A distance-controlled oligopeptide linker as a novel photo-induced energy transfer switch by secondary structural transition.

Article abstract of DOI:10.1039/b212851a

By using an oligopeptide chain as a functional linker and introducing coumarin 2 and coumarin 343 at the chain ends, an effective photo-induced energy transfer system was constructed and energy transfer from coumarin 2 to coumarin 343 was switched on and

Full text of DOI:10.1039/b212851a

A distance-controlled oligopeptide linker as a novel photo-induced
energy transfer switch by secondary structural transition
Akira Kishimoto, Toshiki Mutai and Koji Araki*
Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan.
E-mail: araki@iis.u-tokyo.ac.jp
Received (in Cambridge, UK) 3rd January 2003, Accepted 11th February 2003
First published as an Advance Article on the web 25th February 2003
By using an oligopeptide chain as a functional linker and
introducing coumarin 2 and coumarin 343 at the chain ends,
an effective photo-induced energy transfer system was
constructed and energy transfer from coumarin 2 to
coumarin 343 was switched on and off by a solvent-induced
helix–coil secondary transition of the oligopeptide chain.
Photo-induced energy transfer systems that are capable of
harvesting light and transferring or transforming absorbed
energy have attracted great interest in recent years because of
their applicability in such fields as light emitting diodes or
photonic devices. For this reason, a wide variety of organic,1
organometallic,2 supramolecular,3 polymeric,4 and dendritic5
systems have been developed. Energy transfer between fluores-
cent probes has also been used as a powerful tool for structural
analysis of DNAs and proteins.6 Efficient photo-induced
electron/energy transfer through a DNA duplex is the subject of
current interest.7 Biomacromolecules like DNAs and polypep-
tides are prone to have rigid and highly ordered structures due
to intramolecular hydrogen bonds and, therefore, are suitable
for use as linkers in energy transfer systems. Several groups
have previously reported energy transfer systems containing
chromophores along oligopeptides.8 However, few have re-
ported highly efficient, switchable, and distance controlled
systems by making the best use of the oligopeptide properties.
Here, we report a simple and facile method for constructing
efficient and distance-controlled energy transfer systems by
introducing chromophores at each end of the oligopeptide
linker. The distance between the chromophores can easily be
tuned by controlled ring-opening polymerization of amino acid
N-carboxyanhydride (NCA), and energy transfer between the
chromophores through the oligopeptide is found to be switched
on and off by a solvent-induced helix–coil transition of the
oligopeptide linker.
Scheme 1 Synthesis of donor–PBLG20–acceptor model. (a) g-BLG–NCA,
DMF, 96%. (b) 1, EDC, DMAP, CH2Cl2, 92%.
The absorption spectrum of 4 in CH2Cl2 shows two
absorption maxima at 345 and 420 nm, which correspond to
those of the model donor 1 and acceptor 2, respectively (Fig.
1(a)). This result indicates that 4 contains both the donor and the
acceptor units at the PBLG chain ends. Note that 3 has little or
almost negligible absorption around 345 nm, which confirms
that it is possible to excite the donor unit of 4 selectively at this
wavelength.
The secondary structure of 4 was examined by means of
circular dichroism (CD) and infrared spectroscopy. In the CD
spectrum of 4 in CH2Cl2, a negative Cotton effect was observed
around 230 nm (glutamate unit), indicating that 4 forms a right-
handed a-helix (Fig. 1(b)). This fact was further confirmed by
the IR spectrum of the cast film of 4 from CH2Cl2, which
Chromophores coumarin 2 and coumarin 343 were used as a
donor and an acceptor set, respectively, as recently employed by
Fréchet and coworkers.9 This set meets the requirement for
photo-induced energy transfer that the donor emission band has
a large overlap with the acceptor absorption band. In addition,
coumarin 2 displays a large Stokes’ shift, which lowers the
probability of self-quenching. Acceptor 2 bearing an aminopro-
pyl group was used as an initiator for ring-opening polymeriza-
showed amide I and II bands at 1651 and 1548 cm21
,
respectively, which is typical for a-helical conformation.11 On
the other hand, a cast film of 4 from DMSO, which is known as
a helix-breaking solvent,12 showed blue-shifted amide I and
red-shifted amide II bands at 1674 and 1510 cm21, respectively,
tion of g-benzyl-
L
-glutamate NCA, and the N-terminal of the
-glutamate) (PBLG) (3) was capped
resultant poly(g-benzyl-
L
by donor 1 which bears a benzoic acid moiety.† Thus, PBLG (4)
having a set of donor and acceptor chromophores at the N- and
C-terminals of the chain, respectively, was obtained (Scheme
1). The number-average degree of polymerization can be
controlled simply by adjusting the NCA/initiator molar ratio
when primary amines are used as the initiator.10 In this study,
this was set at 20, and the average molecular weight of the
peptide 3 was confirmed to be 4710 by GPC with a single and
sharp molecular weight distribution.‡ To ensure the absence of
low molecular weight components, the obtained PBLG (4) was
also purified by GPC. PBLG is known to have stable a-helical
rigid conformation in most common solvents,11 and the average
distance between the donor and the acceptor is about 3 nm when
PBLG (n = 20) takes the a-helix conformation.
Fig. 1 (a) Absorption spectra of 1 (– – –), 3 (…) and 4 (—) in CH2Cl2, (b)
CD spectrum of 4 in CH2Cl2.
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CHEM. COMMUN., 2003, 742–743
This journal is © The Royal Society of Chemistry 2003

