Internucleobase-interaction-directed self-assembly of nanofibers from homo- and heteroditopic 1,ω-nucleobase bolaamphiphiles

Article abstract of DOI:10.1021/ja010201i

The complementary 1,ω-thymine, 1,ω-adenine, and 1,ω-(thymine, adenine) bolaamphiphiles, [N,N′-bis[3-(2,4-dihydroxy-5-methylpyrimidine-1-yl)propionyl]1, n-diaminoalkane [T-n-T (n = 10, 11, 12)], N, N′-bis[3-(6-aminopurine-9-yl)propionyl]1,n-diaminoalkane [

Full text of DOI:10.1021/ja010201i

J. Am. Chem. Soc. 2001, 123, 5947-5955
5947
Internucleobase-Interaction-Directed Self-Assembly of Nanofibers
from Homo- and Heteroditopic 1,ω-Nucleobase Bolaamphiphiles
Toshimi Shimizu,*,†,‡,§ Rika Iwaura,| Mitsutoshi Masuda,†,‡,§ Takeshi Hanada,†, and
Kiyoshi Yase†,#
Contribution from the National Institute of Materials and Chemical Research, 1-1 Higashi,
Tsukuba, Ibaraki 305-8565, Japan, Japan Chemical InnoVation Institute, NIMC, 1-1 Higashi,
Tsukuba, Ibaraki 305-8565, Japan, and CREST, Japan Science and Technology Corporation (JST),
1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
ReceiVed January 23, 2001
Abstract: The complementary 1,ω-thymine, 1,ω-adenine, and 1,ω-(thymine, adenine) bolaamphiphiles, [N,N′-
bis[3-(2,4-dihydroxy-5-methylpyrimidine-1-yl)propionyl]1,n-diaminoalkane [T-n-T (n ) 10, 11, 12)], N, N′-
bis[3-(6-aminopurine-9-yl)propionyl]1,n-diaminoalkane [A-n-A (n ) 10, 11, 12)], and N-[3-(2,4-dihydroxy-
5-methylpyrimidine-1-yl)propionyl], N′-[3-(6-aminopurine-9-yl)propionyl]1,n-diaminoalkane [T-n-A (n ) 10,
11, 12)], respectively] have been synthesized. The spontaneous homo- and heteroassembly of these nucleobase-
based bolaamphiphiles has been studied by light microscopy, energy-filtering transmission electron microscopy,
FT-IR, and powder X-ray diffraction analyses. The achiral T-10-T bolaamphiphile produced in 10% ethanolic/
aqueous solutions unprecedented double-helical ropes of 1-2 µm in widths and several hundred micrometers
in length, whereas the complementary homologue A-10-A gave only microcrystalline solids of 1-10 µm in
size. In contrast, an equimolar mixture of T-10-T and A-10-A yielded supramolecular fibers of 15-30 nm in
width. 1H NMR, CD, and UV studies of solution photoreactions of T-10-T suggested that under natural light
the chiral rope formation is triggered by photodimerization of trace amounts of the thymine moieties in the
T-10-T assemblies. Complementary hydrogen bond formation between the thymine-adenine heterobase pairs
was found to prevent such a photoreaction and resulted in no chiral rope formation. The heteroditopic T-12-A
bolaamphiphile self-assembled to form supramolecular fibers. Multilamellar organization was proposed for
the homo- and heteroassemblies made of T-n-T and A-n-A.
Introduction
dimensional (2-D) monolayers,4 and supramolecular polymers.5
The most sophisticated system in nature is double-helical DNA
structures stabilized by Watson-Crick hydrogen bond base
pairing and aryl stacking interactions between adjacent base
pairs.6 However, individual base pairing between nucleic acid
components is generally not performed in aqueous solutions
without a double-stranded polynucleotide chain.7 On the other
hand, through the formation of three-dimensional (3-D) multiple
hydrogen bond networks, we and other research groups have
reported the spontaneous homoassembly of well-defined supra-
molecular fibers,8,9 microtubes,10 and crystals11-13 from sym-
metrical bola-form compounds. Considering the advantage of
both the hydrogen-bonding complementarity and multiplicity,
Molecular organization via the formation of complementary
hydrogen bonds has become a powerful strategy for the
construction of well-defined nanostructures.1 Typically studied
self-assembly systems in line with this concept include lyotropic
mesophases,2 supramolecular fibers and membranes,3 two-
† National Institute of Materials and Chemical Research.
‡ CREST, Japan Science and Technology Corp. (JST).
§ Present address: Nanoarchitectonics Research Center, National Institute
of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1
Higashi, Tsukuba, Ibaraki 305-8565, Japan.
| Japan Chemical Innovation Institute.
Present address: The Institute of Scientific and Industrial Research,
Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan.
# Present address: Photonics Research Institute, National Institute of
Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1
Higashi, Tsukuba, Ibaraki 305-8565, Japan.
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10.1021/ja010201i CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/02/2001

5948 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Shimizu et al.
we have designed and synthesized 1,ω-thymine-, 1,ω-adenine-,
and 1,ω-(thymine, adenine)-appended bolaamphiphiles [N,N′-
bis[3-(2,4-dihydroxy-5-methylpyrimidine-1-yl)propionyl]1,n-di-
aminoalkane [T-n-T (n ) 10, 11, 12)], N, N′-bis[3-(6-amino-
purine-9-yl)propionyl]1,n-diaminoalkane [A-n-A (n ) 10, 11,
12)], and N-[3-(2,4-dihydroxy-5-methylpyrimidine-1-yl)pro-
Results
Light Microscopic Observation of Homo- and Hetero-
assemblies. Among the homoditopic 1,ω-nucleobase bola-
amphiphiles, the longer derivatives T-11-T, T-12-T, A-11-A,
and A-12-A proved only sparingly soluble in aqueous solutions
(< ∼9 × 10-4 M at 50 °C), whereas the decamethylene-bridged
complementary components T-10-T and A-10-A showed rela-
tively good solubility (∼1.9 × 10-3 M). The heteroditopic
bolaamphiphiles T-n-A (n ) 10, 11, 12) displayed solubilization
behavior similar to that of homoditopic homologues. Thus, the
self-assembly of the homo- and heterobase-pair combination
of T-10-T and A-10-A was investigated in 10% ethanolic/
aqueous solutions. The bolaamphiphile A-10-A spontaneously
self-assembled to form microcrystalline solids of 1-10 µm in
size, whereas T-10-T produced fibrous assemblies after incuba-
tion for a few days. Dark-field light microscopy revealed the
formation of a swarm of double-helical ropes from T-10-T.
