Prototype of a photoswitchable foldamer

Article abstract of DOI:10.1002/anie.200503849

(Figure Presented) Know where to fold 'em: A foldamer exhibiting a light-induced helix-coil transition (see scheme) can be constructed by introducing a photochromic azobenzene moiety (red) into the center of an amphiphilic phenylene ethynylene backbone (blue). This system gives insight into folding and unfolding mechanisms and promises applications in photoresponsive (bio)materials and "smart" delivery devices based on photoresponsive dynamic receptors.

Full text of DOI:10.1002/anie.200503849

Angewandte
Chemie
DOI: 10.1002/anie.200503849
Communications
Irradiation of a foldamer bearing a central photochromic azobenzene
moietycauses unfolding of the hollow helical secondarystructure—a
first step towards the design of smart deliveryvehicles. For more
details, see the communication byS. Hecht and co-workers on the
following pages. (Graphic generated byRagnar S. Stoll.)
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Foldamers
cant geometrical change of the core caused by photoisome-
rization, which results in denaturation of the helix because
each individual backbone segment is too short to adopt a
stable helical conformation. Thermal (or photochemical)
isomerization reverses the switching process and produces
the initial, helically folded state.
DOI: 10.1002/anie.200503849
Prototype of a Photoswitchable Foldamer**
Our system is based on amphiphilic oligo(meta-phenylene
ethynylene) foldamers, which were pioneered by Moore and
co-workers.[7] The introduction of a kinked and planar, meta-
connected trans-azobenzene, which closely mimics the struc-
ture of a dimeric repeat unit, into the center of a dodecamer
leads to target structure 1 (Scheme 1).[8] The lengths of the
two oligomer segments were deliberately chosen to allow
folding of the entire strand but not the individual parts. The
azobenzene core was optimized so that it could be excited
selectively, and novel enantiomerically pure (S)-a-methylte-
tra(ethyleneglycol) side chains were introduced to bias the
sense of the helical twist and thereby allow monitoring of the
conformational transition by means of circular dichroism
(CD) spectroscopy.[9]
The folding behavior of oligomer 1 was investigated by
using typical solvent-dependent denaturation experiments by
monitoring the conformational equilibrium with UV/Vis
absorption spectroscopy (Figure 2).[7] The sigmoidal shape
of the resultant titration curve is indicative of a cooperative
folding process, and detailed analysis reveals a helix stabili-
zation energy in pure acetonitrile of DG(CH3CN) =
ꢀ1.7 kcalmolꢀ1.[9] Therefore, replacement of the central
dimeric phenylene ethynylene unit in the native tetrade-
camer[7] with the azobenzene core leads to only slight
destabilization of the helix; this destabilization is attributed
to weaker p,p-stacking interactions caused by the presence of
electron-donating methoxy substituents.[10]
Irradiation at 365 nm of the helically folded 1trans to excite
selectively the central azobenzene chromophore led to rapid
conversion into the corresponding 1cis as monitored by UV/
Vis absorption spectroscopy (Figure 3a). A decrease in the
p–p* (350–400 nm) absorption, a small increase in the n–p*
(400–450 nm) absorption, and an increase in the p–p*
(< 265 nm) absorption, as well as two well-defined isosbestic
points at 265 and 418 nm are observed. These absorbance
changes are indicative of the trans!cis photoisomerization
process.[4] Whereas the photochemical 1trans!1cis conversion
takes place within seconds, the thermal 1cis!1trans reversion
occurs over the time frame of several hours at room temper-
ature.
To monitor the conformational changes during both the
forward and reverse isomerization processes, CD spectra of
solutions of 1 were recorded in solvent mixtures that promote
folding (Figure 3b).[11] The CD spectrum of 1trans in aqueous
acetonitrile shows a strong positive Cotton effect, which is
indicative of the helical conformation, while no CD signal has
been observed in denaturing solvents, such as chloroform.
The CD signal arises from exciton coupling within the helical
backbone and is the result of chirality transfer from the chiral
side chains of the core to the helix. Interestingly, the observed
spectral shape, which indicates a P-helical twist sense, is
exactly opposite to that of oligomers carrying side chains with
the chiral methyl group in b position.[9] Although irradiation
Anzar Khan, Christian Kaiser, and Stefan Hecht*
The formation of secondary structures, for example in
proteins, can be mimicked by foldamers,[1] which can adopt
stable, usually helical conformations in solution. Research in
this area has advanced tremendously in recent years and is of
great interest in the bio- and nanosciences. Foldamers are
ideally suited for the design of responsive materials because
of the dynamic nature of the reversible folding reaction.
However, switching between the foldamer conformations
typically involves changes in temperature or solvent compo-
sition.[1] Alternative stimuli such as pH[2] and metal coordi-
nation[3] involve the addition of acid or base and metal
cations, respectively, and are therefore associated with the
