Dynamics and efficiency of electron injection and transport in DNA using pyrenecarboxamide as an electron donor and 5-bromouracil as an electron acceptor

Article abstract of DOI:10.1021/jp9107393

The photophysical and photochemical behavior of a series of hairpin-forming DNA conjugates possessing a 5′-tethered pyrenecarboxamide chromophore and one or two bromouracil bases has been investigated. Quenching of the pyrene fluorescence and transient absorption spectra characteristic of the pyrene cation radical are observed only when bromouracil is located at the first or second base pair position nearest to the point of pyrene attachment. These observations are consistent with an intercalated structure for these conjugates in which pyrene is adjacent to the second base pair. Selective quenching of singlet pyrene by bromouracil but not by thymine is consistent with the free energy for charge separation estimated using Weiler's equation. Low quantum yields for loss of bromide when bromouracil is not adjacent to pyrene are attributed to inefficient charge separation via either a multistep electron transport or a single-step superexchange mechanism. Quantum yields are only weakly dependent upon the distance between pyrene and bromouracil, as expected for a multistep electron transport mechanism. Loss of bromide from conjugates possessing two bromouracils occurs sequentially. For adjacent bromouracils, competitive loss of bromide from both bromouracils is observed, whereas for nonadjacent bromouracils loss of bromide from the proximal bromouracil occurs prior to any loss from the distal bromouracil, consistent with a slower rate constant for electron transport vs loss of bromide.

Full text of DOI:10.1021/jp9107393

J. Phys. Chem. B 2010, 114, 14265–14272
14265
Dynamics and Efficiency of Electron Injection and Transport in DNA Using
Pyrenecarboxamide as an Electron Donor and 5-Bromouracil as an Electron Acceptor†
Pierre Daublain,‡ Arun K. Thazhathveetil,‡ Vladimir Shafirovich,*,§ Qiang Wang,|
Anton Trifonov,| Torsten Fiebig,*,| and Frederick D. Lewis*,‡
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113, Department of Chemistry,
Boston College, Chestnut Hill, Massachusetts 02467, and Department of Chemistry, New York UniVersity,
New York, New York 10003
ReceiVed: NoVember 11, 2009; ReVised Manuscript ReceiVed: January 12, 2010
The photophysical and photochemical behavior of a series of hairpin-forming DNA conjugates possessing a
5′-tethered pyrenecarboxamide chromophore and one or two bromouracil bases has been investigated.
Quenching of the pyrene fluorescence and transient absorption spectra characteristic of the pyrene cation
radical are observed only when bromouracil is located at the first or second base pair position nearest to the
point of pyrene attachment. These observations are consistent with an intercalated structure for these conjugates
in which pyrene is adjacent to the second base pair. Selective quenching of singlet pyrene by bromouracil but
not by thymine is consistent with the free energy for charge separation estimated using Weller’s equation.
Low quantum yields for loss of bromide when bromouracil is not adjacent to pyrene are attributed to inefficient
charge separation via either a multistep electron transport or a single-step superexchange mechanism. Quantum
yields are only weakly dependent upon the distance between pyrene and bromouracil, as expected for a multistep
electron transport mechanism. Loss of bromide from conjugates possessing two bromouracils occurs
sequentially. For adjacent bromouracils, competitive loss of bromide from both bromouracils is observed,
whereas for nonadjacent bromouracils loss of bromide from the proximal bromouracil occurs prior to any
loss from the distal bromouracil, consistent with a slower rate constant for electron transport vs loss of bromide.
Introduction
irreversible methods precludes investigation of the dynamics
of electron injection.
Charge separation over multiple base pairs in DNA can occur
via either the migration of positive charge (holes) or negative
charge (electrons).1 Experimental studies of the dynamics and
efficiency of photoinduced hole transport have been more
numerous than measurements of electron transport. Reasons for
this include the lower potentials for oxidation of the purines A
and G vs reduction of the pyrimidines T, C, and U (Chart 1a)2
and the ability of G to serve as a hole trap capable of undergoing
hole transport between G-containing sites in competition with
oxidative strand cleavage. The pyrimidines have similar reduc-
tion potentials, and thus none can serve as an electron trap.
Thymine dimer,3-6 thymine oxetane,7 and the halouracils8-10
have been employed as electron traps in investigations of
electron transport in DNA. The thymine dimer and thymine
oxetane undergo cycloreversion, and the halouracils undergo
loss of halide ions upon reduction. Most investigations of
distance-dependent DNA electron transport have employed
irreversible methods of electron injection, including photoin-
duced charge shift from a reduced flavin,6,7 ground state electron
transfer from a ketyl radical anion5 or a reduced irridium(II)
complex,10 or pulse radiolysis.11 These studies have established
that the characteristic reactions of electron traps are only weakly
dependent upon the number or sequence of base pairs separating
the trap from the point of injection. However, the use of
We recently reported the results of an investigation of the
dynamics of photoinduced electron injection and the distance
dependence of the efficiency of electron transport in DNA using
a 5′-tethered aminopyrene (APy, Chart 1b) hairpin possessing
bromo- and iodouracil (XU) as electron acceptors separated from
APy by one or more A-T base pairs (Chart 1c).12 APy is a
strong electron donor capable of reducing adjacent T or XU bases
with subpicosecond (ps) time constants for electron injection
(Figure 1a). Charge recombination in the resulting radical ion
pair is more rapid than electron transport and loss of halide
ions, leading to low quantum yields for dehalogenation. These
observations are consistent with previously reported studies of
photoinduced charge separation using XU bases as electron
acceptors in which neither long-lived charge-separated states
nor efficient loss of halide ions were observed.13 It occurred to
us that the use of a less potent electron donor might effect
selective reduction of XU and thus permit investigation of
electron injection via a single-step superexchange process. Use
of a weaker electron donor might also slow down charge
recombination, thus permitting electron transport to compete
more effectively with charge recombination.
We report here the results of a collaborative investigation of
the dynamics of electron injection and the efficiency of BrU
dehalogenation in a family of hairpins (Chart 1c) possessing a
5′-pyrenecarboxamide electron donor (PyA, Chart 1b) and either
one or two BrU-A base pairs separated by a variable number
of A-T base pairs. We have previously observed that singlet
PyA can serve as a hole donor for the selective oxidation of G
vs A (Figure 1b).14 We now find that PyA can serve as a
selective electron donor for the reduction of BrU vs T. The
† Part of the “Michael R. Wasielewski Festschrift”.
