Substituent effects on photosensitized splitting of thymine cyclobutane dimer by an attached indole

Article abstract of DOI:10.1002/cphc.201200652

In chromophore-containing cyclobutane pyrimidine dimer (CPD) model systems, solvent effects on the splitting efficiency may depend on the length of the linker, the molecular conformation, and the oxidation potential of the donor. To further explore the relationship between chromophore structure and splitting efficiency, we prepared a series of substituted indole-T<>T model compounds 2 a-2 g and measured their splitting quantum yields in various solvents. Two reverse solvent effects were observed: an increase in splitting efficiency in solvents of lower polarity for models 2 a-2 d with an electron-donating group (EDG), and vice versa for models 2 e-2 g with an electron-withdrawing group (EWG). According to the Hammett equation, the negative value of the slope of the Hammett plot indicates that the indole moiety during the T<>T-splitting reaction loses negative charge, and the larger negative value implies that the repair reaction is more sensitive to substituent effects in low-polarity solvents. The EDGs of the models 2 a-2 d can delocalize the charge-separated state, and low-polarity solvents make it more stable, which leads to higher splitting efficiency in low-polarity solvents. Conversely, the EWGs of models 2 e-2 g favor destabilization of the charge-separated state, and high-polarity solvents decrease the destabilization and hence lead to more efficient splitting in high-polarity solvents. Two reverse solvent effects were observed in a series of indole-cyclobutane pyrimidine dimer model systems with differently substituted indole moieties. Thus, the splitting quantum yield Φ_spl shows a normal relation with solvent polarity for compounds with an electron-donating group, and vice versa for those with an electron-withdrawing group (see picture; bet=back electron transfer). Copyright

Full text of DOI:10.1002/cphc.201200652

DOI: 10.1002/cphc.201200652
Substituent Effects on Photosensitized Splitting of
Thymine Cyclobutane Dimer by an Attached Indole
[
a]
[a]
[a]
[a]
[a]
Wenjian Tang,* Hongmei Zhou, Jing Wang, Chunxiao Pan, Jingbo Shi, and
Qinhua Song*
[
b]
In chromophore-containing cyclobutane pyrimidine dimer
CPD) model systems, solvent effects on the splitting efficiency
equation, the negative value of the slope of the Hammett plot
indicates that the indole moiety during the T< >T-splitting re-
action loses negative charge, and the larger negative value im-
plies that the repair reaction is more sensitive to substituent
effects in low-polarity solvents. The EDGs of the models 2a–2d
can delocalize the charge-separated state, and low-polarity sol-
vents make it more stable, which leads to higher splitting effi-
ciency in low-polarity solvents. Conversely, the EWGs of
models 2e–2g favor destabilization of the charge-separated
state, and high-polarity solvents decrease the destabilization
and hence lead to more efficient splitting in high-polarity sol-
vents.
(
may depend on the length of the linker, the molecular confor-
mation, and the oxidation potential of the donor. To further ex-
plore the relationship between chromophore structure and
splitting efficiency, we prepared a series of substituted indole–
T< >T model compounds 2a–2g and measured their splitting
quantum yields in various solvents. Two reverse solvent effects
were observed: an increase in splitting efficiency in solvents of
lower polarity for models 2a–2d with an electron-donating
group (EDG), and vice versa for models 2e–2g with an elec-
tron-withdrawing group (EWG). According to the Hammett
1
. Introduction
A type of major damage to DNA caused by UV irradiation is
the formation of base photolesions, which may be mutagenic
mation of the thymine dimer into the original bases. Formation
of the repaired thymine monomers is finally followed by back
electron transfer to produce the FADHC radical and thus restore
[
1,2]
and are a leading cause of skin cancer. The most frequent of
these UV-induced lesions in DNA are cyclobutane pyrimidine
dimers (CPDs), formed by [2+2] cycloaddition of two adjacent
pyrimidine bases, more frequently thymines (Scheme 1). CPDs
are specifically recognized and repaired by CPD photolyase,
a single ꢀ55 kDa protein found in organisms from all king-
the functional form of flavin for a new cycle of catalysis.
[6–8]
Some model compounds
that mimic the action of CPD
photolyase have been designed, such as a chromophore at-
tached to a CPD unit. Studies performed with these com-
pounds have offered useful insights into the electron-transfer
and bond-breaking processes involved in photosensitized CPD
splitting. In the chromophore-containing CPD model systems,
the splitting efficiency of the CPD units displays two reverse
[9]
solvent effects. One is faster splitting in low polarity solvents,
which has been interpreted in terms of possible slowing of the
highly exothermic back electron transfer process due to
Marcus-region inverted behavior. The other is faster splitting in
[10]
higher polarity solvents.
Recently, several model systems
Scheme 1. UV-induced CPD photoproducts in DNA and their photorepair by
DNA photolyases.
[a] Dr. W. J. Tang, H. M. Zhou, J. Wang, C. X. Pan, Dr. J. B. Shi
School of Pharmacy
Anhui Medical University
Hefei 230032 (China)
Fax: (+86)551-5161115
E-mail: tangwjster@gmail.com
doms of life, except for placental mammals such as humans,
which remove CPD lesions through nucleotide excision
[
3]
[4]
[5]
repair. Experimental and theoretical investigations have
provided evidence for the mechanisms of CPD photolyases.
