Molecular monolayer nanotubes having 7-9 nm inner diameters covered with different inner and outer surfaces

Article abstract of DOI:10.1246/cl.2007.896

Novel unsymmetrical bolaamphiphile, bearing glucose- and triglycine-headgroups at both ends, exclusively self-assembled into nanotubes with 7-9-nm inner diameters, which consist of a single monolayer lined by polyglycine-II-type hydrogen-bond networks among the triglycine moieties. Copyright

Full text of DOI:10.1246/cl.2007.896

896
Chemistry Letters Vol.36, No.7 (2007)
Molecular Monolayer Nanotubes Having 7–9 nm Inner Diameters
Covered with Different Inner and Outer Surfaces
Naohiro Kameta,1 Go Mizuno,2 Mitsutoshi Masuda,ꢀ1;3 Hiroyuki Minamikawa,1;3
Masaki Kogiso,1;3 and Toshimi Shimizuꢀ1;3
1SORST, Japan Science and Technology Agency, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565
2Graduate School of Chemistry, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571
3Nanoarchitectonics Research Center (NARC), National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565
(Received April 18, 2007; CL-070425; E-mail: tshmz-shimizu@aist.go.jp)
Novel unsymmetrical bolaamphiphile, bearing glucose- and
end,12 which exclusively give nanotubes with controlled molec-
ular packing. The number of glycine residues in 1(n) influences
not only the morphologies of the resultant self-assemblies but
also the stabilization of the parallel molecular packing based
on hydrogen-bond networks among the glycine moieties.
The self-assembly experiment was carried out as follows:
The synthetic 1(n) (1 mg) was dispersed in water (1 mL) at
pH 7–10 under reflux conditions. The resultant hot aqueous solu-
tions were gradually cooled to room temperature. Transmission
electron microscopic (TEM) observation revealed that the
self-assembled morphologies strongly depend on the number
of glycine residues (Figure 1). Self-assembly of 1(1) and 1(2)
gave helical nanofibers with 10–25-nm width, while 1(3) formed
nanotubes with 7–9-nm inner diameters and 3–4-nm thickness.
The nanotubes are distinguishable from the nanofibers, since
the penetration of the negative staining reagent, phosphotung-
state, into the hollow cylinder allows visualization of the tubular
structure.
triglycine-headgroups at both ends, exclusively self-assembled
into nanotubes with 7–9-nm inner diameters, which consist of
a single monolayer lined by polyglycine-II-type hydrogen-bond
networks among the triglycine moieties.
Lipid nanotubes self-assembled from amphiphilic molecules
have been becoming attractive nanoarchitectures1,2 since the hy-
drophilic hollow cylinder can act as vessels for nanomaterials,3
templates for metal nanowire formation,4 nanochannels for
nanofluidic devices,5 and carriers for drug delivery.6 Rational
functionalization of their surfaces and control of their inner
diameters are currently big issues to achieve effective and selec-
tive encapsulation of target guest substances into the hollow
cylinders.7,8 Unsymmetrical bolaamphiphiles, in which two
hydrophilic headgroups of different sizes are connected to a
hydrophobic spacer at each end, often form nanotubes having
different inner and outer surfaces depending on their parallel
molecular packing within the monolayer lipid membranes.7–10
However, the control of the molecular packing during the self-
assembly procedure has been difficult since the bolaamphiphiles
also packed in favorable antiparallel fashion.7–9,11 Although we
succeeded in the formation of such nanotubes, the obtained
nanotubes actually involved a mixture of parallel and antiparal-
lel molecular packing.7 The preorganization of the molecular
packing before self-assembly was also indispensable for the
control of polymorphism.8 In those studies, we have also found
that the inner diameter of such nanotubes decreases with short-
ing the length of the oligomethylene spacer in the bolaamphi-
philes. However, the bolaamphiphiles having short oligo-
methylene spacers tended to form nanotubes with antiparallel
molecular packing as well as tape-like structures.7,11 Therefore,
the down sizing of the inner diameter into single nm has been
impossible until now, although we and some groups have report-
ed the nanotubes with inner diameters below 10 nm consisting of
bilayer membrane type.1
IR measurements13 were performed to clarify the difference
1(1)
100 nm
1(3)
1(2)
i.d. 8 nm