Figure 1a clearly displays that two independent chiral ropes of
0.2-1.0 µm in width are helically interwoven each other with
the same helical sense. Polarized light microscopy showed that
they extend in length to several hundred micrometers (not
shown). The helical ropes are very stable in the solution and
exhibit no remarkable change in morphology even after one
year. The total numbers of left- and right-handed double-helical
ropes, calculated for three different pictures, showed an ap-
proximately equal preference for each rope (34 and 27 species
for the left- and right-handed helical ropes, respectively). In
contrast, an equimolar mixture of the complementary T-10-T
and A-10-A components gave an amorphous gel (mp ) 202.0-
207.0 °C for the dried sample) (not shown).17 On the other hand,
spontaneous homoassembly of the heteroditopic bolaamphiphile
T-12-A also produced amorphous gel structures in 50% etha-
nolic-aqueous solutions. The shorter homologue T-10-A or
T-11-A self-assembled to form ordinary microcrystalline solids
(not shown).
pionyl],
N′-[3-(6-aminopurine-9-yl)propionyl]1,n-diamino-
alkane [T-n-A (n ) 10, 11, 12)], respectively] in which thymine
or adenine derivative is homoditopically or heteroditopically
connected via amide linkage to both ends of a long n-alkyl chain.
Except for solution studies by Schall and Gokel14 and uracil-
adenine-based self-assembly studies by Yanagawa et al.,15 there
have been no reports of well-resolved hierarchical nano- and
mesostructure construction using thymine-thymine homo- or
thymine-adenine heterobase interactions. In analogy with a
polynucleotide chain of DNA, a linear amide hydrogen bond
chain should facilitate the nucleobase stacking and interactions
in the absence of the normal sugar phosphate structures. We
are also able to freely tune the internucleobase interactions using
either purine-purine, purine-pyrimidine, or pyrimidine-
pyrimidine base pairing.16 In this paper, we describe the
formation of supramolecular nucleobase fibers through the
spontaneous homoassembly of homoditopic T-10-T or hetero-
ditopic T-12-A bolaamphiphiles or through the heteroassembly
of a 1:1 mixture of T-10-T and A-10-A.
Transmission Electron Microscopic Observation. To pre-
cisely characterize the resultant morphologies and their size
dimensions, we observed the individual self-assembled structures
using energy-filtering transmission electron microscopy (EF-
TEM).18 Unlike conventional TEM, EF-TEM provides a useful
tool for observing low-contrast organic samples without the need
for prior staining.19 The EF-TEM for the helical ropes of T-10-T
also showed clear images similar to those observed using light
microscopy (Figure 1b and c). High-resolution EF-TEM re-
vealed that the helical ropes are composed of very thin ribbons
and fibers with widths of more than 14 nm (Figure 1d), with
the nanoscale fibers themselves only loosely twisted or helically
wound. To date, only a few reports on well-resolved double-
helical strands or ribbons formed from chiral amphiphiles
exist,8a,20 describing only metastable, intermediate molecular
assemblies that slowly convert into higher ordered structures.20b
(9) (a) Shimizu, T.; Masuda, M. J. Am. Chem. Soc. 1997, 119, 2812-
2818. (b) Shimizu, T.; Masuda, M.; Kogiso, M.; Asakawa, M. Kobunshi
Ronbunsyu 1997, 54, 815-828. (c) Masuda, M.; Hanada, T.; Okada, Y.;
Yase, K.; Shimizu, T. Macromolecules 2000, 33, 9233-9238.
(10) (a) Shimizu, T.; Kogiso, M.; Masuda, M. Nature 1996, 383, 487-
488. (b) Kogiso, M.; Ohnishi, S.; Yase, K.; Masuda, M.; Shimizu, T.
Langmuir 1998, 14, 4978-4986. (c) Shimizu, T.; Ohnishi, S.; Kogiso, M.
Angew. Chem., Int. Ed. Engl. 1998, 37, 3260-3262.
(17) For 1:1 heteroassemblies formed from T-11-T/A-11-A, T-11-T/A-
12-A, T-12-T/A-12-A, and T-12-T/A-11-A in ethanol, no notable mor-
phologies were observed using light and transmission electron microscopy.
For details, see the Supporting Information.
(18) Reimer, L. Energy-Filtering Transmission Electron Microscopy;
Reimer, L., Ed.; Springer-Verlag: Berlin, 1995.
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Asai, M.; Shimizu, T. Langmuir 1999, 15, 4757-4764. (b) Lieser, G.;
Schmid, S. C.; Wegner, G. J. Microsc. 1996, 183, 53-59. (c) Lienemann,
C. P.; Mavrocordatos, D.; Perret, D. Mikrochim. Acta 1997, 126, 123-
129.
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119, 6209-6210. (b) Kogiso, M.; Masuda, M.; Shimizu, T. Supramol. Chem.
1998, 9, 183-189.
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232-234. (b) Masuda, M.; Shimizu, T. Carbohydr. Res. 1997, 302, 139-
147.
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Fowler, F. W.; Lauher, J. W. J. Am. Chem. Soc. 1997, 119, 86-93.
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Biochemistry 1992, 31, 4757-4765.
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1965, 87, 5241-5247.
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1989, 111, 4567-4570. (c) Sommerdijk, N. A. J. M.; Buynsters, P. J. J.
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R. J. M.; Zwanenburg, B. Chem. Eur. J. 1998, 4, 127-136.

1,ω-Nucleobase Bolaamphiphile Self-Assembly
J. Am. Chem. Soc., Vol. 123, No. 25, 2001 5949
Figure 1. (a) Dark-field light micrograph of double-helical ropes formed from the homoditopic T-10-T (at 25 °C in 10% ethanolic/aqueous
solutions, bar ) 5 µm). The arrows denote representative interwoven portions of the helical ropes. EF-TEM images of (b, c) the double-helical
ropes (bar ) 1 µm), (d) nanofibers as a constituent of the helical ropes (bar ) 1 µm), (e) nanofibers formed from an equimolar mixture of T-10-T
and A-10-A (bar ) 200 nm), and (f) nanofibers formed from the heteroditopic T-12-A (bar ) 1 µm).