The enzymes transfer light energy, initially absorbed by an aux-
iliary chromophore, to a reduced flavin adenine dinucleotide
[
b] Prof. Q. H. Song
Department of Chemistry
University of Science and Technology of China
Hefei 230026 (China)
Fax: (+86)551-3601592
E-mail: qhsong@ustc.edu.cn
ꢁ
ꢁ
(FADH ) coenzyme. The excited FADH then transfers an elec-
tron to the CPD lesion, which subsequently leads to stepwise
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201200652.
splitting of the C5ꢁC5’ and C6ꢁC6’ bonds and thus to transfor-
4
180
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2012, 13, 4180 – 4185

Photosensitized Splitting of Thymine Cyclobutane Dimer
were devised to explore the origin of these two reverse solvent
2
. Results and Discussion
[
11,12]
effects.
In the indole–CPD model systems with different-length link-
The photophysical and photochemical processes of model
[
11]
ers,
the solvent effect on CPD splitting for model com-
compounds 2a–2g (represented as Ch–D) are illustrated by
a simple mechanistic scheme (Figure 2). Under irradiation by
[10i]
pounds with a long and flexible linker was just the reverse of
that with a short linker, due to the much shorter distance be-
tween the indole and CPD owing to spreading to a U-shaped
conformation with increasing solvent polarity because of their
hydrophobic interaction and the specific structure of the CPD.
[
12b]
However, our recent research
showed that in the chromo-
phore-containing CPD model systems with a short linker, the
solvent effect of CPD splitting in phenothiazine model systems
was contrary to that in non-phenothiazine model systems. The
same phenomenon was also observed in two classes of model
[
12a]
systems:
covalently linked CPD– and oxetane–carbazole
compounds with the same short linker. Whether in CPD– and
oxetane–carbazole systems with the same linker and electron
donor or in chromophore-containing CPD systems with the
same linker and electron acceptor, a lower redox potential
should be a key factor leading to back electron transfer lying
in the different Marcus regions and the two reverse solvent ef-
fects. Therefore, many factors influence the solvent depend-
ence of the splitting quantum yields, such as the length of the
linker, the molecular conformation, the redox potential, and so
on.
Figure 2. Photophysical and photochemical processes of the model com-
pounds.
light with wavelengths above 290 nm, the chromophore
moiety absorbs
a photon, producing the excited state
1
( Ch*–D). This excited state has the following relaxation path-
ways: fluorescence (k ), internal conversion (k ), and electron
f
ic
transfer to the linked T< >T (k ). The charge-separated spe-
fet
cies
+
ꢁ
To further explore the relationship between chromophore
structure and CPD splitting, we prepared a series of covalently
linked indole cyclobutane thymine dimer (T< >T) model com-
pounds with different substituents 2a–2g, and investigated
the solvent dependence of the T< >T splitting efficiency in
various solvents. Although trimethylene-bridged T< >T enfor-
ces an almost planar ring, which is different to that of the un-
bridged T< >T, the bridged T< >T displays similar splitting
(ChC –DC ) formed by electron transfer undergoes two competi-
+
tive processes: T< >T splitting (k ) to produce M’ and ChC
spl
ꢁ
–MC , which then becomes Ch–M by charge combination, and
back electron transfer (kbet) resulting in an unproductive rever-
sal. These processes undoubtedly have counterparts in the en-
zymatic repair process. In these processes, kfet and kspl contrib-
ute to the observed splitting quantum yield F of T< >T split-
ting, while kbet, k , and k would reduce the quantum efficiency.
f
ic
[
10c,13]
efficiency.
The quantum yields of the T< >T splitting ex-
Back electron transfer is a key factor leading to low splitting ef-
ficiencies in model systems, but can be efficiently suppressed
in CPD photolyases with a uniformly high repair efficiency
hibited two reverse solvent effects: an increase in the splitting
efficiency in low-polarity solvents for model compounds with
an EDG (2a–2d), and faster splitting in higher polarity solvents
for those with an EWG (2e–2g). The experimental results were
[3]
(F=0.7–0.98). Besides, internal conversion (k ) and the mo-
ic
lecular conformation play important roles in resulting in a low
[
14]
[15]
analyzed by using the Hammett equation.
Furthermore,
splitting efficiency.
based on the solvent effects and the substituent effects on
T< >T splitting, some new insights into the intramolecular
electron-transfer process in indole–T< >T model systems
were gained (Figure 1)
To investigate the mechanisms of the above photoinduced
electron-transfer processes, seven model compounds 2a–2g
with different substituted indoles and a short (trimethylene)
linker between the chromophore and T< >T were synthesized
as shown in Scheme 2. Model compounds 2a–2g were pre-
[11]
pared from the cis-syn thymine dimer 5 alkylated with the
corresponding 1-(3-bromopropyl)indole or its derivatives 1a–
1
g (Scheme 2). The N-alkylation of indole or its derivative with
an excess of 1,3-dibromopropane was carried with tetrabuty-
lammonium bromide (TBAB) as phase-transfer catalyst in tolu-
ene/water.
The splitting properties of these model compounds were in-
vestigated in solvents of different polarity. After UVB irradiation
of 2a–2g in methanol solutions with the corresponding mono-
chromatic light, analysis of the photolysis mixture by reversed-
phase HPLC confirmed that the model compounds react clean-
ly to give 3a–3g, respectively, as no other products could be
detected besides the expected photoproducts (Scheme 3).
Figure 1. Chemical structure of model compounds with a short linker.
