Synthesis and aqueous aggregation properties of amphiphilic surface-block dendrimers

Article abstract of DOI:10.1021/ol051583q

(Chemical Equation Presented) A family of dendritic amphiphiles were synthesized from the natural metabolites of glycerol, succinic acid, and myristic acid. The surfaces of these dendrimers display different numbers of alkyl chains and carboxylic acids, v

Full text of DOI:10.1021/ol051583q

ORGANIC
LETTERS
2005
Vol. 7, No. 22
4863-4866
Synthesis and Aqueous Aggregation
Properties of Amphiphilic Surface-Block
Dendrimers
Nathanael R. Luman and Mark W. Grinstaff*
Departments of Chemistry and Biomedical Engineering,
Metcalf Center for Science and Engineering, Boston UniVersity,
590 Commonwealth AVe., Boston, Massachusetts 02215
mgrin@bu.edu
Received July 6, 2005
ABSTRACT
A family of dendritic amphiphiles were synthesized from the natural metabolites of glycerol, succinic acid, and myristic acid. The surfaces of
these dendrimers display different numbers of alkyl chains and carboxylic acids, varying the hydrophobic-to-hydrophilic ratio over a relatively
broad range. In solution these dendritic amphiphiles form supramolecular structures, and these aggregates have been characterized by light
microscopy, transmission electron microscopy, and tensiometry. These aggregates can entrap the hydrophobic species pyrene.
Amphiphiles are prevalent throughout the physical and life
sciences and, because of their self-assembly properties, are
investigated for various applications from model membranes
to drug delivery.1 The defining characteristic of amphiphilic
molecules is the simultaneous presence of both hydrophilic
and hydrophobic regions. In aqueous solution these mol-
ecules often spontaneously assemble into ordered structures
such as micelles and vesicles. In addition to traditional small
molecular weight amphiphiles such as fatty acids and
phospholipids, there is significant interest in synthetic
amphiphilic polymers.2 In general, synthetic polymers pos-
sess lower critical aggregation concentrations and the ag-
gregation structures tend to be more stable than those formed
with low molecular weight amphiphiles.2a However, it can
be difficult to systematically vary and obtain consistent
aggregate size and/or morphology with polymeric systems
because these properties are dependent on composition.
Dendritic polymer amphiphiles possess the favorable at-
tributes associated with large amphiphiles yet maintain the
monodispersity found in naturally occurring and synthetic
low molecular weight amphiphiles. Amphiphilic dendrimer
polymers have been reported, most notably “unimolecular”
dendritic amphiphiles or dendritic-linear polymers possessing
a hydrophilic dendritic component and a hydrophobic linear
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10.1021/ol051583q CCC: $30.25
© 2005 American Chemical Society
Published on Web 09/29/2005