* Corresponding authors. E-mail: fdl@northwestern.edu; vs5@nyu.edu;
fiebig@bc.edu.
‡ Northwestern University.
§ New York University.
| Boston University.
10.1021/jp9107393  2010 American Chemical Society
Published on Web 02/02/2010

14266 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Daublain et al.
CHART 1: (a) Purines Adenine (A) and Guanine (G)
and Pyrimidines Thymine (T), 5-Bromouracil (BrU), and
Uracil (U) and Their Redox Potentials, (b) Pyrene
Derivatives Aminopyrene (APy), Pyrenecarboxamide
(PyA), Their Singlet Energies, and Redox Potentials, (c)
Pyrene Conjugates and Control Hairpins, and (d) the
Dyad PyA-PI
the BrU proximate to the PyA prior to loss of bromine from the
distal BrU, consistent with the function of BrU as a deep electron
trap on the time scale of electron transport.
Experimental Section
General Procedures. UV absorption, fluorescence, and
circular dichroism spectra were obtained as previously described
on samples contained in 1 cm path length cuvettes.12,15 Quantum
yields for fluorescence were determined using quinine sulfate
in sulfuric acid as a reference standard.16 CD spectra are the
average of two scans with a data interval of 1.0 nm and a time
interval of 2 s per point. The base lines are corrected by
subtraction of the spectrum of the buffer (10 mM phosphate,
0.1 M NaCl). Electrochemical measurements were performed
using a CH Instruments model 622 electrochemical workstation.
The solvent was dimethyl formamide containing a 0.1 M tetra-
n-butylammonium hexafluorophosphate electrolyte. A glassy
carbon working electrode, platinum wire counter electrode, and
Ag/AgCl reference electrode were employed. Ferrocene/ferro-
cinium (Fc/Fc+, 0.47 vs SCE in DMF17) was used as an internal
reference for all measurements.
All samples for kinetic spectroscopic measurements were
prepared in 10 mM sodium phosphate, pH 7.2, buffer with 0.1
M NaCl (standard buffer). Samples were stirred during these
measurements and absorption spectra checked before and after
spectroscopic studies for the occurrence of decomposition.
Hairpin concentrations were adjusted to provide an absorbance
of 0.2 at 354 nm in a 1 cm path length cuvette for fluorescence
measurements and 0.27 in a 1 mm path length cell for
pump-probe measurements. Fluorescence decay measurements,
nanosecond transient absorption measurements, and femtosecond
broadband pump-probe spectra were obtained as previously
described.12,15
Samples of hairpin conjugates for quantitative photochemical
studies having an optical density of 0.02 at 355 nm were
prepared in standard buffer containing 0.1 M isopropyl alcohol
(i-PrOH). Samples (1.7 mL) were placed in quartz cuvettes
equipped with a stirring bar and a rubber septum and were
purged with dry nitrogen for 15 min prior to irradiation. Samples
were irradiated in a thermostatted cuvette holder at 20 °C with
monochromatic 355 nm light provided by a 150 W mercury
arc lamp and a 0.5 M monochromator. For HPLC analysis, 5.0
µL aliquots were withdrawn by syringe at regular time intervals
and analyzed by HPLC using a Varian Microsorb-MV100 C18
column on a Dionex HPLC system equipped with a diode array
detector. Solutions of (A) 0.03 M TEAA and (B) 95%
acetonitrile and 5% 0.03 M TEAA were used as the mobile
phase. A gradient of 5 min 0-16% B, followed by an isocratic
elution (16% B-84% A) with a flow rate of 1 mL/min was
used for the separation. Integration of HPLC peaks using
detection wavelengths of 260 nm (base pair) and 355 nm
(pyrene) provided similar results. For MALDI analysis, the
overlapping reactant and product peaks were collected by HPLC,
and the solvent was evaporated and the residue dissolved in 10
µL of water. Half of this solution was used to prepare five spots
on the MALDI plate. Integrated HPLC or MALDI peak areas
were used to construct plots of % conversion vs irradiation time.
Relative quantum yields were determined from the initial slopes
of these plots. Absolute quantum yields for APy-4BrU were
determined at low conversions (<25%) of reactant to product
(average of three independent HPLC analyses) using potassium
ferrioxalate actinometry to determine the intensity of absorbed
light.18 The resulting quantum yield was used to convert the
relative quantum yields to absolute quantum yields.
dynamics of both charge separation and charge recombination
with neighboring BrU are slower for PyA vs APy, in accord
with the higher oxidation potential for PyA (Chart 1b). However,
the efficiency of superexchange electron transfer is too low to
be detected with either PyA or APy. The distance dependence
and absolute values of the quantum yields for loss of halide
ion from BrU are similar for the two pyrene derivatives.
Conjugates possessing two adjacent BrU bases undergo competi-
tive loss of halide ions, whereas conjugates possessing two Br
U
bases separated by a single T undergo loss of bromine from
Figure 1. Energetics of charge separation for (a) aminopyrene (APy)
and (b) pyrenecarboxamide (PyA) with selected nucleobases (B).
Energies are in electronvolts.

Dynamics, Electron Injection, and Transport in DNA
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14267
TABLE 1: Fluorescence Quantum Yields and Decay Times
and Transient Kinetics and Assignments for PyA Derivatives
and Conjugates
Materials
N-((3-Dioxoisoindolin-2yl)propyl)pyrene-1-carboxamide
(PyA-PI). N-(3-Aminopropyl)phthalimide was prepared from
the reaction of phthalimide with Boc-protected 3-chloro-1-
aminopropane by the method of Alfred et al.19 Reaction of the
acid chloride of pyrene-1-carboxylic acid with the deprotected
N-(3-aminopropyl)phthalimide afforded PyA-PI. Mp 172-174
pyrenea
Φfl
τs, nsb,c
τ (global), psd
470/520 nm, pse
b
PyA-H
PyA-PI
0.30 37
0.013
480, L
PyA-AT 0.38 3.9 (27), 10.7 (73) 6(r), L
PyA-1Br
PyA-2Br
PyA-3Br
PyA-4Br
U
U
U
U
0.031 0.14 (92), 8.0 (8) 10(r), 30(r), 130, L
0.17 2.1 (24), 10.3 (76) 7(r), 240, L
0.37 3.7 (29), 10.7 (71) 9(r), L
44(r), 170
18(r), 330
1
°C. H NMR (500 MHz; CDCl3): δ (ppm) 8.6 (1H, d, J ) 9.2
Hz), 8.0-8.2 (8H, m, Py-H), 7.73 (2H, dd, J1 ) 3.1, J2 ) 5.2
Hz, An-H), 6.95 (1H, bt, N-H), 3.9 (2H, t), 3.8 (2H, m), 3.6
(2H, J ) 6.1, m), 2.09 (2H, J ) 6.1, q). 13C NMR δ (ppm)
170.0, 168.7, 133.9, 132.4, 131.7, 131.1, 130.9, 130.6, 128.6,
128.5, 127.0, 126.2, 125.7, 125.6, 124.7, 124.5, 124.4, 124.3,
123.2, 36.9, 35.2, 28.5.
