Supramolecular expression of chirality in assemblies of gemini surfactants

Article abstract of DOI:10.1039/a703161k

Supramolecular expression of chirality occurs in mixtures of an L-histidine derivative and the (S,S) isomer of a phosphatidic acid-based gemini surfactant, but not in mixtures of this histidine derivative and the (R,R)- or (R,S)-gemini surfactant.

Full text of DOI:10.1039/a703161k

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Supramolecular expression of chirality in assemblies of gemini surfactants
Nico A. J. M. Sommerdijk, Mark H. L. Lambermon, Martinus C. Feiters, Roeland J.M. Nolte*† and
Binne Zwanenburg*
Department of Organic Chemistry, NSR-Institute for Molecular Structure, Design and Synthesis, University of Nijmegen,
Toernooiveld, 6525 ED Nijmegen, The Netherlands
Supramolecular expression of chirality occurs in mixtures of
an L-histidine derivative and the (S,S) isomer of a phosphat-
idic acid-based gemini surfactant, but not in mixtures of this
histidine derivative and the (R,R)- or (R,S)-gemini surfac-
tant.
probably the building blocks from which the helices are
constructed.
Monolayer experiments revealed that 2 forms stable mono-
layers at the air–water interface. The surface area–surface
pressure isotherms displayed a transition to a non-compressible
state at relatively low surface pressure (Fig. 2). This suggests
Gemini surfactants are amphiphilic molecules containing two
hydrophilic head groups and two long aliphatic chains, which
are linked by a rigid1,2 or flexible spacer,3,4 and as such have
potentially interesting properties.1 Theoretical calculations5 and
recent experimental results6,7 indeed indicate that ‘geminis’
behave differently from normal surfactants, which opens the
way for new applications.
O
O
N
Na2O3PO
Na2O3PO
O
O
C17H35
C17H35
H
HN
C
17H35
*
N
H
*
N
H
C18H37
O
O
1
2
In a previous paper we showed that gemini surfactants based
on phosphatidic acid (see 1) can undergo stereodependent
fission or fusion in the presence of calcium ions.8 Here we
report that these surfactants are capable of recognising a long
chain l-histidine derivative, leading to a stereodependent
expression of molecular chirality at the supramolecular level.
The (R,R), (S,S) and (R,S) forms of compound 1 were
synthesised as described elsewhere.8 All three stereoisomers of
1 formed bilayer aggregates when they were dispersed in water.
Histidine surfactant 2 alone could not be dispersed in water to
form distinct aggregates due to its low solubility.‡ When 2 (25
or 50 mol%) was mixed with the (R,S) isomer of 1 and dispersed
in water at pH 6.5, vesicles with diameters ranging from 150 to
750 nm were generated. Increasing the concentration of 2 to 75
mol% led to the formation of extended bilayers and multilayer
structures. In contrast, at all concentrations aqueous dispersions
of 2 and the (R,R) isomer of 1 yielded irregularly shaped, rod-
like structures with an average diameter of 30 nm and a length
of 0.2–2.0 mm [Fig. 1(a)]. When 2 and the (S,S) isomer of 1 were
mixed, helical structures together with planar bilayer aggregates
were found in samples containing 25 mol% of 2 [Fig. 1(b)]. In
samples containing 50 and 75 mol% of 2, the aggregates tended
to form twisted structures [Fig. 1(c)], but no distinct morpholo-
gies could be detected in the bulk of the material.
Further experiments revealed that the helical structures were
only generated when the concentration of 2 was between 25 and
35 mol%, with the optimum lying at 30 mol%. Right-handed
helices with a pitch of approximately 90 nm and a thickness of
ca. 40 nm were observed exclusively [Fig. 1(d)].§ When the
concentration of 1 deviated from 30 mol%, the observed helices
were shorter and many of them contained defects [Fig. 1(c)].
The fact that the pitch and the thickness of the helices were
similar in all cases suggests that their formation occurred in a
precise manner, requiring a distinct ratio of the two surfactants,
rather than a variable one. Since a concentration of 30 mol% of
2 appeared to be optimal, 1:2 complexes of 2 and (S,S)-1 are
Fig. 1 Transmission electron micrographs [Pt shadowing, (a)–(c) bars
represent 400 nm, (d) bar represents 100 nm] of mixtures of (a) (R,R)-1 and
2 (75 mol%), (b) (S,S)-1 and 2 (25 mol%), (c) (S,S)-1 and 2 (50 mol%) and
(d) (S,S)-1 and 2 (30 mol%)
60
(c)
50
(e)
40
(d)
(b)
30
20
10
0
(a)
50
70
0
10
20
30
40
60
80
90
2
–1
A / Å molecule
Fig. 2 Monolayer isotherms recorded at 20 °C of mixtures of (S,S)-1 and 2
on a sub-phase of pH 6.5: (a) 25, (b) 33, (c) 50, (d) 75 and (e) 100 mol%
2
Chem. Commun., 1997
1423

