Formation and helicity control of ssDNA templated porphyrin nanoassemblies

Article abstract of DOI:10.1039/c2cc38150h

We report the formation of left- (M-helix) and right-handed (P-helix) nanoassemblies of a porphyrin-diaminopurine conjugate (Por-DAP) templated by a single stranded oligodeoxythymidine (dT40) via directional hydrogen bonding. The supramolecular helicity can be controlled by the ionic strength, Por-DAP:dT40 ratio, and annealing rate. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cc38150h

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Formation and helicity control of ssDNA templated
porphyrin nanoassemblies†
Cite this: DOI: 10.1039/c2cc38150h
Gevorg Sargsyan, Alexandra A. Schatz, Jan Kubelka and Milan Balaz*
Received 12th November 2012,
Accepted 14th December 2012
DOI: 10.1039/c2cc38150h
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We report the formation of left- (M-helix) and right-handed (P-helix)
nanoassemblies of a porphyrin–diaminopurine conjugate (Por–DAP)
templated by a single stranded oligodeoxythymidine (dT40) via direc-
tional hydrogen bonding. The supramolecular helicity can be controlled
by the ionic strength, Por–DAP : dT40 ratio, and annealing rate.
The importance of chirality in nature is well pronounced by the
preferred handedness and the existence of single enantiomers
of essential biomolecules. Chiral nanostructures have a great
potential for applications in chiroptical devices. For example,
chiral nematic fluids demonstrated distinct light reflecting
properties depending on the pitch and the sense of helical
components.1,2 DNA is one of the most frequently employed
chiral nano-frameworks. Particularly, its molecular structure can
Scheme 1 Synthesis of porphyrin–diaminopurine conjugate Por–DAP and its
self-assembly on oligothymine 50-(dT)40 template dT40 via directional H-bonding
between diaminopurine and thymidine.
To assemble non-covalent helical stacks of porphyrins along
be easily transformed into helical periodically patterned multi- single stranded DNA via directional H-bonding, we used the
chromophoric ladders. Porphyrins found widespread applications complementary pairing of the diaminopurine with thymidine.
as building blocks in functional nanoarchitectures.3–13 The The 2,6-diaminopurine has been shown to form three hydrogen
majority of non-covalent porphyrin–DNA assemblies rely on bonds with thymine.22 The 5,15-diarylporphyrin 1 was prepared
stacking and electrostatic interactions between charged porphyrins in one step by acid-catalysed condensation of dipyrromethane
and the oligodeoxynucleotide (ODN) duplex. These non-specific with 3-triethyleneglycol substituted benzaldehyde followed by
interactions, however, do not allow accurate control over in situ oxidation with DDQ and metallation with zinc acetate
the geometry and size of the porphyrin nanoassemblies.14–18 (Scheme 1 and ESI†).23 Amphiphilic triethylene glycol side chains
Templated self-assembly by hydrogen-bond directed molecular were employed to increase the water solubility of the Por–DAP
recognition provides access to nanostructures of controlled geometry conjugate. Statistical bromination of 1 with NBS gave, after chroma-
and photophysical properties.19,20 External stimuli can dictate tography, mono-meso-brominated porphyrin 2. The triethyleneglycol
the structure and arrangement of ODN templated chromophores. substituted 8-acetyl-2,6-diaminopurine DAP was synthesized in
However, controlling the helical sense of H-bond templated four steps from 2,6-diaminopurine.21 The palladium-catalyzed
assembly built on an ODN with a single absolute stereochemistry Sonogashira cross-coupling between the brominated porphyrin 2
remains a challenge. So far, only one example of H-bonded DNA- and acetylated diaminopurine DAP followed by demetallation
templated multichromophoric nanoassembly with right- and with trifluoroacetic acid yielded the porphyrin–diaminopurine
left-handed helicities has been reported.21 Herein we present conjugate Por–DAP.
left- and right-handed porphyrin nanoladders assembled on
oligodeoxythymidine via directional H-bonding.
The non-self-complementary oligodeoxythymine 50-(dT)40 (dT40)
was selected as a template. Based on the DNA and porphyrin
geometries, the assembly is expected to be approximately 13 nm
long and 4 nm wide. To explore the assembly of DNA templated
Por–DAP supramolecular ladders, a solution of Por–DAP (10 mM) in
DMSO was added to a solution of 50-(dT)40 (dT40, 10 mM, nucleobase
concentration) in Na-cacodylate buffer (1 mM, pH 7.0) at 85 1C. The
DMSO content of the final sample was varied from 10% to 70%.
Department of Chemistry, University of Wyoming, 1000 E. University Avenue,
Laramie, WY 82071, USA. E-mail: mbalaz@uwyo.edu
† Electronic supplementary information (ESI) available: Experimental conditions,
UV-vis, fluorescence, CD, resonance light scattering spectra, TEM micrographs of
Por–DAPꢀdT40 nanoassemblies, and density functional theory (DFT) simulations.
See DOI: 10.1039/c2cc38150h
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Fig. 1 Variable temperature CD spectra of the Por–DAPꢀdT40 nanoassemblies26
from 20 1C to 85 1C (0.5 1C minꢁ1).
Fig. 2 Variable temperature UV-vis spectra of the Por–DAPꢀdT40 nanoassem-
blies26 from 20 1C to 85 1C (0.5 1C minꢁ1). Inset: CD (481.5 nm) and absorption
(439.0 nm) signals plotted as a function of temperature.