0.38 3.8 (26), 10.7 (73) 5(r), L
a See Chart 1 for structures. Values of Φfl and τs for PyA-H and
PyA-PI determined in methanol and values for the PyA conjugates
in aqueous buffer. b Values of Φfl and τs for conjugates determined
in aqueous buffer. Values of Φfl for PyA-5BrU, -6BrU, and -7BrU are
0.38 ( 0.02. c Lifetimes obtained from fluorescence decays with
pre-exponentials in parentheses. The long-lived decay for PyA-1BrU
is the weighted average of two low-amplitude 4.5 (5.2) and 15 (2.6)
ns components. d Rise (r) and decay times obtained from global fits
of the long-wavelength transient absorption bands except for
PyA-1BrU (fit from rise of 470 nm absorption band) attributed to
1S* relaxation. 30 ps rising component obtained from global fits for
PyA-1BrU attributed to formation of PyA+•BrU-•. Shorter decay
times for PyA-1BrU and -2BrU attributed to formation of
PyA-H was prepared and converted to its phosphoramidite
derivative as previously described14 and attached to the 5′-
terminus of an oligonucleotide using conventional solid sup-
ported oligonucleotide synthesis. The conjugates were depro-
tected using 30% ammonium hydroxide and purified by RP
HPLC. Structures were confirmed by MALDI-TOF-MS (Table
S1, Supporting Information), and the purities were confirmed
by analytical HPLC.
1
PyA+•BrU-•. Longer decay times (L > 2 ns) attributed to S* decay.
e Rising (r) and decaying components of band intensity ratio
attributed to charge separation (formation of PyA+•BrU-•) and charge
recombination, respectively.
Results
Electronic Spectra of the Pyrene Derivatives. The prepara-
tion and characterization of PyA-H and PyA-AT have previ-
ously been described.14 Preparation of the dyad Py-PI and the
BrU-containing PyA conjugates is described in the Experimental
Section. Conjugates PyA-AT and PyA-3BrU have melting
temperatures Tm ) 60.0 and 63.0 °C, respectively (Figure S1,
Supporting Information), determined from the derivatives of 260
nm thermal dissociation profiles. The circular dichroism spectra
of the PyA conjugates (Figure S2, Supporting Information)
display very weak bands at wavelengths longer than 300 nm
attributed to induced CD of the pyrene chromophore and
stronger bands between 200 and 300 nm similar to those for
the duplex poly(dA):poly(dT).20
Figure 2. Transient absorption spectra for (a) PyA-H (0-10 ps) from
ref 14 and (b) PyA-PI (0.20-1.8 ns) in methanol. Blue spectra are for
early delay times and red spectra for longer delay times. Color code:
blue, 0-1 ps; yellow-green, 1-5 ps; and red, >5 ps.
The absorption and fluorescence spectra of PyA-H and PyA-
AT have been reported.14 The presence of the amide substituent
has little effect on the appearance of the spectra of PyA-H,
which display vibronic structure similar to that of unsubstituted
pyrene.16 The weak absorption maximum of PyA-H at 380 nm
is assigned to the pyrene band 1 and the stronger maximum at
341 nm to pyrene band 2. The results of semiempirical ZINDO
calculations21 for N-methylpyrene-carboxamide (Figure S3,
Supporting Information) are consistent with the assignment of
both long-wavelength bands to pyrene-localized π-π* transi-
tions. The long-wavelength region (λ > 300 nm) of the UV
spectra of the dyad PyA-PI (Chart 1) is superimposable upon
that of PyA-H, indicative of the absence of ground state
electronic interaction between the pyrene and phthalimide
chromophore in methanol. The long-wavelength portion of the
UV absorption spectra and the fluorescence spectra of the PyA
conjugates in aqueous buffer are broadened and red-shifted by
several nanometers, when compared to the spectra of PyA-H
in methanol.
Fluorescence quantum yields and decay times for PyA-H and
the dyad PyA-PI in methanol are reported in Table 1 along
with fluorescence data for the PyA conjugates in aqueous buffer
(values of Φfl for conjugates PyA-nBrU (n ) 5-7) are the same
as those for PyA-AT and PyA-4BrU, as reported in footnote
b). The dyad PyA-PI is weakly fluorescent, indicative of
efficient intramolecular quenching of PyA fluorescence by the
PI electron acceptor. Fluorescence decay times for the PyA
conjugates determined with a Ti-Sp-based laser system having
a time resolution of ca. 35 ps are best fit as dual exponentials
(Table 1). The nanosecond decay components observed for all
of the conjugates are assigned to PyA singlet radiative and
nonradiative decay. The 140 ps decay component observed for
PyA-1BrU (140 ps) is tentatively assigned to charge separation.
Pump-Probe Spectra of Reference Molecules. Transient
spectra for PyA-H and PyA-PI in methanol obtained using a
pump wavelength of 346 nm are shown in Figure 2. As
previously reported, the spectra of PyA-H display a negative
band at 345 nm assigned to the ground state bleach and a
2
positive band at ca. 575 nm assigned to pyrene S*, both of
which are formed during the pump pulse (Figure 2a).14 The 575
nm band decays within the first picosecond and is replaced by
two bands at shorter wavelengths (370 and 485 nm) assigned
1
to pyrene S*. These bands do not decay appreciably during
the 1.9 ns time window of our measurements. The appearance
2
1
of the transient spectra assigned to S* and S* is similar to
that reported for pyrene and several of its derivatives, as is the
occurrence of ultrafast internal conversion.22,23 The long transient
decay time for PyA-H is consistent with its large fluorescence
quantum yield and long fluorescence decay time (Table 1).
