A Multifunctional Host–Guest Supramolecular Complex
The described experiments demonstrate the validity of
our logical design and justify further research in this direc-
tion with the intriguing prospect to achieve a novel class of
photoactivatable nanoscaled systems based on similar com-
ponents that exhibit an improved photoaction. In this re-
spect, we are currently working on the synthesis of rhoda-
mine-labeled CD-based oligomers. The presence of multiple
CD compartments should lead, in principle, to nanostruc-
tures able to accommodate a larger amount of NO photodis-
pensers, with a consequential increase in both the light-har-
vesting properties and the reservoir of NO available. The re-
sults of this investigation will be reported in due course.
Experimental Section
Figure 4. Cell mortality of HeLa cells in the dark or upon photoinduction
in the absence of photoactive components (a), in the presence of 2
(10 mm) (b), and in the presence of the complex of 1 with 2 (c).
Materials
All cyclodextrin intermediates were synthesized by CycloLab (Budapest,
Hungary). All other reagents were of the highest commercial grade avail-
able and used without further purification. All solvents used (from Carlo
Erba) were analytical grade, and dried by conventional methods and dis-
tilled immediately prior to use. Slide-A-Lyzer Dialysis Casettes G2, with
a cut-off MW of 2000 (Thermo Scientific), were used for dialyses. Thin
layer chromatography (TLC) was performed on aluminum sheets pre-
coated with silica gel 60 F254 (Merck, Art. No.: 1.05554). Plates were de-
veloped in a saturated chamber of 1,4-dioxane/ammonium hydroxide
(25%, 10:7 (v/v)), visualized by UV light at 254 nm and 366 nm, and
charring with a solution of EtOH (96%)/H2SO4 (96%, 9:1) followed by
heating at 105–1108C. Phosphate buffer (10 mm, pH 7.4) was prepared
with biological grade reagents and all solutions were prepared with nano-
pure water (18 MW).
ascribed to the generation of reactive oxygen species (ROS)
by the rhodamine dye, which has been reported to act as
a photosensitizer although with low efficiency.[34]
Conclusions
We have presented here a novel multifunctional, photores-
ponsive supramolecular “Lego system” based on the effec-
tive formation of an inclusion complex between a tailor-
made NO photodonor and an ad hoc devised CD conjugate
carrying a rhodamine fluorophore. Both the guest and the
host behave as independent photoactive centers in the
supramolecular complex, as proven by the excellent preser-
vation of their photochemical and photophysical properties.
As a result, this nanoassembly exhibits the convergence of
photoregulated release of NO and fluorescence imaging in
one single nanostructure. We would like to highlight that, in
contrast to non-photoresponsive compounds, the preserva-
tion of the photobehavior of independent components after
their assembly is not a “trivial result”. In most cases, the re-
sponse to light of single or multiple photoactive units locat-
ed in a confined space can be, in fact, considerably influ-
enced, in both nature and efficiency, by the occurrence of
competitive photoprocesses (i.e., photoinduced energy and/
or electron transfer, hydrogen abstraction, nonradiative de-
activation, etc.),[18,35] which preclude the final goal.
The host–guest complex internalizes in cancer cells, prob-
ably assisted by its cationic nature at physiological pH, can
be easily mapped therein in view of its satisfactory fluores-
cence emission, and is able to induce about 50% cell photo-
mortality. In this regard, it appears that the extent of the
cellular death is not exclusively due to the light-triggered
generation of NO but also to the involvement of ROS pho-
togenerated by the rhodamine unit. Studies addressed to
clarify this point deserve certainly attention.
Synthesis
Compound 1 and the model compound 3 were synthesized according to
previously reported procedures.[21b,23a] 6-Tetradeoxy-6-triamino-6-rhoda-
minylthioureido-tri-O-(2-hydroxypropyl)cyclomaltoheptaose (2) was syn-
thesized in five steps (see Scheme 2A). The key intermediate 6-tetrazido-
6-tetradeoxycyclomaltoheptaose was synthesized by following the meth-
ods reported in the literature.[36a,b] The hydroxypropylation and the reduc-
tion steps were performed as reported by Malanga et al.[37]
The rhodaminylation was performed as follows: Rhodamine B isothio-
cyanate (RBITC, 0.37 g, 0.69 mmol) was dissolved in DMF (5 mL) and
then added dropwise to a stirred solution of 6-tetramino-6-tetradeoxy-tri-
O-(2-hydroxypropyl)cyclomaltoheptaose (0.91 g, 0.69 mmol) and N,N-dii-
sopropylethylamine (DIPEA, 99%, 0.6 mL, 3.4 mmol) in DMF (30 mL).
The solution was stirred at 90–1008C for 24 h, cooled to 808C, and then
the solvent was removed under reduced pressure (T=808C). The crude
product was dissolved in water (20 mL) and extracted with dichlorome-
thane (3100 mL). Subsequently, water was removed under reduced
pressure (T=608C) and the resulting material was dialyzed against de-
ionized water. Freeze-drying yielded the target compound 2 as a violet
powder (0.66 g, 52%); m.p.: 205–2128C (dec.); IR (KBr): n˜ =3349, 2973,
2928, 1670, 1591, 1466, 1340, 1153, 1083, 1039, 946, 754, 607, 580 cmꢀ1
;
1H NMR (D2O): d=1.13–1.14 (m, 9H, hydroxypropyl-methyl-H), 1.29–
1.36 (m, 12H, RBITC-methyl-H), 3.33–4.13 (m, 59H, H2, H3, H4, H5,
H6, hydroxypropyl-methylene-H, hydroxypropyl-methyne-H, RBITC-
methylene-H), 5.09–5.25 (d, 7H, H1) 6.76–7.39 (bs, 6H, aromatic-H),
8.00–8.04 (d, 1H, aromatic-H), 8.11–8–17 (d, 1H, aromatic-H), 8.45 ppm
(m, 1H, aromatic-H); ESI-MS (MeOH, HP=2-hydroxypropyl fragment):
[M0+Na]+ =1826.77; [M1+Na]+ =1768.73; M1 =M0ꢀHP; [M2+Na]+
1710.73; M2 =M1ꢀHP; [M3+Na]+ =1884.82; M3 =M0+HP; [M4+Na]+
1942.91; M4 =M3+HP.
=
=
The DS (degree of substitution) for HP and rhodamine was determined
from the H NMR data.
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Chem. Asian J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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