Columnar assembly formation and metal binding of cyclic tri-β-peptides having terpyridine ligands

Article abstract of DOI:10.1021/ol0629622

(Figure Presented) A novel cyclic tri-β-peptide having terpyridine (tpy) metal ligands was synthesized to investigate its assembly formation and metal complexation. Microscopic observation revealed that this cyclic peptide formed a rod-shaped molecular as

Full text of DOI:10.1021/ol0629622

ORGANIC
LETTERS
2007
Vol. 9, No. 5
793-796
Columnar Assembly Formation and
Metal Binding of Cyclic Tri-
â-peptides
Having Terpyridine Ligands
Futoshi Fujimura† and Shunsaku Kimura*,†
Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Kyoto-Daigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
shun@scl.kyoto-u.ac.jp
Received December 7, 2006
ABSTRACT
A novel cyclic tri-
Microscopic observation revealed that this cyclic peptide formed a rod-shaped molecular assembly. The assembly was able to bind Cu(II)
because the tpy ligands covered the surface of the crystal, keeping the tpy plane parallel to the ring plane of the cyclic tri- -peptide.
â-peptide having terpyridine (tpy) metal ligands was synthesized to investigate its assembly formation and metal complexation.
â
Organic molecular nanowires fabricated by molecular self-
assembly have attracted considerable attention in recent years
for possible applications to nanoelectronics.1 Molecular
nanowires are required to be controllable in size and length.
In light of this point, peptide nanotubes (PNTs) composed
of cyclic peptides stacking in a column via intermolecular
hydrogen bonds are one of the most suitable molecular
objects, because PNTs have several variables for molecular
design such as ring size, R-,2 â-,3 δ-,4 and ꢀ-peptide,5 and
side chains, which make the molecular assemblies adjustable
for each purpose. We previously demonstrated parallel
assembly of dipolar columns of cyclic tri-â-peptide,6 which
is composed of three trans-2-aminocyclohexylcarboxylic acid
(ACHC) residues familiarized by Gellman et al.7 Since all
amide groups of PNTs composed of cyclic â-peptides orient
(2) (a) Ghadiri, M. R.; Granja, J. R.; Milligan, R. A.; McRee, D. E.;
Khazanovich, N. Nature 1993, 366, 324. (b) Hartgerink, J. D.; Granja, J.
R.; Milligan, R. A.; Ghadiri, M. R. J. Am. Chem. Soc. 1996, 118, 43. (c)
Hartgerink, J. D.; Clark, T. D.; Ghadiri, M. R. Chem. Eur. J. 1998, 4,
1367. (d) Horne, W. S.; Ashkenasy, N.; Ghadiri, M. R. Chem. Eur. J.
2005, 11, 1137.
(3) (a) Seebach, D.; Matthews, J. L.; Meden, A.; Wessels, T.; Baerlocher,
C.; McCusker, L. B. HelV. Chim. Acta 1997, 80, 173. (b) Matthews, J. L.;
Gademann, K.; Jaun, B.; Seebach, D. J. Chem. Soc., Perkin Trans. 1 1998,
3331. (c) Gademann, K.; Ernst, M.; Seebach, D.; Hoyer, D. HelV. Chim.
Acta 2000, 83, 16. (d) Jagannadh, B.; Reddy, M. S.; Rao, C. L.; Prabhakar,
A.; Jagadeesh, B.; Chandrasekhar, S. Chem. Commun. 2006, 4847.
(4) Gauthier, D.; Baillargeon, P.; Drouin, M.; Dory, Y. L. Angew. Chem.,
Int. Ed. 2001, 40, 4635.
(5) Horne, W. S.; Stout, C. D.; Ghadiri, M. R. J. Am. Chem. Soc. 2003,
125, 9372.
(6) Fujimura, F.; Fukuda, M.; Sugiyama, J.; Morita, T.; Kimura, S. Org.
Biomol. Chem. 2006, 4, 1896.
(7) (a) Appella, D. H.; Christianson, L. A.; Karle, I. L.; Powell, D. R.;
Gellman, S. H. J. Am. Chem. Soc. 1999, 121, 6206. (b) Raguse, T. L.; Lai,
J. R.; LePlae, P. R.; Gellman, S. H. Org. Lett. 2001, 3, 3963. (c) Murray,
J. K.; Gellman, S. H. Org. Lett. 2005, 7, 1517.
* To whom correspondence should be addressed. Tel: +81-75-383-2400.
Fax: +81-75-383-2401.
† Department of Material Chemistry, Kyoto University.
(1) (a) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew.
Chem., Int. Ed. 2001, 40, 988. (b) Shimizu, T.; Masuda, M.; Minamikawa,
H. Chem. ReV. 2005, 105, 1401. (c) Hill, J. P.; Jin, W. S.; Kosaka, A.;
Fukushima, T.; Ichihara, H.; Shimomura, T.; Ito, K.; Hashizume, T.; Ishii,
N.; Aida, T. Science 2004, 304, 1481. (d) Kamikawa, Y.; Nishii, M.; Kato,
T. Chem. Eur. J. 2004, 10, 5942. (e) Fenniri, H.; Packiarajan, M.; Vidale,
K. L.; Sherman, D. M.; Hallenga, K.; Wood, K. V.; Stowell, J. G. J. Am.
Chem. Soc. 2001, 123, 3854. (f) Yip, H. L.; Zou, J. Y.; Ma, H.; Tian, Y.
Q.; Tucker, N. M.; Jen, A. K. Y. J. Am. Chem. Soc. 2006, 128, 13042.
10.1021/ol0629622 CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/07/2007

