Photomodulated chiral induction in helical azobenzene oligomers

Article abstract of DOI:10.1021/ol8004722

Appending L-alanine to the terminal positions of a helical azobenzene oligomer produced a P helical bias, which increased with oligomer length. Irradiation gave rise to E → Z isomerization of the terminal azo linkages, which displaced the stereogenic center of L-Ala from the helix backbone and suppressed chiral induction. Theoretical simulations of the CD spectrum of the P helical conformation are in qualitative agreement with the experimental spectra.

Full text of DOI:10.1021/ol8004722

ORGANIC
LETTERS
2008
Vol. 10, No. 9
1671-1674
Photomodulated Chiral Induction in
Helical Azobenzene Oligomers
Eric D. King, Peng Tao, Toby T. Sanan, Christopher M. Hadad, and
Jon R. Parquette*
Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210
parquett@chemistry.ohio-state.edu
Received November 18, 2007
ABSTRACT
Appending L-alanine to the terminal positions of a helical azobenzene oligomer produced a P helical bias, which increased with oligomer
length. Irradiation gave rise to E f Z isomerization of the terminal azo linkages, which displaced the stereogenic center of L-Ala from the helix
backbone and suppressed chiral induction. Theoretical simulations of the CD spectrum of the P helical conformation are in qualitative agreement
with the experimental spectra.
The creation of supramolecular structures that recreate the
folding and dynamics of biological molecules has emerged
as a promising strategy to merge structure with function in
abiotic systems.1 In particular, the prevalence of molecular
helices in nature has inspired many diverse strategies to
induce helicity in synthetic oligomers.2 Helical systems offer
the potential to amplify weak chiral influences via coupled
equilibria that propagate local perturbations along the helical
chain resulting in the predominance of a single helical sense.3
This type of chiral amplification requires a dynamic helical
conformation with infrequent, highly mobile helical reversal
points that interconvert the helical antipodes. The ability of
such dynamic systems to amplify small chiral perturbations
appears to be a general phenomenon among fluxional helical
oligomers,4 polymers,5 and supramolecular assemblies.6 In
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10.1021/ol8004722 CCC: $40.75
Published on Web 04/09/2008
2008 American Chemical Society

this Letter, we report that helical oligomers composed of
alternating pyridine-2,6-dicarboxamides and m-(phenyla-
zo)azobenzenes adopt a preferred helical sense upon attach-
ment of L-alanine at the terminal positions. Exposure to 350
nm light induces a predominant E f Z photoisomerization
of the terminal azo linkages, which decouples the helix and
terminal interactions in a manner that suppresses chiral
amplification.
We recently described a series of oligomers composed of
repeating azobenzene chromophores exhibiting conforma-
tional properties ideally suited to permit the induction of a
helical handedness.7 These oligomers adopt remarkably
stable helical conformations lacking an observable unfolded
state in both polar and nonpolar media. However, NMR line
shape studies indicated a highly dynamic equilibria inter-
converting the M and P helical antipodes with energetic
barriers ranging from 11.1 to 13.8 kcal/mol for two- and
four-turn helices, respectively.
Figure 2. Lowest energy conformation of Cbz-L-ala terminated two-
turn helix 2a determined by a Monte Carlo conformational search
using the AMBER* force field (details in Supporting Information).
Replica-exchange molecular dynamics (REMD)8 simula-
tions suggest that helical exchange proceeds via intermediates
composed of right- and left-handed segments separated by
helical reversal points, which circumvents the need to access
any fully unfolded states (Figure 1).9 The nature of this
we reasoned that capping the terminal amines with Cbz-L-
alanine would place a stereogenic center at approximately
this position. The orientation of the methyl and hydrogen
groups relative to the helix backbone on this carbon would
then differentiate the stabilities of the M and P helical forms
(Figure 2). This supposition was supported by Monte Carlo
conformational searching of two-turn helix 2a, which
indicated an energetic preference for a P helical form, thereby
positioning the methyl group away from the helical back-
bone.11
The UV-vis spectra of oligomers 1a-3b shown in Figure
3 feature a strong absorption at ca. 385 nm corresponding
Figure 1. Helical reversal conformation interconverting M and P
helical forms calculated by REMD simulations. M and P conforma-
tions taken from previously reported X-ray data.7 Details are in
the supporting info.
helical exchange resembles the stepwise unfolding process
attributed to related oligoamide foldamers4d,10 and helical
polymers.3 Further details on these calculations are presented
in the Supporting Information.
Previously, we observed that the methylene protons, Ha
and Hb, in Cbz-protected oligomers (A) exhibited chemical
shifts differing by ca. 0.5 ppm at -37 °C (Figure 2).7 This
difference emerged from the disparity in anisotropic shifting
caused by the positioning of one methylene proton toward
and the other away from the helix backbone. On that basis,
Figure 3. Helical oligomers 1a-3b.
to a πfπ* transition of the azobenzene chromophore.
Additionally, a weak nfπ* transition broadened over the
450-490 nm range, typical of E-azobenzene chromophores,
is also present in the spectra.12 Bis-azo oligomer 1a ((Cbz-
L-Ala)2(NdN)2) is too short to adopt a helical conformation
and, therefore, did not exhibit any observable circular
dichroism (CD) transitions above 200 nm (Figure 4). In
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Parquette, J. R. Prog. Polym. Sci. 2005, 30, 423.
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128, 1162.
(8) Cheng, X.; Cui, G.; Viktor Hornak, V.; Simmerling, C. J. Phys.
Chem. B. 2005, 109, 8220–8230.
(9) Details of the REMD study are provided in the Supporting
Information.
(10) Ohkita, M.; Lehn, J.-M.; Baum, G.; Fenske, D. Chem. Eur. J. 1999,
(11) Implemented in Macromodel 8.5 using the AMBER force field.
5, 3471–3481
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(12) Griffiths, J. Chem. Soc. ReV. 1972, 1, 481–493.
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Org. Lett., Vol. 10, No. 9, 2008