To the best of our knowledge, the present finding gives a unique
example of stable micrometer-sized double-helical rope forma-
tion from an achiral component.
in Table 1 together with their assignments. Figure 2 displays
partial FT-IR spectra of the self-assemblies from the homo- and
heteroditopic bolaamphiphiles (n ) 10). For the dried helical
ropes of T-10-T, the amide and imide N-H bands appeared at
3329 and 3160 cm-1, respectively,22 clearly indicating the
formation of hydrogen bonds (Figure 2g). The appearance of
the amide I (CdO stretching) bands at 1655-1649 cm-1 is also
indicative of the formation of an amide hydrogen bond chain
within the homo- and heteroassemblies.23 The frequencies of
the CH2 antisymmetric and symmetric stretching bands are
compatible with high trans conformational populations in the
oligomethylene spacer.9,24 For the 1:1 heteroassembly, the amide
and imide N-H bands are remarkably shifted from those
observed for the single components.25 In particular, the lower-
frequency shift1a of the adenine N-H stretching band from 3137
An equimolar mixture of the complementary T-10-T and
A-10-A bolaamphiphiles was found to produce nanometer-sized
fibers (nanofibers) instead of the chiral double-helical ropes
observed for T-10-T (Figure 1e). The fibers are of uniform width
ranging from 15 to 30 nm, and no significant twisted and helical
morphologies could be detected. This implies that the presence
of A-10-A prevents the helical rope formation of T-10-T. The
EF-TEM also revealed that the resultant gel structure from the
heteroditopic T-12-A bolaamphiphile proved to be well-defined
nanofibers and nanoribbons of 20-100 nm in width and several
hundreds micrometers in length (Figure 1f). Though the base
pairing of the heteroassembly (T-n-T + A-n-A) should be
identical with that of the homoassembly (T-n-A), the T-n-A
bolaamphiphiles exhibited a chain-length effect21 that needs a
relatively longer oligomethylene spacer for the fiber formation.
Infrared Spectroscopy. Characteristic IR absorption bands
of the obtained homo- and heteroassemblies are summarized
(22) Kyougoku, Y.; Lord, R. C.; Rich, A. J. Am. Chem. Soc. 1967, 89,
496-504.
(23) (a) Buchet, R.; Sandorfy, S. J. Phys. Chem. 1983, 87, 275-280.
(b) Graindourze, M.; Smets, J.; Zeegers-Huyskens, T.; Maes, G. J. Mol.
Struct. 1990, 222, 345-364.
(24) Snyder, R. G.; Strauss, H. L. J. Phys. Chem. 1982, 86, 5145-5150.
(25) For FT-IR spectral change of the equimolar mixture of T-11-T/A-
11-A and T-12-T/A-12-A, see the Supporting Information.
(21) Masuda, M.; Vill, V,; Shimizu, T. J. Am. Chem. Soc. 2000, 122,
12327-12333.

5950 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Shimizu et al.
Table 1. FT-IR Absorption Bands and Their Assignment for the Dried Homo- and Heteroassemblies from T-10-T, A-10-A, and T-10-A
homoassemblya
1:1 heteroassemblya
assignment
amide N-H str
T-10-T
A-10-A
T-10-A
T-10-T/A-10-A
3329
3308
3424
3311
3316
amine N-H str
imide N-H str
3137
∼2750
∼2780
3206b
3108b
3160
3038
∼3223
ring C-H str
CH2 antisym str
CH2 sym str
aromatic CdO str
amide CdO str (amide I)
2928
2855
1694
1651
2930
2851
2932
2851
1687
1643
2928
2855
1695
1649
1655
1641c
1603
1543
NH2 scissoring + ring C-H str
amide N-H def (amide II)
1599
1541
1603
1543
1553
1470
CH2 scissoring def
1489
1450
731
1474
1464
729
1478
1461
723
CH2 rocking def
716
a Self-assembly was carried out in 10% ethanolic/aqueous solutions. b Unidentified shoulder. c Hydrogen-bonded NH2 scissoring plus ring C-H
stretching.
The CH2 scissoring bands also showed distinctive features
on moving between the homo- and heteroassemblies (Table 1).
The sharp band at 1470 cm-1 observed for T-10-T indicates
the presence of triclinic packing (T//)24 of the oligomethylene
chain within the helical ropes, whereas two separated bands at
1461 and 1478 cm-1 observed for the T-10-T/A-10-A hetero-
assembly suggest chain packing in an orthorhombic structure
(O ) within the nanofibers. The dried homo- and hetero-
assemblies from T-11-T, A-11-A, T-12-T, and A-12-A also
gave similar FT-IR results (Details are in the Supporting
Information).
Powder X-ray Diffraction. Powder X-ray diffraction dia-
grams of the dried assemblies from T-10-T, A-10-A, a 1:1
mixture of T-10-T/A-10-A, and T-10-A are shown in Figure
3. The small-angle diffraction pattern of the helical ropes from
T-10-T shows at least four ordered reflection peaks with a long
period of 3.48 nm (Figure 3a). The wide-angle reflection peaks
at d ) 0.393 and 0.375 nm are characteristic of triclinic packing
(T//),26 which is consistent with FT-IR results. The diffraction
diagram of the microcrystalline solids from A-10-A can be
similarly characterized by the reflection peaks, which suggest
a layered structure with a 2.36-nm period and orthorhombic
packing (O ) (Figure 3b). Once again, the X-ray diagram of
the heteroassembly (Figure 3c) is remarkably different from
those of its components, with at least two ordered reflection
peaks appearing with 2.53- and 1.62-nm-long periods. For this
heteroassembly, an orthorhombic packing (O ) of the oligo-
methylene chains supported by FT-IR can also be deduced by
three characteristic peaks at d ) 0.425, 0.393, and 0.367 nm.
