Langmuir
Article
the shape persistence of the micelle. One could imagine each
molecule as a tetragonal pyramid that can assemble into a cube
only if it possesses very specific geometrical dimensions.
Hydrophobic interactions between the tail groups and π−π
interactions between the aromatic rings would provide stability
to the assembly.
cylinder appear. The present results indicate that a precise
combination of tail length, head volume, and molecule rigidity
is necessary to attain the shape persistence.
EXPERIMENTAL SECTION
■
5
Synthesis and Materials. Synthesis of 5,11,17,23-Tetrakis[tert-
butyl((1H-1,2,3-triazol-4-yl)methyl)carbamate-25,26,27,28- tetrapro-
poxy-calix[4]arene (IV): A solution of compound 3 (0.131 g, 1.60
Hirsch and his group were the first to report shape
persistence in a calix[4]arene amphiphile with a carboxylic
dendritic head and dodecyl tails. They concluded that the
micelles consist of seven molecules based on cryo-TEM
observations, although they did not determine the aggregation
number with a direct method. Their molecular dynamics
calculations indicate that the major driving force of the
aggregation and the shape persistence is the geometrical
packing of the dendritic carboxyl heads and the interaction
between the carboxyl heads through cation bridges. In fact,
even when they replace calix[4]arene by fullerene and attach
the same carboxylic dendritic head, they observe similar shape
persistence. Therefore, different features of their molecules are
responsible for the shape persistence of their micelles than for
the shape persistence of ours.
−4
−4
×
10 mol), N-Boc-propargylamine (0.125 g, 8.08 × 10 mol),
5
−
copper sulfate pentahydrate (3.31 mg, 1.32 × 10 mol), sodium
−
4
ascorbate (0.0262 g, 1.32 × 10 mol), and anhydrous N,N-
dimethylformamide (15 mL) was stirred at 90 °C for 36 h under
nitrogen atmosphere, and then water was added and the reactant
extracted with EtOAc. The organic layer was washed three times with
saturated NaCl solution and dried over MgSO . The solution was
4
evaporated to dryness, and the residue was purified by flash
chromatography over silica gel using CH Cl :Methanol = 15:1 as
2
2
eluent, IV was obtained after evaporation of the solvent (0.170 g, 1.19
−4
×
10 mol, 73%).
1
H NMR (500 MHz, CDCl ): δ = 7.46 (s, 4H), 6.49 (s, 8H), 5.43
3
(s, 4H), 5.21 (s, 8H), 4.40 (s, 8H), 4.38 (d, J = 13.5 Hz, 4H), 3.80 (t, J
= 7.5 Hz, 8H), 3.06 (d, J = 13.5 Hz, 4H), 1.88 (m, J = 7.5 Hz, 8H),
1
.41 (s, 36H), 0.97 (t, J = 7.5 Hz, 12H).
Synthesis of 5,11,17,23-Tetrakis[(1H-1,2,3-triazol-4-yl)-
Shape Determination from SAXS. There is a lack of
phase information in scattering data, and thus, the inverse
Fourier transform cannot be carried out without assumption to
reconstruct real space images. If there are enough diffraction
peaks, the inverse Fourier transform can be done with less
methanaminehydrochloride-25,26,27,28- tetrapropoxy-calix[4]arene
(
V): A solution of the compound IV (0.17 g, 1.19 × 10− mol) was
4
treated with 4 M HCl/EtOAc for 2 h, and then washed with CH Cl .
2
2
The residue was recovered with methanol and V was obtained after
1
9
evaporation of the solvent (0.157 g, 98%) 1HNMR (500 MHz,
ambiguity; however, it is a generally challenging issue to do
such reconstruction only from form factor scattering. Recently,
several methods for shape reconstruction from SAXS data have
methanol-d ): δ = 8.12 (s, 4H), 6.69 (s, 8H), 5.36 (s, 8H), 4.42 (d, J =
4
13.5 Hz, 4H), 4.29 (s, 8H), 3.81 (t, J = 7.5 Hz, 8H), 3.13 (d, J = 13.5
2
0−22
Hz, 4H), 1.90 (m, J = 7.5 Hz, 8H), 0.99 (t, J = 7.5 Hz, 12H). ESI-MS
emerged.
