Encapsulation of neutral guests by tri(ethylene oxide)-pyrrole-terminated dendrimer hosts in water

Article abstract of DOI:10.1021/ol060288i

This work focuses on the synthesis and guest hosting capabilities of novel, water-soluble tri(ethylene oxide)-pyrrole-functionalized poly-(propylene imine), PPI, dendrimers (DAB-TEOPy_n). The high-generation DAB-TEOPy_n (n = 32 and 64) possesses the ability to encapsulate hydrophobic guest molecules, such as Nile Red, in aqueous media.

Full text of DOI:10.1021/ol060288i

ORGANIC
LETTERS
2006
Vol. 8, No. 10
1999-2002
Encapsulation of Neutral Guests by
Tri(ethylene oxide)-pyrrole-Terminated
Dendrimer Hosts in Water
Amy D. Morara and Robin L. McCarley*
Department of Chemistry, Louisiana State UniVersity, Baton Rouge, Louisiana 70803
tunnel@LSU.edu
Received February 2, 2006
ABSTRACT
This work focuses on the synthesis and guest hosting capabilities of novel, water-soluble tri(ethylene oxide)-pyrrole-functionalized poly-
(propylene imine), PPI, dendrimers (DAB-TEOPyn). The high-generation DAB-TEOPyn (n
hydrophobic guest molecules, such as Nile Red, in aqueous media.
) 32 and 64) possesses the ability to encapsulate
Since Vo¨gtle introduced them in 19781 and Newkome2 and
Tomalia3 proposed their use as small-molecule hosts in 1985,
efforts have targeted new synthetic routes to dendrimers,4,5
with the hopes of readily altering the structure to effect
dendrimer hosting capabilities.6 Although guest entrapment
by poly(amido amine), PAMAM, dendrimers was demon-
strated in 1989 by Goddard,7 it was not until 1994 that
incarceration of guest molecules was attained by Meijer. In
this work, his group succeeded in the “static trapping” of
dye molecules (incarcerated guests) inside a poly(propylene
imine), PPI, dendrimer modified with bulky, hydrogen-
bonding-capable amino acids.8 Release of the incarcerated
guests was demonstrated, but the harsh conditions necessary
for the triggered release of the guests from the “dendritic
box” are not widely applicable for many host-guest chem-
istry applications, such as drug delivery,9 analyte precon-
centration,10 and catalysis.11 However, this study is a hallmark
and provided impetus for the development and study of many
dendrimer-based hosting systems.12,13
A highly desirable characteristic for an ideal host system
is the ability to reversibly switch on or off its hosting
capability (guest encapsulation and release) via application
of an external stimulus. To that end, a select number of
dendrimers have been modified with stimuli-responsive
moieties, such as light-responsive azobenzene14 and temper-
ature-responsive poly(N-isopropylacrylamide),15 to attempt
control over guest retention via steric interactions. Although
(1) Buhleier, E.; Wehner, W.; Vo¨gtle, F. Synthesis 1978, 155-158.
(2) Newkome, G. R.; Yao, Z.; Baker, G. R.; Gupta, V. K. J. Org. Chem.
1985, 50, 2003-2004.
(3) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin,
S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117-132.
(4) Hawker, C.; Frechet, J. M. J. J. Chem. Soc., Chem. Commun. 1990,
1010-1013.
(9) Boas, U.; Heegaard, P. M. H. Chem. Soc. ReV. 2004, 33, 43-63.
(10) Wu, X.; Peng, L.; Pu, Q.; Su, Z. Anal. Lett. 2003, 36, 2229-2241.
(11) Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. J. Phys. Chem. B
2005, 109, 692-704.
(5) de Brabander-van den Berg, E. M. M.; Meijer, E. W. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 1308-1311.
(12) Hecht, S.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74-
91.
(6) Tomalia, D. A.; Fre´chet, J. M. J. J. Polym. Sci. A 2002, 40, 2719-
2728.
(7) Naylor, A. M.; Goddard, W. A., III; Kiefer, G. E.; Tomalia, D. A. J.
Am. Chem. Soc. 1989, 111, 2339-2341.
(8) Jansen, J. F. G. A.; de Brabander-wan den Berg, E. M. M.; Meijer,
E. W. Science 1994, 266, 1226-1229.
(13) (a) Richter-Egger, D. L.; Tesfai, A.; Tucker, S. A. Anal. Chem. 2001,
73, 5743-5751. (b) Sideratou, Z.; Tsiourvas, D.; Paleos, C. M. Langmuir
2000, 16, 1766-1769.
(14) Archut, A.; Azzellini, G. C.; Balzani, V.; De Cola, L.; Vo¨gtle, F. J.
Am. Chem. Soc. 1998, 120, 12187-12191.
10.1021/ol060288i CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/21/2006