Figure 5. (a). Comparison of the experimental CD spectrum of
oligomer 3a (black) and predicted CD spectra of 3a (RI-BP86/
TZVP in orange, B3LYP/SV(P) in blue, and BHLYP/SV(P) in red),
calculated for geometries optimized at the respective levels of
theory. A comparison with oligomer 2a is in the Supporting
Information (Figure S1).
Figure 4. Molar CD spectra of oligomers 1a-3b and UV-vis
spectra of 3a in MeCN at -10 °C.
contrast, two-turn oligomer 2a ((Cbz-L-Ala)2(NdN)4) dis-
played negative CD bands at 320 and 477 nm, and an intense
positive band at 407 nm. The presence of the CD transitions
in this region of the spectrum indicate the induction of a
preferred sense of helicity in the azobenzene oligomer. The
four-turn oligomer 3a ((Cbz-L-Ala)2(NdN)8) also exhibited
these CD bands with intensities greater than 2-fold that of
2a, as might be expected based on the difference in the
number of azobenzene chromophores. This nonlinear en-
hancement of the CD intensity for 3a, compared with 2a,
indicates a more efficient amplification of the terminal chiral
influences.
The UV-vis and CD spectra of the optimized geometry
of oligomer 2a were calculated using time-dependent density-
functional theory (TD-DFT) at the RI-BP86/TZVP, B3LYP/
SV(P), and BH&HLYP/SV(P)13 levels of theory (Figure 5).14
The resulting spectra were uniformly blue-shifted by 0.5,
0.1, and -0.55 eV, respectively, and a 0.3 eV Gaussian line-
broadening was applied to each excitation.
a fully unfolded state. Hence, the calculated CD spectra of
a static M or P helix would qualitatively reproduce the
features of an experimental spectrum. The agreement be-
tween the simulated spectra and experiment was fairly good,
with the B3LYP spectrum most closely matching experiment.
The qualitative properties of the spectrum and, in particular,
the trough at ∼475 nm and the peak at ∼400 nm were
reproduced; however, the trough at 340 nm shows signifi-
cantly more negative polarization in the calculated spectra.
The BH&HLYP and RI-BP86 spectra show larger deviations
than the B3LYP spectrum, suggesting that the latter has the
proper amount of inclusion of Fock-exchange.15 It should
be noted that the geometries optimized using hybrid DFT
most closely resemble the crystal structure,7 with RI-BP86
overestimating the spacing between the helical turns. Inter-
estingly, the molecular mechanics geometry was also similar
to experiment (see Supporting Information).
The calculated spectra most closely resemble the experi-
mental spectrum of the 4-turn oligomer 3a, particularly in
the region around 320-370 nm, compared with the 2-turn
oligomer 2a. The B3LYP spectrum, in particular, matches
exceptionally well. As the CD spectra were calculated using
static, helical structures, it is reasonable that they might
resemble the experimental spectrum of the very strongly
biased oligomer 3a more closely than that of 2a. Although
oligomers 2a and 3a differ in length, the chromophores are
identical and the excited-state properties are likely quite
similar. With the multiple levels of theory employed,
including the generally more accurate hybrid DFT methods,
and a clear similarity with the spectrum of oligomer 3a, the
calculated CD spectra strongly support the conclusion that
the P helical sense is present in the experimental oligomers.
The prediction of CD spectra has only recently been
applied to the analysis of conformations of large molecules,
and our protocol mirrors that utilized successfully else-
where.15 Oligomers 2a and 3a are ideal candidates for CD
calculations because, as suggested by the REMD studies,
the M and P helical forms interconvert without going through
(13) (a) Becke, A. D. Phys. ReV. A. 1988, 38, 3098. Perdew, J. P. Phys.
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Parr, R. G. Phys. ReV. B 1988, 37, 785.
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conformational search was subsequently optimized using the respective DFT
methods prior to UV-vis and CD spectrum calculations. These calculations
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Org. Lett., Vol. 10, No. 9, 2008
1673

