Efficient photosensitized splitting of the thymine dimer/oxetane unit on its modifying β-cyclodextrin by a binding electron donor

Article abstract of DOI:10.1039/b604529d

Two modified β-cyclodextrins (β-CDs) with a thymine dimer and a thymine oxetane adduct respectively, TD-CD and Ox-CD, have been prepared, and utilized to bind an electron-rich chromophore, indole or N,N-dimethylaniline (DMA), to form a supramolecular complex. We have examined the photosensitized splitting of the dimer/oxetane unit in TD-CD/Ox-CD by indole or DMA via an electron-transfer pathway, and observed high splitting efficiencies of the dimer/oxetane unit. On the basis of measurements of fluorescence spectra and splitting quantum yields, it is suggested that the splitting reaction occurs in a supramolecular complex by an inclusion interaction between the modified β-CDs and DMA or indole. The back electron transfer, which leads low splitting efficiencies for the covalently-linked chromophore-dimer/oxetane compounds, is suppressed in the non-covalently-bound complex, and the mechanism has been discussed. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b604529d

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Efficient photosensitized splitting of the thymine dimer/oxetane unit on its
modifying b-cyclodextrin by a binding electron donor†
Wen-Jian Tang, Qin-Hua Song,* Hong-Bo Wang, Jing-yu Yu and Qing-Xiang Guo
Received 28th March 2006, Accepted 15th May 2006
First published as an Advance Article on the web 26th May 2006
DOI: 10.1039/b604529d
Two modified b-cyclodextrins (b-CDs) with a thymine dimer and a thymine oxetane adduct respectively,
TD-CD and Ox-CD, have been prepared, and utilized to bind an electron-rich chromophore, indole or
N,N-dimethylaniline (DMA), to form a supramolecular complex. We have examined the
photosensitized splitting of the dimer/oxetane unit in TD-CD/Ox-CD by indole or DMA via an
electron-transfer pathway, and observed high splitting efficiencies of the dimer/oxetane unit. On the
basis of measurements of fluorescence spectra and splitting quantum yields, it is suggested that the
splitting reaction occurs in a supramolecular complex by an inclusion interaction between the modified
b-CDs and DMA or indole. The back electron transfer, which leads low splitting efficiencies for the
covalently-linked chromophore–dimer/oxetane compounds, is suppressed in the non-covalently-bound
complex, and the mechanism has been discussed.
Introduction
DNA repair has received increased attention in recent years as
ozone depletion threatens to significantly increase DNA damage
by UV radiation. The two major lesions formed in DNA by
this radiation are the cyclobutane pyrimidine dimers (CPDs)
and pyrimidine–pyrimidone (6–4) photoproduct, which constitute
70–80% and 20–30% of total photoproducts, respectively,1 using
thymine base as an example shown in Fig. 1. The two photolesions
can be repaired through DNA photoreactivation catalyzed by
CPD photolyase and (6–4) photolyase, respectively.
CPD photolyase is the monomeric protein that contains
two non-covalently-bound chromophore/cofactors. One chro-
mophore is a fully reduced flavin adenine dinucletide (FADH−),
the catalytic cofactor that carries out the repair function upon
excitation by either direct photon absorption or resonance energy
transfer from another chromophore, which is the antenna cofactor
(methenyltetrahydrofolate or deazaflavin) that harvests sunlight.
The model for the catalytic reaction is that the enzyme binds CPD
Fig. 1 Formation of the two major photoproducts between adjacent
thymines in DNA under UV light, CPDs and (6–4) photoproducts.
in a light-independent reaction, the excited FADH− transfers an
electron to CPD to generate a charge-shifted radical pair (FADH•–
CPD•−), the dimer radical anion (CPD•−) undergoes spontaneous
0.9); and a low efficiency (0.05–0.1) for (6–4) photolyases.2c The
mechanism for the difference remains unknown.
splitting and back electron transfer restores the dipyrimidine and
the functional form of flavin ready for a new catalytic cycle.2 As (6–
4) photolyases have similar structures and the same chromophores,
the same basic reaction mechanism has been proposed.2c Despite
these similarities, however, certain important differences exist
between the two classes of enzymes: such as the repair efficiency,
CPD photolyases with a uniformly high quantum yield (U = 0.7–
The photosensitized repair of photoproducts by a covalently
attached chromophore appears inefficient, such as U < 0.1 for
the indole–dimer system,3 and ca. 0.2 for the tryptophan–oxetane
system,4 in aqueous solutions, and lower values of U for the
corresponding flavin model system.5 The factor limiting repair
efficiency in covalent systems was thought to be back electron
transfer within the charge-separated species formed upon forward
electron transfer from an excited electron donor to the substrate.
