Supramolecular ssDNA templated porphyrin and metalloporphyrin nanoassemblies with tunable helicity

Article abstract of DOI:10.1002/chem.201304153

Free-base and nickel porphyrin-diaminopurine conjugates were formed by hydrogen-bond directed assembly on single-stranded oligothymidine templates of different lengths into helical multiporphyrin nanoassemblies with highly modular structural and chiroptic

Full text of DOI:10.1002/chem.201304153

DOI: 10.1002/chem.201304153
Full Paper
&
Supramolecular Chemistry |Very Important Paper|
Supramolecular ssDNA Templated Porphyrin and
Metalloporphyrin Nanoassemblies with Tunable Helicity
Gevorg Sargsyan, Brian M. Leonard, Jan Kubelka, and Milan Balaz*[a]
Dedicated to Professor Arlette Solladiꢀ-Cavallo and Professor Marta Salisova
Abstract: Free-base and nickel porphyrin–diaminopurine
conjugates were formed by hydrogen-bond directed assem-
bly on single-stranded oligothymidine templates of different
lengths into helical multiporphyrin nanoassemblies with
highly modular structural and chiroptical properties. Large
red-shifts of the Soret band in the UV/Vis spectroscopy con-
firmed strong electronic coupling among assembled porphy-
rin–diaminopurine units. Slow annealing rates yielded prefer-
entially right-handed nanostructures, whereas fast annealing
yielded left-handed nanostructures. Time-dependent DFT
simulations of UV/Vis and CD spectra for model porphyrin
clusters templated on the canonical B-DNA and its enantio-
meric form, were employed to confirm the origin of ob-
served chiroptical properties and to assign the helicity of
porphyrin nanoassemblies. Molar CD and CD anisotropy
g factors of dialyzed templated porphyrin nanoassemblies
showed very high chiroptical anisotropy. The DNA-templated
porphyrin nanoassemblies displayed high thermal and pH
stability. The structure and handedness of all assemblies was
preserved at temperatures up to +858C and pH between 3
and 12. High-resolution transition electron microscopy con-
firmed formation of DNA-templated nickel(II) porphyrin
nanoassemblies and their self-assembly into helical fibrils
with micrometer lengths.
Introduction
in templated bottom-up nanofabrication of multichromophoric
supramolecular polymers owing to their versatile structural
properties.[6] Combination of hydrogen bonding molecular rec-
ognition (nucleobase complementarity) and p–p stacking con-
trols the architecture and characteristics of nanostructures.[7]
We have recently shown that porphyrin–diaminopurine con-
jugate 2HPor-DAP (Scheme 1) could be successfully assembled
along single stranded oligothymidine (5’-dT40) into right- and
left-handed helical nanostructures by directional hydrogen
bonding.[5] The helicity and chiroptical properties of nanoas-
semblies was modulated by ionic strength and annealing rate.
Herein, we explored formation of DNA-templated porphyrin
nanoassemblies using 1) oligothymidine templates of different
length (5’-dT8, 5’-dT16 and 5’-dT40) and 2) free-base porphyrin
(2HPor-DAP), nickel(II)porphyrin (NiPor-DAP), and zinc(II)-por-
phyrin (ZnPor-DAP). Nickel and zinc porphyrins have been se-
lected to evaluate the influence of metalation and axial ligands
on formation of templated porphyrin nanoassemblies. Zinc
porphyrins are five-coordinate, while nickel porphyrins are
four- and six-coordinate in aqueous solutions with water as
(an) axial ligand(s).[8] The DNA templated multiporphyrin nano-
assemblies exhibited modular structural and chiroptical proper-
ties and high thermal stability. We have constructed molecular
models for the 2HPor-DAP nanoassemblies and employed
time-dependent density functional theory (TDDFT) simulations
of UV/Vis and CD spectra for direct comparison with experi-
mental data. The good qualitative agreement of the simula-
tions with the experimental CD patterns confirmed the origin
of observed CD profiles and helped to assign the helicity of
Chiroptical organic and inorganic nanomaterials have great po-
tential for applications in chiral memory, data storage, biologi-
cal sensing, and optical communication.[1] Modulation of their
chiroptical properties can be achieved by inverting the struc-
ture of chiral structural units or by changing the 3D structure
of a chiral supramolecular nanoassembly by means of external
stimuli. The latter approach brings several advantages originat-
ing from cooperativity and enhanced optical and chiroptical
properties of nanoassemblies in comparison to single chiral
molecules. Porphyrins and their derivatives are among the
most-employed building blocks in organic nanoassemblies
owing to their well-developed chemistry, stability, and modular
structural and photophysical properties.[2] Optically active
supramolecular multiporphyrin nanoassemblies of both helici-
ties have previously been prepared through directional stir-
ring,[3] by rotational forces and magnetic levitation,[4] and by di-
rectional templated assembly.[5] However, only the last ap-
proach allows controlling the helicity together with the size of
nanoassemblies, thus controlling their structural and photo-
physical properties. Short DNA sequences gained prominence
[a] G. Sargsyan, Prof. B. M. Leonard, Prof. J. Kubelka, Prof. M. Balaz
Department of Chemistry, University of Wyoming
1000 E. University Avenue, Laramie, WY 82071 (USA)
Fax: (+1)307-766-2807
E-mail: mbalaz@uwyo.edu
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304153.
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pling reaction conditions to yield
the trihexylsilane-protected dia-
minopurine–acetylene conjugate
3. The trihexylsilane protection
group was removed with TBAF
to give the diaminopurine–acet-
ylene conjugate 4. Bis(triethyle-
neglycol)-substituted
3,4-dihy-
droxy-benzaldehyde 5 was pre-
pared according to a previously
reported procedure.[9] The 5,15-
bisaryl-porphyrin 6 was prepared
by Lindsey’s acid-catalyzed con-
densation of dipyrromethane
with aldehyde 5, followed by ox-
Scheme 1. a) Hydrogen bonding between porphyrin–diaminopurine conjugate 2HPor-DAP and thymidine. b) Rep-
resentation of right-handed DNA-templated multiporphyrin nanoassembly.
optically active multiporphyrin nanoassemblies. High-resolu-
tion transmission electron microscopy (HRTEM) has shown for-
mation of DNA-templated nanoassemblies and their self-as-
sembly into fibril structures.
idation with DDQ. Metalation with zinc acetate yielded zinc(II)
porphyrin 7. The bromination of 7 with NBS in chloroform and
pyridine followed by separation yielded the monobrominated
porphyrin 8. Coupling of 8 and diaminopurine–acetylene 4
under Sonogashira cross-coupling conditions gave the diami-
nopurine–porphyrin conjugate ZnPor-DAP. The 2HPor-DAP
was prepared by demetalation of ZnPor-DAP using TFA. Ni-de-
rivative NiPor-DAP was prepared by refluxing the 2HPor-DAP
with nickel acetate in DMF. The structures of porphyrin–diami-
Results and Discussion
Synthesis of porphyrin–diaminopurine conjugates 2HPor-
DAP, NiPor-DAP, and ZnPor-DAP
1
nopurines conjugates were confirmed by H NMR (Supporting
Information, Figures S1,S2) and MALTI-TOF mass spectrometry.
Diaminopurine–acetylene conjugate 4 was prepared in four
steps from commercially available 2,6-diaminopurine
(Scheme 2). The 2,6-diaminopurine DAP was deprotonated by
NaH and reacted with triethylene glycol monomethyl ether to-
sylate. N-triethyleneglycol–diaminopurine 1 was then bro-
minated by dropwise addition of NBS solution in acetonitrile
over 1 h. The brominated diaminopurine 3 was reacted with
mono(trihexylsilane)acetylene under Sonogashira cross-cou-
Assembly of right-handed 2HPor-DAP:dTx and NiPor-
DAP:dTx nanoassemblies by slow annealing
Slow annealing of 2HPor-DAP with dT40 in the presence of
500 mm NaCl resulted in preferential formation of right-
handed 2HPor-DAP:dT40 helix (see section Theoretical simula-
tions of CD spectra for the helici-
ty assignment). The CD spectrum
showed a positive CD band at
423.0 nm (+52.7 mdeg) and two
negative CD bands at 379.3 nm
(ꢀ10.5 mdeg) and 495.6 nm
(ꢀ29.1 mdeg) in the Soret band
region (Figure 1a). Negative CD
bands at 609.6 nm (ꢀ16.1 mdeg)
and 684.5 nm (ꢀ12.9 mdeg)
were detected in the Q-band
region. Strong CD signals are
consistent with formation of hel-
ical nanostacks of 2HPor-DAP
conjugates along an oligothy-
mine template.
