process, where an added surfactant with a head-group containing a
cationic N–CH3 stabilizes an interfacial 1 : CS complex with 2 : 1
stoichiometry. This dynamic recognition model is supported by
recent studies indicating that electrostatic interactions with
aromatic groups can be a dominant factor in interfacial binding.14
In conclusion, we report that hydrophilic, divalent ligand, CS,
can bind cooperatively to a membrane-bound receptor, 1. Ligand
binding is enhanced as the receptor loading level in the membrane
is increased, because the membrane confines the receptor to two-
dimensions and promotes the formation of receptor : ligand
complexes with 2 : 1 stoichiometry. Ligand binding is further
enhanced by the presence of membrane additives that contain a
cationic N–CH3 group. This relatively simple, dynamic molecular
recognition system demonstrates how localization of a membrane-
bound receptor can be utilized as a method to modulate ligand
binding affinity. This work was supported by the NIH (USA).
Fig. 4 Fluorescence intensity of CS (10 mM, ex: 400 nm, em: 480 nm) in
the presence of POPC vesicles containing 20 mol% 1, such that [1]/[CS] =
2 : 1. The independent samples were titrated with: $: TEGME, m:
LDAO, &: DOTAC. All vesicles contained 2 mol% DPPE-PEG-2000 and
were in buffer (pH 7.4, 10 mM TES and 145 mM NaCl).
Notes and references
{ Definitions: K1 = [CS?1]/([CS] [1]) M21; K2 = [CS?12]/([CS?1] [1]) M21
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Scheme 3
recognition process which may be quite complicated because it
involves ionic species on a membrane surface. However, one
intriguing possibility is the cationic DOTAP head-group stabilizes
a high order interfacial 1 : CS complex. When CS binds to
receptor 1 the hydroxyl residues in CS deprotonate and coordinate
to the receptor’s Zn2+ atom.7,13 Thus, after binding to the receptor,
the CS becomes a highly electron-rich, aromatic surface that will
be attracted to the DOTAP trimethylammonium cation. This may
enable the DOTAP to promote receptor clustering and formation
of the 2 : 1 (1 : CS) complex. Evidence that supports this
hypothesis was gained from the following experiments. A solution
of CS (10 mM) was mixed with vesicles composed of POPC : 1 (80 :
20) such that the ratio of 1 : CS was 2 : 1. At this receptor : ligand
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bound to the receptor. The vesicle dispersion was split into three
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didodecyltrimethylammoniunm chloride (DOTAC, cmc
=
16 mM), which resulted in dose dependent quenching of the CS
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weaker effect was observed when the the second vesicle sample was
titrated with dipolar surfactant, lauryl dimethylamine N-oxide
(LDAO, cmc = 2 mM), whereas, titration of the third vesicle
sample with non-ionic surfactant, tetraethyleneglycol monooctyl
ether (TEGME, cmc = 8 mM) had no effect on the sample’s UV
or fluorescence spectra (Fig. 4). The data are consistent with the
rationalization shown in Scheme 3; a three component assembly
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This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 1407–1409 | 1409