Furthermore, it has been discussed that the reasons leading to low
repair efficiencies are back electron transfer for indole/tryptophan
model systems, and an additional factor, non-radiative processes
that include internal conversion and intersystem crossing of
excited flavin, for flavin model systems. 4
Department of Chemistry, University of Science and Technology of China,
and State Key Laboratory of Elemento-Organic Chemistry, Nankai Uni-
versity, Hefei 230026, Anhui, P. R. China. E-mail: qhsong@ustc.edu.cn;
Fax: +86-551-3601592; Tel: +86-551-3607524
† Electronic supplementary information (ESI) available: copies for 1H
and 13C NMR spectra of compounds TD-CD and Ox-CD. See DOI:
10.1039/b604529d
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Comparing the covalently-linked chromophore–substrate mod-
els with the enzyme–substrate complex, a significant difference is
in their modes linking cofactor/chromophore and substrate, that
is, one is the covalent, the nother is the non-covalent. Two research
groups have presented small-molecule recognition units that can
bind a uracil dimer, and the uracil dimer is split through photo-
sensitization by chromophores attaching on the recognition unit.6,7
Using indole-containing marocycles as recognition units, Rose and
Goodman achieved the repair of a uracil dimer by photoinduced
electron transfer from indole within a hydrogen-bonding complex
with U > 0.1 in both protic and aprotic solvents.6 Another
group utilized a zinc-cyclen moiety covalently linked to a reduced
flavin derivative as an artificial photolyase model, by which the
uracil dimer was recognized through a combination of charge
and hydrogen-bonding interactions. The efficiency of the dimer
splitting in the complex is four times as high as the bimolecular
background reaction.7 The distances between the chromophore
and the dimer in two cases are longer than the covalently-linked
models mentioned above.
prepared from the Paterno`–Bu¨chi reaction of the thymine and
benzophenone.4 Mono-6-deoxy-6-amino-b-cyclodextrin was pre-
pared from b-cyclodextrin according to reported methods.8
The compound TD-CD was prepared through the condensation
of the thymine dimer dicarboxylic acid and mono-6-deoxy-
6-amino-b-cyclodextrin using dicyclohexylcarbodiimide (DCC)
and hydroxybenzotriazole (HOBt) as coupling reagent in DMF
(Scheme 1). Under the same conditions, the compound Ox-CD
was gained using oxetane carboxylic acid instead of the dimer
dicarboxylic acid. The reaction residues were isolated by column
of macroporous resin, giving two compounds with yields of 19
and 16%, respectively.
In CPD photolyase, the back electron transfer can be ef-
ficiently suppressed, for CPDs are repaired with a quantum
yield close to unity. Thus, the back electron transfer could also
be suppressed in the simple complexes formed from a small-
molecule recognition unit with the substrate because of higher
splitting efficiencies of dimers in the complex models over the
corresponding covalently-linked models.
Scheme 1 Synthesis of the model compounds. Reagents and conditions:
DCC, HOBt, DMF, rt, 48 h.
To estimate the efficiency in a supramolecular complex, we have
prepared two modified b-cyclodextrins (b-CDs) with a cis-syn
thymine dimer and a thymine oxetane adduct respectively, TD-
CD and Ox-CD, which were intended to include an electron-rich
chromophore such as indole and N,N-dimethylaniline (DMA),
shown in Chart 1. We have observed efficient photosensitized
splitting of the dimer/oxetane unit by the chromophore within
a possible complex. This implies that the back electron-transfer
process is suppressed in a supramolecular complex formed from
the modified b-CD and the chromophore.
Absorption and fluorescence emission spectra
Fig. 2 shows UV absorption spectra of two modified b-CDs and
two photosensitizers, indole and DMA, and fluorescence emission
spectra of the photosensitizers, in aqueous solutions. The spectra
show that there is no overlap between the emission spectra of the
indole and DMA and the absorption spectra of the TD-CD and
Ox-CD, which have no significant absorption at above 290 nm.
Obvious changes of fluorescence emission of DMA were ob-
served when the concentration of Ox-CD was increased gradually
in aqueous solution of DMA (Fig. 3). As the concentration of Ox-
CD increases, the fluorescence spectra show a decrease in intensity
and a blue shift (the peak from 372 to 355 nm). In general, the
fluorescence emission for DMA entering the hydrophobic cavity
of b-CD becomes strong and shows a blue shift. A possible
explanation for the blue shift and the decrease in intensity is
the formation of a possible supramolecular complex by Ox-CD
Results and discussion
Synthesis of the modified b-CDs, TD-CD and Ox-CD
The synthesis of ethyl cis-syn thymine cyclobutane dimer-yl-N,Nꢀ-
dipropionate (TD, shown in Chart 1) was carried out according
to the literature method.3c The thymine oxetane-1-acetate was
Chart 1
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Fig. 2 UV absorption spectra of relative compounds (solid), and
fluorescence emission spectra (dots) of indole and DMA upon excitation
at 295 nm, in aqueous solutions.