Table 1 Energy transfer efficiency of 4
indicating that PBLG was in a random coil state caused by the
destruction of intramolecular hydrogen bonds.
Solvent
Structure of 4
Energy transfer
efficiency (%)
Fig. 2(a) illustrates the fluorescence spectra of 1 and 4 in both
CH2Cl2 and DMSO by selective excitation of the donor unit at
345 nm. In CH2Cl2, the donor 1 shows strong blue fluorescence
at 420 nm, but the acceptor 3 shows practically no fluorescence
around 460 nm. In the case of 4, however, fluorescence from the
donor unit at 420 nm was considerably quenched and emission
from the acceptor unit at 460 nm appeared, indicating that
efficient energy transfer took place from the donor unit to the
acceptor unit. The observed fluorescent properties of 4 were not
affected by the concentration over the range of 1025–1027 mol
dm23, excluding the possible contribution of intermolecular
energy transfer processes. The energy transfer efficiency was
calculated to be 91% from the degree of quenching of the donor
emission, which is high enough for the donor–acceptor distance
of 3 nm.13 Similarly, acceptor fluorescence and quenching of
the donor fluorescence were observed in DMF, a highly polar
solvent. However, only the donor fluorescence at 420 nm was
observed without any sign of quenching in DMSO, indicating
that energy transfer did not take place at all in DMSO. In the
CH2Cl2/DMSO mixture, increasing the DMSO content resulted
in a decrease of the energy transfer efficiency, and no energy
transfer occurred above 80 vol% of DMSO. Decreasing the
DMSO content to less than 10 vol% by dilution with CH2Cl2
induced fluorescence of the acceptor with concomitant quench-
ing of the donor fluorescence again, showing that the effect of
DMSO is reversible. The effect of DMSO on the energy transfer
is in sharp contrast to that of similarly polar DMF (Table 1). In
CH2Cl2, PBLG takes rigid a-helical conformation suitable for
energy transfer, allowing efficient energy transfer (91%) over 3
nm. It is known that DMF is not effective to induce secondary
transition of PBLG into flexible random coil but DMSO can
effectively induce this transition.14 Therefore, observed switch-
ing off of the energy transfer process by DMSO is attributed to
the helix to random coil transition of the PBLG linker (Fig.
2(b)). Since Förster length R0,15 which is the distance when
energy transfer occurs with 50 % efficiency, is estimated to be
2.53 nm in this sets, energy transfer efficiency is below 1%
when average inter-chromophore distance is over 6 nm.
Therefore, it is likely that the donor and acceptor units in 4 are
separated more than 6 nm in the random coil state.
(CH2Cl2/DMSO)
100/0
80/20
60/40
40/60
20/80
0/100
0/100 (DMF)
a-helix
91
84
55
18
0
mixture
random coil
0
82
a-helix
efficient photo-induced energy transfer system which can be
switched on and off by a helix–coil secondary transition of the
oligopeptide linker. These results offer a facile method for
developing energy transfer systems with various peptide
linkers.
We are grateful to Prof. Takashi Kato (Department of
Chemistry and Biotechnology, Faculty of Engineering, Univ. of
Tokyo) for helping with the GPC measurement.