promising, these two dendrimer-based, stimuli-responsive
systems have not yet been shown to allow for controlled
egress of entrapped guest molecules.
3-(3,6,9-trioxadecanyl)-N-(triisopropylsilyl)pyrrole 2. Desi-
lylation with tetra-n-butylammonium fluoride19 was con-
firmed to be a gentle deprotecting route to generate 3-(3,6,9-
trioxadecanyl) pyrrole 3 (Scheme 1).
We are interested in constructing stimuli-responsive den-
drimer systems possessing hosting capabilities that are
controlled by electron-transfer reactions, which lead to
changes in the steric and electronic nature of groups
appended at the periphery of the dendrimer host.16 In one
approach, dendrimer end groups will be electroactive,
conjugated oligomers based on pyrrole that experience
conformational changes caused by alterations in the oligomer
oxidation state.17 To form the oligo(pyrrole)-terminated
responsive dendrimer hosts, pyrrole monomer-terminated
dendrimers must first be constructed and characterized, and
their dynamic hosting capabilities evaluated in aqueous
media. To achieve water solubility and address biocompat-
ibility issues for future biological applications, the pyrrole
monomer-terminated dendrimers that we discuss here possess
tri(ethylene oxide) functionalities at the 3-position of the
pyrrole ring, to allow for oligomerization of pyrrole at the
2- and 5-positions.17 Here, we describe the synthesis of three
unreported 3-substituted pyrrole compounds (2, 4, 5) and a
new dendrimer series (7a,b) possessing dynamic hosting
capabilities.
Scheme 1
To produce 3-substituted pyrroles, a protecting group on
the pyrrole nitrogen was needed to block the R-positions of
the pyrrole ring to prevent the favored substitution at the 2-
and 5-positions of pyrrole.18,19 The tri(ethylene oxide), TEO,
chain has been previously placed at the 3-position of pyrrole
using a tosyl protecting group at the N-position.20,21 We chose
to use the tri(isopropyl)silyl protecting group at the N-
position because previous molecular modeling22 and experi-
mental studies have shown it to be a bulky enough protecting
group to substantially hinder electrophilic attack at the
R-positions, and it is easily deprotected with fluoride salts.18,19
Three substitution of the silylated pyrrole 1 was achieved in
a manner similar to that of Bray19 but with some modifica-
tions, due to the unique solubility/polarity of the TEO group.
Selective bromination of N-(triisopropylsilyl)pyrrole was
achieved at the â-position (<10% R, >90% â by 1H NMR),
after which a halogen-metal exchange reaction was exe-
cuted to produce the lithiated pyrrole species.19 An SN2
reaction was then carried out with the lithiated species using
the Br(CH2CH2O)3CH3, Br-TEO,23 substrate to generate
After deprotection of 2, addition of a linker arm to the
3-substituted pyrrole was required. The linker chosen was
ω-bromo-n-pentanoic acid, protected with a tert-butyl group.24
Potassium metal was used to generate the pyrrole anion
species, which subsequently underwent an SN2 reaction with
the protected linker chain to give 4. Removal of the tert-
butyl group was achieved with potassium hydroxide25 to
generate the carboxylic acid 5.
The activated ester 6 (not isolated) was then formed in
the presence of the amine-terminated PPI dendrimers to give
the pyrrole-terminated dendrimers 7a,b (DAB-TEOPyn; n )
32, 64) in reasonable yields (80% and 76%). On the basis
of 1H NMR and MALDI-MS data, the degree of PPI
derivatization by the activated pyrrole ester 6 ranged between
84% and 100%.26
(15) Kimura, M.; Michinori, K.; Tsuyoshi, M.; Hanabusa, K.; Hirofusa,
S. Macromolecules 2000, 33, 1117-1119.
(16) (a) Noble, C. O.; McCarley, R. L. J. Am. Chem. Soc. 2000, 122,
6518-6519. (b) Ong, W.; McCarley, R. L. Chem. Commun. 2005, 4699-
4701.
(17) Skotheim, T. A. Handbook of Conducting Polymers; Markel
Dekker: New York, 1986; p 281.
The hydrodynamic radius (Rh) values of each dendrimer,
as determined from dynamic light scattering (DLS) experi-
ments in acetone, are shown in Table 1. The Rh values are
in good agreement with those of other oligo(ethylene oxide)-
modified PPI dendrimers,27 qualitatively pointing to a
relatively high degree of pyrrole group derivatization. In
(18) Muchowski, J. M.; Solas, D. R. Tetrahedron Lett. 1983, 24, 3455-
3456.
(19) Bray, B. L.; Mathies, P. H.; Naef, R.; Solas, D. R.; Tidwell, T. T.;
Artis, D. R.; Muchowski, J. M. J. Org. Chem. 1990, 55, 6317-6328.
(20) Delabouglise, D.; Garnier, F. AdV. Mater. 1990, 2, 91-94.
(21) Moon, D.; Padias, A. B.; Hall, H. K., Jr.; Huntoon, T.; Calvert, P.
D. Macromolecules 1995, 28, 6205-6210.
(22) Corey, E. J.; Cho, H.; Rucker, C.; Hua, D. H. Tetrahedron Lett.
1981, 22, 3455-3458.
(23) Perchonock, C. D.; Uzinskas, I.; McCarthy, M. E.; Erhard, K. F.;
Gleason, J. G.; Wasserman, M. A.; Muccitelli, R. M.; DeVan, J. F.; Tucker,
S. S. et al. J. Med. Chem. 1986, 29, 1442-1452.
(24) McCloskey, A. L.; Fonken, G. S.; Kluiber, R. W.; Johnson, W. S.
Org. Synth. 1954, 34, 26-29.
(25) Sunazuka, T.; Shirahata, T.; Yoshida, K.; Yamamoto, D.; Harigaya,
Y.; Nagai, T.; Kiyohara, H.; Yamada, H.; Kuwajima, I.; Omura, S.
Tetrahedron Lett. 2002, 43, 1265-1268.
(26) See Supporting Information.
(27) Baars, M. W. P. L.; Kleppinger, R.; Koch, M. H. J.; Yeu, S.; Meijer,
E. W. Angew. Chem., Int. Ed. 2000, 39, 1285-1288.
2000
Org. Lett., Vol. 8, No. 10, 2006

