Control over unfolding pathways by localizing photoisomerization events within heterosequence oligoazobenzene foldamers

Article abstract of DOI:10.1002/anie.201307378

From the inside out or from the outside in? Two photoswitchable foldamers that incorporate azobenzene moieties as the energy-acceptor units have been designed. The pathway of helix unfolding can be controlled by localizing these photoinduced triggers (sho

Full text of DOI:10.1002/anie.201307378

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DOI: 10.1002/anie.201307378
Smart Foldamers
Control over Unfolding Pathways by Localizing Photoisomerization
Events within Heterosequence Oligoazobenzene Foldamers**
Zhilin Yu and Stefan Hecht*
In the field of foldamers,[1] the dynamics of the helix–coil
transition are of particular importance for molecular recog-
nition processes, such as guest binding,[2] as well as for the
formation of intertwined aggregates (double helices).[3] In
both processes, the unfolding/refolding pathway that pro-
ceeds either from the termini to the helix core (“outside in”)
or vice versa (“inside out”) plays a crucial role. During our
investigations of photoswitchable foldamers,[4,5] we found that
the location of the light-responsive azobenzene units within
the backbone governs their ability to photoisomerize and
hence to locally induce helix unfolding. This position-
dependent photoisomerization behavior[4d,e] should in princi-
ple allow for the desired control over the unfolding/refolding
pathway if the photoexcitation can be exclusively localized at
the specific photoisomerization site, either at the helix
terminus or the core. Inspired by the ability of natural light-
harvesting systems to funnel excitation energy to a specific
site,[6] we sought to incorporate additional azobenzene units,
which act as energy traps, into our established oligoazoben-
zene foldamer scaffold. Upon global (broad band) excitation,
such gradient architectures that incorporate the traps either at
the termini or at the core should allow control over the
unfolding pathway to occur outside in or inside out, respec-
tively. Herein, we report the design and synthesis of a pair of
heterosequence foldamers that are composed of two different
azobenzene photochromes (donor and acceptor) as well as
spectator meta-phenylene units, and the investigation of their
light-induced unfolding processes that are governed by the
localization of the photoisomerization event.
hence the global energy acceptor sites, or traps.[6] The cross-
conjugated meta linkages ensure decoupling of the individual
phenylene/tolane, azobenzene, and DMAB chromophore
repeat units. The DMAB unit does not only localize the
excitation, but also exhibits a photoisomerization efficiency
that is superior to that of the parent azobenzene photo-
chrome.[7]
To test the influence of localizing the photoisomerization
event on the unfolding pathway, the DMAB unit was
incorporated either at the termini or at the core of the
parent foldamer backbone.[4e] Therefore, the oligomers 143-2
and 144-1 were synthesized and compared to the parent
foldamer 145, which solely contains identical azobenzene
repeat units (Figure 1, left). In all three foldamers, individual
azobenzene photochromes are connected through meta-
phenylene units that enforce an arrangement of two azoben-
zene units per helix turn, which results in cofacial p–p
stacking of the azobenzenes along the helix axis (Figure 1,
right).[4e] The introduction of polar oligo(ethylene glycol) side
chains provides the solvophobic driving force for folding in
a polar medium; in addition, their chiral centers induce
helicity, which aids the conformational analysis by circular
dichroism (CD) spectroscopy.[4,8] The syntheses of the oligo-
mers were carried out by iterative routes that mostly relied on
statistical palladium-catalyzed Sonogashira–Hagihara cou-
pling reactions[9] and employed suitable brominated azoben-
zene and DMAB units as well as 3,5-diiodobenzoate[10]
fragments (see the Supporting Information, Scheme S1 and
S2).[11] The synthesized target compounds 144-1 and 143-2 were
fully characterized with regard to their chemical structure and
