and BOD
2 and Modules A1, A2, and D
a
with the same chemical architecture was also synthesized
(Figure 1, BOD 1), which lack halogens on the core BODIPY,
and as a consequence does not generate singlet oxygen
efficiently due to prohibited intersystem crossing (ISC).
Absorption and fluorescence spectra of BOD 1 and BOD 2
and free donor and acceptor modules in acetonitrile are given
in Figure 2. Characteristic absorption peaks of 2,6-ethyl-1,3,5,7-
b
λabs (nm)
ε (M−1 cm−1
)
λF (nm)
ϕF (λexc(nm))
BOD 1
BOD 2
521
647
521
655
525
654
520
61000
65000
59000
97000
60000
95000
59000
535
670
533
681
670
680
0.26
0.58
0.34
0.22
0.53
0.39
0.90
A1
A2
D
534
a
b
All values are measured in acetonitrile. For the emission band
peaking at 521 nm Rhodamine 6G (water, ϕF = 0.95, n = 1.333) and
for the emission band peaking at 670 and 680 nm Cresyl Violet
(MeOH, ϕF = 0.66, n = 1.329) were used as reference compounds for
quantum yield calculations. The refractive index of acetonitrile is taken
as 1.346.
Figure 2. Normalized electronic absorption (a) and normalized
emission (b) spectra of BOD 1, BOD 2, D, A1, and A2 in acetonitrile.
Each spectrum is normalized with respect to maximum absorption or
emission wavelength of each compound.
tetramethylBODIPYs at around 521 nm and distyryl-BODIPYs
at 655 nm are measured. Compounds have two distinct
emission peaks around 535 nm for donor emission, and 670
(BOD 1) or 681 nm (BOD 2) for acceptor emissions. Detailed
photophysical data for the energy-transfer cassettes and the
corresponding modules are given in Table 1. As a consequence
of EET, excitation with a 488 nm light, which is primarily
absorbed by the donor, results in a decreased fluorescence from
the donor module compared to the emission of the free donor
BODIPY (Figure 3) with a decrease in quantum yield from
0.90 to 0.26 for BOD 1 and 0.34 for BOD 2 with a calculated
energy transfer of 71% and 61% for the compounds,
respectively. In BOD 1, emission at 670 nm is much more
intense due to the lack of a competing intersystem crossing
pathway in the acceptor module as it is in the case of
halogenated acceptor module of BOD 2. Energy transfer is also
calculated to be more pronounced in this model compound
(71% in BOD 1 compared to 61% in BOD 2). This may be due
to slightly larger overlap integral between the donor emission
peak and the acceptor absorption peak in this molecule. The
energy transfer processes in both compounds are further
Figure 3. Fluorescence spectra of BOD 1 (a), BOD 2 (b), and equally
absorbing donor module D in acetonitrile, excited at 488 nm.
demonstrated by the excitation spectra. When the emission of
the acceptor moiety is followed, excitation of the donor seems
to contribute to this fluorescence whereas in the acceptor
module itself, there is no corresponding excitability in the
region of donor absorbance (Figures S1 and S2, Supporting
Information).
Showing that the energy transfer is taking place between the
modules in each compound (BOD 1 and BOD 2), the singlet
oxygen dependent recovery of the donor emission is analyzed
1
as an indication of PS activity. Efficient O2 generation by the
halogenated BODIPYs upon irradiation is already well
studied.17−20 Here, each energy transfer systems are irradiated
with a LED lamp (660 nm, 2.5 mW/cm2 fluence rate). Primary
red light absorbing species in the casettes are acceptor modules
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which are photosensitizer, or halogen-free fluorophore in BOD
2 and BOD 1, respectively. Aliquots were taken in 10 min
intervals from the solutions of BOD 1 and BOD 2, and
emission spectra were acquired by excitation at 520 nm. The
solutions were kept under irradiation with red light for the
duration of the experiment. Singlet oxygen generated upon
excitation of the photosensitizer is expected to react readily
with the electron rich (Z)-1,2-bis(alkylthio)ethene linker and
subsequently release donor fluorophore. This reaction can be
monitored with the increased fluorescence of this module. As
shown in Figures 4 and 5, control compound BOD 1 which
lacks heavy atoms to facilitate intersystem crossing, display no
significant change in emission spectra neither in dark, nor under
irradiation at 660 nm.
Just like BOD 1, BOD 2 shows no remarkable change in
emission intensity when kept in dark (Figure 5). However,
exposure to red light results in an almost 2-fold increase in
donor emission, which is attributed to singlet oxygen
dependent cleavage of the linker and subsequent disintegration
of the EET system. Quantum yield of the donor moiety in
BOD 2 restores back to free donor value within 80 min,
indicating complete cleavage of the 1O2 susceptible bond within
the given time scale. There is no significant change in acceptor
emission in BOD 2 during the reaction. Hence, it is a useful
reference point and comparison of the donor emission at 532
nm (which increase throughout the reaction) to this almost
unchanged acceptor emission at 680 nm enables ratiometric
activity sensing of the singlet oxygen (Figure 5).
Absorption spectra of the compounds throughout the singlet
oxygen generation and reaction were also investigated, and no
significant change is observed which can be interpreted as lack
of any chemical change in the core BODIPYs during the
reporting process, which would most likely yield distinct
spectral signatures (Figures S3 and S4, Supporting Informa-
tion).
In conclusion, we report a novel approach to monitor the
activity of a photodynamic sensitizer, using singlet oxygen
dependent release of a fluorescent reporter module of an
energy transfer cassette, upon which fluorescence is restored.
With a ratiometric comparison to the essentially unchanged
acceptor emission, singlet oxygen generation ability of the
photosensitizer can be assessed. The You group investigated16 a
similarly designed system for its drug release potential.
However, to the best of our knowledge, explicit reaction-
based modulation of EET21 toward singlet oxygen-dependent
ratiometric self-activity sensing of a photosensitizer has not
been reported before. In any case, the rate of increase in the
fluorescence of the reporter module is a direct measure of the
rate of singlet oxygen generation under the conditions of the
study. This system can be further developed for direct analysis
of the activity of PDT agents in vivo. This will provide an
unprecedented capability for in vivo assessment of photo-
sensitizers for PDT in terms of singlet oxygen generation. This
in turn, may lead to more advanced photosensitizer based
therapeutics in the very near future.
Figure 4. Change in fluorescence spectra of BOD 1 (a) and BOD 2 in
dark and after irradiation with 660 nm light in acetonitrile (excited at
520 nm).
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, characterizations, and further spec-
troscopic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: eua@fen.bilkent.edu.tr.
Notes
Figure 5. Normalized change in the fluorescence intensity of donor
moiety in BOD 1 and BOD 2 (intensity at 532 nm) compared to
acceptor emission (intensity at 670 nm for BOD 1 and 680 nm for
BOD 2) before (0−20 min) and after irradiation (20−80 min) with
red light (excited at 520 nm).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.E.-C. thanks TUBITAK for financial support.
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