Efficient photocurrent generation by self-assembled monolayers composed of 310-helical peptides carrying linearly spaced naphthyl groups at the side chains

Full text of DOI:10.1021/ja0476011

Published on Web 09/17/2004
Efficient Photocurrent Generation by Self-Assembled Monolayers Composed
of 310-Helical Peptides Carrying Linearly Spaced Naphthyl Groups at the Side
Chains
Kazuyuki Yanagisawa, Tomoyuki Morita, and Shunsaku Kimura*
Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity Kyoto-Daigaku-Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan
Received April 26, 2004; E-mail: shun@scl.kyoto-u.ac.jp
In natural photosynthetic systems, a regular arrangement of
chromophores is a key for highly efficient light harvesting and
charge separation.1 Among secondary structures found in photo-
synthetic protein assemblies, the helical structure is most frequently
observed and should play an important role in the regular arrange-
ment of chromophores.2 Further, the distance decay factor, â, for
Figure 1. Chemical structures of SSN3B, SSA3B, SSNA2B, and SSA2NB.
the electron transfer along a model helical peptide is reported to
be 0.66 (Å-1),3 which is better than alkane chains in the electron-
between naphthyl groups are 6.1 Å for the first and fourth residue
mediating property.4,5 A helical peptide is thus used here as a
pair and 6.4 Å for the fourth and seventh pair, which are longer
scaffold of an artificial regular chromophoric array to attain an
efficient long-range electron transfer. With respect to the distance
between the chromophores, there should be a compromised distance
of ca. 6 Å, in which electron transfer should be promoted by the
“through space” mechanism3 and will not be hindered by energy-
dissipating trap sites due to the formation of dimer or excimer.6
Taken together, the 310-helical structure, which makes one turn with
just three residues, is chosen. When amino acids carrying chro-
mophores at the side chains are introduced into a 310-helical peptide
backbone at every third residue, the chromophores are regularly
spaced in a linear manner along the helical axis. This linear and
proximal arrangement of chromophores is expected to facilitate
electron transfer between the adjacent chromophores. In the present
study, self-assembled monolayers (SAMs) are prepared on gold
from 310-helical peptides carrying naphthyl groups at the side chains,
and photocurrent generation by the monolayers is discussed in terms
of electron hopping among the naphthyl groups. Photocurrent
generation by molecular assemblies has attracted much attention
in relation to an artificial photosynthetic system.1,7,8
than the critical distance for excimer formation (ca. 3.5 Å). The
space arrangement of the naphthyl groups is thus suitable for
electron hopping without an energy-dissipating trap site. Further,
circular dichroism spectroscopy of SSN3B showed an exciton
coupling at around 223 nm in ethanol, which is characteristic of a
regular arrangement of naphthyl groups in proximity in the 310-
helical conformation.
To examine the electronic interaction between the naphthyl
groups of SSN3B in the ground and excited states, absorption and
fluorescence spectra were measured in ethanol. The absorption
spectra of SSN3B showed the same pattern as that of SSA2NB,
indicating that there is no strong electronic interaction in the ground
state such as dimer formation. In the fluorescence spectrum of
SSN3B, excimer emission was hardly observed, showing that there
is no strong interaction in the excited state either. These observations
agreed well with the prediction from geometry optimization. To
summarize these results, the naphthyl groups of SSN3B are linearly
spaced in proximity along the helical axis, but there is no trap site
against electron hopping.
A sequential nonapeptide carrying three naphthyl groups at the
side chains of every third residue and a disulfide group at the N
terminal, the reference peptide without a naphthyl group, and the
peptides carrying one naphthyl group at N or C terminal (SSN3B,
SSA3B, SSNA2B, SSA2NB, respectively; Figure 1) were synthe-
sized. Since all the peptides are designed to contain six R-amino-
isobutyric acid (Aib) residues, these peptides are expected to take
a 310-helical conformation because of their high content of Aib
residues.9 To examine the conformation of SSN3B in solution, its
derivative BN3B, where the lipoic carbonyl group in SSN3B is
replaced by the tert-butyloxycarbonyl group, was investigated by
1H NMR spectroscopy in CDCl3. The addition of (CD3)2SO to the
solution induced the shift to the lower magnetic field of the urethane
amide proton and one Aib amide proton. The result is consistent
with this peptide taking a 310-helical conformation in chloroform.
The same result was obtained for BA2NB as well. Geometry
optimization using a MM2 method was applied to the acetyl-capped
derivative of BN3B and also showed that 310-helical conformation
is an energetically stable conformation as low as an R-helical
conformation. In the energy-minimized 310-helical conformation,
three naphthyl groups are spaced in a linear array along the helical
axis with face-to-face orientation. The center-to-center distances
SAMs were prepared by immersion of a gold substrate into an
ethanol solution of each peptide. The conformation and orientation
of the peptides in the SAMs were investigated by infrared reflection
absorption spectroscopy (IRRAS). Absorptions appeared at around
1670 and 1540 cm-1 in the IRRAS spectra for the SAMs, which
were assigned to amide I and amide II absorptions of helical confor-
mation, respectively.10 Calculations based on the amide I/II absorp-
tion ratio gave the tilt angles of the helical axis from the surface
normal of 31°, 47°, 57°, and 36° for the SSN3B, SSA3B, SSNA2B,
and SSA2NB SAMs, respectively. To examine the molecular
packing of the SAMs, cyclic voltammetry was performed in an
aqueous K4[Fe(CN)6] solution. Especially, the SSN3B SAM showed
negligible peaks of the Fe(III)/Fe(II) redox couple compared with
the case of a bare gold substrate, indicating that the SSN3B SAM
is tightly packed without any defects. Similar results were obtained
for the SSA3B and SSNA2B SAMs. On the other hand, the
SSA2NB showed relatively large peaks, showing that the SSA2NB
SAM is loosely packed. On the basis of these data, it was suggested
that the three naphthyl groups of SSN3B promote vertical orienta-
tion and dense packing of helical peptides in the SAM.
