Hexadecameric self-assembled dendrimers built from 2′-deoxyguanosine derivatives

Article abstract of DOI:10.1021/ol800701j

(Chemical Equation Presented) Herein we describe the construction of hexadecameric self-assembled dendrimers (SADs) using a series of dendronized 8-(m-acetylphenyl)-2′-deoxyguanosine (mAG) subunits. The azido-substituted mAG subunits were covalently linked to alkynyl polyester dendrons using a coppercatalyzed 1,3-dipolar cycloaddition reaction. Discrete SADs are formed with high fidelity and thermal stability even with the increased steric hindrance offered by the dendrons.

Full text of DOI:10.1021/ol800701j

ORGANIC
LETTERS
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2008
Vol. 10, No. 11
2287-2290
Hexadecameric Self-Assembled Dendrimers
Built from 2′-Deoxyguanosine Derivatives
Jose´ E. Betancourt and Jose´ M. Rivera*
Department of Chemistry, UniVersity of Puerto Rico, R´ıo Piedras Campus,
R´ıo Piedras, Puerto Rico 00931
jmriVortz@mac.com
Received March 27, 2008
ABSTRACT
Herein we describe the construction of hexadecameric self-assembled dendrimers (SADs) using a series of dendronized 8-(m-acetylphenyl)-
2′-deoxyguanosine (mAG) subunits. The azido-substituted mAG subunits were covalently linked to alkynyl polyester dendrons using a copper-
catalyzed 1,3-dipolar cycloaddition reaction. Discrete SADs are formed with high fidelity and thermal stability even with the increased steric
hindrance offered by the dendrons.
Dendrimers are “tree-shaped” monodispersed macromol-
ecules characterized by having “branches” or dendrons that
grow from a central core. Their overall size, shape, and
morphology is determined by the branching pattern, the nature
of both the branching and surface groups, as well as the number
of layers surrounding the core.1 These characteristics make
dendritic structures suitable platforms for applications in light-
harvesting, catalysis, cellular imaging, and drug delivery.2 Their
ideal hyperbranched structures also make their synthesis more
challenging than that of most polymers. Although their prepara-
tion has improved significantly over the past decade, the larger
and more complex structures still require significant synthetic
efforts that are often plagued by low yields and polydispersity.
In a seminal work, Zimmerman and co-workers tackled this
problem by using self-assembly to construct a series of discrete
and well-defined hexameric self-assembled dendrimers (SADs).3
The self-correcting nature of SADs have made them increas-
ingly useful in the construction of complex multifunctional
supramolecules.1b,4
The construction of SADs rely primarily on two strategies:
(1) the self-recognition of subunits and (2) the assembly of
multiple subunits onto a core, which in itself can be a
dendrimer.4,5 Most SADs reported to date rely mainly on
(2) (a) Helms, B.; Meijer, E. W. Science 2006, 313, 929–930. (b) Boas,
U.; Heegaard, P. M. H. Chem. Soc. ReV. 2004, 33, 43–63. (c) Dufes, C.;
Uchegbu, I. F.; Schatzlein, A. G. AdV. Drug DeliVery ReV. 2005, 57, 2177–
2202. (d) Hecht, S.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2001, 40,
74–91. (e) D’Emanuele, A.; Attwood, D. AdV. Drug DeliVery ReV. 2005,
57, 2147–2162. (f) Ferrari, M. Nat. ReV. Cancer 2005, 5, 161–171. (g)
Patri, A. K.; Majoros, I. J., Jr. Current Opin. Chem. Biol. 2002, 6, 466–
471.
(1) (a) Newkome, G. R.; Moorefield, C. N.; Vogtle, F. Dendritic
Molecules: Concepts, Synthesis, PerspectiVes; VCH Publishers: New York,
1996. (b) Zeng, F. W.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681–
1712.
