Angewandte
Chemie
DOI: 10.1002/anie.201001442
Photoresponsive Systems
Light-Responsive Molecular Recognition and Adhesion of Vesicles**
Siva Krishna Mohan Nalluri and Bart Jan Ravoo*
One of the main challenges in chemistry is the preparation of
well-defined self-assembled supramolecular architectures
with dynamic and adaptive properties that emulate biological
systems. Numerous reports have demonstrated that sophisti-
cated stimuli-responsive materials and surfaces can be
assembled by the careful combination of orthogonal inter-
actions.[1] In this context, the molecular recognition and
interaction of bilayer vesicles is a versatile model system for
the recognition, adhesion, and fusion of biological cell
membranes.[2,3] The ultimate aim of this research is a
supramolecular approach towards semisynthetic tissue engi-
neering, that is, the development of adaptive materials on the
basis of self-organizing compartments. In this Communica-
tion, the photoisomerization of a bifunctional noncovalent
linker molecule is used as a trigger to induce as well as reverse
the molecular recognition and adhesion of vesicles. To our
knowledge, this supramolecular photoresponsive system is
unprecedented.
The present light-responsive supramolecular system is
based on the host–guest interaction of azobenzenes with
vesicles made up of amphiphilic cyclodextrins (CDs).[3]
Azobenzenes constitute a well-known class of light-respon-
sive compounds that can be reversibly isomerized from trans
to cis by irradiation at 350 nm and from cis to trans by
irradiation at 455 nm. The photoisomerization of azobenzene
is the molecular basis for a range of light-sensitive supra-
Figure 1. A) Structures of hosts 1a and 1b and guests 2 and 3.
B) Light-responsive inclusion of 2 in a-CD and b-CD.
molecular materials,[4] including photoresponsive vesicles.[5,6]
Also the inclusion of azobenzene as a guest into a CD host is
light-responsive: the rodlike trans-isomer forms a stable
inclusion complex with a-cyclodextrin (a-CD) as well as
with b-cyclodextrin (b-CD), while the bent cis isomer does
not fit in either CD. The light-responsive inclusion of
azobenzenes in CDs has been exploited to make light-
responsive hydrogels,[7] micelles and vesicles,[8] ion channels,[9]
surfaces,[10] and drug-delivery vehicles.[11]
In this work we investigate the light-responsive interac-
tion of vesicles composed of amphiphilic CDs 1a and 1b with
bifunctional guest molecules 2 and 3 (Figure 1A). Amphi-
philic a-CD 1a and b-CD 1b were synthesized as described
previously.[3b,d] Unilamellar CD bilayer vesicles with a diam-
eter of ca. 100 nm were prepared in buffer at pH 7.4 by
extrusion.[3] Guest molecules 2 and 3 were synthesized as
reported in the Supporting Information. The analytical and
spectroscopic data for 2 and 3 are consistent with their
molecular structure. Guest molecule 2 is a homobifunctional
noncovalent linker that carries two identical supramolecular
binding sites: an azobenzene group that forms inclusion
complexes with a-CD and b-CD. The formation of the host–
guest complex of 2 should be light-responsive: only the trans-
azobenzene is a suitable guest for a-CD and b-CD, the cis-
azobenzene is not. Hence, 2 can bind two molecules of a-CD
or b-CD when it is in the trans form, but none in the cis form
(Figure 1B). Guest molecule 3 is a heterobifunctional non-
covalent linker that carries two different supramolecular
binding sites: an azobenzene group that forms inclusion
complexes with a-CD and b-CD, and a tert-butylbenzene
group that forms inclusion complexes with b-CD (but not with
a-CD, because it is too small to host tert-butylbenzene). The
formation of the host–guest complex of 3 should be partially
light-responsive: only the trans-azobenzene is a suitable guest
for the CDs, the cis-azobenzene is not. Hence, 3 can bind two
molecules of CD when it is in the trans form, but only one (b-
CD) when it is in the cis form.
[*] S. K. M. Nalluri, Prof. Dr. B. J. Ravoo
Organic Chemistry Institute and Graduate School of Chemistry
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Corrensstrasse 40, 48149 Mꢁnster (Germany)
Fax: (+49)251-839-36557
E-mail: b.j.ravoo@uni-muenster.de
[**] We are grateful for financial support from the Graduate School of
Chemistry in Mꢁnster (fellowship to S. K. M. Nalluri). We thank Dr.
Seda Kehr for help with confocal microscopy and Jens Voskuhl for
providing rhodamine-labeled cyclodextrin.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2010, 49, 5371 –5374
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5371