in parallel to the tube axis, a large dipole will be generated
in the PNTs, which may have a potential application to
molecular devices. However, PNTs generally possess low
electronic conductivity,8 which may restrict their application.
To improve the electronic conductivity of PNTs, incorpora-
tion of π-systems and/or metal ions to the PNT may be
effective. Therefore, the PNT bearing the π-systems with
the ability to form a metal complex was prepared in this
work. 2,2′:6′,2′′-Terpyridine (tpy) was chosen as the ligand.
Tpy is one of the most versatile building blocks in supramo-
lecular chemistry as demonstrated by Lehn et al.9 This
tridentate ligand is able to form complexes with various
transition metal ions with high binding constants.10 The
present paper reports on the molecular assembly formation
and metal binding property of cyclic tri-â-peptide having
tpy ligands at the side chains.
Scheme 1. Synthetic Route of Cyclo[â-Glu(tpy)3]a
Peptide Design and Synthesis. Metal ions such as Ru-
(II) and Fe(II) form complexes with tpy ligands preferably
taking six-coordinate geometry. In the present study, this
geometry will put a severe constraint on a tubular structure
of stacking cyclic tri-â-peptides when two tpy ligands of
adjacent cyclic peptides in the tube are forced to accom-
modate the metal ion. We therefore selected Cu(II), which
prefers four- or five-coordinate geometry with one tpy
ligand.11 The long piperadine spacer was inserted between
the cyclic skeleton and the tpy ligand in the molecule to avoid
steric hindrance and electrostatic repulsion between the
neighboring ligands either in free or complex form. When
these two factors are avoided, the cyclic tri-â-peptide will
take a planar structure including the side chain keeping the
regular PNT structure even after complexation. The cyclic
tri-â-peptide having three tpy ligands was prepared by a
liquid-phase method (Scheme 1). Boc-[â-Glu(OBzl)]-OtBu
and Boc-[â-Glu(OBzl)]-Opac were synthesized from [â-Glu-
(OBzl)]-OH. Chain extension reactions to the linear di- and
tripeptides were carried out using HATU as a coupling
reagent. In the case of the synthesis of the trimer, the Boc
group of Boc-[â-Glu(OBzl)]-OtBu was selectively removed
without cleavage of tert-butoxy ester by 4 N HCl-dioxane
according to the previous report.12 After the chemical
conversion from benzyl ester to the tpy derivative at the side
chain and removal of the protecting groups at the both
terminals of the main chain with treatment of TFA, cycliza-
tion reaction was carried out using HATU, HOAT and DIEA
a Abbreviations: DMAP, N,N-dimethylaminopyridine; HATU,
O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium haxafluo-
rophosphate; DIEA, N,N-diisopropylethylamine; HOAT, 1-hydroxy-
7-azabenzotriazole; tpy-NH, 4′-(2-piperidin-4-yloxy)-2,2′:6′,2′′-
terpyridine.
(H-N-Câ-Hâ) of 172° according to the Karplus equation.13
The dihedral angle indicates the anti relationship between
the NH and Hâ, which is similar to the result of the other
cyclic tri-â-peptide.6,14
Assembly Formation. The trimer showed high solubility
for a variety of mixed solutions of polar and nonpolar
solvents such as CHCl3-MeOH, THF-CH3CN, and even
THF-H2O.15 This unprecedented solubility of the cyclic tri-
â-peptide may be ascribed to high solubility of tpy ligands
and the difficulty in formation of intermolecular hydrogen
bonds to grow into a tubular structure probably due to the
introduction of the bulky tpy ligands at the side chains.
However, we succeeded in preparing a straight needle-like
assembly under a proper assembling condition. Figure 1
shows the crystals of cyclo[â-Glu(tpy)3] which were recrys-
tallized from THF-CH3CN. When the crystals were ob-
1
in DMF (0.1 mM). H NMR spectrum of the trimer in
DMSO-d6 clearly indicates that the predominant conforma-
tion is C3 symmetric on a NMR time scale. The spin coupling
constants between amide proton and Hâ in the cyclic skeleton
was found to be 9.0 Hz, which gives the dihedral angle of θ
(8) (a) Okamoto, H.; Nakanishi, T.; Nagai, Y.; Kasahara, M.; Takeda,
K. J. Am. Chem. Soc. 2003, 125, 2756. (b) Ashkenasy, N.; Horne, W. S.;
Ghadiri, M. R. Small 2006, 2, 99.
(9) (a) Lehn, J. M. Supramolecular Chemistry, Concepts and Perspec-
tiVes; VCH: Weinheim, 1995. (b) Lehn, J. M. Angew. Chem., Int. Ed. 1988,
27, 89. (c) Schubert, U. S.; Eschbaumer, C. Angew. Chem., Int. Ed. 2002,
41, 2893.
(10) Dobrawa, R.; Lysetska, M.; Ballester, P.; Grune, M.; Wurthner, F.
Macromolecules 2005, 38, 1315.
(11) Cardenas, D. J.; Livoreil, A.; Sauvage, J. P. J. Am. Chem. Soc. 1996,
118, 11980.
(13) Karplus, M. J. Chem. Phys. 1959, 30, 11.
(14) Fujimura, F.; Hirata, T.; Morita, T.; Kimura, S.; Horikawa, Y.;
Sugiyama, J. Biomacromolecules 2006, 7, 2394.
(15) General cyclic â-peptides have a severe solubility problem: (a)
Gademann, K.; Seebach, D. HelV. Chim. Acta 1999, 82, 957. (b) Matthews,
J. L.; Overhand, M.; Kuhnle, F. N. M.; Ciceri, P. E.; Seebach, D. Liebigs
Ann. 1997, 1371.
(12) Han, G.; Tamaki, M.; Hruby, V. J. Peptide Res. 2001, 58, 338.
794
Org. Lett., Vol. 9, No. 5, 2007