522 nm for n ) 32 (data not shown)) is characteristic of a
less polar microenvironment for Nile Red, pointing to Nile
Red uptake by the dendrimer host into its relatively
nonpolar13a interior. The absorption maximum for Nile Red
in neat Et3N (506 nm), acetone (532 nm), and tri(ethylene
glycol) monomethyl ether (545 nm) obtained in our labo-
ratories strongly suggests the dye is located in the interior
PPI region of both DAB-TEOPyn dendrimers.
Table 1. Hydrodynamic Radius of Dendrimers as Determined
by Dynamic Light Scattering at 25 °Ca
dendrimerb
Rh, nm
DAB-TEOPy32
DAB-TEOPy64
2.0-2.1
2.3-2.9
a Measurements taken at 30° and 45° angles. b [DAB-TEOPyn] was 9
mg mL-1 for n ) 32 and 2 mg mL-1 for n ) 64 both in acetone.
The microenvironment polarity difference of the two
dendrimers presents the question of the precise location of
the Nile Red guest within the DAB-TEOPyn dendrimer
interior. At this time, it is unclear why the fourth-generation
material appears to provide a slightly less polar environment
than its fifth-generation counterpart. Preliminary fluorescence
quenching studies of Nile Red encapsulated by the two
dendrimers indicate that the dye is located near the tertiary
amines in the case of the fifth-generation material and not
the fourth-generation host, an observation that is in accord
with the Nile Red absorption data. A similar observation has
been noted for the fifth-generation amine-terminated PPI with
the solvatochromic probe phenol blue in water, wherein the
dye was found to reside near the tertiary amines.13a We
speculate that the depth to which the Nile Red is able to
penetrate into the dendrimer is greater for n ) 32 because
of its more accessible interior than n ) 64, thereby resulting
in the dye residing closer to the aliphatic core. Also, it is
possible that the larger interior volume (vide infra) of the
fifth-generation dendrimer results in the presence of more
water molecules, in turn causing the polarity index to be
higher in this case.
addition, preliminary light scattering studies of 7a,b in
aqueous milieu demonstrate that they do not aggregate under
the conditions reported here.
One of the desired characteristics of the system described
here is the ability to host hydrophobic guests in an aqueous
environment. The interest in this capability is due to potential
applications for the targeted delivery of hydrophobic drugs
in mammals or for reagent storage and distribution in “lab-
on-a-chip” technologies. To investigate the ability of the
DAB-TEOPyn to host hydrophobic guests in water, aqueous
solutions of the neutral, hydrophobic dye Nile Red in the
presence of 7a and 7b were investigated by optical absorption
spectroscopy. Nile Red is a well-known environmentally
sensitive dye28,29 that is chemically stable and charge neutral
under the conditions used here (pKa is near 1).30
As seen in Figure 1, the absorption spectrum of Nile Red
in aqueous solution possesses a very broad, low-intensity
The maximum number of Nile Red molecules encapsulated
per DAB-TEOPy64 host was estimated to be 2,31 and for
DAB-TEOPy32, it was 1. It is important to note that this is
the first study to quantify the number of neutral guest
molecules inside a PPI host in aqueous solution. A previous
study indicates that anionic dye guests (4,5,6,7-tetrachlorof-