Reflection peaks around d ) 0.48-0.49 nm, associated with
the interchain distance when linked with hydrogen-bonded
amide groups,27 appear for both homoassemblies (Figure 3a and
c). On the other hand, the powder X-ray diffraction patterns
for the homoassembly from the heteroditopic T-10-A bola-
amphiphile displayed a single reflection peak attributable to a
1.89-nm-long period as well as well-resolved three peaks (d )
Figure 2. FT-IR spectra (1800-4600 cm-1) of the T-10-T homo-
assemblies, the A-10-A homoassemblies, and the heteroassemblies
formed from their mixtures. Molar ratio of T-10-T to A-10-A: (a) 0.0,
(b) 0.2, (c) 0.4, (d) 0.5, (e) 0.6, (f) 0.8, and (g) 1.0. Partial FT-IR spectra
(∼2780 cm-1) of the equimolar mixture of T-11-T/A-11-A and T-12-
T/A-12-A are shown as an inset. FT-IR spectra (1800-4600 cm-1) of
the T-10-A homoassembly is also shown in (h).
to 2780 cm-1 (shoulder) and its broadening strongly suggest
the formation of complementary 2-D hydrogen bonds between
the thymine and adenine moieties (Figure 2a and d). The IR
spectral features of the self-assembled heteroditopic bola-
amphiphiles T-n-A (n ) 10, 11, 12) are compatible with those
of the corresponding heteroassemblies from T-n-T and A-n-A
(for n ) 10, see Figure 2d and h).
(26) (a) Garti, N.; Sato, K. Crystallization and Polymorphism of Fats
and Fatty Acids; Marcel Dekker: New York, 1988. (b) Yamada, N.;
Okuyama, K.; Serizawa, T.; Kawasaki, M.; Oshima, S. J. Chem. Soc., Perkin
Trans. 2 1996, 2707-2714. (c) Parikh, A. N.; Schivley, M. A.; Koo, E.;
K.Seshadri; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997,
119, 3135-3143. (d) Goto, M.; Asada, E. Bull. Chem. Soc. Jpn. 1978, 51,
2456-2459.
(27) Leiserowitz, L.; Tuval, M. Acta Crystallogr., Sect. B 1978, 34,
1230-1247.

1,ω-Nucleobase Bolaamphiphile Self-Assembly
J. Am. Chem. Soc., Vol. 123, No. 25, 2001 5951
thymine used as a starting material or the final product T-10-T
have no significant Cotton effect up to the detection limit of
the CD apparatus (θ < 0.003° for absorbance ∼1).31 However,
pyrimidine bases are known to reversibly photodimerize and
photodissociate by irradiation of higher (λ > 270 nm) and lower
(λ < 270 nm) wavelength UV light, respectively.32 It should
be noted here that the photodimerization between two thymine
derivatives affords four stereoisomers, among which cis-anti-
and trans-syn-type isomers are chiral.33 Under natural light,
however, no significant UV spectral changes were observed for
the aqueous solution containing T-10-T (c ) 9.2 × 10-5 M).
Photodimerization of the 1,ω-thymine bolaamphiphiles at both
ends should afford mixtures such as cyclic dimers, linear trimers,
and linear oligomers. Actually, monochromated UV irradiation
(λ ) 280 nm) of the same aqueous solutions caused a
photoreaction that can be confirmed by the constant decrease
in the 271-nm absorbance. After a 50-h irradiation in the
presence of acetone (20 wt %) as a sensitizer, we successfully
obtained macrocyclic photodimers cyclo(T-10-T)2 (Figure 4)
as a mixture of four isomers. In addition, we also prepared a
photodimeric mixture of 1-(2-carboxyethyl)thymine T-d-T in
a similar manner as that reported elsewhere.34
1
Next we compared H NMR spectra of the isolated double-
helical ropes with those of the photodimerized derivatives
cyclo(T-10-T)2 and T-d-T. The 1H NMR spectra of the distinct
photodimers can be characterized by four singlets around δ )
10.4-10.6 ppm and one singlet at δ ) 6.5 ppm (Figure 5b and
c). The former signals are assignable to non-hydrogen-bonded
imide NH protons of the photodimerized thymine moiety,
whereas the latter is assignable to hydrogen-bonded imide NH
protons of the photodimer. Figure 5a indicates that one small
(δ ) 8.30 ppm) and one tiny (δ ) 6.53 ppm) singlet newly
appear for the dried helical ropes. From the concentration
dependence of 1H NMR measurement using a nonaged T-10-T
sample, the singlet (m) at δ ) 8.30 ppm can be attributable to
hydrogen-bonded imide NH protons of the thymine. Meanwhile,
the tiny signal at δ ) 6.53 ppm (“Y”) appears at identical
chemical shift with the non-hydrogen-bonded imide NH protons
of the photodimer (Figure 5b and c). These findings strongly
suggest that during incubation trace amounts of photodimeric
impurities are generated under natural light. The chemical shifts
for the imide NH protons of the thymine dimers sensitively
reflect their stereoisomeric structures. We should note here that
a very tiny signal (“X”) at 10.6 ppm of the helical rope
corresponds well to the lowest magnetic field signal of the four
singlets. Furthermore, that signal is not attributable to a cis-syn
(signal q) and cis-anti (signal r) isomer.34 Therefore, a chiral
trans-syn isomer could be a plausible candidate of the trigger
substance.35 The isomer would provide enantiomeric chiral
sources in the assemblies and induce the formation of mirror-
imaged double-helical ropes. Indeed, the helical rope structures
were reproducible by the spiking experiment where the T-10-T
samples are incubated in the dark in the presence of 5% molar
macrocyclic photodimer cyclo(T-10-T)2. In contrast, the self-
assembly of the T-10-T molecules in the dark produced no
double-helical ropes.
Figure 3. Powder X-ray diffraction diagrams (reflection mode) of (a)
the T-10-T homoassemblies, (b) the A-10-A homoassemblies, (c) the
equimolar T-10-T/A-10-A heteroassemblies, and (d) the T-10-A
homoassemblies.
0.435, 0.409, and 0.364 nm) based on an orthorhombic
oligomethylene packing (O ) (Figure 3d). Similar layered
structures can also be confirmed for the homo- and hetero-
assemblies of the longer bolaamphiphiles T-11-T, A-11-A,
T-12-T, and A-12-A (Details are in the Supporting Information).
Discussion
A Possible Formation Mechanism of Double-Helical
Ropes. The double-helical rope formation from the achiral
T-10-T bolaamphiphile is incompatible with well-known results
showing that enantiomeric compounds should form mirror-
imaged assemblies.8,9,20,28-30 In fact, careful circular dichroism
(CD) study demonstrated that the aqueous solutions containing
(31) For high-resolution CD spectra of thymine and T-10-T, see the
Supporting Information.
(32) Wang, S. Y. Nature 1963, 200, 879-880.
(33) Wulff, D. L.; Fraenkel, G. Biochim. Biophys. Acta 1961, 51, 332-
339.
(34) Moghaddam, M. J.; Hozumi, S.; Inaki, Y.; Takemoto, K. J. Polym.