(3) Zimmerman, S. C.; Zeng, F. W.; Reichert, D. E. C.; Kolotuchin,
S. V. Science 1996, 271, 1095–1098.
10.1021/ol800701j CCC: $40.75
Published on Web 05/02/2008
2008 American Chemical Society

the use of one type of noncovalent or reversible covalent
interactions. The former include hydrogen bonds and stack-
ing/hydrophobic interactions6 or mechanical bonds,7 whereas
the latter rely most often on the use of coordinative bonds.8
Nevertheless, most SADs reported to date are of relatively
low molecularity (six subunits or less)9 or are dendritic
structures of indefinite molecularity (e.g., liquid crystals).10
Herein we report the synthesis of dendronized 8-(m-
acetylphenyl)-2′-deoxyguanosine (mAG) derivatives and
their use in the construction of well-defined and discrete
SADs of relatively large molecularity (16 subunits).
In particular we have discovered that the mAG scaffold
shows a strong tendency to self-assemble into a hexadecamer
in the presence of KI under a wide variety of conditions. In
order to test the tolerance of the mAG scaffold to substitu-
tions in the sugar we evaluated the self-assembly in CD3CN
of derivatives having ester groups with alkyl chains of
different sizes (Figure 2). Having acetyl (mAGa), straight
Guanosine and related compounds have a propensity to form
planar tetramers (G-tetrads, Figure 1), which are held together
Figure 1. Hierarchical formation of a self-assembled dendrimer
(SAD). (a) Dendronized mAG subunits self-assemble into (b) disk-
like tetramers, which (c) further stack to form hexadecameric SADs.
(d) The resulting SADs have a functional core composed of a
modified lipophilic G-quadruplex.
Figure 2. (a) Chemical structure of the mAG tetramer and (b)
partial 1H NMR spectra (500 MHz, CD3CN, 295 K) of mAG
derivatives with different substitutions on the 2′-deoxyribosyl
moiety. All of the substituents enable the formation of hexadecamers
except mAGDp, which forms an octamer. The hexadecamers show
a signature of doubled set of resonances due to the two sets of
tetrads that are in different chemical environments (o ) outer, i )
inner). The peaks in the region of 11-13 ppm correspond to the
N1H (red), and the broad peak at around 10.8 ppm corresponds to
the N2H (blue) on the Watson-Crick edge of the guanine moiety.
by hydrogen bonds and cation complexation; further stacking
of such tetrads leads to structures known as G-quadruplexes.11
Previously we have demonstrated that the self-assembly of
lipophilic dG-derivatives could be modulated by replacement
of the H8 in the guanine base with a functionalized phenyl
group.12 The position of the functional group in the phenyl ring
controlled the stoichiometry, thermal stability, and in some cases
their selective binding for metal cations.12,13 Taking advantage
of our familiarity with these derivatives, we embarked on
the construction of other nanostructures such as dendrimers.14
alkyl chains (mAGb, mAGh), or even groups with R-branch-
ing (mAGi) have no detrimental effect in the formation of
a hexadecamer. Nonetheless, the increase steric repulsion
caused by the R-branching in mAGi induces a slight decrease
in the thermal stability (3.5 °C lower) of the resulting
hexadecamer as compared to the one formed by mAGa
(unpublished results). Remarkably, although the quaternary
R-carbon and the bulky aromatic group in mAGDp prevent
the formation of a hexadecamer, the compound is still able
to self-assemble into an octamer. This results prompted us
to install hexanoyl spacers between the 2′-deoxyribosyl moiety
and aliphatic polyester dendrons derived from 2,2-bis(hy-
droxymethyl)propionic acid (bis-MPA).15
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Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4782–4787. (c) Zimmerman, S. C.;
Lawless, L. J. Top. Curr. Chem. 2001, 217, 95–120.
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1564–1567.
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Ma, Y. G.; Witmer, M. J.; Zimmerman, S. C. Proc. Natl. Acad. Sci. U.S.A.
2002, 99, 5099–5104.