ChemPhysChem 2012, 13, 4180 – 4185
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
4181

W. J. Tang, Q. H. Song et al.
with the substituent. The splitting quantum yield shows
a normal relation with solvent polarity for the model com-
pounds with an EWG (2e–2g), but an inverse relation for the
model compounds with an EDG (2a–2d). Therefore, in model
compounds 2a–2g with a short linker, those with an EDG
show a contrary solvent effect to those with an EWG.
The two reverse solvent effects on the splitting efficiency
[11,12]
observed previously in different model systems
may be
due to the length of the linker, the molecular conformation,
the oxidation potential of donor, and so on. For the covalently
linked chromophore–dimer systems, fluorescence quenching
efficiency Q of the chromophore can reflect the efficiency of
Scheme 2. Synthesis of model compounds.
[10i]
forward electron transfer (Q=ffet). Data in Table 2 show that
Table 2. Extent of fluorescence quenching (Q) of model compounds 2a–
[
a]
2
g in various solvents .
Solvent
2a
2b
2c
2d
2e
2 f
2g
THF/H
THF/H
THF/H
THF/H
MeCN
MeOH
THF
2
O (10/90)
2
O (25/75)
2
O (50/50)
2
O (75/25)
0.85
0.70
0.50
0.51
0.56
0.46
0.43
0.94
0.96
0.97
0.93
0.85
0.93
0.81
0.74
0.82
0.75
0.68
0.68
0.66
0.06
0.90
0.87
0.84
0.79
0.82
0.81
0.67
0.74
0.74
0.70
0.55
0.53
0.48
0.30
0.75
0.86
0.88
0.77
0.86
0.74
0.85
0.36
0.27
[
b]
–
–
[
b]
0.19
[
b]
–
–
[
b]
Scheme 3. Photosensitized splitting reactions of the T< >T unit of the
model compounds under irradiation with an appropriate wavelength of
light.
[
a] Estimated error ꢂ6%. [b] No fluorescence quenching.
the extent of fluorescence quenching increases with solvent
polarity, that is, forward electron transfer is accelerated with in-
creasing solvent polarity. Thus, it can be excluded that forward
electron transfer is responsible for the two reverse solvent ef-
fects, and back electron transfer, which is a key factor leading
To measure the splitting quantum yields F of the model
compounds, sample solutions were prepared in solvents of dif-
ferent polarity [THF, acetonitrile, methanol, and water/THF
(25/75, 50/50, 75/25, 90/10)], placed in cuvettes with a Teflon
[16]
stopper, and then irradiated with monochromatic light from
a fluorescence spectrometer. After certain time intervals, the
absorption spectra of the irradiated solutions were recorded
with a
to low repair quantum yield of model compounds, may be
the factor controlling the two reverse solvent effects.
The free-energy change DG for back electron transfer is the
+
ꢁ
energy level of the charge-separated state (ChC –DC ), which
can be estimated by using thermodynamic redox potentials.
The free-energy difference between the charge-transfer state
UV/Vis spectrometer. The intensity of irradiation light was mea-
sured three times during the measurement, and the average of
three measurements was employed to obtain a value. Based
on these data, the quantum yields of the splitting of the
model compounds were obtained and are listed in Table 1.
The data in Table 1 show that the quantum yields for
T< >T splitting of models 2a–2g are strongly solvent depen-
dent and that the solvent effect on splitting efficiency varies
[17]
and the ground state is given by Equation (1):
ꢁ
DGbet ¼ E ꢁE þ DEcoul
ð1Þ
ð2Þ
ox
red
ꢀ
ꢁ
e
1
e
2
37:5
DEcoul=eV ¼
ꢁ
4pe0a
where Eox and Ered are potentials for one-electron oxidation of
[
18]
[
a]
a donor (indole or its derivatives, Table 3) and one-electron
Table 1. Splitting quantum yields F of model compounds 2a–2g in
various solvents.
[19]
reduction of an acceptor (T< >T, ꢁ2.20 V vs. SCE ), respec-
tively; DE and DE are the Coulomb term and the energy
coul
0,0
Solvent
2a
2b
2c
2d
2e
2 f
2g
level of the excited state, respectively; the latter can be ob-
tained from the fluorescence peaks of the indole T< >T
model compounds in the corresponding solvents, and e, a are
the static dielectric constant of a solvent and the center-to-
center distance between a donor and an acceptor, respectively.
Table 3 presents the peak oxidation potentials measured for
THF/H
THF/H
THF/H
THF/H
MeCN
MeOH
THF
2
2
2
2
O (10/90) 0.113 0.073 0.098 0.095 0.080 0.073 0.083
O (25/75) 0.146 0.101 0.130 0.123 0.061 0.061 0.033
O (50/50) 0.169 0.130 0.153 0.131 0.028 0.026 0.008
O (75/25) 0.192 0.131 0.156 0.145 0.013 0.015 0.005
0.184 0.157 0.153 0.147 0.012 0.012 0.002
0.213 0.162 0.171 0.181 0.042 0.038 0.004
[
b]
0.180 0.153 0.153 0.219 0.014 0.005
–
[18]
each indole by linear sweep voltammetry (LSV).
Although
[
a] Average of two determinations, experimental error ꢂ5%. [b] No split-
the substituted positions are different for some model com-
pounds, under the same conditions, the variation in the peak
ting detected.
4
182
www.chemphyschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2012, 13, 4180 – 4185

Photosensitized Splitting of Thymine Cyclobutane Dimer
Table 3. Oxidation potentials Eox, Hammett constants s, and changes in
free energy DGbet of compounds 2a–2g.