polymer (or vice versa).3 The dendritic amphiphiles presented
in this manuscript have different numbers of both hydrophilic
and hydrophobic surface moieties. Specifically, we have
synthesized and characterized a family of amphiphilic
surface-block dendrimers as shown in Figure 1.
unusual for traditional linear polymers. Our laboratory is
particularly interested in dendritic macromolecules composed
of biocompatible building blocks for drug delivery and tissue
engineering applications.5 The amphiphilic surface-block
dendrimers 2, 4, 5, and 7 were synthesized as shown in
Scheme 1 using a convergent approach. Surface-block
Scheme 1. Synthesis of Dendritic Compounds
Figure 1. Four amphiphilic surface-block dendrimers.
An advantage of working with dendritic polymers4 is the
stepwise preparation methods, which proceed in either a
divergent or convergent fashion. The convergent approach
allows for a large degree of chemical diversity such that
functional groups can be incorporated at nearly any position
in the dendritic architecture with degrees of precision highly
dendrimers contain at least two different types of peripheral
end groups confined to definite areas, while the interior
branching structure of the dendrimer remains constant.6
These poly(glycerol-succinic acid) (PGLSA) dendrimer
derivatives are similar with respect to their interior composi-
tion, which consists of glycerol and succinic acid. However,
the surface of these PGLSA dendrimers has been modified
(3) (a) Imae, T. Surfactant Sci. Ser. 2003, 112, 525-545. (b) Newkome,
G. R.; Moorefield, C. N.; Baker, G. R.; Johnson, A. L.; Behera, R. K. Angew.
Chem., Int. Ed. Engl. 1991, 30, 1178-1180. (c) Hawker, C. J.; Wooley, K.
L.; Fre´chet, J. M. J. J. Chem. Soc., Perkin Trans. 1 1993, 12, 1287-1297.
(d) Percec, V.; Cho, W.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc.
2001, 123, 1302-1315. (e) van Hest, J. C. M.; Delnoye, D. A. P.; Baars,
M. W. P. L.; Elissen-Roman, C.; van Genderen, M. H. P.; Meijer, E. W.
Chem. Eur. J. 1996, 2, 1616-1626.
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99, 1665-1688. (b) Fischer, M.; Vo¨gtle, F. Angew. Chem., Int. Ed. 1999,
38, 885-905. (c) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic
Molecules: Concepts, Syntheses, Perspectives; VCH: New York, 1996.
(d) Tomalia, D. A.; Fre´chet, J. M. J. J. Polym. Sci., Part A: Polym. Chem.
2002, 40, 2719-2728. (e) Fre´chet, J. M. J. Proc. Natl. Acad. Sci. U.S.A.
2002, 99, 4782-4787. (f) Luman, N. R.; Kim, T.; Grinstaff, M. W. Pure
Appl. Chem. 2004, 76, 1375-1385. (g) Grinstaff, M. W. Chem. Eur. J.
2002, 8, 2838-2846.
(5) (a) Wathier, M.; Jung, P. J.; Carnahan, M. A.; Kim, T.; Grinstaff,
M. W. J. Am. Chem. Soc. 2004, 126, 12744-12745. (b) Morgan, M. T.;
Carnahan, M. A.; Immoos, C. E.; Ribeiro, A. A.; Finkelstein, S.; Lee, S.
J.; Grinstaff, M. W. J. Am. Chem. Soc. 2003, 125, 15485-15489. (c)
Carnahan, M. A.; Middleton, C.; Kim, J.; Kim, T.; Grinstaff, M. W. J. Am.
Chem. Soc. 2002, 124, 5291-5293. (d) Carnahan, M. A.; Grinstaff, M. W.
J. Am. Chem. Soc. 2001, 123, 2905-2906. (e) Carnahan, M. A.; Grinstaff,
M. W. Macromolecules 2001, 34, 7648-7655.
(6) (a) Wooley, K. L.; Hawker, C. J.; Fre´chet, J. M. J. J. Chem. Soc.,
Perkin Trans. 1 1991, 5, 1059-1076. (b) Hawker, C. J.; Wooley, K. L.;
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4864
Org. Lett., Vol. 7, No. 22, 2005