The transient spectra for the dyad PyA-PI (Figure 2b) display
2
a fast rise and decay of the 575 nm S* band, similar to that for
PyA-H (Figure 2a). This is followed by the growth of a band at
450 nm and a weak shoulder at 420 nm attributed to formation of

14268 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Daublain et al.
Figure 3. Transient absorption spectra of conjugates (a) PyA-AT (data from ref 14), (b) PyA-1BrU, and (c) PyA-2BrU in aqueous buffer (0.2-1.9
ns). Blue spectra are for early delay times and red spectra for longer delay times. Color code: blue, 0-1 ps; yellow-green, 1-5 ps; and red, >5 ps.
Figure 4. (a) Conversion of PyA-2BrU to PyA-2U monitored by HPLC and (b) conversion of PyA-4BrU to PyA-4U monitored by MALDI mass
spectrometry.
the PyA+•-PI-• charge-separated state. The 450 nm band has a
band maximum similar to that reported for a urea-linked pyrene-
nitrobenzene dyad,23 and the 420 nm shoulder is similar to the
maximum of the long-wavelength band of electrochemically
generated PI-•.24 Analysis of the transient spectra provides a charge-
separation time of 480 ps for the PyA-PI (Table 1). Charge
recombination is slow on the 1.9 ns time scale of our measurements.
Pump-Probe Spectra for the PyA Conjugates. The
transient spectra for PyA-AT, -1BrU, and -2BrU are shown in
Figure 3. As is the case for PyA-H, formation and decay of the
obtained from global fitting and somewhat faster than the 140
ps fluorescence decay time. Decay times of ca. 150 and 300 ps
assigned to charge recombination are obtained from the 470/
425 nm band intensity ratios and global fitting of the transient
absorption bands of PyA-1BrU and -2BrU, respectively (Table
1). A 470 nm shoulder is not observed in the transient spectra
of PyA-AT, -3BrU, or -4BrU (spectra not shown).
Photochemistry of the PyA Conjugates. Dilute solutions of
the PyA conjugates containing 0.1 M NaCl and 0.1 M i-PrOH
were irradiated on an optical bench with monochromatic 355 nm
light corresponding to the pyrene absorption maxima. The use of
i-PrOH as an efficient scavenger for the uracilyl radical formed
upon loss of bromide from BrU has been described by Sugiyama
and Saito25 and employed in our recent study of APy conjugates.12
Aliquots were removed and analyzed periodically either directly
by HPLC with UV diode array detection or by MALDI-TOF mass
spectrometry analysis of samples collected by HPLC.
Conjugate PyA-AT is stable under our 355 nm irradiation
conditions, as are the control sequences 3BrU and 4BrU (Figure
S4, Supporting Information). Product peaks corresponding to
loss of bromide and formation of uracil-containing conjugates
were observed for all of the BrU-containing PyA conjugates.
Loss of bromide was confirmed by mass spectrometry of the
major product peaks collected by HPLC. In the case of PyA-
1BrU, two product peaks of similar size were observed. For this
reason, this conjugate was not included in our quantitative
photochemical studies. The reactant and product peaks of
conjugates PyA-2-4BrU can be resolved by HPLC under
nondenaturing conditions, as shown in Figure 4a for the
conversion of PyA-2BrU to PyA-2U with increasing irradiation
time. Integrated peak areas display good mass balance (ca. 90%
at 50% conversion). Reactant and product peak separation
decreases as the distance between the pyrene capping group
2
pyrene S* 575 nm band for all of the PyA conjugates occur
during the initial 1 ps following the laser pump pulse and is
accompanied by the growth of the 1S* absorption bands at 375
and 525 nm. The 525 nm bands in the transient absorption
spectra of PyA-AT (Figure 3a) and the other PyA conjugates
are broader and more symmetric than the corresponding band
of PyA-H (Figure 2a). Global fitting of the transient absorption
bands of PyA-AT and PyA-n-BrU (n ) 1-4) provides rising
components having a time constant of 5-10 ps (assigned to
1
relaxation of S*) and either one or two decay components
(Table 1). The longer lived decay components (>2 ns) observed
in both the global fits and 375 and 525 nm single wavelength
1
decays are assigned to S* (see Discussion Section).
The transient spectra of PyA-1BrU and -2BrU (Figure 3b,c),
but not the other PyA conjugates, display formation of 470 nm
bands that are assigned to PyA+• on the basis of comparison
with the transient spectra of PyA-PI (Figure 2b). The ratio of
the 525/345 nm band intensities is also smaller for these
conjugates (and for PyA-PI) than for the other PyA conjugates.
The rise times for the 470 nm bands of PyA-1BrU and -2BrU
determined from the ratio of the 470/520 nm intensities of the
band maxima are 44 and 18 ps, respectively (Table 1). The rise
time for PyA-1BrU is similar to the 30 ps rising component

Dynamics, Electron Injection, and Transport in DNA
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14269
of the reduction of G, but not A,14 and as an electron donor
capable of the oxidation of BrU but not T (Figure 1b).
Pyrene and other aromatic chromophores tethered to the 5′-
or 3′-terminus of a duplex or hairpin usually are assumed to
adopt an end-capping geometry in which one face of the
chromophore is π-stacked with the terminal base pair and the
other face is exposed to water (Figure 8a).27,28 An end-capped
geometry has been established by solution NMR studies for a
5′-tethered stilbene derivative;29 however, alternative geometries
have been observed for larger, more hydrophobic aromatics.30
We recently reported that a PyA conjugate possessing a single
G-C base pair adopts an intercalated hairpin conformation in
which PyA is π-stacked with a G-C at the second base pair in
preference to an end-capped conformation in which it is
π-stacked with the terminal base pair (Figure 8b).14 The
intercalated structure nicely explains the observation of efficient
hole injection to G-C base pairs located in either of the two
terminal base pairs but not elsewhere in the hairpin structure.
The observation of dual exponential fluorescence decay for PyA-
AT and the PyA-nBrU conjugates (Table 1) is also consistent
with the presence of two kinetically nonequivalent conformations.