In this study, 4,4’-dimethoxy-substituted azobenzene
(DMAB) units were used because they display a bathochro-
mic shift of their p!p* absorption band by 30 nm relative to
that of the parent azobenzene repeat units,[4a,b] which renders
them the most red-shifted chromophore in the foldamer and
1
purity by various techniques, including H NMR spectrosco-
py, MALDI-TOF mass spectrometry, and gel permeation
chromatography (GPC).[11]
The conformations of both oligomers as their all-E
isomers were determined in a solvent titration experiment
in which unfolding was induced by increasing the chloroform
content of an acetonitrile solution while monitoring UV/Vis
absorption[12] and recording CD[8] spectra. In the correspond-
ing UV/Vis spectra (Figure S5),[11] the appearance of a new
absorption band at approximately 310 nm upon addition of
chloroform is indicative of the local cisoid!transoid con-
formational transition. Concomitantly, in the CD spectra
(Figure S6),[11] the initially observed Cotton effect decreases
until it completely vanishes in neat chloroform, which
independently confirms helix unfolding.[4d] Both of these
results imply that the oligomer only adopts a stable helical
conformation in the polar medium acetonitrile. From these
denaturation experiments, the helix stabilization energies of
the oligomers in neat acetonitrile [DG (CH3CN)] were
obtained by plotting the ratio between the absorption
[*] Dr. Z. Yu,[+] Prof. Dr. S. Hecht
Department of Chemistry, Humboldt-Universitꢀt zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
E-mail: sh@chemie.hu-berlin.de
[+] Current address: Institute for BioNanotechnology in Medicine
Northwestern University
Chicago, IL (USA)
[**] We thank Dr. Steffen Weidner (BAM) for MALDI-TOF MS charac-
terization. Generous support by the German Research Foundation
(DFG; SFB 765) and the European Research Council (ERC-2012-
STG_308117; “Light4Function”) is gratefully acknowledged. BASF
AG, Bayer Industry Services, and Sasol Germany are thanked for
generous donations of chemicals.
Supporting information for this article, including experimental and
characterization data, is available on the WWW under http://dx.doi.
org/10.1002/anie.201307378.
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Figure 1. Investigated heterosequence foldamers 143-2 and 144-1 as well as reference foldamer[4e] 145: Chemical structures (left) and schematic
representations of their helically folded structure (right). The foldamers are shown as their all-E isomers with R=(CH2CH2O)3CH3) and their
individual repeat units have been marked in specific colors: DMAB (red), azobenzene (orange), and meta-phenylene (green). In the used
nomenclature, the number 14 denotes the overall number of aromatic benzene units, while the subscripts denote the number of regular
azobenzene units followed by the number of incorporated DMAB moieties. TMS=trimethylsilyl.
intensities of the transoid and cisoid conformers as a function
of solvent composition (Figure S7).[11–13] Foldamer 143-2, which
bears DMAB units at its termini, exhibits a slightly lower
helix stabilization energy than foldamer 144-1, which incorpo-
rates the DMAB unit in the core (Table 1). This difference in
stability is not related to the type of p–p stacking interactions
as observed previously[4e] as they remain the same in both
foldamer helices; instead, we assume a slightly weaker
contact in the case of the more electron-rich terminal
DMAB units.[14] Interestingly, the twist sense bias of these
two foldamers is opposite to each other; foldamer 143-2 shows
a negative Cotton effect, whereas foldamer 144-1 displays
a positive Cotton effect in the CD spectra, similarly to 145 and
in fact all other previously investigated tetradecamers (144
and 147).[4e] The opposite handedness observed for foldamer
143-2 most likely originates from the placement of most of the
chiral side chains at the termini (attached to the terminal
DMAB units) as the specific location of chiral side chains in
the backbone likely dictates the twist sense of a helix.[15] These
different interactions between the side chains might also
explain the observed slightly different helix stabilities.