Similarly, dT16 successfully
templated formation of the P-
isomer
of
2HPor-DAP:dT16
Scheme 2. Synthesis of porphyrin–diaminopurine conjugates 2HPor-DAP, NiPor-DAP, and ZnPor-DAP. i) 1) NaH,
DMF, 608C, 2 h; 2) 2-(2-(2-methoxyethoxy)ethoxy)ethyl 4-methylbenzenesulfonate, 08C, 7 h; ii) N-bromosuccini-
mide (NBS), CH3CN, RT, 1.5 h; iii) [Pd2(dba)3], Ph3P, (nhex)3SiꢀCꢁCH, CuI, piperidine, 408C, 4 h; iv) tetra-n-butylam-
monium fluoride (TBAF), CH2Cl2, RT, 30 min; v) triethylene glycol monomethyl ether tosylate, K2CO3, CH3CN; vi) tri-
fluoroacetic acid (TFA), then 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ); vii) Zn(OAc)2·2H2O, CHCl3, CH3OH,
RT; viii) NBS, CHCl3, C5H5N; ix) 4, [Pd2(dba)3], Ph3P, CuI, DMF, triethylamine (TEA), 408C, 15 h; x) TFA, CH2Cl2, RT, 4 h;
xi) Ni(OAc)2·4H2O, DMF, 1208C, 4 h.
with a positive Cotton effect at
427.1 (+51.2 mdeg) and two
negative CD bands at 379.3 nm
(ꢀ10.5 mdeg) and 496.6 nm
(ꢀ17.1 mdeg) in the Soret band
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at 603.5 nm (ꢀ45.6 mdeg) and
557.2 nm (ꢀ15.4 mdeg). Similarly
to 2HPor-DAP, NiPor-DAP assem-
blies formed by slow annealing
on a dT8 template resulted in
a weaker CD signal in the Soret
absorption region with maxima
at 435.1 nm (+12.9 mdeg) and
485.0 nm (ꢀ13.9 mdeg).
The formation of right-handed
2HPor-DAP:dTx (x=8, 16 and
40) and NiPor-DAP:dTx nanoas-
semblies by slow annealing
caused large red-shifts of the
Soret bands (up to 54.4 nm) and
Q-bands (up to 33.4 nm) in the
absorption spectra indicative of
strong electronic coupling be-
tween porphyrin–diaminopurine
chromophores
(Figure 1c–1 h
and Table 1). The red-shifts sug-
gested that porphyrin–diamino-
purine
conjugates
adopted
structures similar to J-aggregates
within the DNA-templated multi-
porphyrin nanoassemblies.
The presence and type of
transition metal in the porphyrin
coordination center also influ-
enced nanoassembly formation.
Free-base and nickel porphyrins
exhibited similar behavior while
Figure 1. CD and UV/Vis spectra of right-handed oligothymidine-templated 2HPor-DAP:dTx and NiPor-DAP:dTx
nanoassemblies prepared by slow annealing. 2HPor-DAP:dTx: slow annealing of 2HPor-DAP (10 mm) with dTx
(10 mm) in 40% DMSO Na-cacodylate buffer (1 mm, pH 7.0, 500 mm NaCl (dT40) and 200 mm NaCl (dT16). NiPor-
DAP:dTx: slow annealing of NiPor-DAP (10 mm) with dTx (10 mm) in 45% DMSO Na-cacodylate buffer (1 mm,
pH 7.0, 100 mm NaCl.
region. Two additional negative Cotton effects were observed
Table 1. Soret band and Q-band shifts upon annealing of porphyrin–dia-
minopurine conjugates with oligothymidine templates.
in the Q-band region at 605.0 nm (ꢀ9.5 mdeg) and 677.0 nm
(ꢀ8.1 mdeg). Similar CD spectra for 2HPor-DAP:dT40 and
2HPor-DAP:dT16 suggested their similar structures. Annealing
of 2HPor-DAP with the shortest template dT8 yielded a right-
handed 2HPor-DAP:dT8 nanostructure characterized by a nega-
tive CD band at 477.7 nm (ꢀ4.4 mdeg) and a positive CD band
at 409.85 nm (+3.97 mdeg). No CD bands were observed in
the Q-band region.
2HPor-DAP:dTx
NiPor-DAP:dTx
(DSoret/DQ-band)
[a]
[a]
(DSoret/DQ-band
)
slow annealing fast annealing slow annealing fast annealing
dT40 54.4/33.4
dT16 52.8/30.9
47.2/28.0
42.4/26.6
47.2/28.2
47.6/20.6
49.0/20.6
49.2/20.4
42.2/16.8
45.0/18.2
47.2/19.2
dT8
50.4/28.2
[a] Soret band and Q-band bathochromic shifts [nm].
Slow annealing of NiPor-DAP with dT40 in the presence of
100 mm NaCl also resulted in the assembly of preferentially
right-handed NiPor-DAP:dT40 nanoarrays characterized by
a negative Cotton effect 478.9 nm (ꢀ120 mdeg) and a positive
Cotton effect at 426.0 nm (+75.0 mdeg) in the Soret band
region (Figure 1b). Negative CD bands at 550 nm
(ꢀ10.7 mdeg) and 595.2 nm (ꢀ40.7 mdeg) and a positive CD
band at 620 nm (+20 mdeg) were detected in the Q-band
region. The chiral right-handed assembly NiPor-DAP:dT16 was
also successfully formed upon slow annealing of NiPor-DAP
with the shorter dT16 template, as evidenced by a positive CD
band at 440.6 nm (+93.3 mdeg) and a negative CD band at
491.0 nm (ꢀ63.9 mdeg) in the Soret region. Two negative
Cotton effects were observed in the Q-band absorption region
all attempts to prepare chiral dT40 templated nanoassemblies
of ZnPor-DAP conjugate failed. We have unsuccessfully tested
slow and fast annealing while varying the DMSO content from
1% to 70% in Na-cacodylate buffer (1 mm, pH 7.0) as well as
increasing the ionic strength (up to 500 mm NaCl). Zinc por-
phyrins are five coordinate in aqueous solutions and have
been shown not to lose the axial water ligand easily.[8] On the
other hand, nickel porphyrins have been shown to easily lose
axial ligands to form four-coordinate species.[10] We hypothe-
sized that the axial ligand on the zinc atom sterically prevent-
ed the efficient p-stacking and hampered formation of ZnPor-
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DAP:dT40 nanoassemblies. Similar behavior has been previous-
ly observed for zinc-porphyrin–DNA capping.[2d,11]
blies displayed positive CD bands at 488.7 nm (43.0 mdeg) and
390.9 nm (23.2 mdeg) and a negative band at 462.2 nm
(ꢀ75.7 mdeg) in the Soret region (Figure 2b, green curve). Pos-
itive Cotton effects at 602.8 nm (+15.9 mdeg) and 556.1 nm
Helicity control of 2HPor-DAP:dTx and NiPor-DAP:dTx nano-
assemblies: Fast annealing
(+5.1 mdeg) and
a negative Cotton effect at 587.3 nm
(ꢀ6.4 mdeg) were observed in the Q-band region. Shorter
NiPor-DAP:dT16 nanoassemblies exhibited a very similar CD
profile to longer NiPor-DAP:dT40. The CD spectrum showed
positive CD bands at 490.6 nm (+53.4 mdeg) and 392.3 nm
(+31.9 mdeg) and a negative band 466.4 nm (ꢀ87.6 mdeg) in
the Soret region (Figure 2b, purple curve). Positive Cotton ef-
fects at 604.8 nm (+23.0 mdeg) and at 557.2 nm (+5.6 mdeg)
and a negative Cotton effect at 586.4 nm (ꢀ5.6 mdeg) were
observed in the Q-band region. The dT8 sequence yielded
a similar CD profile with smaller negative and positive CD sig-
nals at 468.4 nm (ꢀ5.3 mdeg) and 395.1 nm (+6.8 mdeg), re-
spectively. Fast annealing of left-handed 2HPor-DAP:dTx and
NiPor-DAP:dTx nanoassemblies resulted in large red-shifts of
the Soret bands (up to 47.2 nm) and Q-bands (up to 28.2 nm)
in the absorption spectra (Figure 2c–h and Table 1). Fast an-
nealing yielded nanoassemblies with smaller red-shifts than
those formed by slow annealing. Similarly, 2HPor-DAP:dTx ex-
hibited larger red-shifts of the Soret and Q-bands than the
nickel derivatives indicating differences in angles and distances
We have shown previously that the handedness of the oligo-
thymidine templated porphyrin–diaminopurine conjugate
(2HPor-DAP:dT40) can be controlled and opposite helicities
can be obtained by controlling the ionic strength and anneal-
ing rate of the precursors.[5] The slow annealing (linear cooling
at 0.58Cminꢀ1; cooling time from 858C to 208C=260 min) in
the presence of NaCl has been shown to form predominately
right-handed nanoassemblies. On the other hand, the fast an-
nealing (non-linear cooling; cooling time from 858C to 208C
<5 min) in the presence of NaCl provided predominantly left-
handed nanostructures. We have therefore explored the effect
of annealing rates on the helicity of assembled nanostructures
while varying the oligothymidine lengths and metal in the co-
ordination center of the porphyrin (free-base and nickel).