Sci. Polym. Chem. Ed. 1988, 26, 3297-3308.
(35) Isolation and identification of the photoreaction products from the
double-helical ropes were unsuccessful because of their formation in trace
amounts under these experimental conditions.
(28) (a) Tachibana, T.; Kambara, H. J. Am. Oil. Chem. Soc. 1965, 87,
3015-3016. (b)Nakashima, N.; Asakuma, S.; Kim, J.-M. Chem. Lett. 1984,
1709-1712.
(29) Fuhrhop, J.-H.; Schnieder, P.; Boekema, E.; Helfrich, W. J. Am.
Chem. Soc. 1988, 110, 2861-2867.
(30) Yamada, N.; Sasaki, T.; Murata, H.; Kunitake, T. Chem. Lett. 1989,
205-208.

5952 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Shimizu et al.
Figure 4. Molecular structures of (a) thymine bolaamphiphile (T-10-T), (b) macrocyclic photodimers [cyclo(T-10-T)2] of T-10-T, and (c) four
photodimerized stereoisomers (T-d-T) of the thymine carboxyethyl derivative.
Table 2. Molar Absorptivity (ꢀ) and Absorption Maximums (λmax
)
of T-10-T, A-10-A, a 1:1 Mixture of T-10-T/A-10-A, and Related
Compounds in 10% Ethanolic Solutions and Their Hypochromicity
UV spectral data
reference
λmax
λmax
nucleobase derivative
ꢀ
(nm)
ꢀR
(nm) H (%)
T-10-T
A-10-A
11500
9100
13650
17200
271
262
262
270
8700b 271
13300c 262
20600d
34a
66a
34e
11
47.6
47.3
30
T-10-T/A-10-A
bis(thymine)f
9700
272
purinophaneg
purinophaneh
pyrimidinopurinophanei
pyrimidinopurinophanej
25
a The value of hypochromicity H (%) was evaluated according to
the following equation: H (%) ) 100(1 - ꢀ/2ꢀR). b The molar
absorptivity of thymine. c The molar absorptivity of adenine. d Sum-
mation of the molar absorptivity of T-10-T and A-10-A. e The values
of hypochromicity H(%) were evaluated according to the following
equation: H (%) ) 100(1 - ꢀ/ꢀR). f 1,1′-(1,3-Propanediyl)bis[thy-
mine].37 g 1,3-Dithia[3,3](6,9)(9′,6′)purinophane in water.36 h 1,3-
Dithia[3,3](6,9)(9′,6′)purinophane in ethanol.38 i In water.39 j In etha-
nol.39
inhibition by adenine has already been reported for the photo-
dimerization of thymine-carrying polymers.36 Thus, the present
results may give an interesting example of photochemical
mutation within supramolecular assemblies by UV light. In
DNA, this has a fatal effect on biological systems.
Base Stacking in Homo- and Heteroassemblies. Inter-
nucleobase stacking and complementary hydrogen bond forma-
tion play an important role in stabilizing the double-helix
formation of polynucleotide chains in DNA. To confirm the
base stacking interaction in the homo- and heteroassemblies
from T-10-T and A-10-A, we examined their UV spectra in
diluted aqueous solutions (c ) 7 × 10-5 M). The resulting
hypochromism (decrease in UV absorption intensity) was com-
pared with that of related compounds such as bis(thymine)s,37
-(purinophane)s,38 and -(pyrimidinopurinophane)s.39 The UV
absorption data of T-10-T, A-10-A, a 1:1 mixture of T-10-T/
A-10-A complex, and related compounds are summarized in
Figure 5. Partial 1H NMR spectra (600 MHz, at 25 °C in DMSO-d6)
of (a) dried double-helical ropes of T-10-T, (b) macrocyclic photodimers
[cyclo(T-10-T)2], and (c) photodimers (T-d-T) of the carboxyethyl
thymine derivative. Thymine imide N-H, thymine ring C-H, and
amide N-H protons of T-10-T are denoted by the symbols l, p, and n,
respectively. New signals generated by the photoreaction are denoted
by the symbols X and Y.
Once the complementary base pairing of the thymine and
adenine is formed, such a situation will hinder the thymine
moiety from taking an appropriate orientation for the photo-
dimerization. Consequently, the heteroassembly of T-10-T and
A-10-A will result into no helical rope formation. Osmotic and
1H NMR studies indicate that purine-pyrimidine interactions
are more favorable than those between pyrimidine bases.16 This
means that the thymine photodimerization is prevented in the
presence of the complementary adenine component. Similar
(36) Kita, Y.; Inaki, Y.; Takemoto, K. J. Polym. Sci., Polym. Chem. Ed.
1980, 18, 427-439.
(37) Itahara, T. Bull. Chem. Soc. Jpn. 1997, 70, 2239-2247.

1,ω-Nucleobase Bolaamphiphile Self-Assembly
J. Am. Chem. Soc., Vol. 123, No. 25, 2001 5953
Figure 6. A possible molecular packing and hydrogen bond scheme for (a) the heteroassembly formed from an equimolar mixture of T-10-T and
A-10-A and (b) the homoassembly from T-10-A. (a, b) Top view of a layered structure composed of linear polymolecular arrays (“reversed”
Hoogsteen base pair configuration is employed here for the thymine-adenine heteroassociation). (c) Front view showing 2-D complementary and
1-D amide hydrogen bond network. (d) Side view of the polymolecular arrays. In (d), the one-dimensional amide hydrogen bond chain contributes
to the stabilization of the base stacking and the formation of complementary hydrogen bonds.
Table 2. The hypochromicity values H (%) were calculated by
a comparison of the absorption intensity with that of a
monomeric reference. The H value of T-10-T was evaluated to
be 34%, which is, to the best our knowledge, the largest
hypochromicity value reported for thymine-teriminated bola-
form compounds such as 1,1′-(R,ω-alkanediyl)bis(thymine)s.37
The 1,ω-adenine bolaamphiphile A-10-A also indicated a much
larger H value than those so far reported.38,39 In general, the
stacking interaction between pyrimidine bases is considerably
smaller than that between purine bases.16 Therefore, the obtained
H value of T-10-T strongly supports the effective stacking of
the pyrimidine bases in the solution. This situation is presumably
due to the fixation and stabilization of the bases through the
formation of the amide hydrogen bond chain.