formation of by-products. Clearly, the use of light as a
noninvasive stimulus that can be applied with precise control
over timing, location, and intensity of exposure would be
highly desirable. Photochromic molecules[4] have frequently
been utilized to switch a variety of molecular and materials
properties,[5] including, for example, photomodulation of the
helix–coil equilibrium in linear peptides.[6]
Herein, we report the first example of a photoswitchable
foldamer. Our concept is based on the incorporation of a
photoisomerizable core into a foldamer strand (Figure 1).
Two backbone segments are joined by the photoresponsive
core such that the entire strand is long enough to be able to
fold into a helical conformation. Irradiation causes a signifi-
Figure 1. Photoswitchable foldamers: Irradiation of the photoisomeriz-
able kinked unit (red) in the middle of a helically folded backbone
(blue) induces a reversible helix–coil transition.
[*] A. Khan, C. Kaiser, Dr. S. Hecht
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
Fax: (+49)208-306-2979
E-mail: hecht@mpi-muelheim.mpg.de
[**] Synthesis of the chiral side chains was carried out by Christian
Kaiser at Freie Universität Berlin. The authors thank Dr. Eckhard Bill
(MPI für Bioanorganische Chemie, Mülheim/Ruhr) for the extensive
use of the CD spectrometer. Generous support by the Alexander von
Humboldt Foundation (Sofja Kovalevskaja Program sponsored by
the Federal Ministry of Education and Research and the Program for
Investment in the Future (ZIP) of the German government), the
German Research Foundation (DFG: SFB 448, GK 788, HE 3675/2-
1), and the Max Planck Society (MPG) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
Scheme 1. Synthesis of photoswitchable foldamer 1. DIB=dibromoisocyanuric acid, TMSA=trimethylsilylacetylene, TBAF=tetrabutylammonium
fluoride, TDIB=triethyleneglycol 3,5-diiodobenzoate.
Figure 2. UV/Vis absorption spectra of 1 in acetonitrile (4.7”10ꢀ6 m)
with increasing chloroform content (100 vol% CH3CN!100 vol%
CHCl3) at 258C. The inset shows the absorbance ratio A303nm/A288nm as
a function of solvent composition.
leads to rapid decrease of the CD signal, which indicates
depletion of the helical conformation through unfolding,
thermal reversion leads to complete recovery of the initial CD
signal intensity, thus indicating refolding of the backbone.[12]
The presence of a distinct isodichroic point at 295 nm suggests
a clean conversion between the two conformations. Surpris-
ingly, the observed conformational transition is not evident
from the band shape of the UV/Vis absorption spectra. The
composition of the mixture in the photostationary state (PSS)
can be deduced directly from the ratio of the CD signals (1cis/
(1cis+1trans) ꢁ 40%).[8] Analysis of the data also provides the
rate constant for the thermal cis!trans isomerization kcis!trans
at 258C of approximately 3.8 ” 10ꢀ5 sꢀ1, which corresponds to
a half-life t1/2 of around 5 h and an activation energy DG° of
approximately 23.5 kcalmolꢀ1; these values are typical for
azobenzene-based macromolecules in solution.[13]
Figure 3. a) UV/Vis absorption spectra obtained during photochemical
trans!cis isomerization of 1 caused by irradiation at 365 nm in
acetonitrile (5.9”10ꢀ6 m) at 258C (t=0, 1, 3, 7, 15, 31, 63 s). The inset
shows a magnification of the decreasing p–p* absorption band of the
azobenzene unit. b) CD spectra obtained during thermal cis!trans
isomerization of 1 in 60 vol% H2O in CH3CN (5.7”10ꢀ6 m) at 228C
(starting at PSS: t=0, 1.5, 3, 4.5, 7.5, 10, 48, 53, 72 h).
We have designed a photochromic foldamer and demon-
strated switching of the helix–coil folding transition. Light-
triggered systems such as 1 can provide fundamental insight
into the folding and unfolding mechanisms by enabling time-
resolved measurements[14] and promise applications in photo-
responsive (bio)materials[5,15] and smart delivery devices
based on photoresponsive dynamic receptors.[16] Our work
in progress is focused along these directions, in particular, on
the extension of the described system to photoresponsive
polymeric folding backbones with increased cis content in the
PSS.
Received: October 31, 2005
Published online: January 20, 2006
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Keywords: azo compounds · foldamers · helical structures ·
.
photochromism
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hydrophobic driving force and therefore ensure more accurate
kinetic data.
[12] An alternative explanation to account for the CD spectral
changes would be a transition to a more loosely packed helical
conformation. Although this conformation is rather unlikely
because of the inherent strain caused by the incorporation of the
nonmatching and nonplanar azobenzene core, it cannot be ruled
out at present time. Fluorescence emission spectroscopy, which
is a complementary experimental technique for monitoring the
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system, and both HPLC and gel permeation chromatography
have not been able to completely resolve the 1trans/1cis mixture.
[13] For example, see: L.-X. Liao, F. Stellacci, D. V. McGrath,J. Am.
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