Fast annealing of 2HPor-DAP with dT40 in the presence of
100 mm NaCl resulted in preferential formation of left-handed
2HPor-DAP:dT40 nanoassemblies, characterized by a negative
CD band at 461.6 nm (ꢀ57.7 mdeg) and positive CD bands at
400.1 nm (46.9 mdeg) and at
500.0 nm (+21.8 mdeg) in the
Soret band region (Figure 2a,
green curve). The Q-band
absorption
region
revealed
positive CD bands at 626.1 nm
(+8.4 mdeg) and 693.5 nm
(+7.8 mdeg) and
a
negative
band at 598.1 nm (ꢀ4.0 mdeg).
The shorter template dT16 also
promoted formation of left-
handed 2HPor-DAP:dT16, as in-
dicated by appearance of a nega-
tive Cotton effect at 462.2 nm
(ꢀ27.9 mdeg)
and
positive
Cotton effects at 397.1 nm
(+30.6 mdeg) and 497.3 nm
(+19.7 mdeg) in the Soret
region (Figure 2a, purple curve).
The Q-band region displayed
positive CD bands at 619.7 nm
(+7.8 mdeg)
and
682.5 nm
(+7.1 mdeg). Shortest dT8 also
promoted assembly of 2HPor-
DAP, although with weaker CD
signals in Soret region and no
CD signal in the Q-band region
(Figure 2a, cyan curve). Fast an-
nealing of NiPor-DAP in the
presence of oligothymidine tem-
plates promoted preferential for-
mation of left-handed helices.
Figure 2. CD and UV/Vis spectra of left-handed oligothymidine-templated 2HPor-DAP:dTx and NiPor-DAP:dTx
nanoassemblies prepared by fast annealing. Conditions: [MPor-DAP]=10 mm (M=2H or Ni), [dTx]=10 mm, 40%
(M)-NiPor-DAP:dT40 nanoassem- DMSO Na-cacodylate buffer (1 mm, pH 7.0, 100 mm NaCl).
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between porphyrin–diaminopur-
ine conjugates in right- and left-
handed nanoassemblies.
All three of the oligothymidine
templates successfully assem-
bled 2HPor-DAP and NiPor-DAP
conjugates into chiral nano-
stacks under both annealing
rates. The length of the template
and the annealing rate had a pro-
nounced effect on the CD signa-
ture of assembled nanostruc-
tures. The longest oligothymi-
dine template, dT40, provided
the most intense CD spectra
under slow as well as fast an-
nealing conditions for 2HPor-
DAP and NiPor-DAP. The nanoas-
semblies formed along the
shortest template, dT8, gave rise
to weaker CD spectra with
a slightly different CD profile
than their longer counterparts.
The annealing rate controlled
the helicity of formed nanoas-
semblies with slow annealing
yielding preferentially right-
handed assemblies (P-helix) and
the fast annealing yielded favor-
ably the left-handed assemblies
(M-helix) when using dT40 and
Figure 3. Comparison of CD spectra of right-handed and left-handed a) 2HPor-DAP:dT16 nanoassemblies,
b) NiPor-DAP:dT16 nanoassemblies, c) 2HPor-DAP:dT40, and d) NiPor-DAP:dT40 nanoassemblies, prepared by
slow (red) and fast (green) annealing. Slow annealing of 2HPor-DAP (10 mm) with dT16 (10 mm) in 40% DMSO, Na-
cacodylate buffer (1 mm, pH 7.0, 200 mm NaCl). Fast annealing of 2HPor-DAP (10 mm) with dT16 (10 mm) in 45%
DMSO, Na-cacodylate buffer (1 mm, pH 7.0, 100 mm NaCl). Slow and fast annealing of NiPor-DAP (10 mm) with dTx
(10 mm) in 40% DMSO, Na-cacodylate buffer (1 mm, pH 7.0, 100 mm NaCl).
dT16 oligothymidine templates (Figure 3). The M-isomers of
the 2HPor-DAP:dT40 and 2HPor-DAP:dT16 nanoassemblies, P-
isomers of 2HPor-DAP:dT40 and 2HPor-DAP:dT16 nanoassem-
blies, and M-isomers of the NiPor-DAP:dT40 and NiPor-
DAP:dT16 nanoassemblies, had very similar CD signatures for
each pair, which differed only in their intensities. The CD spec-
troscopic data suggested that the dT8 template does not
allow both helicities to be successfully accessed.
2HPor-DAP tetramer is depicted in Figure 4a. Excited-state cal-
culations were carried out at time-dependent density function-
al theory (TDDFT) level, using a B3LYP hybrid functional. Dipo-
lar and rotational strengths were computed using both dipole-
length and dipole-velocity gauge formalisms and found to be
essentially identical; only dipole-velocity results are presented.
The computed rotational strengths were converted to molar
extinction coefficients and convoluted with Gaussian band
shapes of a uniform half-width of 25 nm. Owing to the size of
the model systems, along with the large number of excited
states (>500) that has to be computed to adequately repre-
sent the CD spectra, small basis sets are necessary to keep the
calculations tractable. Basis-set dependence was tested on
2HPor dimers, as shown in Figure 4b. Larger basis sets result-
ed in a red-shift on the spectrum along with an increase of the
CD intensity, but the overall sign pattern and shape of the CD
were essentially independent of the basis set. Furthermore, the
effect of solvent (water) on the computed spectra was tested
using conductor-like polarized continuum model (CPCM) as im-
plemented in Gaussian 09 (Supporting Information, Figure S3).
Including implicit solvent likewise resulted in a slight red-shift
of the spectrum, but it only had a minor effect on the CD.
The CD spectra calculated for the model P- and M-helix
structures are shown in Figure 4c. The simulations were carried
out for 2HPor-DAP tetramers using the TD B3LYP/3-21G level
of theory. The predicted CD patterns in the Soret region (com-
Theoretical simulations of CD spectra
Theoretical simulations of UV/Vis and CD spectra for model
porphyrin assemblies[5] were carried out using the Gaussian 09
program.[12] The model structures were constructed from
a 2HPor-DAP monomer whose geometry was fully optimized
at B3LYP/6-31G(d,p) level of theory. The right-handed 2HPor-
DAP assemblies (P-helix) were formed by attachment of the in-
dividual 2HPor-DAP conjugates to the poly(dT) DNA strand in
a B-form, to form canonical (Watson–Crick) hydrogen bonds.
This resulted in the vertical distance and angle between the
successive 2HPor-DAP units of 3 ꢁ and 348, respectively. Supra-
molecular 2HPor-DAP oligomers of two to six units were built.
The left-handed structures were obtained simply by inverting
the angle of the neighboring 2HPor-DAP conjugates (ꢀ348).
The 2HPor structures were obtained from 2HPor-DAP by delet-
ing the diaminopurine. A model structure for the P-helix of the
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duced the experimentally ob-
served CD very well. All tests of
the necessary approximations,
such as the neglect of solvent
(Supporting Information, Figure
S3), small basis sets, and limita-
tion to
a small number of
2HPor-DAP units (Figure 4),
showed correct trends toward
better correspondence with the
experiment. While further work
is necessary to determine the
definite structural arrangements
adopted by these nanoassem-
blies, the structures based on M-
and P-helices are very reasona-
ble models.
Dialysis: Purification of porphy-
rin–diaminopurine nanoassem-
blies 2HPor-DAP:dT40 and
NiPor-DAP:dT40
To characterize the structural
and photophysical properties of
DNA-templated porphyrin nano-
assemblies, we separated them
from porphyrin monomers. We
used dialysis (2kDa cassettes),
a widely used technique for re-
moval of small contaminants
Figure 4. a) 2HPor-DAP model structure for the P-helix. Comparison of simulated (B3LYP/3-21G) CD spectra for
model P-helix (red) and M-helix (green) structures of 2HPor-DAP tetramer. b) Dependence of the simulated
(B3LYP/STO-3G) CD spectra on the number of 2HPor-DAP units stacked in the P-helix model structure. c) TD-DFT
simulations of CD spectra for model porphyrin nanoassemblies (from dimer to hexamer). Intensities are normal-
ized to a single 2HPor unit. d) Basis set dependence of the CD spectra simulated with B3LYP density functional
for the P-helix 2HPor dimer.
puted between 250–450 nm) are in a good qualitative agree-
ment with the experimental data (compare to Figure 3a,c). An
intense negative couplet predicted for the model M-helix is
seen experimentally. However, essentially an oppositely signed
couplet is predicted for the P-helix with only a small additional
negative CD on the short wavelength side, while experimental
data show a distinct ꢀ/+/ꢀ pattern. This suggests that the
model structures, in particular of the P-helix, may not be accu-
rate. The model structures are likely also responsible, at least
in part, for a too high CD signal computed at longer wave-
lengths (500–800 nm) in 2HPor-DAP models, which is not seen
experimentally. Close stacking of DAP rings (Figure 4a) over-
emphasizes their contributions to the spectra over that of the
2HPor. As the calculations are necessarily limited to small
number of stacked rings, another possible source of discrepan-
cy between the experiment and simulations may be the size of
the stacked assembly. The dependence of the CD on the
number of stacked 2HPor-DAP was tested at TD-B3LYP/STO-3G
level (Figure 4d). These simulations show that the Soret band
CD intensity does grow more significantly with the assembly
size than do the 500–800 nm signals, and that relative intensity
of the negative CD to the red of the main couplet increases.