Molecular Packing. The X-ray diffraction of the T-10-T
homoassembly displayed typical reflection peaks (d ) 0.340
and 0.349 nm, in Figure 3a) indicative of the formation of
vertical base stacking.6 In addition, its small-angle region
suggests that the helical ropes consist of multiple monolayers
with a period of 3.48 nm. From the CPK molecular modeling,
a fully extended molecular length of T-10-T was estimated to
be 3.33 nm. If T-10-T takes an extended conformation, the
molecules should be arranged parallel with respect to the normal
to the layer plane. Similarly, the IR and X-ray diffraction studies
of the heteroassemblies from T-10-T and A-10-A indicate the
formation of stable layered structures. Considering the two long
periods of 2.53 and 1.62 nm (Figure 3c), we could depict
lamellar organization in which the molecules should be arranged
with tilting to the normal to the layer plane (Figure 6a).
Furthermore, the molecules should form linear polymolecular
chains through 2-D complementary hydrogen bonds (Figure 6c).
The 0.489-nm methylene chain distance supported by X-ray
diffraction necessitates tilting of the base plane to achieve the
0.34-nm distance favored for internucleobase stacking (Figure
6d). The above features well resemble the layered 3-D organiza-
tion of related 1-glucosamide bolaamphiphiles.9a,12
X-ray diffraction data indicated the 1.89-nm-long period for
the homoassembly of the heteroditopic T-10-A bolaamphiphile
(Figure 6b), which supports a molecular packing similar to that
of the heteroassembly from T-10-T and A-10-A. Complemen-
tary hydrogen bond formation between the heteroditopic mo-
lecular ends can also be supported by the FT-IR measurement.
In addition, the formation of a linear amide hydrogen bond chain
should direct and stabilize the supramolecular nucleobase fiber
formation. It is of great interest that the heteroditopic T-12-A
bolaamphiphile prefers the high aspect ratio structures rather
than the microcrystalline solids. This chain-length dependence
on the self-assembled morphologies is consistent with that
observed for the fiber formation from valylvaline-based bola-
amphiphiles.21,40 In the case of shorter T-n-A derivatives, they
will need relatively stronger internucleobase interactions in order
(38) Seyama, F.; Akahori, K.; Sakata, Y.; Misumi, S.; Aida, M.; Nagata,
C. J. Am. Chem. Soc., 1988, 110, 2192-2201.
(39) Doyama, K.; Higashii, T.; Seyama, F.; Sakata, Y. Misumi, S. Bull.
Chem. Soc. Jpn. 1988, 61, 3619-3627.

5954 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Shimizu et al.
trichloroacetate (1.2 g, 2.9 mmol) and triethylamine (0.4 mL) were
added. The reaction mixture was stirred at room temperature for 20 h.
The resulting mixture was then evaporated in vacuo to give a solid,
which was thoroughly washed with cold water. Recrystallization of
the raw product from a 1:1 mixture of chloroform and methanol (v/v)
gave pentachlorophenyl 3-(2, 4-dihydroxy-5-methylpyrimidine-1-yl)-
propionate as a colorless needle crystal (0.4 g, yield 36%). This activated
ester (0.3 g, 0.7 mmol) and 1,10-diaminodecane (0.06 g, 0.35 mmol)
were then dissolved in DMF (20 mL), and the mixture was stirred at
room temperature overnight. The reaction mixture was then evaporated
in vacuo to give a solid, which was thoroughly washed with acetone.
Purification by silica gel column chromatography (eluent: chloroform/
methanol ) 2/1, v/v) gave T-10-T as a white powder (0.09 g, 50%):
mp 210.0-212.5 °C; TLC (silica gel, 1:1 CHCl3/MeOH) Rf ) 0.8; 1H
NMR (600 MHz, DMSO-d6, 25 °C) δ 1.20-1.35 (m, 16H, -CH2-),
1.71 (s, 6H, CH3-5), 2.41 (t, J ) 6.6 Hz, 4H, -CH2CH2CONH-), 3.00
(dt, J ) 6.6, 6.6 and 7.2 Hz, 4H, -CONHCH2-), 3.81 (t, J ) 6.6 Hz,
4H, -CH2CH2CONH-), 7.37 (s, 2H, H-6), 7.92 (t, J ) 5.4 Hz, 2H,
-CONH-), 11.2 (s, 2H, NH-3). Anal. Calcd for C26H40N6O6: C, 58.63;
H, 7.57; N, 15.78. Found: C, 58.77; H, 7.82; N, 15.89.
to compensate weak van der Waals forces between the alkyl
chains. As a result, strong interlayer interaction between the
monolayers might have resulted in 3-D crystalline solids. Thus,
the self-assembled morphologies, such as solids, fibers, and
double-helical ropes, are found to be strongly dependent on the
force strength of the internucleobase interaction. We should note
here that four arrangements of the base pairs including Watson-
Crick41 and Hoogsteen types42 are possible for the configuration
of the adenine-thymine base pairs. At present we have,
however, no definite evidence for the lateral or vertical
arrangement of the individual polymolecular chains.
In summary, spontaneous homoassembly of the 1,ω-thymine
bolaamphiphile T-10-T produced unprecedented double-helical
ropes. The formation may be triggered by trace amounts of
photodimerization products within the assemblies. Spontaneous
heteroassembly of the complementary 1,ω-thymine and -adenine
components and homoassembly of the heteroditopic homologues
yielded stable nanofibers instead of the helical ropes. Inter-
nucleobase interaction between the resultant monolayers domi-
nated a variety of self-assembled morphologies. The linear amide
hydrogen bond chain, like a polynucleotide chain within DNA,
was found to stabilize the base stacking and their complementary
double hydrogen bonds by fixing them in the internal hydro-
phobic environment.
N,N′-Bis[3-(6-aminopurine-9-yl)propionyl]1,10-diaminodecane (A-
10-A). Into a solution of 9-(2-carboxyethyl)adenine (0.4 g, 1.9 mmol)
and p-nitrophenol (0.3 g, 2.2 mmol) in pyridine (20 mL), p-nitrophenyl
trifluoroacetate (2.0 g, 8.5 mmol) was added with stirring. The reaction
mixture was then stirred at 50 °C for 2 h and then evaporated in vacuo.