[
a]
[b]
Compound
Substituents
5-OMe
E
ox
ꢁDGbet (MeCN)
s
2
2
2
2
2
2
2
a
b
c
d
e
f
0.83
0.90
0.94
1.10
1.10
1.30
1.64
2.97
3.04
3.08
3.24
3.24
3.44
3.78
ꢁ0.268 (p)
ꢁ0.239 (p+m)
ꢁ0.069 (m)
0
0.232 (p)
0.373 (m)
0.56 (m)
[
c]
2,3-Me
3-Me
H
2
5-Br
6-Cl
4-CN
g
[a] From ref. [17]. [b] From ref. [19], p and m denote para and meta sub-
stituent, respectively. [c] Eox of 2,3-dimethylindole estimated.
oxidation potential should mirror changes in the oxidation po-
tential. With increasing oxidation potential, from 0.83 eV for
methoxylindole to 1.64 eV for cyanoindole, the value of ꢁDGbet
increases, which is favorable for back electron transfer and
leads to the quantum yields of T< >T splitting decreasing in
the sequence from 2a to 2g in acetonitrile. The variational
trend in other solvents is identical.
Figure 3. Plot of the splitting quantum yields F of model compounds
versus Hammett constants s in various solvents.
rated state, leading a significant competition between T< >T
splitting and futile back electron transfer. The larger 1 value in
low-polarity solvents suggests more charge transfer in the
The data in Table 3 also show that the splitting efficiency of
indole T< >T model 2d is affected by the substituents at the
indole ring. These effects are often quantified by means of
substituent constants, which were first introduced by Ham-
+
ꢁ
charge-separated intermediate (X–ChC –DC , where X denotes
the substituted group). When X is an EDG, the zwitterion can
be stabilized by charge delocalization, while an EWG leads to
destabilizing effect. Thus, in low-polarity solvents, the EDGs of
models 2a–2d can delocalize the charge-separated intermedi-
ate and leave more time to cleave the T< >T, so that a high
splitting efficiency results, whereas the EWGs of models 2e–2g
destabilize the intermediate and thus favor back electron
transfer and inhibit T< >T splitting. Since solvent has a large
effect on the magnitude of 1, the 1 value clearly decreases
with increasing solvent polarity. In high-polarity solvents, the
small 1 value implies that there is less charge transfer in the
charge-separated state and the reaction rate is almost unaf-
fected by substituent effects, because solvation effects in high-
polarity solvents can decrease the charge delocalization of
EDGs or the destabilization of EWGs, which leads to decreased
splitting efficiency of models 2a–2d and increased splitting ef-
ficiency of models 2e–2g in high-polarity solvents. Therefore,
the EDGs of models 2a–2d can stabilize the charge-separated
state by charge delocalization, and the larger 1 value in low-
polarity solvents makes the intermediate more stable, which
leads to higher splitting quantum yields in low-polarity solvent.
Conversely, the EWGs of models 2e–2g favor destabilization of
the charge-separated state, and high-polarity solvents decrease
the destabilization, and hence splitting is more efficient in
high-polarity solvents.
[
20]
mett. There are two s values for each substituent relative to
the nitrogen atom of the indole ring, whereby R1 and R4 are
para substituents, and R2, R3, and R5 meta substituents. The
s value of model 2b is the sum of para and meta values. Al-
though the s values vary with the solvents, this variation is
usually small enough to be ignored. Reaction rates for a substi-
tuted molecule relative to the rate for an unsubstituted mole-
cule are plotted against the Hammett constants in order to
gain insight into the reaction mechanism. Thus, in the present
T< >T-splitting reaction, we can relate the splitting quantum
yields to the Hammett constants, as the logarithm of F is
a function of the free energy of activation. These relations can
be exploited to deduce the mechanism of the T< >T-splitting
reaction.
In Figure 3, the logarithm of the ratio of the measured F for
the substituted and unsubstituted indole–T< >T is plotted
against the Hammett parameter from ref. [20b]. The Hammett
plots are linear correlations with 1 values of ꢁ0.20 (r=0.87),
ꢁ
0.69 (r=0.92), ꢁ1.57 (r=0.95), ꢁ1.98 (r=0.95), ꢁ2.39 (r=
0
.95), and ꢁ2.46 (r=0.90) in THF/H O (10/90), THF/H O (25/75),
2
2
THF/H O (50/50), THF/H O (75/25), acetonitrile, and THF respec-
2
2
tively. Figure 3 shows a clear trend in that the slope of the
Hammett plot becomes more negative as the solvent polarity
decreases. This negative slope indicates that the indole moiety
in the T< >T-splitting reaction is positively charged (or loses
negative charge), which corresponds with the indole radical
cation of the electron-transfer reaction. Moreover, with de-
creasing solvent polarity, the negative 1 value increases gradu-
ally, and this implies that the repair reaction is more sensitive
to substituents in low-polarity solvents.