Table 1. Calculated Molecular Weights (MW), Mass Spectrometry (MALDI-MS), Size Exclusion Chromatography (SEC), Thermal
Transition Temperature, Critical Aggregation Concentration (cac), and Area Per Molecule (ApM) for Surface-Block Dendrimers
MW
(g/mol)
MALDI
(M+Na+)
SEC
(Mw)
transition
temp (°C)
cac
(mol/L)
ApM
(Å2)
compd
PDI
2
4
5
7
1137.13
1435.55
1656.12
3301.46
1158.72
1458.34
1678.37
3322.85
1482
1872
2160
3944
1.02
1.02
1.01
1.03
-22
-10
18
2.0 × 10-4
1.1 × 10-5
440
210
47
1.1 × 10-5
360
with various numbers of alkyl chains and carboxylic acids
in a controlled fashion. The 14-carbon chain of myristic acid
adds sufficient hydrophobicity to the macromolecules while
the carboxylic acid groups are hydrophilic. Compounds 2
and 4 both display four carboxyl groups, whereas 2 contains
a single myristic chain and 4 possesses two. Compounds 5
and 7 both display four myristic chains; 7 contains eight
carboxyl groups and 5 possesses two. All compounds shown
in Figure 1 and Scheme 1 have been fully characterized by
1H and 13C NMR, MALDI-MS, size exclusion chromatog-
raphy (SEC), and elemental analysis (EA) as reported in
Supporting Information.
When dissolved in aqueous solution, dendrimers 2, 4, and
7 spontaneously form supramolecular aggregates. Because
of the poor solubility of 5, no hydration structures are
observed. The morphology of the structures formed by 2, 4,
and 7 can be altered depending upon external factors such
as temperature, concentration, and sample preparation. A
selection of the supramolecular assemblies formed from these
amphiphiles is shown in Figure 2 (additional pictures are in
The following description of the synthesis for compound
4 is a representative example, and using slight modifications,
each of the macromolecules shown in Figure 1 can be
prepared in high yield (details in Supporting Information).
First, compound 87 was coupled with 4 equiv of monoben-
zylated succinic acid in the presence of N,N-dicyclohexyl-
carbodiimide (DCC) and 4-(dimethylamino)pyridinium p-
toluenesulfonate (DPTS) in 79% yield. Treatment of this
intermediate with tetrabutylammonium fluoride (TBAF)
afforded compound 12 in 75% yield. Compound 12 was then
coupled with 1,3-di-O-tetradecanoylglycerol,8 in the presence
of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydro-
chloride (EDCI) and 4-(dimethylamino)pyridine (DMAP),
to afford 3 in 74% yield. Finally, compound 3 was subjected
to hydrogenolysis conditions to cleave the benzyl esters and
afford compound 4 in 97% yield.
The data presented in Table 1 show good correlation
between calculated molecular weight and the MW observed
by MALDI-MS. The low polydispersity indices obtained by
SEC indicate a nearly monodisperse system. The masses
obtained by SEC were calibrated against linear polystyrene
standards and show an increase in MW from 2 to 7 for the
synthesized compounds. The transition temperatures were
determined by differential scanning calorimetry (DSC) and
show a trend that correlates increased MW with increased
transition temperatures. The data were collected using 2 mg
of sample in an aluminum pan, and the temperature was
equilibrated for 10 min at -60 °C. The temperature was
increased at 3 °C/min to 100 °C where it was held for 5
min. The heating-cooling cycle was repeated two times, and
the data on the third cycle was analyzed.
Figure 2. (A) TEM image of compound 7. (B) TEM image of
compound 4 after extrusion.
Supporting Information). The micrographs displayed in
Figure 2A and 2B are obtained via transmission electron
microscopy (TEM) with a negative staining solution of 2%
uranyl acetate and a concentration of 1 × 10-3 M of dendritic
amphiphile dissolved in 200 mM buffered HEPES solution
(pH ) 7.4). Figure 2A shows multilamellar aggregates of
compound 7 after sonication with an intralamellar distance
of approximately 7 nm. Figure 2B shows vesicles of
compound 4 with an intralamellar distance of approximately
18 nm following sonication and extrusion through a 0.1 µm
nylon filter. Preparation of compound 4 without extrusion
provided results similar to those shown in Figure 2A.
Multilamellar aggregates of compound 7 are observed by
TEM for both extruded and nonextruded solutions. Com-
pound 2 does not display aggregation structures via TEM.
Additional studies were performed to quantify the solution
aggregation behavior of these macromolecules. Tensiometric
determination of the critical aggregation concentration (cac)
for compounds 2, 4, and 7 was performed under isothermal
conditions (24 °C). The surface tension (σ, mN/m) was
plotted as a logarithmic function of surfactant concentration
in PBS buffer (8.3 mmol, pH ) 7.2), and a break in the
(7) Luman, N. R.; Smeds, K. A.; Grinstaff, M. W. Chem. Eur. J. 2003,
9, 5618-5626.
(8) Cockman, S. J.; Joll, C. A.; Mortimer, B. C.; Redgrave, T. G.; Stick,
R. V. Aust. J. Chem. 1990, 43, 2093-2097.
Org. Lett., Vol. 7, No. 22, 2005
4865