Both these trends are in the direction of experimental P-helix
CD.
from macromolecular systems. Dialysis allowed us to remove
non-assembled porphyrin conjugates together with the DMSO
and NaCl. The DMSO concentration was decreased from 40%
to 0.0028%, whereas the NaCl concentration was decreased
from 100 mm to 0.007 mm. The dialyzed 2HPor-DAP:dT40 solu-
tion was characterized by UV/Vis absorption, emission, and CD
spectroscopy. The absorption spectrum displayed a maximum
at 491.0 nm with a shoulder at about 435 nm (Figure 5c, blue
curve). The intensity of 440.0 nm absorption band correspond-
ing to non-assembled 2HPor-DAP conjugates notably de-
creased. A CD spectrum of dialyzed 2HPor-DAP:dT40 revealed
an identical profile with reduced intensity in comparison to
the pre-dialysis sample (Figure 5a). CD spectrum confirmed the
presence of chiral right-handed 2HPor-DAP:dT40 nanoassem-
blies after dialysis.
Emission spectra of dialyzed 2HPor-DAP:dT40 displayed
a single emission band at 685 nm, which was independent of
excitation wavelength (483.0 nm or 438.0 nm; Figure 5e). UV/
Vis absorption and emission spectra confirmed that dialysis
successfully removed the non-assembled 2HPor-DAP units
from the 2HPor-DAP:dT40 nanoassemblies. NiPor-DAP:dT40
was dialyzed using the same procedure as 2HPor-DAP:dT40.
The dialyzed solution was characterized by UV/Vis absorption
and CD spectroscopy. The absorption spectrum displayed
a maximum at 482.8 nm with a shoulder at about 420 nm (Fig-
ure 5d, blue curve). The 438.0 nm absorption band present in
All combined, considering the limitation of the theoretical
simulations particularly in regard to size, the simulations repro-
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Molar CD and CD anisotropy of
porphyrin nanoassemblies
After purifying the 2HPor-
DAP:dTx and NiPor-DAP:dTx
nanoassemblies by dialysis, the
concentrations of 2HPor-DAP
and NiPor-DAP conjugates in the
nanoassembly solutions were
determined by melting the
nanoassemblies by addition of
DMSO and subsequent heating
to 858C (see the Supporting In-
formation). The Soret band ab-
sorption was then compared
with the absorption of DNA-free
porphyrin
conjugates
with
known extinction coefficients
under identical conditions. In
a typical dialysis experiment, the
concentration of porphyrin–dia-
minopurine conjugate was de-
creased from 10 mm to 3.0 mm.
Knowing the concentrations of
the porphyrins, the CD elliptici-
ties (in millidegrees) were then
converted into molar CD (De,
Lmolꢀ1 cmꢀ1), which allowed us
to compare CD spectra of the
different nanoassemblies at dif-
ferent concentrations. Molar CD
spectra of dialysis purified
2HPor-DAP:dTx
and
NiPor-
DAP:dTx nanoassemblies are
plotted in Figure 6.
Slow annealing and longer oli-
gothymidine template dT40 pro-
duced chiral nanostructures with
stronger molar CD than fast an-
nealing and shorter templates
Figure 5. a),b) CD, c),d) normalized UV/Vis absorption, and e) normalized emission spectra of 2HPor-DAP:dT40
(left column) and NiPor-DAP:dT40 (right column) before dialysis (red) and after dialysis (blue). Nanoassemblies
were prepared by slow annealing in Na-cacodylate buffer (1 mm, pH 7.0). 2HPor-DAP:dT40 (40% DMSO, 500 mm
NaCl), NiPor-DAP:dT40, (45% DMSO, 100 mm NaCl).
(dT16,
dT8).
Right-handed
the spectrum of the NiPor-DAP:dT40 assembly before dialysis
significantly diminished. Dialysis therefore successfully re-
moved the non-assembled NiPor-DAP conjugates from the hel-
ical NiPor-DAP:dT40 nanoassembly. CD spectrum of the dia-
lyzed NiPor-DAP:dT40 solution revealed identical features, with
a reduced intensity in comparison to the pre-dialysis sample
(Figure 5b). A CD spectrum confirmed the presence of chiral
NiPor-DAP:dT40 nanoassemblies after dialysis. Similar results
have been observed for the dialysis of left-handed 2HPor-
DAP:dT40 and NiPor-DAP:dT40 prepared by fast annealing and
right- and let-handed handed 2HPor-DAP:dT16 and NiPor-
DAP:dT16 nanoassemblies prepared by slow and fast anneal-
ing, respectively (Supporting Information, Figures S5–S7).
2HPor-DAP:dT40 nanoassemblies prepared by slow annealing
displayed one of the highest reported molar CD values per as-
sembly
unit
in
supramolecular
structures
(e= +
850 Lmolꢀ1 cmꢀ1 at 428.0 nm). Molar CD confirmed very effi-
cient and strong chiral coupling between helically arranged
2HPor-DAP and NiPor-DAP conjugates.
To further evaluate the chiroptical characteristics of the
nanoassemblies, their CD anisotropy factors (also referred as
CD dissymmetry or g-factor) in the Soret band region were cal-
culated. The CD anisotropy factor is defined as g=De/e=
(ALꢀAR)/A, where A represents the conventional absorbance of
non-polarized light and AL and AR are the absorptions of left
and right circularly polarized light, respectively. The CD aniso-
tropy factor is independent of the concentration and of the
path length if the CD and absorbance spectra are taken on the
same sample. Large anisotropy factors of up to 8.5ꢂ10ꢀ3 for
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nanowires showed that the
widths of the nanowires varied
between 15 to 16 nm (Fig-
ure 7b; Supporting Information,
Figure S4). These dimensions are
in good agreement with the
length predicted by the molecu-
lar model of NiPor-DA P:dT40
nanoassembly based on
a
B-DNA geometry (40 base pairs:
length 13.2 nm). It is reasonable
to assume that triethyleneglycol
(TEG) chains (five TEG chains per
each porphyrin–diaminopurine
unit)
promoted
side-to-side
interactions between NiPor-
DAP:dT40 nanoassemblies and
mediated the formation of
supramolecular polymeric nano-
wires. A molecular representa-
tion of the NiPor-DAP:dT40
nanoassembly and its side-by-
side assembly into a nanowire
are demonstrated in Figure 7c.
The nanowires were further
elongated into helical polymeric
Figure 6. Molar CD spectra of a),b) NiPor-DAP:dTx and c),d) 2HPor-DAP:dTx nanoassemblies prepared by slow
cooling (left column) and fast cooling (right column).
Table 2. CD anisotropy factors of porphyrin nanoassemblies prepared by
slow and fast annealing measured for CD Cotton effects in the Soret
band region.
2HPor-DAP:dTx
fast[a]
NiPor-DAP:dTx
slow[a] fast[a]
dTx tem- slow[a]
plate
dT40
+8.5ꢂ10ꢀ3
+4.7ꢂ10ꢀ3
-6.7ꢂ10ꢀ3
(481.0 nm)
+10.0ꢂ10ꢀ3
(438.0 nm)
+1.4ꢂ10ꢀ3
(418.0 nm)
-0.7ꢂ10ꢀ3
(471.0 nm)
(428.0 nm)
+8.0ꢂ10ꢀ3
(427.2 nm)
(398.0 nm)
+1.17ꢂ10ꢀ3
(392.6 nm)
dT16
[a] Annealing rate of porphyrin nanoassembly preparation.[5]
right-handed 2HPor-DAP:dT40 and g=10ꢂ10ꢀ3 for right-
handed NiPor-DAP:dT16 further confirmed intense dissymme-
try of the DNA-templated multi-porphyrin nanoassemblies
(Table 2).
HRTEM microscopy of dialyzed right-handed NiPor-
DAP:dT40 nanoassemblies
Figure 7. Bright field (a,b,d) and dark field (e) TEM images of right-handed
NiPor-DAP:dT40 nanoassemblies. b) Inset: grey scale cross-section of the
nanowire. c) Model representation of NiPor-DAP:dT40 nanoassembly and its
side-to-side self-assembly into a nanowire. dT40 (red), NiPor-DAP (black).
