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Electrostatic Complex of DNA with Starburst Dendrimer: Self-Assembled Structures
and Its Application as Templater for Nanoparticle Assembly
Yi-Chun Liu1,2, Chun-Jen Su1, Hsin-Lung Chen1*, Hsien-Kuang Lin2, Wen-Lian Liu2 and U-Ser Jeng3
1
Department of Chemical Engineering, National Tsing Hua University, Taiwan
2
Materials Research Laboratories, Industrial Technology Research Institute, Taiwan
3
National Synchrotron Radiation Research Center, Hsin-Chu, Taiwan
ABSTRACT
The self-assembly mechanism and the resultant
structures of the complexes of DNA with surface
protonated poly(amidoamine) (PAMAM) dendrimers of
generation two (G2) and four (G4) have been studied as a
function of the molar ratio of dendrimer surface charge to
DNA base pair (x). For both dendrimer systems soluble
undercharged complexes were formed at x < 1.5 without
obvious sign of DNA ordering in the complexes.
Significant aggregation of the complexes resulting in
precipitation occurred at x > 1.5, yielding condensed
mesomorphic phases in which the DNA was
orientationally and/or positionally ordered. The
condensed phase in G2 dendrimer complexes was
characterized by the nematic ordering of DNA. DNA/G4
dendrimer complexes exhibited a square columnar
structure at x > 3 in addition to the condensed nematic
phase appeared at smaller x. Complexation with DNA
deformed the dendrimer molecules due to mismatch
between the surface curvature of DNA and that of
dendrimer molecules under the condition of maximizing
their electrostatic interactions. Upon incorporating
metallic nanoparticles into the core region of dendrimer,
the subsequent complexation with DNA may generate
ordered nanoparticle arrays.
INTRODUCTION
Starburst dendrimers constitute a special class of
macromolecule characterized by their compact and
highly symmetric molecular structure composed layers of
monomer units irradiating from a central core.
Polyamidoamine (PAMAM) starburst dendrimers, for
instance, are obtained by covalently attaching
amidoamine units to an amino or ethylenediamine core.
Each complete grafting cycle is called a “generation”.
Dendrimers are biocompatible and can be used in the
biochemical fields as for example vechicles of biological
materials and probes for oligonucleotide arrays. In
particular, positively charged dendrimers have been
complexed with DNA to act as gene vectors for gene
therapy.
Cationic dendrimers are effectively macroions; thus
their complexation with DNA may induce “DNA
condensation” through which the molecularly dissolved
worm-like DNA chains in aqueous solution aggregate
into compact ordered condensates. Various models have
been proposed for the self-assembled structures of
dendrimer/DNA complexes. It was used to believe that
the DNA chains tended to coil around the dendrimer
molecules in the complexes, resembling the
supramolecular
configuration
of
DNA/histone
complex[1-2]. However, in a recent study of the phase
behavior of DNA complexes with poly(propylene imine)
(PPI) dendrimers of intermediate sizes (i.e.,G4 and G5)
[3], Heather et al. revealed the structural transitions
between columnar mesophases with in-plane square and
hexagonal symmetries, which greatly modified the
commonly assumed “beads-on-string” model.
In spite of the previous efforts, further studies are
necessary to resolve the detailed mechanism associated
with the structural formation of dendrimer/DNA
complexes. In this study, the effects of several key
parameters such as the dendrimer-to-DNA molar ratio
and the generation of the dendrimer on the binding
mechanism and the self-assembled structure of the
complexes are examined. In contrast to the study by
Heather et al., the present study centers on the PAMAM
dendrimers of both small (G2) and intermediate (G4)
sizes to reveal the size effect on the self-assembly
behavior of the complexes. Furthermore, we will
demonstrate how the complexation may be used to direct
the assembly of gold nanoparticles into ordered arrays.
EXPERIMENTAL
Linear DNA type XIV from herring testes sodium salt
was purchased from Sigma and used without further
purification. PAMAM dendrimers of generation 2 (20
wt% in methanol) and generation 4 (10 wt% in methanol)
were obtained from Aldrich. To complex with the
polyanionic DNA, the surface amino groups of PAMAM
dendrimers were first protonated by adding prescribed
amount of 0.2 M HCl. Because the basicity of the surface
primary amines is significantly larger than the interior
tertiary amines, it is reasonable to assume that the surface
amino groups of the dendrimer were fully protonated first.
The protonated dendrimer aqueous solution was mixed
with the aqueous solution containing prescribed amount
of DNA to obtain the complex. The occurrence of
complexation was visually identifiable by precipitation
for most compositions. The complex composition, x, is
expressed by the molar ratio of dendrimer surface charge
to base pairs in DNA.
RESULTS AND DISCUSSION
Self-Assembled Structures. For both dendrimer systems
soluble undercharged complexes are formed at x < 1.5
and the system is said to form a “non-condensed phase”
Significant aggregation of the complexes resulting in
visually observable precipitation occurred at x > 1.5, and
the system is said to form “condensed phase”. The SAXS
profiles of the complexes in the non-condensed (x < 1.5)
and condensed (x > 1.5) phases are displayed in Figure 1.