After confirming the helical backbone conformation of
143-2 and 144-1 in their all-E configuration in acetonitrile, these
solutions were subjected to UV irradiation, which induced
E!Z photoisomerization of the embedded azobenzene units
to trigger helix unfolding. Upon exposure to 358 nm light,
a decreasing p!p* absorption band at 287 nm and a slightly
increasing n!p* absorption band at approximately 440 nm
were observed for both foldamers (Figure 2, top; Fig-
ure S9),[11] indicating the occurrence of the photoinduced
E!Z isomerization in azobenzene photochromes.[7b]
Whereas UV/Vis spectra provide information about the
local photochemistry, the CD spectra allow for evaluation of
the overall chain conformation; indeed, the significant
decrease of the Cotton effect (Figure 2, bottom) indicated
an efficient helix–coil transition, or unfolding, as a conse-
quence of E!Z photoisomerization.
Table 1: Folding and photoswitching parameters of foldamers 143-2 and
144-1 and reference foldamer 145 in acetonitrile at 258C.
Parameter
143-2
145
144-1
DG (CH3CN)[a]
ꢀ2.68ꢁ0.18
ꢀ3.38ꢁ0.20
ꢀ2.95ꢁ00.20
[kcalmolꢀ1
]
k (unfold)[b]
[102 sꢀ1
3.84ꢁ0.09
1.21ꢁ0.02
3.62ꢁ00.09
]
!
Zterm
Zcore
Zav
62.5ꢁ0.5%
22.3ꢁ0.3%[d]
38.4ꢁ0.6%
45.5ꢁ0.6%!
39.6ꢁ0.4%
37.5ꢁ0.5%[d]
33.6ꢁ0.6%
Z content
!
31.3ꢁ0.3%!
at PSS [%][c]
To gain insight into the localization of the individual
photoswitching event(s), 1H NMR spectroscopy was used and
greatly facilitated by different substitution patterns of the
azobenzene repeat units, which allowed their spectral sepa-
ration and unambiguous identification (Figure S12 and
S13).[11] At the photostationary state (PSS), NMR analysis
37.0ꢁ0.6%
[a] Derived from UV/Vis spectral changes. [b] Derived from CD spectral
changes. [c] Determined by 1H NMR spectroscopy (for terminal, internal,
and core azobenzenes as shown in Figure S12,13; Zav =mathematical
average). [d] The internal azobenzene could not be distinguished from
the core azobenzene; hence, their average value is given.
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conformation on localizing the E!Z photoisomerization
events, the Z content of azobenzene units was also analyzed at
the PSS in chloroform; in this solvent, the backbone adopts
a random-coil conformation (Table S3).[11] Whereas in 145 an
almost identical Z content was observed at different positions,
a higher Z content of DMAB units remained in foldamers 143-
2 and 144-1. However, the Z content at the positions occupied
by the DMAB moieties was revealed to be significantly
greater in acetonitrile than in chloroform (Figure S15).[11] This
important finding of improved localization of the E!Z
photoisomerization events demonstrates the superior perfor-
mance of the helix structure in the energy-transfer process. In
light of these results, and considering the decrease of the
Z content of the parent azobenzene units relative to foldamer
145 (Table 1), we speculate that energy transfer between the
two different azobenzene units (and possibly the tolane
units)[17] is largely responsible for the observed preferential
DMAB isomerization. The better inherent switching ability
of the DMAB unit[7] certainly plays an additional role.
Further investigations of the energy transfer pathway require
the use of time-resolved spectroscopic studies, which suffer
from fluorescence quenching for helical conformations and
strong solvent background signals for random-coil struc-
tures.[7,18]
Although a detailed microscopic insight into the unfolding
dynamics is currently not available, monitoring the changes in
the UV/Vis and CD spectra during irradiation provides
important kinetic information. Analysis of the UV/Vis
spectral evolution is not conclusive (Figure S10 and
Table S1)[11] as the spectra are the result of superimposed
photoisomerization events in all azobenzene subunits of every
folded and unfolded species present in solution, and hence
neither unfolding rates nor rates of the initial E!Z photo-
isomerization can be deduced. However, CD spectroscopy
proved to be particularly useful in this case as it is only
sensitive to conformational changes and therefore can be
used as a direct measure of the helix–coil transition.[4d]
Analysis of the decay of the CD signal over time (Fig-
ure S11)[11] provided the unfolding rates, which showed that
foldamer 143-2 isomerizes more quickly than 144-1 (Table 1).
