Rosette Nanotubes: Factors Affecting the Self-assembly of the Monobases Versus the Twin Base System
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Rosette Nanotubes: Factors Affecting the Self-assembly of the Monobases Versus the Twin Base System Usha Hemraz, and Hicham Fenniri National Institute for Nanotechnology, Chemistry Department, University of Alberta, 11421 Saskatchewan Drive, Edmonton AB, T6G2M9, Canada ABSTRACT Rosette Nanotubes (RNTs) are formed by the self-assembly of a guanine-cytosine motif (G∧C), a hybrid of the DNA bases guanine and cytosine, to give a six membered macrocycle maintained by 18 H-bonds. In theory, any moiety covalently attached to the G∧C base can be expressed on the nanotubes surface. However we anticipate that the self-assembly and stability of these functionalised RNTs will also be governed by steric effects. Herein we describe the synthesis and the self assembly of the Twin Base Lysine (TBL-K) and its monobase (MBL-K). While TBL-K self-assembles readily in water and methanol to give nanotubular structures, MBL-K does not form nanotubes. Various techniques were used to characterize the RNTs and the factors, preventing self-assembly in the case of MBL-K, were investigated. INTRODUCTION Self-assembly has emerged as a very powerful way to rationally build large and chemically complex supramolecular structures, which are barely accessible by traditional covalent chemistry [1]. The G∧C motif [2], possessing the Watson-Crick donor-donor-acceptor of guanine and acceptor-donor-donor of cytosine (Figure 1) has been shown to undergo a selfassembly process by an array of H-bonds [3]. While one may imagine it is theoretically possible to express any moiety covalently attached to the G∧C motif onto the nanotube surface; it is still a challenge to balance enthalpic and entropic factors to generate a stable supramolecular architecture. O N N D
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N G^C motif N
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Rosette
Figure 1: The G∧C motif self-assembles to form a 6-membered supermacrocycle (rosette), maintained by 18 H-bonds.
In an earlier report, we have shown how compound 1 (Figure 2) was a better candidate for self-assembly in water due to lower charge density, less steric repulsions and twice the number of hydrogen bonds per module relative to its monobase analogue (12 instead of 6) [4].
Figure 2: Self assembly of 1 into a twin rosette held by 36 H-bonds, followed by stacking to form RNTs. Here we describe the synthesis and characterization of a number of molecules and the conditions under which they were made to undergo nanotube formation. While in these cases, the steric repulsions and the number of H-bonds (18) were intact, we managed to stabilize the net positive charge on the molecules, thus leading to self-assembled nanotubes. EXPERIMENTAL DETAILS NH2 HCl
FmocHN
HBTU, DIEA
6 O
FmocHN NHBoc
HO
9 BocN
O
NHBoc O
8
OBn
NBoc2
N
N N
NHBoc H N
DMF, 90 % H2N NHBoc
NHBoc 7 OBn
Piperidine
NHBoc H N
DMF, 88 %
N
O
NBoc2
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