Fig. 3 (a)–(f) Transmission electron micrographs of a 0.1% (m/m) dispersions of 1 containing 0.25 molar equivalents of copper(ii) triflate. (a) Pt shadowing,
bar 5 mm, (b) negative staining, bar 1 mm, (c) Pt shadowing, bar 250 nm, (d) Pt shadowing, bar 500 nm, (e) Pt shadowing, bar 500 nm, (f) Pt shadowing, bar
200 nm. Insets: graphic representations of the observed structures. (g) Schematic representation of the process of boomerang formation is deduced from the
different observed structures, (h) optical micrograph of protoplasts at different stages of growth after addition of fusicoccin, kindly provided by Dr A. N. M.
van Amstel.6
When copper triflate was added to 1 in a 1:4 ratio,‡
boomerang-like scrolls were generated [Fig. 3(a), (b)]. These
had different thicknesses, but all showed a left-handed turn
[Fig. 3(d), (f)]. Electron micrographs taken of samples stained
with 2% uranyl acetate indicated that these scrolls were not
massive, but contained an aqueous interior [Fig. 3(b)]. The
different stages of scroll formation shown in Fig. 3 suggest that
these boomerangs are formed by stretching and twisting of the
vesicle membranes in a stereospecific manner. According to the
overall shape of the scrolls, this twisting starts initially at
opposite sides of the vesicle [Fig. 3(g)], resulting in a structure
with a thickened centre and thin twisted ends [Fig. 3(c)], and
eventually in thinner species which are also helical in the central
part of the aggregate [Fig. 3(e)]. The aggregate structures in
Fig. 3 show features that strongly resemble the spiral growth of
protoplasts induced by the action of fusicoccin as reported by
van Amstel [Fig. 3(h)].6
The authors thank H. P. M. Geurts for his assistance in
performing electron microscopy experiments, W. Wong for
providing artwork and Professor G. I. Tesser and P. J. H. M.
Adams for the kind donation of Boc-protected l -histidine and
helpful discussions.
Footnotes
† Surfactant 1 was conveniently prepared from Boc-protected l -histidine.
One alkyl chain was introduced by a DCC coupling with octadecylamine
(yield 60%). Deprotection of this Boc–amide with trifluoroacetic acid and
subsequent acylation with N-stearoyloxy–succinimide resulted in the
introduction of the second alkyl chain (yield 75%). Anal. 1 Calc. C, 74.9; H,
12.0; N, 8.3. Found: C, 74.2; H, 11.5; N, 8.4%.
‡ Comparable ligand systems have been shown to form copper complexes
of the type [Cu(Him)4] (Him = imidazole), see ref. 4b.
References
Remarkably, complexation of copper triflate leads to a
monolayer which displays an even higher degree of compressi-
bility, as compared to the monolayers formed on subphases of
pure water adjusted to pH 6.5 and 2.5 (Fig. 2). This implies that
addition of copper triflate leads to a lower degree of pre-
organisation in the two-dimensional surfactant layer. This is
most likely due to a preference of hydrogen-bond formation
between molecules within the same complex, as opposed to the
formation of long linear arrays of intermolecular hydrogen
bonds. A similar situation may be present in the three-
dimensional aggregates allowing the molecules to pack in such
a way that their chirality is expressed at the supramolecular
level. The boomerang shaped aggregates may be considered as
being metastable states in the transformation of closed bilayer
structures of helical ribbons. Further studies are in progress.
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Received, 19th December 1996; Com. 6/08500H
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Chem. Commun., 1997