3
. Conclusions
We have measured the splitting quantum yields of a series of
substituted indole–T< >T model compounds with a short
linker 2a–2g and observed two reverse solvent effects: an in-
crease in splitting efficiency in solvents of lower polarity for
models 2a–2d with an EDG and faster splitting in higher po-
larity solvents for models 2e–2g with an EWG. According to
The reaction constant 1 has been interpreted as a measure
of the susceptibility to substituent effects of the charge-sepa-
ChemPhysChem 2012, 13, 4180 – 4185
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
4183

W. J. Tang, Q. H. Song et al.
Aex
the Hammett equation, the negative value of the slope of the
Hammett plot indicates that the indole moiety loses negative
charge during the T< >T-splitting reaction, and the larger
negative value implies that the repair reaction is more sensitive
to substituent effects in low-polarity solvents. The EDGs of
models 2a–2d can delocalize the charge-separated state and
low-polarity solvents make it more stable, which leads to
higher splitting efficiency in low-polarity solvents. Conversely,
the EWGs of models 2e–2g favor destabilization of the
charge-separated state and high-polarity solvents decrease the
destabilization, and hence splitting is more efficient in high-po-
larity solvents. Analysis of solvent effects and substituent ef-
fects on the charge-separated state can interpret the two re-
verse solvent effects on the splitting efficiency. It can be de-
duced that the repair efficiency in CPD photolyase can be in-
Beer’s law, I
a
¼ I
0
ð1 ꢁ 10ꢁ Þ. The change in the molar extinction
coefficients (De ) was obtained from the UV absorption spectra of
the model compounds and the fully split products. These values al-
lowed calculation of the quantum yield, F=BV /De273Ia, wherein V0
is the volume of irradiation solution (3ꢁ10 L). The experimental
error was within 10%.
273
0
ꢁ3
The quantum yields of splitting did not significantly change with
and without N bubbling prior to irradiation. Hence, nondeaerated
2
solutions were employed in all measurements of the quantum
yield. To limit the competition for absorption of the irradiated light
between model compounds and photoproducts, the splitting
extent of the model compounds was controlled within 10% in all
of the measurements of the quantum yield.
General Method for the Synthesis of 2a–2g (Scheme 2): A mixture
of the appropriate indole or its derivative, 1,3-dibromopropane
(
(
15.0 equiv), 50% NaOH aqueous solution (10 mL), and TBAB
7.2 equiv) in 20 mL of toluene was stirred at room temperature for
3.5 h. The mixture was diluted with water and extracted with
EtOAc. The organic layers were dried with Na SO , filtered, and
concentrated in vacuo. The 3-(1-bromopropyl)indole derivatives
a–1g were obtained as oils after purification by column chroma-
[
4]
creased by tuning the solvent polarity and the redox poten-
[
21]
tial of the flavoproteins, and this work helps us to screen
more efficient photosensitizers based on structure optimiza-
tion.
2
4
1
tography on silica gel. Then, sodium tert-butoxide (2.0 equiv) was
added to a solution of cis-syn thymine dimer 5 (62 mg, 0.20 mmol)
in 5 mL of DMF and the reaction mixture stirred at room tempera-
ture for 30 min. The above oils 1a–1g (2.0 equiv) were added, and
stirring was resumed at room temperature for 3 h. The mixture
was diluted with water and extracted with EtOAc. The organic
layer was dried with Na SO , filtered, and concentrated in vacuo.
Experimental Section
General: Melting points are uncorrected. All materials were ob-
tained from commercial suppliers and used as received. Solvents
of technical quality were distilled prior to use. DMF was dried over-
night with K CO and distilled. THF was dried with sodium metal
2
3
2
4
and distilled before use for the photosplitting measurements on
the model compounds. Acetonitrile and methanol were of spectro-
scopic grade from commercial suppliers and used without further
purification.
The crude product was purified by flash chromatography and re-
crystallization. The products were obtained as white powders.
Model Compound 2a: By the above procedure, 1a (1.8 g, 78%)
was obtained as light yellow oil. 2a (54 mg, 55%) was prepared
from 1a and 5 and purified by column chromatography on silica
Measurement of Steady-State Fluorescence Emission: Fluorescence
emission spectra were measured at room temperature on a fluores-
cence spectrometer. To determine the extent of fluorescence
quenching Q, the fluorescence intensities Findole–D of 2a–2g were
compared to that (Findole) of the corresponding indole without an
attached T< >T (4a–4g, see the Supporting Information), that is,
Q=1ꢁFindole–D/Findole. The concentrations of the indole moiety of
the indole-T< >T models and the free indole were controlled
within 0.05 for absorbance at the corresponding excitation wave-
length and the fluorescence intensities were normalized with the
absorbances.
1
gel (EtOAc as eluent) as a white powder. M.p. 252–2538C; H NMR
(300 MHz, CDCl ): d=1.32 (s, 3H, CH ), 1.41 (s, 3H, CH ), 1.64–1.67
3
3
3
(m, 1H), 2.01–2.17 (m, 2H), 2.27–2.39 (m, 2H), 2.80 (s, 3H, NCH3),
3.46 (d, J=7.1 Hz, 1H, CH), 3.60 (d, J=7.1 Hz, 1H, CH), 3.85 (s, 3H,
OCH ), 3.91–3.96 (m, 2H), 4.02–4.27 (m, 5H), 6.43–7.26 ppm (m,
3
13
5H, ArH); C NMR (75 MHz, CDCl ): d=18.3 (CH ), 18.7 (CH ), 22.9,
3
3
3
28.8, 36.5, 41.7, 41.8, 44.1, 46.2, 50.7 (C), 51.0 (C), 56.1 (OCH ), 59.0
3
(CH), 60.7 (CH), 101.5, 103.0, 110.1, 112.4, 128.4, 129.3, 131.3, 151.9,
152.0, 154.4, 170.1, 170.2 ppm; HRMS (ESI-TOF) calcd for C H N O
5
2
6
32
5
+
[M+H] : 494.2415, found: 494.2398.