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The synthesis of the dendronized derivatives was performed
as described in Scheme 1. The mAG derivative was prepared
noninteracting monomeric species and expected with such a
highly competitive solvent. Similarly, in CD3CN all peaks are
sharp and well-resolved with the exception of the N1H peaks,
which are slightly exchange-broadened, indicating the formation
of loosely bound aggregates (Supporting Information, Figures
S1 and S2). For both compounds, addition of 0.0625 equiv of
KI induces the appearance of a new set of peaks that are sharper
and correspond to the formation of an octamer. Further addi-
tion of KI (0.125-0.5 equiv) leads in both cases to the
appearance of a hexadecamer as evidenced by the splitting of
every signal of the spectrum into pairs of peaks. For both
mAGD2 and mAGD3, the difference in the chemical shift for
the imino (N1H) protons of the inner and outer tetrads is about
1.6 ppm, reflecting the anisotropic shielding of the inner tetrads
relative to the outer tetrads (Figure 3).
Scheme 1. Synthesis of Dendronized mAG Derivatives
in high yield via a Suzuki-Miyaura coupling as previously
described.12 Esterification of mAG with 6-azidohexanoic acid
afforded the azide-containing derivative (mAGhaz), and sub-
sequent reaction with alkynyl-containing dendrons via the
efficient copper-catalyzed Huisgen 1,3-dipolar cycloaddition
reaction (aka “click” reaction) furnished the dendronized
derivatives mAGD2 and mAGD3 in good yields.15–17 As a
result of the bifunctional nature of the mAG scaffold, the
resulting dendronized derivatives are one generation number
higher than the alkynyl-containing dendrons used. For
example, coupling of two alkyne dendrons Dg1 leads to the
formation of mAGD2. This is very attractive as it implies
that we could build large dendrimers starting from relatively
small dendrons.
Figure 3.
1H NMR spectra (500 MHz, CD3CN, 298 K, 0.5 equiv
of KI) of (a) (mAGD2)16 and (b) (mAGD3)16.
After addition of 0.5 equiv of KI, both mAGD2 and
mAGD3 form hexadecamers with high fidelity, and their
spectra is very reminiscent of that of the mAG derivatives
shown in Figure 2. This underscores the fact that the mAG
scaffold is very robust to substitutions in the 2′-deoxyribosyl
moiety.
To confirm the molecularity of the resulting SADs we
performed vapor pressure osmometry (VPO) measurements.
Although VPO is not as accurate as other techniques such
as mass spectrometry (MS), it does offer the advantage of
performing measurements in solution under conditions
(concentration, temperature, etc.) that are very close to those
used in NMR experiments. As seen in Table 1 (using mAGi
Both derivatives mAGD2 and mAGD3 show sharp and well-
defined signals in DMSO-d6, which is indicative of mostly
(7) (a) Elizarov, A. M.; Chang, T.; Chiu, S. H.; Stoddart, J. F. Org.
Lett. 2002, 4, 3565–3568. (b) Leung, K. C.; Arico´, F.; Cantrill, S. J.;
Stoddart, J. F. Macromolecules 2007, 40, 3951–3959.
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(b) Yamamoto, K.; Imaoka, T. Bull. Chem. Soc. Jpn. 2006, 79, 511–526.
(c) Chase, P. A.; Gebbink, R. J. M. K.; van Koten, G. J. Organomet. Chem.
2004, 689, 4016–4054.
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C. H.; Chow, H. F.; Hui, S. K.; Sze, K. H. Org. Lett. 2006, 8, 1811–1814.
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Huang, B. Q. Prog. Polym. Sci. 2005, 30, 220–293.
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Soc. 2007, 129, 11265–11278. (b) Donnio, B.; Buathong, S.; Bury, I.;
Guillon, D. Chem. Soc. ReV. 2007, 36, 1495–1513. (c) Kato, T.; Mizoshita,
N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38–68. (d) Marcos,
M.; Mart´ın-Rapu´n, R.; Omenat, A.; Serrano, J. L. Chem. Soc. ReV. 2007,
36, 1889–1901.
Table 1. Molecular Weight (MW) for Assemblies As
Determined by VPO in CH3CN (40 °C)
(11) (a) Davis, J. T.; Spada, G. P. Chem. Soc. ReV. 2007, 36, 296–313.