Furthermore, both foldamers unfolded about three times
faster than the parent homoazobenzene foldamer 145. The
CD-derived rates describe the overall unfolding process,
which includes both initial E!Z photoisomerization and
subsequent helix!coil transition, which are strongly coupled
events.
Figure 2. UV/Vis absorption (top) and CD (bottom) spectra indicate
photochemical E!Z isomerization of oligomers 143-2 and
144-1 during irradiation (lirr =358 nm) in CH3CN at 258C.
provided an average distribution of E!Z photoisomerization
throughout the backbone, which is expressed as the Z content
(Table 1). In comparison with the parent foldamer 145,[4e] the
occurrence of E!Z photoisomerization is specifically
enhanced in the positions occupied by DMAB units
(Table 1). The relative increase of the Z content at the
terminal and central DMAB positions amounts to 37% and
20% in 143-2 and 144-1, respectively, relative to foldamer 145
(Table 1; see also Figure S15).[11]
These findings indicate that E!Z photoisomerization
preferably occurs at the DMAB sites, which implies that the
localization of individual isomerization events at the desired
positions within these heterosequence foldamers has been
improved. In conjunction with the improvement of the
Z content of the DMAB units, the Z content of the parent
azobenzene units is decreasing.
The enhanced localization of E!Z photoisomerization
events can in principle be attributed to an intrinsically higher
photoreactivity of the DMAB units and/or an efficient
mechanism to funnel the excitation energy to the DMAB
sites. The first option results from a combination of a higher
extinction coefficient and a higher quantum yield of the
DMAB photochrome upon 358 nm excitation. However,
exposure to 320 nm irradiation, where the parent azobenzene
absorbs more strongly, showed the same tendency of local-
izing E!Z photoisomerization at the DMAB sites
(Table S2),[11] which indicates that the latter option might be
the operating mechanism. In this alternative or additional
scenario, an efficient energy-transfer cascade that involves
various absorbing chromophores,[16] such as the tolane and
parent azobenzene donors and the DMAB acceptor(s), would
cause localization of excitation at the DMAB sites, which is
followed by their isomerization. Such an energy-transfer
mechanism would largely benefit from a chromophore
arrangement that allows for their strong coupling, for
example, in a compact helix conformation with a small pitch
leading to p–p stacking. To investigate the effect of the chain
Shifting the initial E!Z photoisomerization events to
either terminal or core DMAB sites results in two different
unfolding pathways, namely outside in or inside out, which are
associated with different kinetic barriers and rates. In
foldamers that contain identical azobenzene units, denatura-
tion of the helical structure predominantly arises from
terminal isomerization processes as described previously;[4c]
in foldamers 143-2 and 144-1, however, the location of the
DMAB units has a significant impact on their unfolding
processes (Scheme 1). Most notably, in 143-2, terminal isomer-
ization processes occur much more frequently than central
isomerization, and therefore, unfolding should mainly pro-
ceed through an outside–in pathway. In its foldamer counter-
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Keywords: azobenzenes ·
.
circular dichroism ·
energy transfer · foldamers ·
photochromism
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Scheme 1. Two competing unfolding pathways that are triggered by the localization of photoisomerization
events at the DMAB sites.
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localized in desired positions by the site-specific incorpora-
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Received: August 22, 2013
Revised: October 12, 2013
Published online: November 19, 2013
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[17] Unpublished results in collaboration with the group of J.
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