To further investigate the DNA-templated porphyrin nanoas-
semblies, we have utilized high-resolution transmission elec-
tron microscopy (HRTEM) and high-angle annular dark-field
imaging (HAADF). Figure 7 shows representative TEM micro-
graphs of drop-casted dialyzed aqueous solutions of NiPor-
DAP:dT40. Nanowires and helical fibrils with lengths from
a few hundred nanometers to several micrometers have been
observed. A detailed analysis of individual one-dimensional
fibrils. Helical twisting of wires around each other can clearly
be seen in Figure 7a. The presence of nickel (II) in the coordi-
nation center of the porphyrin allowed us to study the nano-
wires using the HAADF mode, which is highly sensitive to
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changes in atomic number. Dark-field imaging confirmed that
all of the nanowires contained nickel atoms (Figure 7e), which
are visible as bright areas compared to the carbon back-
ground. TEM studies thus confirmed the formation of NiPor-
DAP:dT40 nanoassemblies and showed their self-assembly into
helical fibrils of micrometer lengths.
Importantly, the 2HPor-DAP:dT40 remained a right-handed
helical structure, with CD features similar to the original freshly
prepared nanoassembly (Figure 1a) after five heating–cooling
cycles (cycle: 208C!858C!208C; Figure 8b, dashed curve).
On the other hand, heating the non-dialyzed right-handed
2HPor-DAP:dT40 nanoassembly from 208C to 858C caused its
complete melting, as evidenced by CD, UV/Vis absorption, and
emission spectra (Figure 8a,c,e). The CD signal completely dis-
appeared for temperatures above 708C. Sigmoidal fit of the
change of the 440 nm CD signal as a function of temperature
allowed us to calculate the melting temperature Tm as 53.78C.
At 858C, the absorption spectrum showed a disappearance of
the 500 nm band, while the emission spectrum was altered
into a single band at 700 nm.
Thermal and acid–base stability of Por-DAP:dT40 porphyrin
nanoassemblies
Next we explored the influence of temperature and pH on the
stability, spectroscopic, and conformational behavior of the of
the dialyzed and non-dialyzed right- and left-handed DNA-tem-
plated 2HPor-DAP:dT40 and NiPor-DAP:dT40 porphyrin nano-
assemblies. The structural stability of supramolecular nanoas-
semblies is one of key features for their future applications in
nanomaterials. Because of rela-
Similarly to its free-base counterpart, the dialyzed right-
handed NiPor-DAP:dT40 nanoassembly displayed very high
tively weak interactions, such as
p–p stacking and hydrogen
bonding, stability formation of
functional supramolecular archi-
tectures with well-controlled
morphology remains a challenge.
Thermal stability was evaluated
by heating a solution of dialyzed
nanoassemblies in Na-cacodylate
buffer (1 mm, pH 7.0) and also
by heating a solution of non-dia-
lyzed nanoassemblies in 40%
DMSO, Na-cacodylate buffer
(1 mm, pH 7.0 500 mm NaCl).
The
dialyzed
right-handed
nanoassem-
2HPor-DAP:dT40
blies displayed a very high level
of thermal stability and the
nanoassemblies did not melt
even at 858C (Figure 8). The
shape of the CD spectrum did
not change, although the CD in-
tensity decreased. The 2HPor-
DAP:dT40 thus remained in the
helical structure after heating
from 208C to 858C (Figure 8b).
For comparison, Stulz and co-
workers reported the complete
disassembly of zipper porphy-
rin–DNA nanoassemblies with
porphyrins covalently attached
to two complementary DNA
strands with temperatures of
above 708C.[2c] Absorption spec-
tra revealed a decrease of the
shoulder at 430 nm without
a
shift of the lmax (488 nm),
Figure 8. Variable-temperature a),b) CD, c),d) normalized UV/Vis absorption, and e),f) normalized emission spectra
of the right-handed 2HPor-DAP:dT40 nanoassembly before dialysis (left spectra) and after dialysis (right spectra)
at 208C (red), 858C (blue), and after five heating–cooling cycles (dashed curve). Assembly formation: [2HPor-
while the emission spectra
stayed virtually identical upon
heating to 858C (Figure 8d,f). DAP]=[dT40]=10 mm; slow annealing in 40% DMSO Na-cacodylate buffer (1 mm, pH 7.0, 500 mm NaCl).
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dashed curve). In contrast, the
non-dialyzed left-handed 2HPor-
DAP:dT40 nanoassembly fully
melted upon heating and the
CD signal completely disap-
peared above 708C (Figure 10a).
Sigmoidal fit of the change of
the 462 nm CD signal as a func-
tion of temperature yielded the
Tm=50.38C. The absorption
spectrum showed the disappear-
ance of the 486.2 nm band and
a small blue-shift (D=7.1 nm) of
the 445.0 nm band. The emission
spectrum displayed a new band
at 673.2 nm after heating to
858C.
The
dialyzed
left-handed
NiPor-DAP:dT40 nanoassembly
exhibited temperature induced
structural changes as evidenced
by the CD spectrum. The CD
spectrum at 858C displayed pos-
itive CD bands at 391.4, 492.8,
and 602.4 nm and a negative CD
band at 465.9 nm (Figure 11b).
The UV/Vis absorption showed
Figure 9. Variable-temperature a),b) CD and c),d) normalized UV/Vis absorption spectra of the right-handed NiPor-
DAP:dT40 nanoassembly before dialysis (left spectra) and after dialysis (right spectra) at 208C (red), 858C (blue),
and after five heating–cooling cycles (dashed curve). Assembly formation: [NiPor-DAP]=[dT40]=10 mm; slow an-
nealing in 45% DMSO, Na-cacodylate buffer (1 mm, pH 7.0, 100 mm NaCl).
a
red-shift of the lmax from
472.5 nm to 486.8 nm. The helic-
thermal stability and analogous spectroscopic behavior upon
heating. The intensity of the Soret CD bands decreased; how-
ever, the overall CD profile did not change when NiPor-
DAP:dT40 nanoassembly was heated from 208C to 858C (Fig-
ure 9b). The UV/Vis absorption experienced a red-shift of the
lmax from 480.2 nm to 487.5 nm. Importantly, NiPor-DAP:dT40
remained a chiral right-handed supramolecular structure with
even after five heating–cooling cycles. On the other hand,
heating the non-dialyzed NiPor-DAP:dT40 from 208C to 858C
caused complete melting, which is evident by the disappear-
ance of the CD signal (Figure 9a). The Soret band showed
a small blue-shift (7.1 nm) to 437.9 nm. Sigmoidal fit of the CD
signal change at 436.0 nm as a function of temperature al-
lowed us to calculate the Tm=56.08C.
The dialyzed left-handed 2HPor-DAP:dT40 nanoassemblies
displayed larger spectroscopic and structural changes upon
heating-cooling. The profile of the CD spectrum of the left-
handed 2HPor-DAP:dT40 nanoassembly changed after heating
from 208C to 858C, though the assembly retained its counter-
clockwise helical structure (Figure 10b). The CD spectrum at
858C exhibited significant similarities with original freshly pre-
pared left-handed 2HPor-DAP:dT40 (Figure 2a) with negative
CD band 461.2 nm and positive CD bands at 398.5 nm and
505.0 nm. Absorption spectrum exhibited a red-shift from
481.5 nm to 488.8 nm. The emission spectrum stayed virtually
identical upon heating to 858C.
ity of the left-handed NiPor-DAP:dT40 was preserved after five
heating–cooling cycles (Figure 11b, dashed curve). On the
other hand, heating of the non-dialyzed left-handed NiPor-
DAP:dT40 from 208C to 858C caused its complete disassembly
as evident by the absence of a CD signal and the disappear-
ance of the absorption band at 476.2 nm (Figure 11a,c). Tm
was calculated to be 46.98C using CD signal at 462 nm. Right-
handed nanoassemblies prepared by fast cooling showed an-
nealing-induced structural changes towards thermodynamical-
ly most stable structure. The thermal studies determined that
all dialyzed nanoassemblies exhibited a high degree of thermal
stability.