(b) Davis, J. T. Angew. Chem., Int. Ed. 2004, 43, 668–698.
(12) Gubala, V.; Betancourt, J. E.; Rivera, J. M. Org. Lett. 2004, 6,
4735–4738.
MW
derivativea
theoretical
VPO
|MW|
(13) Gubala, V.; De Jesus, D.; Rivera, J. M. Tetrahedron Lett. 2006,
47, 1413–1416.
(14) Alberti, P.; Bourdoncle, A.; Sacca, B.; Lacroix, L.; Mergny, J. L.
Org. Biomol. Chem. 2006, 4, 3383–3391.
(mAGi)16·3KI
(mAGD2)16·3KI
(mAGD3)16·3KI
8907
17908
26622
8661
18432
26048
246
523
574
(15) Malkoch, M.; Malmstrom, E.; Hult, A. Macromolecules 2002, 35,
8307–8314.
a Measured in a concentration range of 35-50 mM.
(16) (a) Wu, P.; Malkoch, M.; Hunt, J. N.; Vestberg, R.; Kaltgrad, E.;
Finn, M. G.; Fokin, V. V.; Sharpless, K. B.; Hawker, C. J. Chem. Commun.
2005, 5775–5777. (b) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.;
Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596–2599
.
(17) Note: A similar strategy was reported recently for the preparation
of self-assembling oligothiophene-nucleoside conjugates. For details, see:
Jatsch, A.; Kopyshev, A.; Mena-Osteritz, E.; Bauerle, P. Org. Lett. 2008,
as a reference), samples of both mAGD2 and mAGD3 give
results that are consistent with a hexadecamer templated by
three potassium cations. Furthermore, although the difference
10, 961–964
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Org. Lett., Vol. 10, No. 11, 2008
2289

between the theoretical and the measured values (∆MW) may
seem large, it is less than half the molecular weight of one
subunit.
Nevertheless, we performed PFG-NMR experiments to
further confirm the results from VPO.18 As expected, the
hexadecamer formed by mAGi gives a diffusion coefficient
(D) that is higher than those of mAGD2 and mAGD3 (Table
2). Surprisingly, the values of D for both SADs are
rapidly with the concomitant appearance of a monomeric
species. Nonetheless, even at 1 mM there is a visible amount
of hexadecamer (25% for mAGD2 and 23% for mAGD3)
indicating the robustness of the system. Although at the
lowest concentrations the presence of octamers and dodecam-
ers are visible, it seems that the disassembly occurs coop-
eratively and most of the subunits become monomeric
without populating either one of those states.12
Parallel VT experiments were performed with 5 mM solu-
tions in CD3CN. We chose this particular concentration as it
offers a good mixture of monomers to SADs that is suitable to
NMR analysis (Supporting Information, Figures S7 and S8,
Table S2). Although the hexadecameric SADs formed by both
mAGD2 and mAGD3 essentially disappear at temperatures
above 323 K, at 328 K there is still a significant amount of
octameric SADs visible. Both VC and VT experiments confirm
that the increased steric bulk of the SAD formed by mAGD3
relative to the one formed by mAGD2 induces slight decrease
in the thermodynamic stability.
Table 2. Diffusion Coefficients (D) and Hydrodynamic Radii (r)
of SADs and Other Substances As Determined by PFG-NMR in
CD3CN
substance
D (10-10 m2 s-1
)
r (Å)
Dsub/Dstd
adenosine standarda 12.704 ( 1.358
5.04 ( 0.47
9.96 ( 0.05
(mAGi)16·3KI
6.430 ( 0.114
0.506
0.351
0.362
(mAGD2)16·3KI
(mAGD3)16·3KI
4.463 ( 0.199 14.43 ( 0.19
4.600 ( 0.345 13.93 ( 0.34
a See Supporting Information for structural details.