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nanoparticles embedded within the dendrimer. Gold
nanoparticles with the size of 1 ~ 5 nm were first
synthesized using PAMAM dendrimer as the template by
reducing HAuCl4.salt incorporating in the core of
dendrimers. The surface of the dendrimer molecules
enclosing the nanoparticles was subsequently protonated
and finally mixed with DNA to generate the complex
forming condensed phase. It is hoped in this case that the
ordering of DNA induced by the complexation can direct
the assembly/organization of the dendrimer-embedded
nanoparticles. Figure 3 displays the morphology of the
resultant complex viewed under AFM. It can be seen
that nanoparticle arrays are indeed formed. It is
interesting to note that the inter-particle distance along
the long axis of the array is about 3.4 nm, which
corresponds well to the pitch of a B-form DNA duplex.
REFERENCES
[1] Ottaviani, M. F. et al. Macromolecules 1999, 32,
2275.
[2] Chen, W. et al. Langmuir 2000, 16, 15.
[3] Heather M. et al. Phys. Rev. Lett. 2003,91,7,075501.
5
10
1
(a)
X
2
1/2
X
1
3
10
4.0
2
4
1/2
(b)
4
10
10.0
10.0
1/2
5.0
1
3.0
1/2
2
4.0
2
10
3.0
I(q)(a.u.)
2.0
1
10
I(q)(a.u.)
Let us consider the scattering patterns of G4 system first.
The SAXS profiles associated with the non-condensed
phase is characterized by a monotonically decayed curve,
indicating the lack of coherent inter-chain correlation of
DNA. As x is increased to 1.5 and 2 (i.e., as the system
enters the condensed phase regime), a rather broad
DNA-DNA correlation peak at q * =1.5 nm-1 becomes
visible. The location of this scattering peak corresponds
to the interhelical distance of DNA, dDNA= 2/q*  4.2
nm. It is noted that these compositions display optically
birefringerent patterns characteristic to liquid crystalline
phases; therefore the corresponding condensed phase is
attributed to a nematic phase (cf. Figure 2(a)) in light of
the lack of higher-order diffraction peaks in the SAXS
profiles. Higher-order diffraction peaks appear at x > 3,
signaling the formation of long-range ordered
morphology. The relative positions of the scattering
peaks closely follow the ratio of 1:21/2:41/2, corresponding
to the lattice scattering from a square columnar phase (cf.
Figure 2(b))with a lattice constant of 4.2 nm.
In the condensed phase regime, G2/DNA complexes
exhibit a single broad scattering peak (corresponding to
dDNA = 3.7 nm) associated with a nematic phase
irrespective of x. The surface groups of the smaller G2
dendrimers are indeed more mobile than those of G4
dendrimers due to the more opened structure. The larger
flexibility in structure allows for greater electrostatic
interaction with DNA which is unfavorable in terms of
dynamics for the interior reorganization of DNA to attain
a more ordered structure. Therefore, a compact and tight
structure such as square columnar phase was not formed
under the present experimental time scale.
It is noted that the hexagonal symmetry reported by
Heather et al. for polypropylene (imine) (PPI) G4
dendrimer/DNA complex [3] was not observed here. The
surface positive charges of PAMAM G4 dendrimer and
PPI G4 dendrimer are 64e and 32e, respectively. The
stronger dendrimer-dendrimer repulsion due to highly
charged surface groups of PAMAM dendrimer is likely
to stabilize the square columnar phase over the hexagonal
phase19.
dDNA in G4 complexes is about 4.2 nm. The size of a
G4 dendrimer molecule in the complex is then 2.2 nm
after deducting the diameter of DNA (2.0nm) from the
observed interhelical distance. However, the diameter of
a G4 dendrimer molecule is 4.5 nm. This means that the
dendrimer molecules are somehow deformed to a
spheroid upon binding to DNA. Similar degree of
deformation is observed for the G2 molecules in the
complex. The deformation may be driven by the
enhancement of the effectiveness of charge matching
with the DNA phosphate groups in that DNA is a planar
molecule which is not geometrically favorable for the
adhesion of the sphere-like dendrimer molecules with
large curvature. The dendrimer molecules hence deform
and become flatter to enhance the electrostatic interaction
with DNA.
Nanoparticle Assembly Directed by the Complexation.
In addition to its potential use as gene carrier,
DNA/dendrimer complex is also an attractive material for
nanostructure construction. Let us consider a case where
the complexation directs the assembly of gold
1.5
1.0
2.0
-1
10
0
0.25
10
0.5
0.25
-3
10
-2
10
0.5
0.5
1.0
1.5
2.0
2.5
1.0
1.5
2.0
2.5
-1
q(nm )
3.0
-1
q(nm )
Figure 1. SAXS profiles of (a) G4/DNA and (b) G2/DNA
complexes
(a)
(b)
Figure 2. Schematic illustrations of (a) nematic and (b)
square columnar phases in dendrimer/DNA complexes
Figure 3. Arrays of dendrimer-embedded
nanoparticles in the dendrimer/DNA complex.
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gold
3.0