Photocurrent generation by these peptide SAMs was investigated
by photoexcitation of the naphthyl group in an aqueous solution
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10.1021/ja0476011 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. Energy diagram for anodic photocurrent generation by the SSN3B
SAM in the case that the naphthyl group at the site nearest to gold is excited
by photoirradiation.
peptide backbone3,11 and might be accelerated by the dipole moment
of the helix, since it has been shown that the dipole moment with
the same direction as that of electron transfer accelerates the electron
transfer in R-helical peptides.5,12 In addition, energy migration
among the naphthyl groups may promote the photocurrent genera-
tion, because the distance between the neighboring naphthyl groups
(ca. 6 Å) is shorter than the critical distance between naphthyl
groups for energy migration of the Fo¨rster type (8.3 Å). These
effects on the photocurrent generation are to be examined in a future
report.
In summary, we prepared SAMs from a novel 310-helical peptide
carrying three naphthyl groups of a regular and linear arrangement
with the reference peptides carrying no or one naphthyl group at
the terminal residue. The SAM prepared from the peptide carrying
three naphthyl groups was shown to have a well-packed structure
and vertical molecular orientation. The photocurrent generation
experiment revealed that the linearly spaced naphthyl groups along
the helical axis effectively increase the photocurrent by photo-
sensitization and electron hopping between the naphthyl groups.
To our knowledge, this is the first example of a three-dimensional
arrangement (linear array) of functional groups in a self-assembled
monolayer system accomplished by utilizing the regular structure
of a 310-helical peptide, that cannot be realized by conventional
sulfur-terminated compounds. We are now examining the distant
electron-transfer reactions in 310-helical peptides carrying chro-
mophores at the side chains.
Figure 2. Photocurrent generation upon light irradiation at 280 nm (1.2 ×
1014 photons s-1) at applied potential of 0 V vs Ag/AgCl reference electrode.
The irradiated area of electrode was ca. 0.2 cm2. The measurements were
carried out in an aqueous 50 mM TEOA solution.
containing triethanolamine (TEOA) as an electron donor. Significant
anodic photocurrent generation was successfully observed on the
SSN3B SAM in response to photoirradiation (Figure 2). The action
spectrum of the SSN3B SAM agreed well with the absorption
spectrum of SSN3B in ethanol, showing that the naphthyl groups
act as sensitizer for the photocurrent generation. The quantum
efficiency of the photocurrent generation was found to be 2.1%,
which is a moderate value for the long-range electron transfer.7,8
On the other hand, no anodic photocurrent was generated by the
SSA3B or SSNA2B SAM, and only indistinct photocurrent was
observed in the SSA2NB SAM, which was at most one-third of
that by the SSN3B SAM. The low signal/noise ratio in the
photocurrent of the SSA2NB SAM should arise from the loose
packing of the monolayer because the bare gold area exposed to
the aqueous solution is sensitive to the environmental electric noises
since the bare gold substrate shows large noises.
Noticeably, the SSNA2B SAM showed almost no photocurrent
signal, but a weak photocurrent generation was observed in the
SSA2NB SAM. The reason for this difference between the reference
SAMs (with one naphthyl group) may be explained as follows.
There are two steps for the anodic photocurrent generation; one is
the photoinduced electron transfer from the photoexcited naphthyl
group to gold, and the other is the subsequent electron donation
from TEOA to the oxidized naphthyl group. It is considered that
the photoexcited naphthyl group in the both SAMs can donate an
electron easily to gold because of the large energy difference
between the redox potential of the photoexcited naphthyl group
and the Fermi level of gold. However, the subsequent electron
donation from TEOA to the radical cation of the naphthyl group
needs diffusion of TEOA in aqueous phase to the naphthyl group
in the SAMs. The diffusion is strongly suppressed in the case of
the SSNA2B SAM to generate no photocurrent. The other possible
reference monolayer, where one naphthyl group is located at the
middle site, was not examined in this study; however, the
photocurrent can be roughly estimated to be in between those by
SSNA2B and SSA2NB SAMs due to the intermediate distance
between TEOA in aqueous phase and the naphthyl group in the
SAM. The photocurrent by the SSN3B SAM is then at least two
times larger than the sum of the photocurrents by these three
reference SAMs. It is therefore concluded that electron hopping
among the linearly-spaced naphthyl groups effectively promotes
the photocurrent generation in the SSN3B SAM. A representative
case is shown in Figure 3, where the naphthyl group at the site
nearest to gold is photoexcited. In this case, the radical cation, which
is generated by photoinduced electron transfer from the naphthyl
group to gold, subsequently hops away from gold via the naphthyl
groups and finally is quenched by electron donation from TEOA
in an aqueous phase. Further, the electron hopping process should
be promoted by effective mediation of electron transfer by a helical
Acknowledgment. This work is partly supported by a Grant-
in-Aid for Young Scientists B (14750694), Priority Areas Research
B (Construction of Dynamic Redox Systems Based on Nano-Space
Control), and 21st Century COE program, COE for a United
Approach to New Materials Science, from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology.
Supporting Information Available: Details in synthesis, spectro-
scopic and electrochemical measurements. This material is available
free of charge via the Internet at http://pubs.acs.org.
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