Further, it suggests that the difference in experimental CD
spectra is the result of changes in the helical bias.
Figure 2. However, the exact nature of this interaction
remains to be determined.
Previously, we observed that EfZ photoisomerization of
the azo bonds within the helix was progressively suppressed
with increasing oligomer length.7 Conversely, the azo bonds
at the termini of the helices experienced ca. 40% isomer-
ization to the Z form in both the two-turn and four-turn
oligomers. Accordingly, irradiation of the oligomers induces
EfZ isomerization mainly at the terminal positions without
disruption of the helical structure (Figure 8). Irradiating the
A large decrease in the amplitude of the CD spectrum was
observed for four-turn oligomer 3b ((H2N-L-Ala)2(NdN)8),
which lacked the terminal Cbz groups (Figure 6). To explore
Figure 6
-10 °C).
. Effect of terminal group: CD spectra of 2a-2e (MeCN,
Figure 8. Predominant isomerization of terminal azo linkages
decreases chiral induction.
the impact of the terminal acyl groups on the extent of helical
bias, we varied the nature of the acyl groups in analogs of
the two-turn oligomer 2a. The intensity of the CD band at
407 nm was highest for 2a, lowest for amino-terminated 2b
and generally increased as the acyl group became more
electron-withdrawing (Figure 6). The source of this effect
is likely due to a stabilizing hydrogen bond between the NH-
CO and another donor group that is enhanced by electron-
withdrawing groups or a favorable π-stacking effect between
electron-rich and electron-poor aromatic rings as seen in
two- and four-turn oligomers, 2a and 3a, with 350 nm light
afforded a photostationary state within 10 min exhibiting
significantly diminished CD spectra (Figure 7). The decrease
in helical bias induced by exposure to light emerges from
isomerization of the terminal azo linkages which displaces
the controlling stereogenic center of the Cbz-L-ala group from
the helix backbone (Figure 8).
In conclusion, appending Cbz-L-alanine to the terminal
position of azobenzene oligomers biases the twist-sense of
their helical structures. Comparison of the experimental and
computationally predicted CD spectra indicate the presence
of a P helical bias. Molecular modeling studies suggest that
that the energetic benefit of projecting the methyl group of
L-alanine away from the helix backbone produces the P
helical bias. Exposure to 350 nm light suppresses chiral
induction within the helical structure by inducing isomer-
ization of the terminal azo linkages, which orients the chiral
terminal groups away from the helix.
Acknowledgment. This work was supported by the
National Science Foundation (CHE-0239871 and CHE-
0526864) and the Ohio Supercomputer Center. Acknowledg-
ment is also made to the donors of the Petroleum Research
Fund, administered by the American Chemical Society for
partial support of this work.
Supporting Information Available: Experimental pro-
cedures and compound characterization for 1-3, details of
UV and CD calculations for 2a.
Figure 7. CD spectra in CH3CN at -10 °C before (solid lines)
and after (dashed lines) irradiation with 350 nm light.
OL8004722
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Org. Lett., Vol. 10, No. 9, 2008