As right-handed 2HPor-DAP:dT40 nanoassemblies prepared
by slow annealing exhibited the highest thermal stability we
have explored the effect of pH on their stability. In two sepa-
rate experiments the pH of the 2HPor-DAP:dT40 solution was
decreased to 1.7 using HCl or increased to a pH 12.5 using
NaOH while measuring UV/Vis and CD spectra (Figure 12). In-
creasing the pH to 12.5 did not change the shape of the UV/
Vis absorption and CD spectra of 2HPor-DAP:dT40 but de-
creased their intensity. The UV/Vis Soret band signal at
490.7 nm dropped by 23.5%. Ellipticities at 428.5 nm and at
493.4 nm diminished from +31.6 mdeg to +17.0 mdeg
(ꢀ46.2%) and from ꢀ18.5 mdeg to ꢀ8.2 mdeg (ꢀ55.6%), re-
spectively. Decreasing the pH of right-handed 2HPor-
DAP:dT40 solution from 7 to 6 had no effect on the structure
of the assemblies as indicated by identical CD and UV/Vis spec-
tra (Figure 12a–c). Decreasing the pH to 3 caused Soret band
Five heating-cooling cycles caused very little change of the
CD spectrum of the left-handed 2HPor-DAP:dT40 (Figure 10b,
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Conclusion
We have shown that free-base
and nickel porphyrin–diamino-
purine conjugates 2HPor-DAP
and NiPor-DAP can be success-
fully assembled on oligothymi-
dine templates of different
lengths into helical supramolec-
ular multiporphyrin nanostruc-
tures by hydrogen bonding be-
tween complementary thymine
and diaminopurine groups. UV/
Vis spectroscopy showed strong
electronic coupling between
porphyrin units, as shown by
a strong red-shift of the Soret
band (up to 54 nm). The hand-
edness of templated multipor-
phyrin nanoassemblies could be
controlled by the annealing rate
and ionic strength during the
nanoassembly formation. Slow
annealing rates yielded preferen-
tially right-handed nanostruc-
tures, whereas fast annealing
yielded left-handed nanostruc-
tures. The helicities have been
assigned with the help of theo-
retical CD calculations. We have
successfully used dialysis to sep-
arate porphyrin nanoassemblies
from porphyrin monomers. All of
the dialyzed DNA-templated por-
phyrin nanoassemblies had very
high values of molar CD (e of up
to +850 Lmolꢀ1 cmꢀ1) as well as
CD anisotropy (g of up to +1.0ꢂ
10ꢀ2). HRTEM studies confirmed
formation of DNA-templated
Figure 10. Variable-temperature a),b) CD, normalized c),d) UV/Vis absorption, and e),f) emission spectra of the
left-handed 2HPor-DAP:dT40 nanoassembly before dialysis (left spectra) and after dialysis (right spectra) at 208C
(red), 858C (blue), and after five heating–cooling cycles (dashed curve). Assembly formation: [2HPor-
DAP]=[dT40]=10 mm; fast annealing in 40% DMSO Na-cacodylate buffer (1 mm, pH 7.0, 100 mm NaCl).
broadening and 28.7% hypochromicity. CD signal at 428.5 nm
dropped to 17.0 mdeg without a wavelength shift. Interesting-
ly, further decrease to pH 1.7 led to increase of the ellipticity
together with CD band shifts. The positive Cotton effect shift-
ed from 428.5 nm to 439.5 nm and its intensity increased to
28.5 mdeg. The negative Cotton effect shifted from 493.4 nm
to 479.8 nm and its intensity increased to ꢀ13.1 mdeg. UV/Vis
absorption spectrum showed a blue-shift of the Soret band
maximum to 463.0 nm. The absorption peak at 489.0 nm corre-
sponding to original structure of right-handed 2HPor-
DAP:dT40 assembly completely disappeared. The right-handed
2HPor-DAP:dT40 nanoassemblies thus possessed excellent
acid–base stability with minimal structural changes between
pH 3 and 13.
nickel(II)porphyrin nanoassemblies and showed their elonga-
tion into helical fibrils with micrometer lengths. The chiral
DNA-templated multiporphyrin nanoassemblies exhibited high
thermal and pH stability. The handedness of all assemblies was
preserved at temperatures up to +858C and pH between 3
and 12. We anticipate that chiral DNA-templated porphyrin
nanoassemblies will find applications in chiroptical nanomateri-
als owing to their modular chiroptical and structural properties
and high stability.
Experimental Section
All of the commercially available reagents were used as received
without purification unless otherwise indicated. Water was ob-
tained from Milli-Q system with a resistivity of 18.2 MWcm. DNA
concentrations are reported as nucleobase concentration and they
were quantified by UV/Vis absorption spectroscopy. All DNA-tem-
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plated porphyrin nanoassemblies
were prepared following previous-
ly reported procedures.[5] CD spec-
tra were recorded on a Jasco J-815
spectropolarimeter equipped with
a
Peltier temperature control
system. Conditions were as fol-
lows: scanning speed 50 nmminꢀ1
,
data pitch 0.5 nm, DIT 2 s, and
bandwidth 1 nm. UV/Vis absorp-
tion spectra were recorded on
a Jasco V-630 double-beam spec-
trophotometer equipped with
a single-position Peltier tempera-
ture control system. Fluorescence
emission spectra were recorded
on the Varian Cary Eclipse
fluorescence spectrophotometer
equipped with a Peltier tempera-
ture-control system. Conditions
were as follows: scan rate
600 nmminꢀ1
,
excitation
slit
5.0 nm, and emission slit 5.0 nm. A
quartz cuvette with a 1 cm path
length was used for all CD, UV/Vis,
and emission spectroscopy experi-
ments. 1H NMR spectra were ob-
tained on a Bruker DRX-400 spec-
trometer (1H: 400 MHz). Chemical
shifts are quoted as parts per mil-
lion (ppm) relative to the solvent
residual peak and coupling con-
stants (J) are quoted in Hz. A solu-
tion of dialyzed NiPor-DAP:dT40 in
water was drop-cast onto carbon-
coated copper grids and dried in
air. Imaging was performed on an
FEI Tecnai G2 F20 scanning trans-
Figure 11. Variable-temperature a),b) CD and c),d) normalized UV/Vis absorption spectra of the left-handed
NiPor-DAP:dT40 nanoassembly before dialysis (left spectra) and after dialysis (right spectra) at 208C (red curves),
858C (blue curves), and after five heating–cooling cycles (dashed curve). Assembly formation: [NiPor-
DAP]=[dT40]=10 mm; fast annealing in 45% DMSO Na-cacodylate buffer (1 mm, pH 7.0, 100 mm NaCl).
mission
electron
microscope
(STEM) operating at 200 kV.
Dialysis
A solution of DNA-templated por-
phyrin
nanoassembly
(10 mm,
1.6 mL) in 40% DMSO Na-cacody-
late buffer (1 mm, pH 7.0, 100 mm
NaCl) was placed into
a 2000
Dalton dialysis cassette. The cas-
sette was submerged into the Na-
cacodylate buffer (250 mL, 1 mm,
pH 7.0) and gently stirred for 12 h.
The cassette was then moved to
a new buffer solution and stirred
for additional 2 h.
9-Methoxy-trietyleneglycol-dia-
minopurine-2,6 (1)
2,6-diaminopurine
(DAP)
(500.0 mg, 3.33 mmol) was added
to a freshly distilled DMF (15.0 mL)
and the resulting suspension was
heated to 608C under nitrogen
Figure 12. Effect of pH on CD and UV/Vis absorption spectra of right-handed 2HPor-DAP:dT40. Assembly forma-
tion: [2HPor-DAP]=[dT40]=10 mm; slow cooling in 40% DMSO Na-cacodylate buffer (1 mm, pH 7.0, 500 mm
NaCl).
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until a solution was formed. NaH (96.0 mg, 4 mmol) was added
and the mixture was stirred for 2 h followed by a dropwise addi-
tion of triethylene glycol monomethyl ether tosylate (1.059 g,
3.32 mmol) in DMF (5.0 mL) at 08C. The reaction was monitored by
TLC (CH2Cl2/MeOH, 9:1) until no further change was observed (7 h).
DMF was evaporated, and the product was purified by flash
column chromatography on silica using CH2Cl2/MeOH (9:1) as the
eluent. Evaporation of the solvent gave (1) as a colorless crystalline
lene), 3.48–3.50 (m, 2H, O-CH2), 3.55–3.63 (m, 6H, O-CH2), 3.84 (t,
J=6.0 Hz, 2H, O-CH2), 4.31 (t, J=6.0 Hz, 2H, O-CH2), 4.82 (bs, 2H,
NH2), 5.48 ppm (bs, 2H, NH2); m/z ESI C14H20N6O3 [MH+] calcd
321.167; found 321.133.
5,15-Diaryl-porphyrin (6)
Dipyrromethane (1.80 g, 12.36 mmol) and 3,4-bis-substituted alde-
hyde 5 (5.32 g, 12.36 mmol) were dissolved in CH2Cl2 (2.4 L) under
nitrogen. TFA (600 mL) was added to the resulting solution by a sy-
ringe. The flask was shielded from light with aluminum foil and the
solution stirred at room temperature for 3 h. 2,3-Dichloro-5,6-dicya-
no-1,4-benzoquinone (DDQ; 3.60 g, 15.80 mmol) was added and
the solution stirred for additional 30 min. The mixture was neutral-
ized with triethylamine (TEA, 12.0 mL, 0.086 mol) and poured di-
rectly onto a silica gel pad (4 cm x 7 cm). Fast-running DDQ resi-
dues were removed by CH2Cl2 and the product eluted with CH2Cl2/
MeOH (95:5). The product was further purified by a flash silica gel
column chromatography using CH2Cl2/MeOH (98:2) as the eluent.