Although the approaches of each method and
+
(
m/z): [M−Na] calcd for C H N NaO4 1056.27; found 1055.6.
the respective programs are different, they all rely on the same
principle. Given minimal initial constraints, random shapes are
generated, the scattering pattern is calculated and compared to
the experimental data, and subsequently the shapes are changed
accordingly in small steps in order to improve the fit.
Although the fitting parameters to sphere, cylinder, or vesicle
models give a general idea of the shape of the molecule or
aggregate of interest, they are simplifying and not very
informative. In this study, we reconstructed the shape of the
CaL[4]C3 low pH micelle and showed that a hexamer arranged
in octahedral symmetry is indeed compatible with the
experimental SAXS data. Moreover, the shape of the produced
dummy atom model is far from the perfect sphere implied by
the fitting parameter models. This allows for a better estimation
of the relative positions of the monomers that comprise the
aggregate and subsequently the interactions that lead to the
specific micelle formation.
Concluding Remarks. A new calix[4]arene-based cationic
lipid with varying alkyl tail length was synthesized and the
micellar architecture exhibits strong dependence on pH and tail
length. Spherical micelles were formed at low pH (protonated
state of the amine head) for both CaL[4]C3 and C6. Upon
deprotonation, CaL[4]C3 showed a sphere to cylinder
transition, while CaL[4]C6 changed from sphere, to cylinder,
to monolayer vesicle. SAXS from the CaL[4]C3 micelle for
both spherical and cylinder states exhibited shape monodisper-
sity. The monodispersity of the CaL[4]C3 sphere was
confirmed with analytical ultracentrifugation. The aggregation
numbers of the protonated states of CaL[4]C3 and CaL[4]C6,
as determined by SAXS, static light scattering, and ultra-
centrifugation, are 6 and 12, respectively. When carbon tail
length was increased to 9 (CaL[4]C9), there was no clear
morphology observed at high pH, and only at low pH did
56 72 16
The compounds I, II, and III were synthesized with the reported
2
3
method. Calix[4]arene and synthesis chemicals and solvents were
purchased from Tokyo Chemical Industry Co., Sigma-Aldrich Co.,
Wako Chemical Industries and used without further purification.
Water was purified with a Millipore Milli-Q water purification system.
Light Scattering. Light scattering (LS) was carried out with a
Dawn-Heleos-II (Wyatt) coupled with a Shodex GPC-101 system. A
400 μL sample of unfiltered solution was injected into a system
consisting of a Waters 590 programmable HPLC pump (Waters,
Milford, MA) and a Shodex degassing unit (ERS-3000). The
chromatogram was measured with an Optilab DSP interferometric
differential refractive index detector (Wyatt) and a UV absorbance
detector SPD-10A (Shimazu). The column was a Shodex IC SI-90 4E
(
polyvinyl alcohol quaternary ammonium, particle size; 9 μm) which is
generally used for chromatographic analysis or separation of anions.
We used this column to optically purify the sample solution. The
elution of sufficient amount of intact micelles required the injection of
an untypically large amount of CaL[4]C3 sample in order to saturate
the hydrophobic groups of the column resin (that otherwise destroy
the micelles). The mobile phase was a 50 mM NaCl solution with a
pH of 2.8. Data acquisition and analysis was performed using Wyatt’s
ASTRA for Windows software (v 4.73.04). Scattered light intensities at
scattering angles 14−163° were measured and the weight averaged
molecular mass (M ) was determined.
W
The specific refractive index increment (∂n/∂c) of CaL[4]C3 and
CaL[4]C6 in a 50 mM NaCl solution (pH = 3.0) were 0.202 and
3
−1
0.198 cm g , respectively, determined with a DRM-1021 differential
refractometer (Otsuka Electronics) at 633 nm and 25 °C (Supporting
Information S8) .
Synchrotron SAXS Measurements. SAXS measurements were
performed at BL-40B2 of SPring-8, Japan. A 30 cm × 30 cm imaging
plate (Rigaku R-AXIS VII) detector was placed at 0.7 or 1.8 m away
from the sample. The wavelength of the incident beam (λ) was either
0
.071 or 0.10 nm. The 0.7 and 1.8 m set-ups provided a q range of
−
1
−1
0.2−10 nm and 0.07−4.0 nm , respectively, where q is the
magnitude of the scattering vector defined by q = 4π sin θ/λ with the
3
099
dx.doi.org/10.1021/la2037668 | Langmuir 2012, 28, 3092−3101