New water-soluble polyanionic dendrimers and binding to acetylcholine in water by means of contact ion-pairing interactions

Article abstract of DOI:10.1039/b710391c

A new water-soluble polyanionic dendrimer containing 81 benzoate termini (diameter: 11 ± 1 nm from DOSY NMR spectroscopy) has been synthesized; it interacts with acetylcholine cations in water-soluble assemblies in which each carboxylate terminus reversib

Full text of DOI:10.1039/b710391c

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COMMUNICATION
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New water-soluble polyanionic dendrimers and binding to acetylcholine
in water by means of contact ion-pairing interactions{
Ca´tia Ornelas, Elodie Boisselier, Victor Martinez, Isabelle Pianet, Jaime Ruiz Aranzaes and Didier Astruc*
Received (in Cambridge, UK) 9th July 2007, Accepted 19th September 2007
First published as an Advance Article on the web 8th October 2007
DOI: 10.1039/b710391c
catalyst, regioselectively giving the nona-chloromethyldimethylsilyl
intermediate that, upon reaction with NaI, provides the nona-
iodide 3. The dendritic progression was achieved using the known
phenoltriallyl dendron p-(HO)C4H4C(CH2CHLCH2)3, obtained
by one-pot reaction of [FeCp(g6-p-chlorotoluene][PF6] with allyl
bromide and t-BuOK (Scheme 1).13
A new water-soluble polyanionic dendrimer containing 81
benzoate termini (diameter: 11 ¡ 1 nm from DOSY NMR
spectroscopy) has been synthesized; it interacts with acetylcho-
line cations in water-soluble assemblies in which each carboxyl-
ate terminus reversibly forms contact ion pairs and aggregates
at the tether termini, as shown by 1H NMR spectroscopy.
Williamson’s reaction of the dendri-81-iodide, 6, with methyl
4-hydroxybenzoate yielded the dendri-81-benzoate, 7, that was
characterized by its molecular peak at 29 471 (M+) in the MALDI
TOF mass spectrum (calcd for C1611H2352O279Si117: 29 469.75).
The supramolecular facets of dendrimers1 have been largely
considered for uses as molecular boxes,2 exoreceptors,3 and
sensors.4 Indeed, applications of water-soluble dendrimers as drug
vectors are most promising.5,6 A possible drawback of polycationic
dendrimers, however, is their toxicity. On the other hand, it has
been recognized that polyanionic dendrimers usually display
acceptable biocompatibility.7,8 Therefore, we have investigated
the route to polybenzoate-terminated dendrimers synthesized from
polyiodomethylsilyl precursors, and their ability to transport
cations of biomedical interest such as acetylcholine.
1
The H NMR spectrum of the dendrimer-81-acid, 8, in MeOD
shows all the signals of the molecule, including all the protons of
the dendritic core, confirming the expected structure. When this
dendrimer is solubilized in water upon addition of a stoichiometric
amount of NaOH yielding 9, the 1H NMR spectrum in D2O
shows the proton signals of the periphery only.
The dendrimer 9, containing 81 sodium carboxylate groups,
reversibly reacts with AC chloride to form water-soluble
assemblies whose structure can be examined by 1H NMR
spectroscopy.15 The interaction between the dendrimer 9 and
AC is characterized in the 1H NMR spectrum by the large upfield
shift of the four AC signals upon interaction with 9. The
dendrimer signals also move, but to a lesser extent, the average
shift being 0.06 ppm in water for the peripheral protons numbered
from 5 to 8. The titration of AC was achieved in order to
tentatively quantify the number of AC that can be transported by
9.{
When the first 20 equivalents of AC are added, the AC signals
are shifted from 4.56 ppm to 4.04 ppm for the CH2–CH2–N
protons numbered 3, from 3.75 ppm to 3.18 ppm for the CH2–N
protons numbered 2, from 3.23 ppm to 2.78 ppm for the CH3–N
numbered 1, and from 2.16 ppm to 1.81 ppm for the CH3–COO
protons (see Fig. 1). These results correspond to an average
displacement of 0.5 ppm. The four signals of AC are shifted during
this titration because of the interactions of the whole molecule with
9. Indeed, when AC interacts with 9, the ammonium AC group
should be located at the dendrimer periphery, reversibly forming
contact ion pairs and aggregates with the carboxylate ion.
The number n of AC molecules bound to the dendrimer is
determined as a function of the variation Dd of chemical shifts
according to eqn (1):15
Acetylcholine (AC) chloride is produced naturally by the
nervous system. It is also used as an active ingredient in some
drugs, but it is not very active upon oral ingestion because of
hydrolysis in the digestive tube. AC chloride has various
pharmacological properties: it can be used as a parasympathomi-
metic,9a a peripheral vasodilator, an antihypertensive, a miotic or a
coronarodilator.9b Its muscarinic parasympathomimetic action
consists in contracting the smooth fibre in the digestive tube,10b the
eye and the bronchi.10a It is used in a drug marketed as a
parasympathomimetic preparation for intraocular use, although
aqueous solutions are unstable and must thus be prepared just
before use.11
We now report the assembly of polyanionic dendrimers and
dendrimer–acetylcholine molecular architectures, including their
water-solubility properties and ion-pair behavior in water.
For the dendritic construction, we used the 1 A 3 C
connectivity, pioneered by Newkome et al.,12 as shown in
Scheme 1. It starts with the known nona-allylation of [FeCp(g6-
mesitylene][PF6], 1, that quantitatively yields the nona-allyl
dendritic core 1,3,5-[C(CH2CHLCH2)3]C6H3, 2, on a large scale
subsequent to visible-light photolysis that removes the metal
moiety.13 Hydrosilylation of the terminal olefinic bonds, a reaction
pioneered in dendrimer synthesis by van Leeuwen et al.,14 is
carried out on 2, using chloromethyldimethylsilane and Karsted
Dd = KDdmax[(1 + Kd/n[D0] + [AC]/n[D0]) 2
{(1 + Kd/n[D0] + [AC]/n[D0])2 2 4[AC]/n[D0]}K]
(1)
Institut des Sciences Mole´culaires, UMR CNRS Nu5255, Universite´
Bordeaux 1, 33405 Talence Cedex, France. E-mail: d.astruc@
ism.u-bordeaux1.fr; Fax: +33 54040 6646
{ Electronic supplementary information (ESI) available: Synthesis and
detailed 1H NMR data including DOSY and ROESY experiments. See
DOI: 10.1039/b710391c
n: number of AC molecules bound to 9; [D0]: total concentra-
tion of 9; [AC]: total concentration of AC; Kd: dissociation
constant.
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Chem. Commun., 2007, 5093–5095 | 5093