The energetics of hole and electron injection from PyA to a
neighboring electron acceptor or donor can be estimated using
Weller’s equation (eq 1)
Figure 5. Time evolution for the formation of PyA-nU conjugates
upon 355 nm irradiation of PyA-nBrU conjugates in aqueous buffer
containing 0.1 M i-PrOH.
Figure 6. Quantum yields for loss of bromide from APy-nBrU and
PyA-nBrU conjugates in aqueous buffer containing 0.1 M i-PrOH.
∆Get ) -Es - (Erdn - Eox) + (2.6/ε - 0.13 eV)
(1)
and BrU increases, necessitating the use of MALDI analysis for
the PyA-5-7BrU conjugates. In the case of PyA-4BrU, analysis
by both HPLC and MALDI (Figure 4b) provides essentially
identical data for conversion vs irradiation time (Figure S5,
Supporting Information).
Plots of conversion vs irradiation time for the conjugates PyA-
2-7BrU obtained from HPLC and MALDI data are shown in
Figure 5. The initial slopes of these plots provide relative
quantum yields for the formation of PyA-nU. Absolute quantum
yields (ΦU) for PyA-4BrU were determined using monochro-
matic irradiation of the pyrene chromophore and potassium
ferrioxalate chemical actinometry.18 The resulting values of ΦU
for the PyA conjugates are shown in Figure 6 along with our
previously published results for APy conjugates.12 The absolute
values of ΦU are small; however, our analytical methods permit
accurate determination of ΦU.
where Es is the singlet energy of PyA-H; Eox and Erdn are
oxidation and reduction potentials of PyA and the nucleobases
(Chart 1a,b); and the final term is an empirical correction for
the solvent-dependent Coulomb attraction energy (-0.06 eV
in acetonitrile).31 The redox potentials of both PyA and the bases
are not expected to be precisely the same for the hairpin
structures in aqueous solution as for the isolated molecules in
polar aprotic solvents. Thus, the free energies for formation of
the charge-separated states obtained using eq 1 and shown in
Figure 1 are approximate values. However, the estimated
energies of the contact radical ion pairs (Figure 1b) are consistent
with the observation of fluorescence quenching of PyA by G
but not A14 and by BrU but not T (Table 1). Similarly, the value
of ∆Get ) -0.5 eV for the dyad PyA-PI calculated from the
phthalimide reduction potential (-1.40 V) is consistent with
the observation of efficient PyA fluorescence quenching by PI
(Table 1).
The photochemical behavior of the conjugates PyA-2,3BrU
and PyA-2,4BrU (Chart 1c) which possess two BrU bases has
also been investigated. HPLC traces for irradiation of these
conjugates are provided in Figure S6 and S7, Supporting
Information, and plots of conversion vs time are shown in Figure
7. In the case of PyA-2,3BrU, the two monobromides PyA-
2U,3BrU and PyA-2BrU,3U are both primary photoproducts
formed in an initial ratio of ca. 2:1 (Figure 7a). Upon further
irradiation, both products are converted to PyA-2,3U. In the
case of PyA-2,4BrU, the monobromide PyA-2U,4BrU is the only
primary photoproduct. It is converted to PyA-2,4U upon further
irradiation (Figure 7b).
Dynamics of Electron Injection and Charge Recombina-
tion. The transient absorption spectra for all of the PyA
conjugates display rapid decay (<1 ps) of the 2S* 575 nm band
1
1
and formation of S* 375 and 525 nm bands. The S* band
intensities continue to increase with rise times of 6-10 ps
attributed to 1S* relaxation, independent of the location of Br
U
(Table 1). In addition to these bands, the transient absorption
spectra of PyA-1BrU and -2BrU (Figure 3b,c) but not the other
PyA conjugates display bands at 470 nm which are assigned to
the formation of the PyA+•-BrU-• contact radical ion pair, on
the basis of their similarity to the 450 nm band in the transient
absorption spectrum of the dyad PyA-PI (Figure 1b). The rise
times for the 470 nm bands determined from the 470/520 nm
band intensity ratios are 44 ps for PyA-1BrU and 18 ps for PyA-
2BrU (Table 1). These rise times for electron injection are similar
to the hole injection times for PyA-1G and -2G, in accord with
the similar energetics for electron injection to BrU and hole
injection to G (Figure 1b). Faster hole and electron injection
for PyA-2G vs -1G and PyA-2BrU vs -1BrU plausibly reflects
Discussion
Conjugate Structure and Energetics of Hole and Electron
Injection. Pyrene derivatives have been widely employed in
studies of DNA electron transfer.12,13,26,27 The singlet states of
pyrene derivatives can serve as an electron acceptor and/or
donor, depending upon the pyrene substituent and the choice
of nucleobase quencher. For example, we find that the pyren-
ecarboxamide PyA can serve as an electron acceptor capable

14270 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Daublain et al.
Figure 7. Time-dependent composition of reduction products formed during 355 nm irradiation of (a) PyA-2,3BrU and (b) PyA-2,4BrU in standard
buffer with 0.1 M isopropanol.
Figure 8. Cartoon from ref 14 showing a duplex having a 5′- or 3′-
tethered chromophore (yellow oblong box) serving as a capping group
(a) or intercalated between the first and second base pairs (b).
Figure 9. Kinetic scheme for photoinduced charge separation in (a)
APy and (b) PyA conjugates. Bold arrows indicate major decay
pathways.
better π-stacking of PyA with the second base pair in an
intercalated structure (Figure 8b). The long-lived components
of PyA-1BrU and -2BrU transient decay and fluorescence decay
are attributed to conformations in which PyA is not well-stacked
with the BrU-A base pair.
radical ion pair transient absorption spectra, which was not
possible in the case of the PyA-•-G+• contact radical ion pair
as a consequence of inverted kinetics.14
Photochemical Behavior of the PyA-nBrU Conjugates. The
protocol developed for determining the quantum yields for
conversion of the APy-nBrU conjugates to APy-nU has been
employed in our studies of the PyA-nBrU conjugates.12 To
briefly summarize, photoinduced charge separation followed by
loss of bromide from BrU-• results in formation of the uracilyl
radical, which is scavenged efficiently by 0.1 M i-PrOH with
formation of the PyA-nU conjugate. In the absence of either
BrU (PyA-AT) or PyA (3BrU and 4BrU, Chart 1c), no products
are formed upon irradiation. Direct irradiation of BrU-containing
duplexes at 308 nm results in loss of bromide;25 however, BrU
does not absorb the 355 nm irradiation used in our studies.