Solid residues obtained were dissolved in a small amount of chloroform
and reprecipitated by addition of diethyl ether. Further reprecipitation
from a 1:1 mixture of chloroform and diethyl ether gave p-nitrophenyl
3-(6-trifluoroacetoamidopurine-9-yl)propionate as a white powder (0.6
g, yield 75%). Into a solution of this material (0.5 g, 1.25 mmol) in
DMF (20 mL), 1, 10-diaminododecane (0.1 g, 0.58 mmol) and
imidazole (0.1 g, 1.47 mmol) were added. The mixture was stirred at
room temperature for 3 days. The resultant mixture was then evaporated
in vacuo to give a raw product, which was thoroughly washed with
diethyl ether. Purification by silica gel column chromatography
(eluent: CHCl3/MeOH ) 9/1-4/1, v/v, gradient) afforded N, N′-bis-
[3-(6-trifluoroacetoamidopurine-9-yl)propionyl]1,10-diaminodecane (0.1
g, 40%). Deprotection of trifluroacetyl group was carried out by stirring
this material in an queous methanolic solution (H2O/MeOH ) 2/5, v/v,
20 mL) of potassium carbonate (7 wt %) overnight. Evaporation of
the solvent gave a solid residue which was thoroughly washed with
water. Reprecipitation from a 1:1 mixture of ethanol and water (v/v)
afforded A-10-A as a white powder (0.07 g, 93%): mp 220.1-221.5
Experimental Section
Materials and General Methods. The nucleobases thymine (>98%)
and adenine (>99%) were purchased from Merck and were used as
received. 1-(2-Carboxyethyl)thymine and 9-(2-carboxyethyl)adenine
were synthesized according to a previously described method.43 Other
chemicals were commercially available high-purity grades and were
used without further purification. The structures of intermediates and
the final products were confirmed by FT-IR, NMR spectroscopy, and
1
elemental analysis. H NMR spectra were recorded on a JEOL 600
spectrometer using tetramethylsilane as an internal standard. For the
FT-IR measurement, a Jasco FT-IR-620 (resolution 4 cm-1) was used.
The UV absorption spectra were recorded at 25 °C on a Hitachi U-3300
spectrometer. CD measurement was performed using a Jasco J-725
spectrometer (sensitivity 0.005°, cell length 5 and 10 mm, temperature
25-70 °C).
1
°C; TLC (silica gel, 1:1 CHCl3/methanol) Rf ) 0.5; H NMR (600
Synthesis of 1,ω-Nucleobase-Appended Bolaamphiphiles. The
homoditopic 1,ω-thymine bolaamphiphiles T-n-T (n ) 10, 11, 12) were
prepared via the coupling of pentachlorophenyl ester of 1-(2-carboxy-
ethyl)thymine with the corresponding long-chain diamines. Similarly,
the 1,ω-adenine bolaamphiphiles A-n-A (n ) 10, 11, 12) were
synthesized via the coupling of p-nitrophenyl ester of 6-trifluoroacety-
lated 9-(2-carboxyethyl)adenine with the corresponding diamines. On
the other hand, the 1,ω-(thymine,amine) bolaamphiphiles T-n-NH2 (n
) 10, 11, 12) were prepared via the coupling of pentachlorophenyl
ester of 1-(2-carboxyethyl)thymine with an excess amount of corre-
sponding long-chain diamines (10 equiv). Consequently, the heterodi-
topic T-n-A bolaamphiphiles (n ) 10, 11, 12) were obtainable by the
coupling of T-n-NH2 with p-nitrophenyl ester of 6-trifluoroacetylated
9-(2-carboxyethyl)adenine. All the final products were then purified
by silica gel column chromatography (Yamazen medium-pressure
column chromatography system YFLC-6004-FC-GR) to yield white
solids. A typical synthetic procedure and related analytical data are as
follows.
MHz, DMSO-d6, 25 °C) δ 1.17-1.30 (m, 16H, -CH2-), 2.65 (t, J )
6.6 Hz, 4H, -CH2CH2CONH-), 2.97 (dt, J ) 6.0, 6.0, and 7.2 Hz,
4H, -CONHCH2-), 4.33 (t, J ) 6.6 Hz, 4H, -CH2CH2CONH-), 7.21
(s, 4H, NH2-6), 7.88 (t, J ) 5.4 Hz, 2H, -CONH-), 7.98 (s, 2H, H-8),
8.13 (s, 2H, H-2). Anal. Calcd for C26H38N12O2: C, 57.61; H, 6.96; N,
30.52. Found: C, 57.55; H, 7.07; N, 30.49.
N-[3-(2,4-Dihydroxy-5-methylpyrimidine-1-yl)propionyl], N′-[3-
(6-aminopurine-9-yl)propionyl]1,10-diaminodecane (T-10-A). The
thymine carboxyethyl derivative, pentachlorophenyl 3-(2, 4-dihydroxy-
5-methylpyrimidine-1-yl)propionate (0.3 g, 0.7 mmol), and 1, 10-
diaminodecane (1.20 g, 7.0 mmol) were then dissolved in DMF (20
mL), and the mixture was stirred at room temperature overnight. The
reaction mixture was then evaporated in vacuo to give a solid, which
was thoroughly washed with acetone. Purification by silica gel column
chromatography (eluent: chloroform/methanol ) 2/1, v/v) gave T-10-
NH2 as a white powder (0.09 g, 50%). Into a solution of p-nitrophenyl
3-(6-trifluoroacetoamidopurine-9-yl)propionate (0.5 g, 1.25 mmol) in
DMF (20 mL), T-10-NH2 (0.1 g, 0.58 mmol) and imidazole (0.1 g,
1.47 mmol) were added. The mixture was stirred at room temperature
for 3 days. The resultant mixture was then evaporated in vacuo to give
a raw product, which was thoroughly washed with diethyl ether.
Purification by silica gel column chromatography (eluent: CHCl3/
MeOH ) 9/1∼4/1, v/v, gradient) afforded N-[3-(2,4-dihydroxy-5-
methylpyrimidine-1-yl)propionyl]-N′-[3-(6-aminopurine-9-yl)-
propionyl]1,n-diaminodecane (0.1 g, 40%): mp 206.0-206.7 °C; TLC
(silica gel, 1:1 CHCl3/MeOH) Rf ) 0.6; 1H NMR (600 MHz, DMSO-
d6, 25 °C) δ 1.20-1.32 (m, 16H, -CH2-), 1.71 (s, 3H, Thy-CH3-5),
N,N′-Bis[3-(2,4-dihydroxy-5-methylpyrimidine-1-yl)propionyl]-
1,10-diaminodecane (T-10-T). Into a solution of 1-(2-carboxyethyl)-
thymine (0.5 g, 2.6 mmol) in DMF (10 mL), pentachlorophenyl
(40) (a) Kogiso, M,; Hanada, T.; Yase, K.; Shimizu, T. Chem. Commun.