On removal of solvent and drying under high vacuum, product 6
(3.40 g, 49%) was obtained as purple solid glass. 1H NMR (CDCl3,
400 MHz) d=ꢀ3.07 (br. s, 2H, N-H pyrrole), 3.21 (s, 6H, O-CH3),
3.33 (t, J=12.0 Hz, 4H), 3.42 (bs, 6H, O-CH3), 3.49 (dd, J=5.0 Hz,
4 Hz, 4H), 3.64–3.70 (m, 8H), 3.77–3.84 (m, 8H), 3.80 (m, 4H), 3.90–
3.96 (m, 8H), 4.10 (m, 4H), 4.35–4.42 (m, 4H), 4.50 (m, 4H), 7.29 (d,
J=8.0 Hz, 2H, Ar), 7.40 (d, J=8.0 Hz, 2H, Ar), 7.78 (s, 2H, Ar), 9.09
(d, J=4.0 Hz, 4H, b-H), 9.35 (d, J=4.0 Hz, 4H, b-H), 10.23 ppm (s,
2H, meso-H); m/z ESI C60H78N4O16 [MH+] calcd 1111.55; found
1111.80.
1
solid (514.6 mg, 52%). H NMR (CDCl3, 400 MHz) d=3.36 (s, 3H, O-
CH3), 3.51–3.54 (m, 2H, O-CH2), 3.57–3.60 (m, 6H, O-CH2), 3.76 (dd,
J=5.2 Hz, 4.8 Hz, 2H, O-CH2), 4.18 (dd, J=5.2 Hz, 4.8 Hz, 2H, O-
CH2), 4.76 (bs, 2H, NH2), 5.49 (bs, 2H, NH2), 7.67 ppm (s, 1H,
H8ꢀpurine); m/z ESI C12H20N6O3 [MH+] calcd 297.167; found 297.200.
8-Bromo-9-methoxy-trietyleneglycol-diaminopurine-2,6 (2)
TEG-purine 1 (200.0 mg, 0.675 mmol) was added to MeCN (5.0 mL)
under nitrogen and the solution was placed in an ice bath. NBS
(138 mg, 0.776 mmol) dissolved in MeCN (3.0 mL) was added drop-
wise over 1 h. The ice bath was removed and the reaction was
stirred for 30 min at room temperature. The solvent was evaporat-
ed and the product purified by flash column chromatography on
silica using CH2Cl2/MeOH (97:3) as the eluent. Evaporation of the
solvent gave 218.0 mg (87%) of product 2 as a colorless solid.
1H NMR (CDCl3, 400 MHz) d=3.35 (s, 3H, O-CH3), 3.48–3.50 (m, 2H,
O-CH2), 3.54–3.63 (m, 6H, O-CH2), 3.81 (t, J=6.0 Hz, 2H, O-CH2),
4.24 (t, J=6.0 Hz, 2H, O-CH2), 4.78 (bs, 2H, NH2), 5.36 ppm (bs, 2H,
NH2); m/z ESI C12H19BrN6O3 [MH+] calcd 375.0783; found 375.0660.
8-Trihexylsilyl-ethynyl-9-methoxy-trietyleneglycol-diamino-
purine-2,6 (3)
5,15-Diaryl-Zn-porphyrin (7)
A solution of zinc acetate dihydrate (5.96 g, 27.18 mmol) in MeOH
(50 mL) was added to a stirred solution of porphyrin 6 (3.40 g,
3.02 mmol) in a 1:1 mixture of CH2Cl2/MeOH (100 mL). The reaction
mixture was stirred for another 45 min, after which TLC (CH2Cl2/
MeOH, 95:5) confirmed completion. Evaporation of the solvent and
flash chromatography on silica, eluted with CH2Cl2/MeOH (95:5),
was carried out to remove the excess zinc salts. Evaporation of the
8-Bromopurine 2 (200.0 mg, 0.534 mmol) was dissolved in piperi-
dine (10.0 mL) and the solution was deoxygenated using
a
vacuum/nitrogen cycle. Triphenylphosphine (140.0 mg,
0.534 mmol) and [Pd2(dba)3] (122.2 mg, 0.133 mmol) were added
subsequently against positive pressure of nitrogen. Mono(trihexyl-
silyl)acetylene (492.0 mg, 1.600 mmol) was added to piperidine
(3.0 mL) and the solution was deoxygenated using a vacuum/nitro-
gen cycle. The resulting suspension was deoxygenated, then
heated to 408C for 10 min. Copper(I) iodide (50.8 mg, 0.267 mmol)
was added and reaction was monitored by TLC (CH2Cl2/MeOH,
95:5). The starting material disappeared after 4 h. The reaction mix-
ture was cooled to room temperature, the solvent was evaporated,
and the product purified by flash column chromatography on silica
using ethyl acetate as eluent. Evaporation of the solvent yielded 3
as a white solid (279.0 mg, 87%). 1H NMR (CDCl3, 400 MHz) d=
0.70–0.74 (m, 6H, hexyl), 0.88–0.92 (m, 9H, CH2, hexyl), 1.24–1.45
(m, 24H, hexyl), 3.34 (s, 3H, O-CH3), 3.46–3.48 (m, 2H, O-CH2), 3.53–
3.62 (m, 6H, O-CH2), 3.84 (t, J=6.0 Hz, 2H, O-CH2), 4.29 (t, J=
6.0 Hz, 2H, O-CH2), 4.81 (bs, 2H, NH2), 5.39 ppm (bs, 2H, NH2); m/z
ESI C32H58N6O3Si [MH+] calcd 603.441; found 603.666.
1
solvent gave zinc porphyrin 7 as a red glass (2.81 g, 78%). H NMR
(CDCl3, 400 MHz) d=3.21 (s, 6H, O-CH3), 3.33 (m, 4H), 3.42 (s, 6H,
O-CH3), 3.49 (m, 4H), 3.64 (m, 8H), 3.75 (m, 8H), 3.80 (m, 4H), 3.90–
3.96 (m, 8H), 4.09 (m, 4H), 4.35 (m, 4H), 4.50 (m, 4H), 7.23 (d, J=
8.0 Hz, 2H, Ar), 7.79 (dd, J=8.0 Hz, 2 Hz, 2H, Ar), 7.92 (dd, J=
8.0 Hz, 2.0 Hz, 2H, Ar), 9.14 (d, J=2.0 Hz, 2H, b-H), 9.15 (d, J=
2.0 Hz, 2H, b-H), 9.38 (d, J=2.0 Hz, 2H, b-H), 6.39 (d, J=2.0 Hz, 2H,
b-H), 10.24 ppm (d, J=2.0 Hz, 2H, meso-H); m/z ESI C60H76N4O16Zn
[MH+] calcd 1173.46; found 1173.70.
Monobrominated Zn-porphyrin (8)
Zinc porphyrin 7 (1.90 g, 1.62 mmol) was dissolved in chloroform
(200 mL) with pyridine (375 mL). A solution of NBS (288 mg,
1.62 mmol) in chloroform (40 mL) and pyridine (190 mL) was added
dropwise over 60 min. The reaction was stirred for 15 min and
then quenched with acetone (5 mL). The solvents were removed
under reduced pressure, and the product 8 was purified by flash
silica chromatography using 50:35:15 Hexane/Toluene/MeOH.
Evaporation of the solvents gave monobrominated zinc porphyrin
8-Ethynyl-9-methoxy-trietyleneglycol-diaminopurine-2,6 (4)
THS-diaminourine
3 (108.8 mg, 0.180 mmol) was dissolved in
CH2Cl2 (5 mL) under nitrogen. TBAF (550.0 mL, 1m solution in THF,
0.550 mmol) was added. The reaction mixture was stirred at room
temperature for 30 min followed by addition of anhydrous granu-
lar calcium chloride (ca. 200 mg). The solvent was reduced by
evaporation and the reaction mixture passed through a silica gel
column using CH2Cl2/MeOH (9:1) as eluent. Evaporation of the sol-
vent gave 52.0 mg (90%) of diaminopurine 4 as a white solid.
1H NMR (CDCl3, 400 MHz) d=3.35 (s, 3H, O-CH3), 3.41 (s, 1H, acety-
1
8 (547 mg, 27%). H NMR (CDCl3/1% C5D5N, 400 MHz) d=3.21 (s,
6H, O-CH3), 3.33–3.35 (m, 4H), 3.41 (s, 6H, O-CH3), 3.48–3.50 (m,
4H), 3.59–3.62 (8H, m, J=9 Hz, 4.5 Hz), 3.72–3.80 (m, 12H), 3.88–
3.94 (m, 8H), 4.07 (t, J=4.5 Hz, 4H), 4.33 (t, J=4.5 Hz, 4H), 4.48 (t,
J=4.5 Hz, 4H), 7.27 (d, J= 8.0 Hz, 2H, Ar), 7.68 (d, J=8.0 Hz, 2H,
Chem. Eur. J. 2014, 20, 1878 – 1892
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Ar), 7.79 (s, 2H, Ar), 8.97–8.99 (m, J=4.0 Hz, 4H, b-H), 9.25 (d, J=
4.5 Hz, 2H, b-H), 9.71 (d, J=4.5 Hz, 2H, b-H), 10.06 ppm (s, 1H,
meso-H); m/z ESI C60H75N4O16Zn [M-Na]+ calcd 1273.36; found
1273.54.