Measurement of Splitting Quantum Yields of Model Compounds:
To measure the quantum yields of T< >T splitting of the model
compounds [F=(rate of T< >T splitting)/(rate of photons ab-
sorbed)], sample solutions (ꢀ5ꢁ10 m, 3 mL) were prepared and
placed in quartz cuvettes with a Teflon stopper and then irradiated
with light of the corresponding wavelength (295 nm for 2d,
Model Compound 2b: By the above procedure, 1b (450 mg, 25%)
was obtained as light yellow oil. 2b (70 mg, 71%) was prepared
from 1b and 5 and purified by column chromatography on silica
gel (petroleum ether/EtOAc 4/1!1/1 as eluent) as a white powder.
ꢁ
5
1
M.p. 258–2598C; H NMR (300 MHz, CDCl ): d=1.34 (s, 3H, CH ),
3
3
1.40 (s, 3H, CH ), 1.62 (m, 1H), 1.83–2.13 (m, 2H), 2.25 (s, 3H, CH ),
3
3
3
10 nm for 2a, 320 nm for 2g, 300 nm for 2b, 2c, 2e, 2 f) from
2.28–2.29 (m, 1H), 2.33 (s, 3H, CH ), 2.36–2.42 (m, 1H), 2.76 (s, 3H,
3
a fluorescence spectrometer operated with a 10 nm slit. The absor-
bances at 273 nm (A273) and the corresponding wavelength (Aex)
were recorded at certain intervals of time after irradiation. The
extent of T< >T splitting was measured by monitoring the in-
crease in A273 due to regeneration of the thymine bases. The A273
change (DA273) of the solution depends on the extent of splitting
of the model compounds. The plot of DA273 against irradiation
time (t/min) is well fitted as a straight line, whereby the slope of
the straight line B reflects the splitting rate of the model com-
NCH ), 3.36 (d, J=7.1 Hz, 1H, CH), 3.46 (d, J=7.1 Hz, 1H, CH), 3.91–
3
3.95 (m, 2H), 4.02–4.27 (m, 5H), 7.07–7.51 ppm (m, 4H, ArH);
13
C NMR (75 MHz, CDCl ): d=9.0 (CH ), 10.4 (CH ), 18.3 (CH ), 18.6
3
3
3
3
(CH ), 22.9, 29.3, 36.3, 40.7, 41.7, 41.8, 45.8, 50.7 (C), 50.8 (C), 58.6
3
(CH), 60.6 (CH), 107.4, 108.7, 118.4, 119.1, 121.0, 128.9, 132.1, 136.0,
151.9 (2C), 170.1, 170.2 ppm; HRMS (ESI-TOF) calcd for C H N O
4
27
34
5
+
[M+H] : 492.262, found 492.2605.
Model Compound 2c: By the above procedure, 1c (1.8 g, 79%)
was obtained as colorless oil. 2c (60 mg, 63%) was prepared from
1c and 5 and purified by column chromatography on silica gel
pound. The intensity of the excitation light beam (I , unit: einstein
0
[22]
ꢁ1
min ) was measured by ferrioxalate actinometry. The intensity
(EtOAc as eluent) as a white amorphous powder. M.p. 260–2638C;
of light absorbed (I ) by the solution was calculated in terms of
a
4
184
www.chemphyschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2012, 13, 4180 – 4185

Photosensitized Splitting of Thymine Cyclobutane Dimer
1
H NMR (300 MHz, CDCl ): d=1.27 (s, 3H, CH ), 1.40 (s, 3H, CH ),
.62–1.63 (m, 1H), 2.01–2.03 (m, 1H), 2.14–2.16 (m, 1H), 2.31 (s,
30870581), Anhui Province and China Postdoctoral Science Foun-
dation (Grant No. 20100470823).
3
3
3
1
3
1
H, CH ), 2.33–2.40 (m, 2H), 2.79 (s, 3H, NCH ), 3.41 (d, J=7.1 Hz,
3
3
H, CH), 3.54 (d, J=7.1 Hz, 1H, CH), 3.92–3.96 (m, 2H), 4.05–4.27
13
Keywords: DNA
photorepair
·
electron
transfer
·
(
m, 5H), 6.85–7.62 ppm (m, 5H, ArH); C NMR (75 MHz, CDCl ): d=
3
9
4
.8 (CH ), 18.2 (CH ), 18.7 (CH ), 22.9, 28.8, 36.4, 41.7, 41.8, 43.7,
3 3 3
6.1, 50.7 (C), 51.0 (C), 58.8 (CH), 60.7 (CH), 109.2, 111.1, 119.1,
photochemistry · solvent effects · substituent effects
1
19.5, 121.9, 125.4, 129.1, 136.3, 151.8, 152.0, 170.1, 170.2 ppm;
+
HRMS (ESI-TOF) calcd for C H N O [M+H] : 478.2467, found:
26
32
5
4
4
78.2449.
[1] E. C. Friedberg, G. C. Walker, W. Siede, DNA Repair and Mutagenesis,
ASM, Washington, DC, 1995.
[2] K. M. Lima-Bessa, C. F. M. Menck, Curr. Biol. 2005, 15, R58–R61.
[3] A. Sancar, Chem. Rev. 2003, 103, 2203–2237.