The initial slopes of plots of conversion of PyA-nBrU
conjugates to PyA-nU conjugates vs irradiation time (Figure
5) provide relative quantum yields, which can be converted to
absolute quantum yields (ΦU) by comparison with the quantum
yield for PyA-4BrU determined using ferrioxalate actinometry.
A plot of ΦU vs the position of BrU in the hairpin conjugate is
remarkably similar to that previously reported for the APy-nBrU
conjugates (Figure 6). The low values of ΦU for the APy-nBrU
conjugates were attributed to efficient charge separation followed
by fast charge recombination in the APy+•-T-• contact ion pairs
(Figure 9a).12 The absence of fluorescence quenching or
formation of the PyA+• in the transient absorption spectrum of
PyA-3BrU and -4BrU suggests that formation of PyA+•-T-•
contact ion pairs is inefficient (Figure 9b), presumably due to
slow, reversible endergonic electron injection to neighboring
T-A base pairs. Thus, the dominant energy-wasting processes
for the PyA conjugates are radiative and nonradiative decay of
singlet PyA. As is the case for APy conjugates, loss of bromide
in the PyA systems presumably occurs via inefficient electron
transport that competes with reversible electron injection (k-i).
A plot of ln(ΦU) vs the distance between PyA and BrU for
PyA-nBrU (n > 2) calculated assuming an end-capping confor-
mation and a π-stacking distance of 3.4 Å has a slope of ꢀ )
The observation of efficient charge separation by transient
absorption spectroscopy only when PyA and BrU (or G14) are
nearest neighbors is reminiscent of the photooxidation of G-C
base pairs by an intercalated acridium dye32 or by a perylene-
dicarboxamide hairpin linker,33 both of which are relatively weak
electron acceptors. All of these systems display strong distance
dependence with little or no electron transfer when BrU or G is
separated from the chromophore by even one A-T base pair.
Such strong distance dependence is indicative of a very large
value of ꢀ, the distance dependence for single-step superex-
change electron transfer, in these systems. Strong distance
dependence has also been observed for the ethidium-
nitroindole donor-acceptor system studied by Fiebig and
Wagenknecht.34 This system displays a minor component of
moderately rapid transient decay (50 ps, 22% amplitude) and a
major component of long-lived decay (>2 ns), attributed to
poorly stacked conformations, when a single A:T base pair
separates the donor and acceptor.
The PyA conjugates all have long-lived transient decay
components (>2 ps) assigned to 1S* radiative and nonradiative
decay, in accord with the observation of long fluorescence decay
times (Table 1). PyA-1BrU and PyA-2BrU also have shorter-
lived transient decay components assigned to charge recombina-
tion of the PyA+•-BrU-• contact radical ion pairs. The decay
times of 130 and 240 ps obtained from global fitting are similar
to those obtained from the 470/520 nm band intensity ratios
(Table 1). These decay times are substantially longer than those
for the corresponding APy conjugates which have decay times
of 2.4 and 4.4 ps, respectively.12 Thus the higher energy of the
PyA+•-BrU-• vs APy+•-BrU-• radical ion pairs results in slower
recombination for the PyA conjugates, as expected for an
electron transfer process in the Marcus inverted region.35 The
somewhat longer charge recombination vs charge separation
times for PyA-1BrU and PyA-2BrU permit observation of their

Dynamics, Electron Injection, and Transport in DNA
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14271
0.26 ( 0.4 Å-1 (Figure S8, Supporting Information), similar to
the value for the APy-nBrU conjugates (ꢀ ) 0.22 ( 2 Å-1).12
Both values are somewhat larger than those reported for several
other electron transport systems including flavin-sensitized
cleavage of the T < >T dimer (ꢀ ) 0.11 Å-1),4 flavin-sensitized
cleavage of thymine oxetane (ꢀ ) 0.16 Å-1),7 and Ir(III)-
sensitized loss of bromide from BrU (ꢀ ) 0.12 Å-1).10 However,
the linear nature of these plots and their similar slopes is
suggestive of a common mechanism for electron transport in
duplex DNA. We previously noted that the distance dependence
for ΦU in the APy system is similar to that for the quantum
yield for A-tract hole transport in capped hairpins having a
stilbenedicarboxamide capping group and a stilbenediether linker
which serve as electron acceptor and donor, respectively (ꢀ )
0.16 Å-1).12 The similar distance dependence for DNA hole and
electron transport may reflect a similar rate-determining process
such as conformational gating for both processes.
Photochemical Behavior of the PyA-2,3BrU and PyA-
2,4BrU Conjugates. The dibromides PyA-2,3BrU and PyA-
2,4BrU undergo loss of both bromides upon prolonged irradiation
resulting in the formation of PyA-2,3U and PyA-2,4U, respec-
tively (Figures S6 and S7, Supporting Information, and Figure
7). Induction periods are observed for the formation of PyA-
2,3U and PyA-2,4U, indicating that that a single photon effects
the formation of a single uridine, as expected for a mechanism
in which loss of bromide removes the excess negative charge
leaving an uncharged conjugate. Giese et al. observed that a
single photoinjected electron can repair more than one thymine
dimer as the negative charge is retained after dimer cleavage.5
The loss of the second bromide from PyA-2,4BrU is slower than
that from PyA-2,3BrU (Figure 7b), in accord with a consecutive
reaction process, and with the lower quantum yield for the
monobromide PyA-4BrU vs PyA-3BrU.
Concluding Remarks. The singlet state of PyA can serve
as a weak electron donor, capable of selective electron injection
to BrU vs the natural pyrimidines, as well as a weak electron
acceptor, capable of hole injection to G, but not A (Figure 1b).14
Electron and hole injection are observed only when the bases
BrU or G, respectively, are nearest neighbors to PyA. The
absence of spectroscopic evidence for electron transfer over even
a single intervening A-T base pair indicates that single-step
superexchange (tunneling) is inefficient in these systems even
at relatively modest donor-acceptor distances (6.8 Å). This in
turn suggests that superexchange is strongly distance dependent
(ꢀ > 1 Å-1), as previously observed for other systems in which
electron or hole injection into the bridging base pairs is highly
endergonic.15,32-34 These observations are consistent with the
proposal of Beratan et al. that the tunneling energy gap
determines ꢀ, a large gap resulting in a large value of ꢀ.40 The
higher energies of charge separated vs contact radical ion pairs
may also account for the large values of ꢀ in these systems.