align along the long axis to contribute to the refractive index
along the long axis. Then, in order to explain the larger
refractive index along the short axis of the crystal, the tpy
ligands should orient perpendicular to the long axis of the
crystal with keeping the tpy plane in the ring plane of the
cyclic tri-â-peptide. The latter condition is indispensable for
the cyclic peptides to stack up without steric hindrance.
Further, the columnar stacking of the tpy ligands with this
style should be stabilized by π-π interaction in the tubular
structure.
Metal Binding Property. To examine the ability of the
tpy ligands in cyclo[â-Glu(tpy)3] to bind Cu(II), the complex
formation was studied by UV-visible absorption spectros-
copy. A typical spectral change in the UV-vis absorbance
of the peptide solution upon the addition of CuCl2 is shown
in Figure 2. When each tpy ligand in cyclo[â-Glu(tpy)3]
forms complex with Cu(II) independently, the stoichiometry
of the complex should be molpeptide/molmetal ) 1:3. However,
the absorbance increase at 330 nm with the addition of Cu-
(II) shows saturation at the addition of two equivalents of
Cu(II) as shown in the Figure 3 inset. On the other hand,
Figure 1. Optical microscopic image of cyclo[â-Glu(tpy)3] crystals
under the cross-nicol configuration with a sensitive tint plate. The
double-headed arrow shows the orientation of z′ axis of a tint plate.
Scale bar ) 100 µm.
served under the cross-nicol configuration with a sensitive
tint plate, they displayed the orange or yellow color in the
case that the long axis of the crystal coincides with the z′
axis of the tint plate and blue color in the case that they are
perpendicularly oriented. The result indicates that the refrac-
tive index along the short axis of the crystal is larger than
that along the long one. There are two possible chromophores
in the molecule, making the refractive index large: amide
groups and tpy ligands.
The crystal structure was analyzed by electron diffraction.
The diffraction pattern obtained by the incident electron beam
perpendicular to the rod axis showed a unit cell with an axial
spacing of ca. 4.8 Å, which is agreeable with the interpreta-
tion that the cyclic tri-â-peptides stack up to form a tubular
structure through 1-D intermolecular hydrogen bond network
as described with other cyclic â-peptides in the previous
reports.6,14 Amide groups in the crystals should therefore
Figure 3. Cross-polarized optical (left) and fluorescence (right)
microscopic images of cyclo[â-Glu(tpy)3] crystals with (a) and
without (b) Cu(II). The double-headed arrow shows the orientation
of z′ axis of a tint plate. Scale bar ) 100 µm.
the titration using a reference compound (4′-(2-piperidin-4-
yloxy)-2,2′:6′,2′′-terpyridine/tpy-NH) and Cu(II) shows com-
plexation of a 1:1 ratio at the same concentration range. The
tpy groups of the cyclic tri-â-peptide are confirmed to be
involved in the complexation with Cu(II), because the
absorbance around 710 nm, which is assigned to the metal-
ligand charge-transfer band of the complex type of [Cu(tpy)]-
Cl2, appeared and increased the intensity until the addition
of two equivalents of Cu(II).16 The unusual stoichiometry
of the 1:2 complex was also observed by MALDI-TOF/MS
spectroscopy, which showed a signal of [(M + (CuCl2)2 +
H)+] (m/z ) 1638) as a highest molecular mass component
Figure 2. UV-vis absorption spectra obtained during the com-
plexation reaction between cyclo[â-Glu(tpy)3] and Cu(II). The inset
shows the increases in absorbance at 330 nm.
(16) Kwik, W. L.; Ang, K. P. Transition Met. Chem. 1987, 12, 58.
Org. Lett., Vol. 9, No. 5, 2007
795