Thickness
3 nm
Here, we describe novel unsymmetrical bolaamphiphiles,
1(n) (Scheme 1), having both glucose and oligoglycine-
headgroups connected to a short oligomethylene spacer at each
O
OH
50 nm
100 nm
H
N
H
N
HO
OH
OH
O
N
H
H
n
Figure 1. TEM images of the self-assemblies formed from 1(n)
negatively stained with phosphotungstate. The hollow cylinder
space of the nanotubes for 1(3) can be visualized by relatively
darker image than surrounding.
O
Scheme 1. Unsymmetrical bolaamphiphiles 1(n) (n ¼ 1, 2,
and 3).
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.7 (2007)
897
Table 1. IR absorption band for the self-assemblies from 1(n)
1(1)
1(n)
n ¼ 1
n ¼ 2
n ¼ 3
nanofiber
nanofiber
nanotube
1(2)
1(3)
NH str./cmꢁ1
Amide I/cmꢁ1
Amide II/cmꢁ1
ꢁs (CH2)/cmꢁ1
ꢀ (CH2)/cmꢁ1
r (CH2)/cmꢁ1
3288
1655
3298
1653
3290
1642
1548
2850
1465 (13.4)1
1548
2849
1473 (10.1)1
1561
2850
1465 (8.3)1
718
717
719
1Full width half-maximum.
1500
1450
1400
1350 1150
1100
1050
1000
Wavenumber / cm-1
ꢁs(CH2) stretching vibration band is sensitive for the gauche/
trans conformation in the oligomethylene spacer.11 The oligo-
methylene spacer in the nanotube and nanofibers takes an
all-trans conformation since the band frequencies appear at less
than 2850 cmꢁ1 (Table 1). The membrane thickness (3–4 nm)
of the nanotubes estimated from TEM image is similar to the
extended molecular length (3.57 nm) of 1(3), indicating the
nanotube consists of a single monolayer. Powder X-ray diffrac-
tion measurement for the nanotubes showed no information
about d spacing because of the thin layer. All results suggest
that 1(3) forms single monolayer with parallel molecular
packing and produces nanotubes with spontaneous membrane
curvature based on the size difference of the hydrophilic head-
groups (Figure 3).
Figure 2. The CH deformation and skeletal vibration IR bands
for the oligoglycine moieties in the self-assembled 1(n).
Upper
NH
O
≡
HN
O
NH
O
Side
H N
2
3.57 nm
Self-assembly of unsymmetrical bolaamphiphile 1(n)
proved to give separately nanotubes and nanofibers depending
on the number of the glycine residues. Especially, the specific
hydrogen-bond networks among the triglycine in 1(3) allowed
the nanotubes to bring not only different inner and outer surfaces
but also the inner diameters below 10 nm.
Figure 3. A schematic model for the nanotube consisting of the
single monolayer lined by pseudo polyglycine-II-type hydrogen-
bond networks among triglycine in 1(3).
References and Notes
1
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in the molecular packing between the nanotubes and the nanofib-
ers. The nanotube from 1(3) indicates two peaks at 1420 and
1026 cmꢁ1, whereas the nanofibers from 1(1) and 1(2) have
one of them (Figure 2). The two bands are observable in the
CH deformation and skeletal vibration bands of polyglycine,14
showing that the triglycine of 1(3) in the nanotube form the
intermolecular hydrogen bonding similar to the polyglycine-II-
type hydrogen-bond networks of polyglycine.15 Pseudo hexago-
nal-type hydrogen-bond networks16,17 should induce the parallel
molecular packing in the nanotube from 1(3) having triglycine
moieties (Figure 3). On the other hand, the nanofibers from
1(1) and 1(2) will allow the coexistence of the antiparallel pack-
ing partly due to the incomplete hydrogen-bond networks among
the mono- or di-glycine moieties. Actually, the frequencies of
the C=O stretching (amide I), N–H deformation (amide II),
and N–H stretching vibration bands support the view that the
amide hydrogen bond of the nanotube was stronger than that
of the nanofibers (Table 1).
The ꢀ(CH2) scissoring and r(CH2) rocking vibration bands
reflecting the lateral chain packing of the oligomethylene spacer
in 1(n), the so-called ‘‘subcell structure,’’ support the difference
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(Table 1).18 A single sharp peak of each band for the nanotubes
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peak of each for the nanofibers is compatible with a distorted
hexagonal (Hex) structure. These results are in good accord with
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13 Before IR measurement, the self-assembled nanotubes and nanofibers were
completely lyophilized.
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Published on the web (Advance View) June 16, 2007; doi:10.1246/cl.2007.896