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HO
P
HO
P
(a)
HO
P
O–
O–
O–
HN
N
HN
N
HO
P
HO
P
HO
P
O–
O–
O–
HO
P
HO
P
(b)
O–
O–
HN
N
HO
P
HO
HO
P
HO
P
O–
O–
P
O–
HN
N
O–
HO
P
HO
P
O–
O–
2–
HO
P
O
O
CH2OPO3
H
C
17H35
O
O
O–
(S,S)-1
2
HN
N
HO
P
H
C17H35
2–
O–
CH2OPO3
Fig. 3 Schematic representation of (a) a 1:1 complex and (b) a 1:2 complex of 2 and (S,S)-1
that in the monolayer the molecules of 2 have a high degree of
organisation, probably because they are linked by strong
intermolecular hydrogen bonds via their imidazole groups,
assisted by the formation of so-called amide polymers, i.e.
linear arrays of hydrogen bonds between the amide groups of 2.
Mixing 2 with an increasing amount of (S,S)-1 led to a gradual
increase in the compressibility of the monolayer and to a
gradual change in the overall shape of the isotherm. At a
concentration of 33 mol% of 2—this is the concentration at
which helices start to be observed—the isotherm did not change
further and showed the highest degree of compressibility.¶
These results can be rationalised as follows: the (S,S) isomer of
1 preferentially adopts an extended conformation in which the
phosphate groups have an anti orientation.8∑ It is likely
therefore that in a 1:1 mixture highly ordered, linear arrays of
pairs of (S,S)-1 and 2 are present, interconnected by hydrogen
bonds, both between the head groups and the neighbouring
amide groups [Fig. 3(a)]. In this way the molecules will be
confined to their positions in a two dimensional lattice. In a
mixture containing 33 mol% of 2, however, individual 2:1
complexes of (S,S)-1 and 2 may be formed, allowing both
translational and rotational freedom of the complexes with
respect to each other [Fig. 3(b)]. The complexes are thus free to
adopt orientations in which the average molecular area is
minimised, and in which chirality is accumulated by close
packing of the stereogenic centres. This asymmetric packing of
the building blocks probably extends over long distances,
leading to the formation of helical aggregates.
Footnotes
† E-mail: tijdink@sci.kun.nl
‡ Surfactant 2 was prepared from Boc-protected l-histidine. One alkyl chain
was introduced by a DCC coupling with octadecylamine (yield 60%).
Deprotection of the Boc-amide with trifluoroacetic acid and subsequent
acylation with N-(stearoyloxy)succinimide resulted in the introduction of
the second alkyl chain (yield 75%). Calc. for 2 (C42H80O2N4): C, 74.9; H,
12.0; N, 8.3. Found: C, 74.2; H, 11.5; N, 8.4%.
§ In mixtures containing 1–10 mol% of 2 planar bilayers and a small amount
of fibres were observed, suggesting that only a small fraction of (S,S)-1 is
interacting with 2, leading to the formation of these fibres.
¶ Monolayers of mixtures of 1 and 2 were inspected by Brewster angle
microscopy. No chiral domains were detected.
∑ The pKa values of (S,S)-1 were determined to be 1.8 and 10.2, implying
that at pH 6.5 the molecules are in the monoprotonated state (ref. 9).
References
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The authors thank H. P. M. Geurts for assistance in
performing the electron microscopy experiments and Professor
G. I. Tesser and P. J. H. M. Adams for the kind donation of Boc-
protected l-histidine and for helpful discussions.
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Received in Cambridge, UK, 8th May 1997; 7/03161K
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Chem. Commun., 1997