Chiral Self-Assembly of
Designed Amphiphiles: Optimization
for Nanotube Formation
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Abstract
Four amphiphiles with l-aspartic acid headgroups
(Asp)
and a diphenyldiazenyl group (Azo) contained within the hydrophobic
tails were designed and synthesized for self-assembly into helically
based nanotubes. The amphiphiles of the form <i>R</i>′<i>-</i>{4-[(4-alkylphenyl)diazenyl]phenoxy}alkanoyl-l-aspartic acid (where <i>R</i>′ is 10 or 11) varied
only in alkyl chain lengths either side of the azo group, having 4,
7, or 10 carbon distal chains and 10 or 11 carbon proximal chains
(<i>R</i>-Azo-<i>R</i>′-Asp, where <i>R</i> denotes the number of carbons in the distal chain and <i>R</i>′ denotes the number of carbons in the proximal
chain). Despite the molecular similarities, distinct differences were
identified in the chiral order of the structures self-assembled from
hot methanolic aqueous solutions using microscopy and spectroscopic
analyses. This was reflected in dominant thermodynamic aggregate morphologies
that ranged from amorphous material for 10-Azo-10-Asp, through twisted
ribbons (196 ± 49 nm pitch) for 7-Azo-11-Asp, to the desired
helically based nanotubes for 4- and 7-Azo-10-Asp (81 ± 11 and
76 ± 6 nm diameters, respectively). Another key variable in the
self-assembly of the amphiphiles was the use of a second method to
precipitate aggregates from solution at room temperature. This method
enabled the isolation of thermodynamically unstable and key transitional
structures. Helical ribbons were precursor structures to the nanotubes
formed from 4- and 7-Azo-10-Asp as well as the wide, flattened nanotube
structures (587 ± 85 nm width) found for 4-Azo-10-Asp. Overall,
the results highlighted the interplay of influence of the headgroup
and the hydrophobic tail on self-assembly, providing a basis for future
rational design of self-assembling amphiphiles