Notes and references
† Compounds 1 and 2 were prepared as follows: reaction of coumarin 2 and
methyl 4-(bromomethyl)benzoate following hydrolysis yielded 1 (25%),
high-resolution MS (FAB): m/z calcd: 352.1471; found: 352.1556.
Coumarin 343 was reacted with N-(3-aminopropyl) carbamic acid tert-butyl
ester, which following deprotection of the Boc group afforded 2 (64%),
high-resolution MS (FAB): m/z calcd: 342.1739; found: 342.1818.
‡ GPC analysis was performed at 25 °C with a TSKgel GMHHR-N column
(300 3 7.8 mm), DMF as eluent, and narrow-polydispersity poly(styrene)
standards as a reference. The calculated molecular weight of the 20mer of
peptide 3 is 4727, which is in agreement with the observed value (4710).
1 M. Beggren, A. Dodabalapur, R. E. Slusher and Z. Bao, Nature, 1997,
389, 466.
2 A. Nakano, A. Osuka, I. Yamazaki, T. Yamazaki and Y. Nishimura,
Angew. Chem., Int. Ed., 1998, 37, 3023; T. Akasaka, H. Inoue, M.
Kuwabara, T. Mutai, J. Otsuki and K. Arai, Dalton Trans., 2003, 815.
3 M. D. Ward, Chem. Soc. Rev., 1997, 26, 365.
4 D. Ng and J. E. Guillet, Macromol., 1982, 15, 724; A. Adronov, D. R.
Robello and J. M. J. Fréchet, J. Polym. Sci. A, 2001, 39, 1366.
5 A. Adronov, S. L. Gilat, J. M. J Fréchet, K. Ohta, F. V. R. Neuwahl and
G. R. Fleming, J. Am. Chem. Soc., 2000, 122, 1175; A. Adronov, P. R.
L. Malenfant and J. M. J. Fréchet, Chem. Mater., 2000, 12, 1463.
6 A. I. H. Murchie, R. M. Clegg, E. von Kitzing, D. R. Duckett, S.
Diekmann and D. M. J. Lilley, Nature, 1989, 341, 763; M. P. Lillo, J. M.
Beecham, B. K. Szpikowska, M. A. Sherman and M. T. Mas,
Biochemistry, 1997, 36, 11261; D. M. J. Lilley and T. J. Wilson, Curr.
Opin. Chem. Biol., 2000, 4, 507.
In summary, we developed a simple method to construct a set
of chromophores separated by a distance-controlled oligopep-
tide linker, and demonstrated that the system serves as an
7 G. Taubes, Science, 1997, 275, 1420; C. J. Murphy, M. R. Arkin, Y.
Jenkins, N. D. Ghatilia, S. H. Bossmann, N. J. Turro and J. K. Barton,
Science, 1993, 262, 1025; K. Fukui and K. Tanaka, Angew. Chem., Int.
Ed., 1998, 37, 158.
8 L. Stryer and R. P. Haugland, Proc. Natl. Acad. Sci. USA, 1967, 58, 720;
G. Gabor, Biopolymer, 1968, 6, 809; M. Kuragaki and M. Sisido, J.
Phys. Chem., 1996, 100, 16019.
9 S. L. Gilat, A. Adronov and J. M. J. Fréchet, Angew. Chem., Int. Ed.,
1999, 38, 1422.
10 E. R. Blout and R. H. Karlson, J. Am. Chem. Soc., 1956, 78, 941; E. R.
Blout and A. Asadourian, J. Am. Chem. Soc., 1956, 78, 955; T. J.
Deming, Adv. Mater., 1997, 9, 299.
11 H. A. Klok, J. F. Langenwalter and S. Lecommandoux, Macromole-
cules, 2000, 33, 7819.
12 M. Niwa, M. Morikawa, T. Nabeta and N. Higashi, Macromolecules,
2002, 35, 2769.
13 Y. Ohya, K. Yabuki, M. Komatsu and T. Ouchi, Polym. Adv. Technol.,
2000, 11, 845.
14 H. Ushiki and I. Mita, Polym. J., 1984, 16, 751; H. Ushiki and I. Mita,
Polym. J., 1981, 13, 837.
15 W. G. McGimpsey, L. Chen, R. Carraway and W. N. Samaniego, J.
Phys. Chem. A, 1999, 103, 6082.
Fig. 2 (a) Fluorescence spectra of 1 (— -), 3 (– – –) and 4 (—) in CH2Cl2
and DMSO, (b) schematic illustration of the secondary structure of 4 in both
CH2Cl2 and DMSO.
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