for the sample prepared by mixing peptide 1 and CuCl2 at
molar ratio of 1:3.3. Since replacement reaction of two Cl
atoms in [Cu(tpy)]Cl2 with another tpy is considered unlikely
to occur under the normal condition,11 the complex formation
with more than two cyclic peptides can be excluded. The
third tpy ligand of cyclic peptide after complexation with
two Cu(II) ions is thus somehow disturbed. Increase of the
total plus charges of the complex may reduce the binding
ability of the third ligand. Or the carbonyl groups in the cyclic
skeleton could participate in the complexation. The reason
remains to be solved. However, it is notable that [Cu(tpy)]-
Cl2 takes a trigonal bipyramidal geometry with ca. 4 Å in
length between two Cl atoms, which is less likely to affect
the molecular stacking of cyclic peptides because the distance
between adjacent cyclic peptides in the column is 4.8 Å.
Construction of PNT with Metal Complexes on the
Surface. The surface of nanotube assemblies of cyclic tri-
â-peptide should be covered with tpy ligands, which will
provide binding sites for Cu(II). The complex of the peptide
nanotube with Cu(II) is interesting in terms of the conductive
property in a shape of the nanorod. The complex formation
was analyzed by fluorescence microscopy. As shown in
Figure 3, the crystals of cyclic tri-â-peptide were observed
as fluorescent rods with excitation around 330 nm due to
the tpy ligands. However, after the crystals were immersed
in a MeOH solution of CuCl2, they were weakly fluorescent,
keeping the rod structure, indicating complexation of Cu(II)
on the surface of the nanorods to quench the emission from
tpy ligands. Since the observed rod structure was an assembly
of many PNTs, tpy ligands at the interior of the bundle could
not form complex with Cu(II). However, Figure 3(b) shows
that the fluorescence of the assembly was quenched drasti-
cally. The result suggests that the tpy ligands located only
at the external surface may be intensively fluorescent and
the tpy ligands at the internal region are self-quenched due
to high local concentration.
optical microscope observation with polarizers and a tint
plate, the anisotropic optical retardation properties were not
changed upon complexation with Cu(II), indicating that the
orientations of amide groups and tpy ligands in the assembly
were not affected by metal binding at the exterior of the
nanorods. Because Cu(II) prefers four- or five-coordinate
geometry with a tpy ligand as described above, the binding
of this metal to the columnar assembly did not destroy the
intermolecular hydrogen bonds in the assembly. We tried to
prepare a PNT with metal complex from Cu(II)-coordinated
cyclo[â-Glu(tpy)3], but regular assemblies were not obtained,
suggesting that the carbonyl groups in the cyclic skeleton
may participate in the complexation with Cu(II) in a solution
to disturb formation of a PNT when the cyclic peptide was
in advance incubated with Cu(II).
In summary, we synthesized the novel cyclic â-peptide
having tpy ligands and investigated its assembly formation
and metal binding property. The optical microscopic obser-
vation reveals that the trimer forms rod-shaped crystals and
that amide groups and tpy ligands orient parallel and
perpendicular to the rod axis, respectively. Moreover,
fluorescence microscopic observation suggests that tpy
ligands in the assembly bind Cu(II) ions. Since the prepared
assembly may possess both high electronic conductivety and
a large dipole, this assembly may have a variety of
applications in the organic electronics field. This is now
under investigation.
Acknowledgment. This work is partly supported by
Grants-in-Aid for Scientific Research B (18350063) and the
21st century COE program, COE for an approach to New
Materials Science, from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan.
Supporting Information Available: Synthetic details,
1
characterization data, and H NMR spectra for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
The observation suggests clearly that the molecular column
of cyclic peptide having tpy ligands can bind Cu(II) without
disruption of the original shape. Interestingly, under the
OL0629622
796
Org. Lett., Vol. 9, No. 5, 2007