Photoreduction of BrU in the conjugates PyA-nBrU when n
g 3 most likely occurs via endergonic charge separation
followed by electron transport from the PyA+•-T-• contact
radical ion pair to BrU (Figure 9b). Reversible electron injection
followed by slow electron transport can account for the absence
of fluorescence quenching for PyA-nBrU when n g 3 (Table 1)
as well as the low quantum yields for product formation from
these conjugates (Figure 6). When BrU is adjacent to PyA, the
PyA+•-BrU-• contact radical ion pair presumably is formed
irreversibly. However, charge recombination is more rapid than
loss of bromide, resulting in a low quantum yield for product
formation from PyA-2BrU (Figure 6). The quantum yields for
loss of bromide from the PyA conjugates decrease by a factor
of 2 for each additional A-T base pair separating PyA and
BrU, a distance dependence similar to that observed for other
DNA electron and hole transport systems.4,7,10,14 This suggests
that both hole and electron transport occur by relatively slow
multistep hopping mechanisms, the dynamics of which may be
subject to conformational control.
In the case of PyA-2,3BrU, the primary products PyA-
2U,3BrU and PyA-2BrU,3U are formed with an initial ratio of
ca. 2:1 (Figure 7a). The preference for loss of the 5′-bromide is
analogous to the preference for oxidative cleavage at the 5′-G
in a GG step.36 This result is consistent with either a localized
anion radical with relatively rapid electron migration from
reduced 2BrU to 3BrU or a delocalized anion radical with faster
loss of bromide from 2BrU vs 3BrU. The distinction between
localized and delocalized BrUBrU steps remains to be explored,
while the debate among theoreticians over the extent of hole
delocalization in GG steps continues.37
Acknowledgment. This research is supported by grants from
the Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences, Office of Science, U.S.
Department of Energy (DE-FG02-96ER14604 to FDL), the
National Science Foundation (CHE-0628130 to TF).
Supporting Information Available: MALDI-TOF analysis
for capped hairpins, circular dichroism spectra, thermal dis-
sociation profiles, ZINDO-derived UV spectra, plots of conver-
sion vs irradiation time obtained from HPLC and MALDI
analysis of PyA-4BrU, HPLC traces for irradiated solutions of
conjugates containing two bromouracils, and a plot of the
distance dependence of the quantum yield for product formation.
This material is available free of charge via the Internet at http://
pubs.acs.org.
In the case of PyA-2,4BrU, the major primary product is PyA-
2U,4BrU (Figure 7b). This indicates that electron transport in
the 5′-2BrUT4BrU triad is unable to compete with loss of bromide
from reduced 2BrU-•, as previously observed by Ito and co-
workers for the 2BrUC 4BrU triad using a covalently linked
phenothiazine to effect photochemical electron injection.9 Rivera
and Schuler report a rate constant of 1.4 × 108 s-1 for loss of
bromide from reduced BrU.38 Assuming a similar rate constant
for loss of bromide in the base paired hairpin, the rate constant
for 2BrU T 4BrU electron transport can be no faster than 107 s-1
and still be consistent with our experimental results (Figure 7b).
We previously reported that hole transport in the a GAG triad
References and Notes
(1) (a) Lewis, F. D. Electron Transfer in Chemistry; Balzani, V., Ed.;
Wiley-VCH: Weinheim, Germany, 2001; Vol. 3, pp 105-175. (b) Schuster,
G. B., Ed. Long-Range Charge Transfer in DNA, I and II; Springer, 2004;
Vol. 236, 237. (c) Wagenknecht, H. A. Charge Transfer in DNA; Wiley-
VCH: Weinheim, 2005.
(2) Seidel, C. A. M.; Schulz, A.; Sauer, M. H. M. J. Phys. Chem. 1996,
100, 5541–5553.
(3) (a) Schwo¨gler, A.; Burgdorf, L. T.; Carell, T. Angew. Chem., Int.
Ed. 2000, 39, 3918–3920. (b) Breeger, S.; Hennecke, U.; Carell, T. J. Am.
Chem. Soc. 2004, 126, 1302–1303.
(4) Behrens, C.; Burgdorf, L. T.; Schwogler, A.; Carell, T. Angew.
Chem., Int. Ed. 2002, 41, 1763–1766.
occurs with a rate constant of ca. 107 s-1 39
Thus, the rate
.
constant for single-step electron transport in a triad system is
comparable to or slower than that for hole transport. Further-
more, since loss of bromide terminates the electron transport
process, BrU cannot be used to study multiple-step hopping
processes analogous to those observed for hole hopping between
G hole traps.

14272 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Daublain et al.
(5) Giese, B.; Carl, B.; Carl, T.; Carell, T.; Behrens, C.; Hennecke,
U.; Schiemann, O.; Feresin, E. Angew. Chem., Int. Ed. 2004, 43, 1848–
1851.
(6) Carell, T.; Behrens, C.; Gierlich, J. Org. Biomol. Chem. 2003, 1,
2221–2228.
(24) Gosztola, D.; Niemczyk, M. P.; Svec, W.; Lukas, A. S.; Wasielews-
ki, M. R. J. Phys. Chem. A 2000, 104, 6545–6551.
(25) (a) Sugiyama, H.; Tsutsumi, Y.; Saito, I. J. Am. Chem. Soc. 1990,
112, 6720–6721. (b) Fujimoto, K.; Sugiyama, H.; Saito, I. Tetrahedron Lett.
1998, 39, 2137–2140.
(7) Stafforst, T.; Diederichsen, U. Angew. Chem., Int. Ed. 2006, 45,
5376–5380.
(8) (a) Ito, T.; Rokita, S. E. J. Am. Chem. Soc. 2003, 125, 11480–
11481. (b) Ito, T.; Rokita, S. E. J. Am. Chem. Soc. 2004, 126, 15552–
15559. (c) Manetto, A.; Breeger, S.; Chatgilialoglu, C.; Carell, T. Angew.
Chem., Int. Ed. 2006, 45, 318–321.