The solution was kept at 85 1C for 1 h to ensure full homogeneity
of the sample. No CD signal was observed at 85 1C for any DMSO
concentration studied. The solution was then cooled to 20 1C
(slow annealing, gradient: ꢁ0.5 1C minꢁ1, 1 min equilibration
time, cooling time 260 min) to promote the formation of tem-
plated Por–DAPꢀdT40 assembly via hydrogen bond interactions
between the Por–DAP and dT40.24 The most intense CD spectrum
was observed in 40% aqueous DMSO (Fig. 1, blue curve) with
negative Cotton effects at 678 nm (ꢁ2.6 mdeg), 601 nm (ꢁ8.8 mdeg)
and 481.5 nm (ꢁ56.5 mdeg) and positive Cotton effects at
694 nm (+7.2 mdeg), 637 nm (+7.4 mdeg), 515 nm (+8.0 mdeg),
406 nm (+45.0 mdeg), and 292 nm (+5.2 mdeg). The negative
CD couplet observed in the Soret band region arises from the
counterclockwise arrangement of porphyrins within the assembly
(M-helix).25 This is confirmed by excellent agreement of the
experimental CD with simulations using time-dependent den-
sity functional theory (TD-DFT) for a model porphyrin tetramer
(see ESI,† Fig. S23 and S24). No CD signal was observed at 20 1C
for the samples with r20% and Z 50% of DMSO (Fig. S1,
ESI†). The CD signal below 240 nm was not monitored because
Fig. 3 CD spectra of the Por–DAPꢀdT40 nanoassemblies formed by slow cool-
ing in the absence (blue curve, M-helix) and presence (500 mM, P-helix, red curve)
of NaCl.26 Inset: TEM image of Por–DAPꢀdT40 nanoassemblies formed in the
presence of NaCl (scale bar = 20 nm).
of the DMSO absorption overlap, hence limited data were for model clockwise Por–DAP arrangements are consistent
obtained for the conformation of the DNA strand. with the observed spectral sign patterns (Fig. S24, ESI†). Lastly,
The slow annealing of the Por–DAP + dT40 mixture from 85 1C TEM images of a solution of Por–DAPꢀdT40 assembled in the
to 20 1C revealed a 60% hypochromicity of the Soret band and presence of 500 mM NaCl revealed structures with lengths of
appearance of a new band at 491.0 nm indicating a columnar 8–16 nm and a width of 4–6 nm, both within theoretically predicted
stacking of porphyrins (Fig. 2). Analysis of CD and UV-vis melting values (Fig. S18, ESI†).
curves gave melting temperature Tm = 57.3 1C and Tm = 65.3 1C,
The molar fraction of Por–DAP–dT40 as well as the annealing
respectively (Fig. 2, inset). Variable temperature fluorescence and rate of the sample had a strong effect on the helicity of Por–DAPꢀ
resonance light scattering (RLS) experiments corroborated the dT40 (Fig. 4). Various concentrations of Por–DAP and dT40 were
CD and UV-vis results (Fig. S3–S10, ESI†). Annealing of Por–DAP mixed at 85 1C in the presence of NaCl, while the total concentration
with a non-complementary oligoadenosine (dA40) did not yield of dT40 and Por–DAP was kept constant (20 mM), followed by slow
an induced CD signal (Fig. S11, ESI†).
annealing (Fig. S13, ESI†). The annealing of Por–DAP (4 mM)
Next we explored the effect of ionic strength on the helicity of the with dT40 (16 mM) in the presence of 500 mM NaCl gave rise to
Por–DAPꢀdT40 assembly. The CD spectrum of the Por–DAPꢀdT40 a CD spectrum of opposite sign though of smaller intensity
nanoassemblies formed by slow annealing in the presence of (Fig. 4a, green curve) compared to annealing of the 10 + 10 mM
500 mM NaCl yielded a nearly mirror-imaged CD profile (Fig. 3, Por–DAP–dT40 mixture (Fig. 4a, red line).
red curve) compared to the assemblies formed in the absence of
To evaluate the effect of annealing rate on Por–DAPꢀdT40 helicity,
NaCl (M-helix, Fig. 3, blue curve). The virtually inverted CD we compared two different cooling rates, (i) slow annealing reported
signal is strong evidence of helicity inversion of Por–DAPꢀdT40. in the above, and (ii) a fast annealing with a cooling time of o5 min
Post-assembly removal of NaCl from the solution by dialysis (inset, Fig. 4b). For the latter, a sample heated to 85 1C was quickly
did not have any effect on the CD spectrum of Por–DAPꢀdT40 placed in a holder pre-equilibrated at 20 1C. Even though the sample
nanoassemblies (Fig. S25, ESI†). TD-DFT calculations of the CD temperature dropped from 85 1C to 22 1C in 4 minutes, it took
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templated on ssDNA via directional H-bonding was controlled
and switched by environmental conditions. We believe that
these highly modular systems have excellent potential to be
utilized in chiral memory and chirophotonic applications.
The work was supported by the UW Start-up Funds, School
of Energy Resources Graduate Assistantship (G.S.), UW REU
program (A.A.S) and NSF Career 0846140 (J.K.). We thank
F. Ngwa and Prof. B. Leonard (UW).
Notes and references
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diaminopurine conjugate Por–DAP can be assembled into right-
and left-handed nanoladders of a defined size via directional
hydrogen bonding using oligodeoxythymine dT40 as a template
(Fig. 5). UV-vis absorption, fluorescence, and RLS data together
with TEM corroborated the formation of DNA templated por-
phyrin nanoassemblies. The handedness of Por–DAPꢀdT40
nanoassemblies could be controlled by the ionic strength,
chromophore–template molar ratio, and the annealing rate.
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Na-cacodylate buffer (1 mM, pH = 7.0).
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.