1998, 1791-1792. (b) Kogiso, M.; Okada, Y.; Hanada, T.; Yase, K.;
Shimizu, T. Biochim. Biophys. Acta 2000, 1475, 346-352.
(41) Seeman, N. C.; Rosenberg, J. M.; Suddath, F. L.; Kim, J. J. P.;
Rich, A. J. Mol. Biol. 1976, 104, 109-144.
(42) Hoogsteen, K. Acta Crystallogr. 1963, 16, 907-916.
(43) Kondo, K.; Sato, T.; Takemoto, K. Chem. Lett. 1973, 967-968.

1,ω-Nucleobase Bolaamphiphile Self-Assembly
J. Am. Chem. Soc., Vol. 123, No. 25, 2001 5955
2.41 (t, J ) 6.6 Hz, 2H, Thy-(CH2)2CONHCH2CH2-), 2.65 (t, J )
6.6 Hz, 2H, Ade-(CH2)2CONHCH2CH2-), 2.98 (m, 4H, -CONHCH2-
), 3.81 (t, J ) 6.6 Hz, 2H, Thy-CH2CH2CONH-), 4.33 (t, J ) 6.6 Hz,
2H, Ade-CH2CH2CONH-), 7.16 (s, 2H, Ade-NH2-6), 7.34 (s, 1H,
Thy-H-6), 7.89-7.93 (m, 2H, Thy-(CH2)2-CONH-, Ade-(CH2)2-
CONH-), 7.98 (s, 1H, Ade-H-8), 8.13 (s, 1H, Ade-H-2), 11.2 (s, 1H,
Thy-NH-3). Anal. Calcd for C26H39N9O4: C, 57.65; H, 7.26; N, 22.27.
Found: C, 57.89; H, 7.56; N, 22.12.
RAD-C powder diffractometer (40 kV and 35 mA) and Rigaku Rotaflex
generator RU-200 (40 kV and 150 mA), respectively. The Cu KR beam
was obtained via a graphite monochromator (reflection mode) or a
nickel filter (transmission mode). The spectra (reflection method) were
measured at room temperature between 1° and 36° in the 2θ/θ-scan
mode with steps of 0.01° in 2θ and 0.6-s measurement time per step.
The diffraction pattern (transmission method) was recorded with an
imaging plate (Fuji Film Ltd., CR ST-VA) in a flat camera.
Spontaneous Homo- and Heteroassembly. Unless otherwise noted,
self-assembly experiments were carried out in either 10% ethanolic
deionized aqueous solutions for T-10-T, A-10-A, and T-10-A or in
ethanol for T-n-T, A-n-A, and T-n-A (n ) 11, 12). Ethanol was
spectroscopic grade from Wako Chemicals. For the homo- and hetero-
self-assemblies of T-10-T, A-10-A, and T-10-A, the aqueous solutions
(∼5 × 10-4 M) were prepared by sonication of a weighed bola-
amphiphile sample or their equimolar mixtures at 30 °C (Branson
ultrasonicator model 1200, 47 kHz, 60 W). Each solution was allowed
to stand at room temperature for 2-3 days. Self-assembled molecular
objects in the solutions were then subjected to light microscopy.
Preliminary differential scanning calorimetry (DSC) and polarized light
microscopic study showed that no thermotropic mesophases were
obtained for the dried 1:1 mixtures of T-n-T and A-n-A (n ) 10, 11,
12).44
Photoreaction of 1,ω-Thymine Bolaamphiphiles. Photoreactions
of T-10-T in 10% ethanolic-aqueous solutions (c ) 9.2 ×10-5-4.7 ×
10-4 M) were carried out in a 10-mm quartz cell at room temperature.
The solutions containing acetone (5-20 wt %) as a sensitizer were
irradiated with a monochromated high-pressure mercury lamp (λ )
280 nm). The photodimerization was monitored by UV absorbance at
271 nm. Exposure of more than 100 h showed complete disappearance
of the 271-nm absorption band. The production of an isomeric mixture
1
of macrocyclic photodimeric derivatives was confirmed by H NMR
and FAB-MASS (m/z ) 1064 [M + 2Na]+). Photodimers of 1-(2-
carboxyethyl)thymine were also prepared in a manner similar to that
described by Inaki et al.34
Acknowledgment. We thank Prof. Y. Inaki of Osaka
University for helpful discussions and Dr. T. Takakuwa of Jasco
Corp. for CD measurements.
This paper is dedicated to Naresh Dalal on the occasion of
his 60th birthday.
Light Microscopy and Energy-Filtering Transmission Electron
Microscopy. Light microscopic measurement was carried out in a way
similar to those described in detail elsewhere.9a Unstained specimens
for electron microscopy were prepared by placing a 3-µL drop of the
supernatant of the dispersion on an amorphous carbon supporting film
mounted on a standard TEM grid. The drop was blotted off with filter
paper. The dried specimens were examined by electron spectroscopic
imaging using an electron microscope (Carl Zeiss EM902) operated at
80 keV with a Castain-Henry-type electron energy filter at room
temperature. Zero-loss bright-field images were recorded on an imaging
plate (Fuji Photo Film Co., Ltd. FDL 5000) with 20-eV energy windows
at 3000-250000× and digitally enlarged.
Supporting Information Available: Additional experimen-
tal data (analytical data, FT-IR spectra, XRD diagrams, and
tables showing spectral peak positions) for the homo- and
heteroassemblies from the longer T-11-T, A-11-A, T-12-T, and
A-12-A bolaamphiphiles, which are not discussed above; Light
micrographs of the T-10-T or A-10-A homoassembly; CD
spectra of thymine and T-10-T bolaamphiphile (PDF). This
material is available free of charge via the Internet at http://
pubs.acs.org. See any current masthead page for ordering
information and Web access instructions.
Powder X-ray Diffraction. Dried samples of the homo- and
heteroassemblies were prepared by the careful isolation of the fibers
and ropes and their subsequent drying in vacuo. All powder spectra
were taken by the reflection or the transmission method with a Rigaku
(44) Paleos, C. M.; Michas, J. Liq. Cryst, 1992, 11, 773-778.
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