helical peptide component. In the LB10 layer, the two com-
ponents have a similar horizontal orientation to the surface.
Under this geometrical constraint, the two components in the
conjugate should favor antiparallel arrangement. Further-
more, head-to-tail arrangement in the layer may be prevailing
because of stabilization of dipole–dipole interaction as
depicted in Fig. 7a. This type of head-to-tail arrangement
was previously reported in a LB monolayer of a 24mer helical
peptide.28 The red-shift of 10 nm is thus caused by the electric
field generated by the peptide dipoles neighboring the OPE. In
the LB20 layer, the conjugates are piled up to double with
keeping the horizontal and the antiparallel arrangement
(Fig. 7b). The electric field strength around the OPE thus
becomes double to induce another red-shift of 10 nm. On the
other hand, in the SAM, the peptide components orient
randomly on the surface. Although the orientation of the
OPE component in the SAM could not be measured, it is
plausibly considered that the relative orientation of the OPE to
the peptide is random because of random distribution of the
surrounding dipoles.
layers using scanning probe microscopies of STM and AFM.
Another novel conjugate is under investigation, where the two
components are connected at two sites to fix the antiparallel
arrangement. The dipole effect on the OPE will appear more
prominent than in the present case.
Acknowledgements
This work is partly supported by Grant-in-Aids for Explora-
tory Research (17655098), and for Scientific Research B
(15350068), and Global COE program, International Center
for Integrated Research and Advanced Education in Materials
Science, from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan.
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Conclusion
A novel conjugate of OPE and a helical peptide was synthe-
sized and studied on the dipole effect of the helical peptide on
the electronic structure of the OPE and a dipole–dipole
interaction in regulation of the molecular structure. In chloro-
form, the conjugate showed
a red-shifted absorption
compared to a reference OPE derivative, indicating that the
electric field effect of the helical peptide dipole on the electro-
nic structure of the OPE in the conjugate appeared, where
both components favored an antiparallel arrangement due to
the dipole–dipole interaction. This interpretation was
supported by ab initio calculations. In the LB layers of the
conjugate, the red-shifts of the lmax became larger than that in
chloroform because of the additive dipole effects from the
helical peptides neighboring the OPE. We are now working on
clarification of the precise molecular alignment in the LB
ꢀc
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Phys. Chem. Chem. Phys., 2009, 11, 3967–3976 | 3975