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additional stronger, presumably interstrand H bonds pointing to a Notes and references
b-hairpin-like structure.
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CD spectra in the 195–250 nm range were determined for
trans-AzoChig1–3 peptides at concentrations of c = 76–78 mM in
MeOH at 5 1C (vide supra). Solvent-dependent CD spectra of
AzoChig1 and AzoChig2 where recorded at concentrations of
c = 82–112 mM in MeOH–H2O mixtures at 5 1C. For the cis-azo
isomers, CD spectra were recorded after irradiation at 350 nm
for 180 s, i.e., at the cis-photostationary state (Fig. 3D). All three
cis-azo peptides feature distinct maxima at 231 nm – assigned
to stacking of Trp9 to Tyr2 – as well as minima at 212 nm and
200 nm, and pronounced positive signals below 195 nm.
Hence, they strongly resemble the spectra of native chignolin
suggesting a folded, hairpin-like structure. The enhanced
hydrophobic interactions between Tyr2 and 5FTrp9 in AzoChig2
become visible by the increase of the CD signal in the 231 nm
range. Temperature-dependent CD spectra of cis-AzoChig1 and cis-
AzoChig2 in the 5–60 1C range (data not shown, see ESI†) reveal a
thermal unfolding process to be working at higher temperatures,
similar to native chignolin. The CD spectra of trans-AzoChig1–3
display small maxima at 231 nm and minima at 213 nm but deviate
in the 200 nm range. The deep minimum found for AzoChig2 at
200 nm can be tentatively interpreted in terms of a better ordering
of the peptide part due to the increased hydrophobic interaction of
5FTrp9 with Tyr2.
Moreover, solvent-dependent CD measurements were per-
formed using mixtures of MeOH–H2O (Fig. 3E and F, see ESI†),
with ratios from 10–100% MeOH. The MeOH–H2O spectra of
the cis-/trans-AzoChig1 and cis-/trans-AzoChig2 peptidomi-
metics show increasing values for the maximum around
230 nm and deeper minima at 200 nm with increasing water
amounts. Presumably, the hydrophobic turn region of the cis-
peptides becomes more stabilized by hydrophobic interactions
between Tyr2:Trp9 in aqueous surroundings, which leads to the
observed increase in molar ellipticity at 230 nm. Furthermore,
interstrand interactions are stabilized by water-mediated
hydrogen bonding as reflected by the decreased ellipticity
around 200 nm. Finally and at higher water ratios, the observed
CD signals of AzoChig1 and AzoChig2 approximate nicely the
reported molar ellipticity characteristics of the parent peptide
chignolin.14
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´
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28 T. E. Schrader, W. J. Schreier, T. Cordes, F. O. Koller, G. Babitzki,
We have presented a novel class of photoswitchable b-hairpin
model peptides derived from the designer mini protein chignolin
by substitution of two central amino acids from the turn
sequence by the known azobenzene chromophore AMPP. The
resulting AzoChig1–3 peptidomimetics were assembled by SPPS
and carefully characterized at both photoisomeric states using
UV/VIS, IR, CD, and NMR spectroscopy. In the trans-state of
AMPP, the peptides mostly exhibit a disordered structure, while
trans - cis photoisomerization of the azobenzene chromophore
induces folded b-hairpin-like structures.
¨
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and W. Zinth, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 15729–15734.
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30 In consequence of the limited solubility of peptidomimetics AzoChig1
and AzoChig2 in aqueous solutions and their intrinsic aggregation
tendency at very high concentrations (45 mM), spectroscopic confor-
mational analyses were performed in MeOH, similar to related model
´
´
studies (see e.g. ref. 12 and 28 and following references: (a) M. Erdelyi,
A. Karlen and A. Gogoll, Chem. – Eur. J., 2006, 12, 403–412; (b) M. Erdelyi,
V. Langer, A. Karlen and A. Gogoll, New J. Chem., 2002, 26, 834–843;
(c) Y. J. Chung, B. R. Huck, L. A. Christianson, H. E. Stanger,
´
´
¨
S. Krauthauser, D. R. Powell and S. H. Gellman, J. Am. Chem. Soc.,
Support from LMU Munich, the Deutsche Forschungsge-
meinschaft (SFB 749, project A5) and the Excellence Cluster
CIPSM – Center of Integrated Protein Science Munich – is
gratefully acknowledged.
2000, 122, 3995–4004). Finally, it must be emphasized that so far, the
AzoChig peptides represent first model compounds and will be further
optimized for folding studies under more physiological conditions.
These optimizations require greater divergement from the original
chignolin design and are subject to extended future studies.
4004 | Chem. Commun., 2015, 51, 4001--4004
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