curve occurs at the threshold aggregation concentration.9 As
shown in Table 1, the critical aggregation concentrations for
compounds 2, 4, and 7 are 2.0 × 10-4, 1.1 × 10-5, and 1.1
× 10-5 M, respectively. Compounds 4 and 7 have double
the ratio of hydrophobic alkyl chains to hydrophilic carboxyl
groups as compound 2. The larger degree of hydrophobicity
likely plays a significant role in the lower cac’s observed
for compounds 4 and 7 as compared with 2. Dendritic-linear
hybrids and modified poly(propylene imine) dendrimers have
been reported to have cac’s of 10-6-10-7 M and modified
poly(amido amine) dendrimers have been reported with cacs
of 2-6 × 10-4 M.10
2, 4, or 7 was measured above and below the cac.
Fluorescence intensities of 20, 23, and 32 au were observed
with compounds 2, 4, and 7 at concentrations below the cac
(1 × 10-5 M), respectively. Above the cac (1 × 10-3 M),
the intensity of the 373 nm peak increases to 246, 289, and
361 au with compounds 2, 4, and 7, respectively. Addition-
ally, the change in emission intensity ratio of the first (I1 )
373 nm) and third (I3 ) 383 nm) vibrational bands is
regarded as a reliable indicator of the surrounding polarity
of pyrene. The I1/I3 ratio of pyrene in aqueous solution is
∼1.6; however, when pyrene is incorporated within the
interior of surfactant aggregates, this value typically decreases
by ∼30-40%.14 Below the cac of compounds 2, 4, and 7 (1
× 10-5 M) the I1/I3 ratio of pyrene was determined to be
1.7, 1.5, and 1.4, respectively. Above the cac of compounds
2, 4, and 7 (1 × 10-3 M) the I1/I3 ratio decreased to 0.8, 1.2,
and 1.1, respectively. The pyrene concentration was 5 × 10-7
M in all solutions. The observed changes in the I1 and I1/I3
emission intensity are consistent with pyrene being located
in a more nonpolar environment supporting pyrene incor-
poration within the dendritic aggregates.
In conclusion, a family of myristylated poly(glycerol-
succinic acid) dendritic amphiphiles were synthesized that
show a wide range of aqueous aggregation behavior. The
synthetic approach reported can be applied to the preparation
of a range of amphiphilic molecules and allows control over
the hydrophilic-to-hydrophobic ratio. The thermal transition
temperatures and critical aggregation concentrations were
determined, which provide quantitative information on the
relatively few reported examples of dendritic amphiphiles.
Additionally, dendritic amphiphile aggregates were able to
entrap a hydrophobic dye, pyrene. Such dendritic polyesters
are likely to be of interest for medical, biotechnological, and
biological applications due to their biocompatible building
blocks and the interesting structures formed in aqueous
solution.
The area per molecule (ApM) values of 440, 210, and 360
Å2 were calculated from the Gibbs absorption equation11 for
compounds 2, 4, and 7, respectively (Table 1). Compound
2 forms the most densely packed monolayers, followed by
7 and 4, respectively. The ApM values for these dendritic
amphiphiles are roughly 10 times the reported values for
myristic acid12 and coincide with the range of values reported
for other amphiphilic species.11,13
A particularly interesting functional aspect of surfactant
systems is the ability to solubilize hydrophobic molecules
within the nonpolar environments of multimolecular ag-
gregates. To demonstrate the solubilization capability of these
amphiphilic dendrimers, we selected the hydrophobic fluo-
rescent probe pyrene. The photophysical properties of pyrene
are well documented, and the fluorescence spectrum is highly
dependent on the local environment. For example, the
intensity of the first vibrational band (0-0 band, I1 ∼373
nm) of pyrene increases when pyrene transitions from bulk
aqueous solution to the hydrophobic regions of surfactant
aggregates.14 The fluorescence intensity of pyrene (5 × 10-7
M, λex ) 320 nm, λem ) 373 nm) in buffered HEPES
solutions (200 mM, pH ) 7.4) with dendritic amphiphiles
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Press: Boca Ranton, FL, 1997. (b) Hiemenz, P. C. Principles of Colloid
and Surface Chemistry; Marcel Dekker: New York, 1997.
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York, 1978.
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Acknowledgment. This work was supported in part by
the Johnson & Johnson Foundation and the NIH. N.R.L.
thanks Prof. P. Barthe´le´my and Prof. B. Pucci from the
University of Avignon for the use of their TEM, hydrogen-
ator, and tensiometer. M.W.G. thanks the Dreyfus Foundation
for a Camille Dreyfus Teacher-Scholar and the A. P. Sloan
Foundation for a Research Fellowship.
(13) (a) Sui, G.; Micic, M.; Huo, Q.; Leblanc, R. M. Langmuir 2000,
16, 7847-7851. (b) Felder, D.; Gallani, J.; Guillon, D.; Heinrich, B.; Nicoud,
J.; Nierengarten J. Angew. Chem., Int. Ed. 2000, 39, 201-204.
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26, 7339-7352. (b) Wilhelm, M.; Zhao, C. L.; Wang, Y.; Xu, R.; Winnik,
M. A.; Mura, J. L.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24,
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Supporting Information Available: Experimental pro-
cedures and characterization data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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