[11]
Model Compound 2d:
By the above procedure, 1d (1.67 g,
2%)was obtained as light yellow oil. 2d (60 mg, 65%) was pre-
8
pared from 1d and 5 and purified by column chromatography on
silica gel (EtOAc/MeOH 1/0!5/1 as eluent) as a white amorphous
powder. M.p. 244–2478C; HRMS (ESI-TOF) calculated for C H N O
4
[4] a) Y.-T. Kao, C. L. Saxena, J. Wang, A. Sancar, D. Zhong, Proc. Natl. Acad.
Sci. USA 2005, 102, 16128–16132; b) Z. Liu, C. Tan, X. Guo, Y.-T. Kao, J.
Li, L. Wang, A. Sancar, D. Zhong, Proc. Natl. Acad. Sci. USA 2011, 108,
14831–14836; c) V. Thiagarajan, M. Byrdin, A. P. Eker, P. Mꢂller, K. Brettel,
Proc. Natl. Acad. Sci. USA 2011, 108, 9402–9407.
25
30
5
+
[M+H] : 464.2304, found: 464.2292.
Model Compound 2e: By the above procedure, 1e (630 mg, 80%)
was obtained as light yellow oil. 2e (43 mg, 41%) was prepared
from 1e and 5 and purified by column chromatography on silica
[5] a) F. Masson, T. Laino, U. Rothlisberger, J. Hutter, ChemPhysChem 2009,
1
2
0, 400–410; b) A. A. Hassanali, D. Zhong, S. J. Singer, J. Phys. Chem. B
011, 115, 3848–3859.
[6] P. F. Heelis, R. F. Hartman, S. D. Rose, Chem. Soc. Rev. 1995, 24, 289–297.
[7] K. Heil, D. Pearson, T. Carell, Chem. Soc. Rev. 2011, 40, 4271–4278.
[8] K. Brettel, M. Byrdin, Curr. Opin. Struc. Biol. 2010, 20, 693–701.
[9] a) S.-T. Kim, R. F. Hartman, S. D. Rose, Photochem. Photobiol. 1990, 52,
789–794; b) S.-T. Kim, S. D. Rose, J. Photochem. Photobiol. B 1992, 12,
179–191; c) D. G. Hartzfeld, S. D. Rose, J. Am. Chem. Soc. 1993, 115,
850–854.
gel (petroleum ether/EtOAc 1/3 as eluent) as a white powder. M.p.
1
2
3
2
1
7
37–2388C; H NMR (300 MHz, CDCl ): d=1.36 (s, 3H, CH ), 1.44 (s,
3
3
H, CH ), 1.63–1.68 (m, 1H), 2.01–2.18 (m, 2H), 2.27–2.39 (m, 2H),
.82 (s, 3H, NCH ), 3.53 (d, J=7.1 Hz, 1H, CH), 3.66 (d, J=7.1 Hz,
H, CH), 3.91–3.98 (m, 2H), 4.04–4.27 (m, 5H), 6.46 (d, 1H, ArH),
.09–7.77 ppm (m, 4H, ArH); C NMR (75 MHz, CDCl ): d=17.4
3
3
13
3
(
(
1
CH ), 17.6 (CH ), 22.1, 28.0, 35.5, 40.5, 40.8, 43.2, 44.9, 50.0 (C), 50.2
[10] a) R. Epple, E.-U. Wallenborn, T. Carell, J. Am. Chem. Soc. 1997, 119,
3
3
7
6
440–7451; b) J. Butenandt, A. P. Eker, T. Carell, Chem. Eur. J. 1998, 4,
42–654; c) T. Carell, R. Epple, Eur. J. Org. Chem. 1998, 1245–1258; d) R.
C), 57.4 (CH), 59.2 (CH), 100.2, 111.4, 111.6, 122.5, 123.3, 129.7,
29.8, 134.2, 151.2, 151.3, 169.8, 169.9 ppm; HRMS (ESI-TOF) calcu-
+
Epple, T. Carell, Angew. Chem. 1998, 110, 986–989; Angew. Chem. Int.
Ed. 1998, 37, 938–941; e) R. Epple, T. Carell, J. Am. Chem. Soc. 1999,
121, 7318–7329; f) J. Butenandt, L. T. Burgdorf, T. Carell, Angew. Chem.
lated for C H BrN O [M+H] : 542.1395, found: 542.1397.
2
5
29
5
4
Model Compound 2 f: By the above procedure, 1 f (1.8 g, 78%)
was obtained as light yellow oil. 2 f (60 mg, 60%) was prepared
from 1 f and 5 and purified by column chromatography on silica
1
999, 111, 718–721; Angew. Chem. Int. Ed. 1999, 38, 708–711; g) J. Bute-
nandt, R. Epple, E.-U. Wallenborn, A. P. Eker, E.-U. Gramlich, T. Carell,
Chem. Eur. J. 2000, 6, 62–72; h) M. K. Cichon, S. Arnold, T. Carell, Angew.
Chem. 2002, 114, 793–796; Angew. Chem. Int. Ed. 2002, 41, 767–770;
i) Q. H. Song, W. J. Tang, X. M. Hei, H. B. Wang, Q. X. Guo, S. Q. Yu, Eur. J.
Org. Chem. 2005, 1097–1106; j) S. Breeger, M. Meltzer, U. Hennecke, T.
Carell, Chem. Eur. J. 2006, 12, 6469–6477.