(9) Ito, T.; Kondo, A.; Terada, S.; Nishimoto, S. J. Am. Chem. Soc.
2006, 128, 10934–10942.
(10) Elias, B.; Shao, F. W.; Barton, J. K. J. Am. Chem. Soc. 2008, 130,
1152–1153.
(26) (a) Shafirovich, V. Y.; Dourandin, A.; Luneva, N. P.; Geacintov,
N. E. J. Phys. Chem. B 1997, 101, 5863–5868. (b) Geacintov, N. E.;
Solntsev, K.; Johnson, L. W.; Chen, J. X.; Kolbanovskiy, A. D.; Liu, T. M.;
Shafirovich, V. Y. J. Phys. Org. Chem. 1998, 11, 561–565.
(27) Zahavy, E.; Fox, M. A. J. Phys. Chem. B 1999, 103, 9321–9327.
(28) (a) Mann, J. S.; Shibata, Y.; Meehan, T. Bioconjugate Chem. 1992,
3, 554–558. (b) Schuster, G. B. Acc. Chem. Res. 2000, 33, 253–260. (c)
Lewis, F. D.; Zhang, L.; Liu, X.; Zuo, X.; Tiede, D. M.; Long, H.; Schatz,
G. S. J. Am. Chem. Soc. 2005, 127, 14445–14453.
(29) Tuma, J.; Connors, W. H.; Stitelman, D. H.; Richert, C. J. Am.
Chem. Soc. 2002, 124, 4236–4246.
(30) (a) Siegmund, K.; Maheshwary, S.; Narayanan, S.; Connors, W.;
Riedrich, M.; Printz, M.; Richert, C. Nucleic Acids Res. 2005, 33, 4838–
4848. (b) Tuma, J.; Paulini, R.; Stu¨tz, J. A. R.; Richert, C. Biochemistry
2004, 43, 15680–15687.
(31) Weller, A. Zeit. Phys. Chem. Neue. Folg. 1982, 133, 93–98.
(32) (a) Hess, S.; Go¨tz, M.; Davis, W. B.; Michel-Beyerle, M. E. J. Am.
Chem. Soc. 2001, 123, 10046–10055. (b) von Feilitzsch, T.; Tuma, J.;
Neubauer, H.; Verdier, L.; Haselsberger, R.; Feick, R.; Gurzadyan, G.;
Voityuk, A. A.; Griesinger, C.; Michel-Beyerle, M. E. J. Phys. Chem. B
2008, 112, 973–989.
(33) Lewis, F. D.; Zhang, L.; Kelley, R. F.; McCamant, D.; Wasielewski,
M. R. Tetrahedron 2007, 63, 3457–3464.
(34) Valis, L.; Wang, Q.; Raytchev, M.; Buchvarov, I.; Wagenknecht,
H. A.; Fiebig, T. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10192–10195.
(35) Marcus, R. A. J. Chem. Phys. 1965, 43, 679–701.
(36) (a) Saito, I.; Takayama, M.; Sugiyama, H.; Nakatani, K. J. Am.
Chem. Soc. 1995, 117, 6406–6407. (b) Sugiyama, H.; Saito, I. J. Am. Chem.
Soc. 1996, 118, 7063–7068.
(37) (a) Conwell, E. M.; Basko, D. M. J. Am. Chem. Soc. 2001, 123,
11441–11445. (b) Kurnikov, I. V.; Tong, G. S. M.; Madrid, M.; Beratan,
D. N. J. Phys. Chem. B 2002, 106, 7–10.
(38) Rivera, E.; Schuler, R. H. J. Phys. Chem. 1983, 87, 3966–3971.
(39) Lewis, F. D.; Liu, J.; Zuo, X.; Hayes, R. T.; Wasielewski, M. R.
J. Am. Chem. Soc. 2003, 125, 4850–4861.
(40) Lewis, F. D.; Liu, J.; Weigel, W.; Rettig, W.; Kurnikov, I. V.;
Beratan, D. N. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12536–12541.
(11) Kawai, K.; Kimura, T.; Kawabata, K.; Tojo, S.; Majima, T. J. Phys.
Chem. B 2003, 107, 12838–12841.
(12) Daublain, P.; Thazhathveetil, A. K.; Wang, Q.; Trifonov, A.; Fiebig,
T.; Lewis, F. D. J. Am. Chem. Soc. 2009, 131, 16790–16797.
(13) (a) Kaden, P.; Mayer-Enthart, E.; Trifonov, A.; Fiebig, T.;
Wagenknecht, H.-A. Angew. Chem., Int. Ed. 2005, 44, 1636–1639. (b)
Gaballah, S. T.; Collier, G.; Netzel, T. L. J. Phys. Chem. B 2005, 109,
12175–12181.
(14) Siegmund, K.; Daublain, P.; Wang, Q.; Trifonov, A.; Fiebig, T.;
Lewis, F. D. J. Phys. Chem. B 2009, 113, 16276–16284.
(15) Lewis, F. D.; Zhu, H.; Daublain, P.; Fiebig, T.; Raytchev, M.; Wang,
Q.; Shafirovich, V. J. Am. Chem. Soc. 2006, 128, 791–800.
(16) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic
Molecules, 2nd ed.; Academic Press: New York, 1971.
(17) Astruc, D. Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, 2001; Vol. 2.
(18) Murov, S. L. Handbook of Photochemistry; Marcel Dekker: New
York, 1973.
(19) Alfred, J.-C.; Daunis, J.; Jacquier, R. Macromol. Chem. Phys. 1996,
197, 389–401.
(20) Johnson, W. C. Circular Dichroism, Principles and Applications;
Berova, N., Nakanishi, K., Woody, R. W., Eds.; Wiley-VCH: New York,
2000; pp 741-768.
(21) CAChe CAChe Release 6.1.10, 2000.
(22) Raytchev, M.; Pandurski, E.; Buchvarov, I.; Modrakowski, C.;
Fiebig, T. J. Phys. Chem. A 2003, 107, 4592–4600.
(23) Lewis, F. D.; Daublain, P.; Delos Santos, G.; Liu, W.; Asatryan,
A. M.; Markarian, S. A.; Fiebig, T.; Raytchev, M.; Wang, Q. J. Am. Chem.
Soc. 2006, 128, 791–800.
JP9107393