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Scheme 1 Synthesis of the water soluble dendrimer-81-carboxylate G2 (9).
[Na(H2O)x]+[Cl(H2O)y]2 is also formed in eqn (2).
The best fit is that for which dendrimer 9 interacts with 2 (¡0.1)
molecules of AC per dendritic branch. Indeed, the first AC
molecule (per branch) interacts at the dendrimer periphery by
electrostatic interaction with a dissociation constant Kd1 of 17
(¡2) 6 1023 M. Then, the other AC molecule (per branch) also
interacts, with seemingly weaker interactions, with a dissociation
constant Kd2 of 230 (¡20) 6 1023 M (eqn (2), R = dendrimer,
DOSY experiments were carried out in order to follow
the evolution of the diffusion coefficient of the dendrimer
without AC and with an increasing concentration of AC. The
dendrimer without AC in water has a diffusion coefficient of 4.4 6
10211 m2 s21, and a hydrodynamic diameter of 11 ¡ 1 nm; a
molecule of AC has a diffusion coefficient of 5.9 6 10210 m2 s21
,
R9 = AC).
and a hydrodynamic diameter of 1.2 ¡ 0.1 nm (see the ESI{). The
diffusion coefficient of 9 does not vary during the titration,
meaning that the dendrimer has approximately the same size
whether it is free or bound. This lack of dendrimer size increase
upon AC binding, together with the upfield shift of the AC NMR
Kd1
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z
z
RCO{NazzR0N Me3Cl{ RCO R N Me
{
2
0
/
3
2
Kd2
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/
À
Á
z
(2)
{
2
RCO R0N Me3
2
5094 | Chem. Commun., 2007, 5093–5095
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comparison, PhCO22Na+ hardly shows any interaction with AC
(Dd , 0.1 ppm), which demonstrates the positive dendritic effect.
Notes and references
{ In an NMR tube, 5 mg of 9 were introduced in 0.5 mL D2O, then AC
was progressively added. The titration spans from 0 to 320 equivalents of
AC per dendrimer 9 (Fig. 1, the shifts of the peripheral protons of 9 are
framed). The shifts of the four signals of AC are represented and numbered
from 1 to 4.
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Fig. 1 1H NMR titration of AC with 9: (a) 9 as its Na+ salt; (b) 9 + 20 eq.
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Chem. Commun., 2007, 5093–5095 | 5095