1
gel (EtOAc as eluent) as a white powder. M.p. 245–2478C; H NMR
(
(
300 MHz, CDCl ): d=1.36 (s, 3H, CH ), 1.43 (s, 3H, CH ), 1.63–1.65
m, 1H), 2.01–2.17 (m, 2H), 2.31–2.40 (m, 2H), 2.84 (s, 3H, NCH3),
3
3
3
3
(
.55 (d, J=7.1 Hz, 1H, CH), 3.65 (d, J=7.1 Hz, 1H, CH), 3.92–3.97
13
m, 2H), 4.03–4.28 (m, 5H), 6.48–7.56 ppm (m, 5H, ArH); C NMR
[11] W. J. Tang, Q. X. Guo, Q. H. Song, J. Phys. Chem. B 2009, 113, 7205–
7210.
(
75 MHz, CDCl ): d=18.3 (CH ), 18.7 (CH ), 22.8, 30.5, 41.7, 41.8,
3
3
3
4
1
4.0, 46.1, 50.7 (C), 51.0 (C), 59.1 (CH), 60.7 (CH), 102.2, 109.4, 120.5,
[12] a) Q. Q. Wu, Q. H. Song, J. Phys. Chem. B 2010, 114, 9827–9832; b) H. M.
Zhou, W. J. Tang, H. Zhang, X. X. Li, J. Li, J. Photochem. Photobiol. A
2012, 246, 60–66.
22.3, 127.4, 128.0, 128.6, 136.3, 151.8, 152.1, 170.1 ppm (2C);
+
HRMS (ESI-TOF) calcd for C H ClN O [M+H] : 498.1904, found:
4
25
29
5
4
[
[
[
13] T. Carell, R. Epple, V. Gramlich, Helv. Chim. Acta 1997, 80, 2191–2203.
14] K. Fukui, T. Yonezawa, H. Shingu, J. Chem. Phys. 1952, 20, 722–725.
15] a) Q. H. Song, H. B. Wang, W. J. Tang, Q. X. Guo, S. Q. Yu, Org. Biomol.
Chem. 2006, 4, 291–298; b) W. J. Tang, Q. H. Song, H. B. Wang, J. Y. Yu,
Q. X. Guo, Org. Biomol. Chem. 2006, 4, 2575–2580; c) Q. H. Song, W. J.
Tang, X. B. Ji, H. B. Wang, Q. X. Guo, Chem. Eur. J. 2007, 13, 7762–7770.
16] Y.-T. Kao, Q. H. Song, C. Saxena, L. Wang, D. Zhong, J. Am. Chem. Soc.
98.1903.
Model Compound 2g: By the procedure above, 1g (400 mg, 86%)
was obtained as light yellow oil. 2g (40 mg, 41%) was prepared
from 1g and 5 and purified by column chromatography on silica
1
gel (EtOAc as eluent) as a white powder. M.p. 268–2708C; H NMR
[
(
300 MHz, CDCl ): d=1.41 (s, 3H, CH ), 1.46 (s, 3H, CH ), 1.66 (m,
3
3
3
2
012, 134, 1501–1503.
1
H), 2.04–2.19 (m, 2H), 2.32–2.41 (m, 2H), 2.84 (s, 3H, NCH ), 3.62
3
[17] A. A. Weller, Z. Phys. Chem. 1982, 133, 93–98.
(
2
d, J=7.0 Hz, 1H, CH), 3.74 (d, J=7.0 Hz, 1H, CH), 3.93–3.98 (m,
H), 4.11–4.28 (m, 5H), 6.74–7.55 ppm (m, 5H, ArH); C NMR
[
18] P. Jennings, A. C. Jones, A. R. Mount, A. D. Thomson, J. Chem. Soc. Fara-
day Trans. 1997, 93, 3791–3797.
13
(
4
1
1
75 MHz, CDCl ): d=17.4 (CH ), 17.7 (CH ), 22.2, 28.2, 35.7, 40.7,
1.0, 43.4, 44.9, 50.1 (C), 50.4 (C), 57.3 (CH), 59.2 (CH), 99.0, 101.5,
[19] M. R. Scannell, D. J. Fenick, S. R. Yeh, D. E. Falvey, J. Am. Chem. Soc.
997, 119, 1971–1977.
3
3
3
1
[
20] a) L. P. Hammett, J. Am. Chem. Soc. 1937, 59, 96–103; b) C. Hansch, A.
Leo, R. W. Taft, Chem. Rev. 1991, 91, 165–195.
15.4, 118.6, 121.0, 124.7, 128.9, 132.0, 135.3, 151.4, 151.5, 170.1,
+
70.2 ppm; HRMS (ESI-TOF) calcd for C H N O [M+H] : 489.2247,
26 29 6 4
[
21] a) A. Mees, T. Klar, P. Gnau, U. Hennecke, A. P. Eker, T. Carell, L.-O. Essen,
Science 2004, 306, 1789–1793; b) Y. M. Gindt, J. P. Schelvis, K. L. Thoren,
T. H. Huang, J. Am. Chem. Soc. 2005, 127, 10472–10473.
found: 489.2245.
[
22] S. L. Murov, I. Carmichael, G. L. Hug, Handbook of Photochemistry, 2nd
ed., Marcel Dekker, New York, 1993.
Acknowledgements
The authors are grateful for the financial support by the National
Natural Science Foundation of China (Grant Nos. 20802003 and
Received: August 10, 2012
Published online on October 4, 2012
ChemPhysChem 2012, 13, 4180 – 4185
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
4185