luorescein and Rose Bengal) can be hosted in an oligo-
(ethylene oxide)-aryl-functionalized PPI dendrimer at dye-
to-host ratios of 20:1 and 40:1; however, this enhanced level
of loading is greatly impacted by acid-base interactions that
result between the cationic, tertiary amine interior of the PPI
interior and the carboxylate dyes.27 The modest level of dye
encapsulation found here is therefore expected based on the
fact that the guest is neutral. Preliminary studies of Nile Red
retention upon size-based membrane separation indicate that
the dye is retained in DAB-TEOPy64 for up to 3 days,
whereas it is completely released after only 1 day with DAB-
TEOPy32. These dynamic trapping outcomes are encouraging
for our future work involving static trapping of guests
through oligomerization of the pyrrole periphery.16a
Figure 1. Absorption spectra of aqueous 40 µM Nile Red with
and without 2.5 µM DAB-TEOPy64.
band centered near 609 nm, indicating that the Nile Red is
present in an aqueous (polar) microenvironment.28,29 Upon
addition of Nile Red to aqueous DAB-TEOPyn solutions, a
distinct blue shift in the absorption energy is observed, as
well as a significant absorption intensity increase. This shift
to a distinctly shorter wavelength (527 nm for n ) 64 and
In summary, the synthesis of a new water-soluble host
and three new 3-substituted pyrrole intermediates has been
reported. The hosts have great potential as a molecule
delivery system because of their demonstrated ability to
(28) Watkins, D. M.; Sayed-Sweet, Y.; Klimash, J. W.; Turro, N. J.;
Tomalia, D. A. Langmuir 1997, 13, 3136-3141.
(31) The molar absorptivity of Nile Red in acetone (36 900 M-1 cm-1
)
(29) Deye, J. F.; Berger, T. A.; Anderson, A. G. Anal. Chem. 1990, 62,
615-622.
was used, as previously reported for microenvironmental studies of
polymeric micelles.32
(30) Davis, M. M.; Hetzer, H. B. Anal. Chem. 1966, 38, 451-461.
(32) Chen, G.; Guan, Z. J. Am. Chem. Soc. 2004, 126, 2662-2663.
Org. Lett., Vol. 8, No. 10, 2006
2001

encapsulate hydrophobic neutral guest molecules in aqueous
media. Future work in our laboratory will focus on more
elaborate characterization of this new class of hosts through
dynamic light scattering, fluorescence, and nuclear mag-
netic resonance relaxation studies. Furthermore, oligomer-
ization of the pyrrole-terminated periphery16a will also be
studied using IR spectroscopy and mass spectrometry to learn
how to effect the stimuli-responsive hosting nature of the
DAB-TEOPyn.
Tracy McCarley for ESI-HRMS analysis, and Dr. Dale
Treleaven for obtaining the high-field H NMR data. We
also thank Dr. Grigor Bantchev for DLS help and Dr. Paul
Russo for use of his DLS facility.
1
Supporting Information Available: Experimental pro-
cedures and characterization of compounds 2, 4, 5, and 7a,b
1
are included. Characterization includes H NMR and 13C
NMR for all molecules, ESI-HRMS for 2, 4, and 5, and
MALDI-MS for 7a,b. Details regarding encapsulation and
DLS are also described. This material is available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. This research was supported by the
National Science Foundation (CHE-0108961). We thank
Rebecca M. Brauch for assistance with the MALDI-MS, Dr.
OL060288I
2002
Org. Lett., Vol. 8, No. 10, 2006