The versatility of this approach for the construction of
dendrimers is illustrated by the fact that a hexadecameric SAD
of mAGD3 (obtained from the coupling of Dg2) displays 128
protected oxygens at its periphery. In contrast, to achieve the
equivalent with a fully covalent dendrimer will require, for
example, the synthesis and coupling of four fifth-generation
dendrons (e.g., Dg5) to a tetrafunctional core (e.g., a porphy-
rin).20 Furthermore, these SADs are discrete, well-defined, easy
to make, chiral, and thermally stable and have a functional
core.21 The fact that these SADs are sustained by multiple
noncovalent interactions enables the fine-tuning of their structure
and dynamics by using a wide variety of external stimuli.
Studies aimed at doing the latter as well as evaluating the
encapsulation of guest molecules are currently underway.
practically identical within the margin of error. This phe-
nomenon might be explained by the reported back-folding
of the terminal groups in dendrimers, which are known to
distribute themselves throughout the volume of the den-
drimer.19 This causes larger dendrimers to become more
compact with increasing generation numbers.
Moreover, 2D-NOESY experiments provide evidence that
the hexadecameric SADs formed by mAGD2 and mAGD3
are isostructural to those shown in Figure 2. Although, as
mentioned above, the dendrons have a tendency to ag-
glomerate throughout the volume of the dendrimer, the
quadruplex core remains largely unchanged. This is apparent
in the signature cross peaks observed in the 2D-NOESY
spectra (Supporting Information, Figure S4). The core of the
resulting SADs result from the arrangement of 16 subunits
packed into four stacked tetrads with three collinear potas-
sium cations located down a central channel. The preferential
formation of such resilient supramolecular structures requires
the concerted work of multiple noncovalent interactions
(hydrogenbonds,cation-dipole,andVanderWaalsinteractions).
The thermodynamic stability of such SADs can be assessed
by variable concentration (VC) and variable temperature
(VT) experiments. Dilution experiments in CD3CN allowed
us to evaluate the range of concentrations over which the
SADs remain largely assembled with high fidelity. We use
the ratio (f) of ordered species formed by mAGD2 and
mAGD3 to simplify the comparison between supramolecules
of different molecularities (Supporting Information, Figures
S5 and S6, Table S1). In the concentration range between
30 and 10 mM (concentration of the individual subunits), f
remains effectively constant for both SADs. However,
dilution below 10 mM causes the hexadecamer to vanish
Acknowledgment. We thank Dr. Vladimir Gubala (UP-
RRP), Andre´s Gonza´lez (UPRRP), and Medardo Rosario
(UPRRP) for helpful technical assistance during the initial
stages of this work. J.E.B. thanks the AGEP-PR program
(No. 0302696) for financial support. This publication was
made possible by Grant No. P20 RR016470 from the
National Center for Research Resources (NCRR), a com-
ponent of the National Institutes of Health (NIH). Its contents
are solely the responsibility of the authors and do not
necessarily represent the official views of the NCRR or NIH.
Supporting Information Available: Detailed synthetic
procedures, characterization data for all new compounds,
experimental protocols, and NMR titration data. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL800701J
(20) To calculate the number of surface groups (Z) one can use the
formula Z ) NcNbG, where Nc is the branching of the core, Nb is the
branching of the repeat unit (two for the bifunctional bis-MPA), and G is
the generation number. For (mAGD3)16, Z ) (16)(2)3 ) 128, and for a
hypothetical covalent dendrimer using a tetrafunctional core and a fifth
generation bifunctional repeat unit, Z ) (4)(2)5 ) 128. For more details
see ref 1a.
(18) (a) Cohen, Y.; Avram, L.; Frish, L. Angew. Chem., Int. Ed. 2005,
44, 520–554. (b) Kaucher, M. S.; Lam, Y. F.; Pieraccini, S.; Gottarelli, G.;
Davis, J. T. Chem. Eur. J. 2004, 11, 164–173.
(21) (a) Smith, D. K.; Diederich, F. Chem. Eur. J. 1998, 4, 1353–1361.
(b) Diederich, F.; Felber, B. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4778–
4781.
(19) Ballauff, M.; Likos, C. N. Angew. Chem., Int. Ed. 2004, 43, 2998–
3020.
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