NiPor-DAP
The free-base porphyrin–diaminopurine (2HPor-DAP) (11.0 mg,
0.0076 mmol) was dissolved in freshly distilled DMF (3.0 mL) under
nitrogen followed by addition of nickel(II) acetate tetrahydrate
(19.1 mg). The solution was heated to 1208C for 4 h when TLC
(CH2Cl2/MeOH, 95:5) showed completion. DMF was evaporated
under reduced pressure and the residue was dissolved in CH2Cl2
(20 mL) and washed with water (10 mL). NiPor-DAP was purified by
flash silica gel chromatography with CHCl3/MeOH/TEA (90:9:1).
Yield 9.6 mg, 83%. 1H NMR (CDCl3, 400 MHz) d=3.04 (s, 3H, O-
CH3), 3.11–3.13 (m, 2H), 3.21 (s, 6H, O-CH3), 3.26–3.36 (m, 6H), 3.41
(s, 6H, O-CH3), 3.48–3.50 (m, 4H), 3.53–3.55 (m, 2H), 3.59–3.62 (m,
8H), 3.68–3.76 (m, 14H), 3.87–3.92 (m, 8H), 4.05 (dd, J=5.5 Hz,
5.0 Hz, 4H), 4.12 (t, J=5.5 Hz, 2H), 4.27–4.29 (m, 4H), 4.45 (m, 4H),
4.71 (t, J=5.5 Hz, 2H), 4.84 (bs, 2H, NH2), 5.53 (bs, 2H, NH2), 7.24
(d, J=8.0 Hz, 2H, Ar), 7.55 (dd, J=8.0 Hz, 2.0 Hz, 2H, Ar), 7.61 (d,
J=2.0 Hz, 2H, Ar), 8.90 (d, J=4.5 Hz, 2H, b-H), 8.94 (d, J=4.5 Hz,
2H, b-H), 9.11 (d, J=4.5 Hz, 2H, b-H), 9.71 (d, J=4.5 Hz, 2H, b-H),
9.76 ppm (s, 1H, meso-H); Triethylamine coordinated on Ni: d=
1,20 (t, J=7.0 Hz, 9H, CH3), 3.47 ppm (q, J=7.0 Hz, 6H, CH2). Coor-
dination of solvents (for example DMF, MeOH, water) to metal
center porphyrins has been previously reported in magnesium por-
phyrins.[13] MALDI/TOF: m/z C74H94N10NiO19 [MꢀH+] calcd
1485.6128; found 1485.6212. UV/Vis lmax (DMSO) (log e)=436.6 nm
(5.05), 544.4 nm (4.25), 591.2 nm (4.37).
ZnPor-DAP
The monobrominated Zn-porphyrin 8 (360.0 mg, 0.287 mmol) was
dissolved in freshly distilled DMF (10.0 mL) under nitrogen fol-
lowed by addition of TEA (8.0 mL). The solution was deoxygenated
(vacuum/nitrogen cycle) and triphenylphosphine (75.2 mg,
0.287 mmol) and [Pd2(dba)3] (65.0 mg, 0.071 mmol) were added
subsequently. Next, acetylene–diaminopurine
4
(165.1 mg,
0.515 mmol) was added in DMF (5.0 mL). The resulting mixture was
deoxygenated (vacuum/nitrogen cycle), then heated to 408C for
10 min. Copper(I) iodide (27.3 mg, 0.143 mmol) was added and re-
action was stirred at 408C overnight (15 h) when TLC (CH2Cl2/
MeOH, 95:5) confirmed completion. The mixture was cooled and
the solvents removed. Porphyrin–diaminopurine ZnPor-DAP was
obtained as a green glass (290.0 mg, 67%) by flash silica chroma-
tography using CH2Cl2/MeOH/pyridine (90:9:1). 1H NMR (CDCl3,
400 MHz) d=3.07 (s, 3H, O-CH3), 3.16 (dd, J=4.6 Hz, 4.4 Hz, 2H),
3.21 (s, 6H, O-CH3), 3.34–3.40 (m, 6H), 3.41 (s, 6H, O-CH3), 3.49–
3.52 (m, 4H), 3.58–3.63 (m, 10H), 3.73–3.81 (m, 14H), 3.90–3.96 (m,
8H), 4.09 (t, J=5.0 Hz, 4H), 4.21 (t, J=5.0 Hz, 2H), 4.34 (t, J=
8.0 Hz, 4H), 4.49 (t, J=8.0 Hz, 4H), 4.82 (t, J=5.5 Hz, 2H), 4.86 (bs,
2H, NH2), 5.59 (bs, 2H, NH2), 7.27 (d, J=8.0 Hz, 2H, Ar), 7.68 (dd,
J=8.0 Hz, 2.0 Hz, 2H, Ar), 7.80 (d, J=2.0 Hz, 2H, Ar), 8.95 (d, J=
4.5 Hz, 2H, b-H), 9.00 (d, J=4.5 Hz, 2H, b-H), 9.25 (d, J=4.5 Hz, 2H,
b-H), 9.79 (d, J=4.5 Hz, 2H, b-H), 10.11 ppm (s, 1H, meso-H);
MALDI/TOF: m/z C74H94N10O19Zn [M-H+] calcd 1491.6066; found
1491.6025. UV/Vis lmax (DMSO) (log e)=451.8 nm (5.33), 576.4 nm
(4.18), 636.0 nm (4.62).
Acknowledgements
This work was supported by the UW Start-up Funds (M.B.), Uni-
versity of Wyoming, School of Energy Resources Graduate As-
sistantship (M.B., G.S.), SER Center for Photoconversion and
Catalysis (M.B., G.S.), and NSF Career 0846140 (J.K.).
Keywords: circular dichroism · porphyrins · self-assembly ·
supramolecular chemistry · TDDFT calculations
2HPor-DAP
ZnII porphyrin–diaminopurine (15.0 mg, 0.010 mmol) was dissolved
in CH2Cl2 (2.5 mL). The solution was stirred in a flask in an ice bath
for 15 min, and then TFA (100 mL) was added dropwise. After
10 min the ice bath was removed and the reaction was stirred at
room temperature for 4 h. Then the flask was placed in an ice bath
and a saturated solution of NaHCO3 (20.0 mL) was added. The ice
bath was removed and solution was stirred at room temperature
for 30 min. The mixture was extracted with CH2Cl2 (3ꢂ20 mL) and
combined organic layers washed with water (20 mL). The CH2Cl2
was evaporated and the product was purified by flash column
chromatography on silica using CH2Cl2/MeOH (9:1) as the eluent.
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2HPor-DAP was obtained as a green glass (10 mg, 70%). H NMR
(CDCl3, 400 MHz) d=ꢀ2.52 (s, 2H, pyrrole), 3.03 (s, 3H, O-CH3),
3.12–3.15 (m, 2H), 3.19 (s, 3H, O-CH3), 3.20 (s, 3H, O-CH3), 3.31–
3.38 (m, 6H), 3.42 (s, 6H, O-CH3), 3.49–3.52 (m, 4H), 3.58–3–64 (m,
10H), 3.74–3.81 (m, 14H), 3.91–3.97 (m, 8H), 4.09–4.11 (m, 4H),
4.19 (t, J=5.0 Hz, 2H,), 4.32–4.35 (m, 4H), 4.49–4.51 (m, 4H), 4.80
(t, J=5.5 Hz, 2H), 4.85 (bs, 2H, NH2), 5.53 (bs, 2H, NH2), 7.32 (d, J=
8.0 Hz, 2H, Ar), 7.74 (dd, J=8.0 Hz, 2.0 Hz, 2H, Ar), 7.82 (d, J=
2.0 Hz, 2H, Ar), 8.97 (d, J=4.5 Hz, 2H, b-H), 9.02 (d, J=4.5 Hz, 2H,
b-H), 9.27 (d, J=4.5 Hz, 2H, b-H), 9.84 (d, J=4.5 Hz, 2H, b-H),
10.19 ppm (s, 1H, meso-H); HR-ESI: C74H96N10O19, m/z [M-H+] calcd
1429.6931; found 1429.6920. UV/Vis: lmax (DMSO) (log e)=
437.0 nm (5.18), 592.0 nm (4.44), 673.0 nm (4.15).
Chem. Eur. J. 2014, 20, 1878 – 1892
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Received: October 24, 2013
Published online on January 23, 2014
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim