3854
J. Am. Chem. Soc. 2001, 123, 3854-3855
Helical Rosette Nanotubes: Design, Self-Assembly,
and Characterization
Hicham Fenniri,* Packiarajan Mathivanan, Kenrick L. Vidale,
Debra M. Sherman,‡ Klaas Hallenga,† Karl V. Wood,§ and
Joseph G. Stowell|
1393 H. C. Brown Chemistry Laboratory
Purdue UniVersity West Lafayette, Indiana 47907-1393
ReceiVed December 14, 2000
Organic,1 inorganic,2 and surfactant-derived3 discrete tubular
architectures have been the subject of intense investigation in the
fields of materials science, nanotechnology, molecular electronic
and photonic devices, sensor and artificial channel systems. Here
we present evidence in support of a modus operandi for the
hierarchical self-assembly of organic nanotubes from self-
assembled supermacrocycles (rosettes)3f,g,4 of low-molecular
weight synthetic modules, under physiological conditions.
For a system based on hydrogen bonds to self-assemble in water
one has to balance the enthalpic loss (H-bonds) with a consequent
entropic gain (stacking interactions and hydrophobic effect). If
preorganized, ionic H-bonds could also add to the enthalpic term.
Nature has ingeniously taken advantage of these design principles
to compartmentalize the cell (membranes), and to create thermo-
dynamically favorable pathways for protein and nucleic acids
folding. With this in mind, self-assembling L-module 1 (Figure
1a) was designed and synthesized with the following features:
(a) a hydrophobic base unit possessing the Watson-Crick donor-
donor-acceptor (DDA) H-bond array of guanine and acceptor-
acceptor-donor (AAD) of cytosine. The spatial arrangement of
these arrays constrains 1 to form a six-membered supermacrocycle
(rosette, Figure 1b).4a,b (b) A methyl group (AHNCH3) was
introduced to minimize peripheral access of water and to enforce
the formation of an intramolecular ionic hydrogen bond between
the side chain secondary ammonium and the neighboring ring
carbonyl. (c) An ethylene spacer unit linking the base component
to the chiral center was chosen to allow for said intramolecular
Figure 1. Hierarchical self-assembly of rosette nanotubes from L-moldule
1. (a) Modules 1-4 synthesized and investigated. (b) Molecular model
of the rosette structure resulting from 1. The thin yellow lines show the
hydrogen-bond network. The thick yellow lines highlight unique inter-
modular NOEs recorded. (c) Molecular model of the proposed nanotube.
Eighteen rosettes were arranged in a tubular fashion with a starting inter-
plane distance of 4.5 Å and 30° rotation along the tube’s main axis.6 The
inner solvent-accessible surface area of the tube is highlighted in red.
ionic H-bond. (d) An amino acid moiety that dictates the
supramolecular chirality of the resulting assembly was chosen.
This tri-block design endows 1 with elements essential for the
sequential self-assembly into stable nanotubular architectures
(Figure 1c). Although a few natural5 and synthetic3f,g,4 compounds
were shown to form supermacrocyclic assemblies in organic
solvents, and in solid and liquid crystalline states, none are known
to self-assemble into supermacrocycles or discrete nanotubular
assemblies in water.
A synthetic scheme was devised to allow for oligomerization
and functionalization at virtually any position of 1.6 2-4 were
prepared to establish the role of the stacking interactions,
hydrophobic effect, and the chirality of the side chain in the
assembly of the tubular architectures.
‡ Electron Microscopy Facility-Agriculture, Purdue University.
† NMR Center, Purdue University.
§ Mass Spectrometry Center, Purdue University.
| School of Pharmacy, Purdue University.
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other NOEs or imino proton signals resulting from nonassembled
1 or nonspecific aggregates thereof were observed. In agreement
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of dilute aqueous solutions of 1 displayed all the peaks corre-
sponding to the noncovalent intermediate species (1-mer to 6-mer)
of the parent rosette.6 Under the same conditions an equimolar
aqueous mixture of the complementary, rosette forming, 2 and 3
did not undergo self-assembly (data not shown).
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(6) See Supporting Information.
10.1021/ja005886l CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/31/2001