Scheme 2
is no overlap between the emission spectra of the indole and DMA
and the absorption spectra of TD-CD and Ox-CD. Since the TD-
CD and Ox-CD have no significant absorption at above 290 nm,
an internal filter effect is insignificant. A control experiment shows
that no splitting of the oxetane unit for a 0.5 mM Ox-CD aqueous
solution was detected upon irradiation with 310 nm light for
10 min in the absence of DMA. Hence, the splitting reaction may
undergo an electron transfer from the excited indole/DMA to the
oxetane/dimer. The driving force for the electron transfer can be
estimated in terms of the Rehm–Weller equation,9
DG (eV) = [Eox − Ered − e2/eRD ] − DE0,0
+
−
A
where DE0,0 is the energy level of an excited photosensitizer, which
is obtained from its fluorescence emission spectra, and Eox and
Ered are the oxidation potential of a donor and the reduction
potential of an acceptor, respectively. The Ered of the dimer was
reported to be between ca. −1.4 and −1.9 V,10 and its average
value, −1.7 V, was used in the calculation. Data in Table 1 showed
that the electron-transfer reaction from the excited indole or DMA
to the dimer would spontaneously occur. Although the value of
Fig. 3 Dependence of the fluorescence of 0.05 mM DMA on the
concentration of Ox-CD (0–6.0 × 10−4 M, kex = 295 nm).
E
red for the oxetane is unavailable, the excited oxidation potentials
(Eox* = Eox − DE0,0) for both indole and DMA are in the range,
where the electron-transfer reaction has been confirmed, from −
2.45 to − 3.32 V.13
including DMA, and in the complex, electron transfer from the
excited DMA to the oxetane unit occurs to lead to fluorescence
quenching instead of fluorescence enhancement.
Two groups of control experiments revealed that the splitting
reactions in the systems of Ox-CD/TD-CD and indole/DMA
are not simple biomolecular reactions, for the splittings of the
dimer/oxetane unit in the modefied b-CDs are more efficient than
TD or Ox in the presence of indole or DMA (compare entries 1 and
Photosensitized splitting of the dimer/oxetane unit of
TD-CD/Ox-CD by indole/DMA
To examine the photochemical behavior of the complex formed
from Ox-CD and DMA, a photolysis experiment was performed.
The cycloreversion of thymine oxetane to the thymine monomer
and benzophenone was confirmed through measurement of the 1H
NMR spectra (Scheme 2). Likewise, photolysis of Ox-CD/TD-
CD solutions containing indole led to the splitting of the
Table 1 Excited oxidation potentials of photosensitizers and calculated
DG values (eV) from the Rehm–Weller equation
Eox/V (SCE)
DE0,0
Eox*
DG
1
oxetane/dimer, which was detected by H NMR spectroscopy.
+0.96a
3.42
3.34
−2.46
−2.81
−0.79
−1.14
The splitting reactions were clean conversions because no side
products were detected.
Indole
DMA
+0.53a
The singlet–singlet energy transfer from the excited indole/
DMA to the substrate TD/Ox unit should be ruled out, for there
a From ref. 11, 12 respectively.
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Table 2 Splitting quantum yields of the dimer/oxetane unit in various
a maximum, for it was limited by a low solubility of Ox-
CD. According to the inclusion interaction in a ratio of 1 : 1, the
double reciprocal plot of 1/U versus 1/cOx-CD (Michaelis–Menten
equation) gave the association constant Ks from the ratio of the
intercept to the slope of the straight line fit: 780 M− 1 for the Ox-
CD–DMA system, and 730 M− 1 the for Ox-CD–indole system.
It is evident that the quantum efficiencies in the complexes
must be much higher than that in the covalently-linked models
(Chart 2)3c,4 composed of oxetane/dimer and a tryptophan
residue, such as 0.093 for TD-Trp3c and 0.24 for Ox-Trp4 in
aqueous solutions. For example, the splitting quantum efficiencies
are 0.16 for TD-CD/indole, and 0.24 for Ox-CD/indole in
aqueous solution (at a ratio of 0.5 mM/0.05 mM), in which
part indole molecules are included such as only 27% indole in
the Ox-CD/indole complex in the solution. This implies that
the photosensitized splitting reaction of the dimer/oxetane by
an electron donor in a complex is highly efficient. Thus, back
electron transfer of the radical ion pair upon electron transfer
from an electron donor within a complex would be suppressed.
systemsa
Entry
System
U
1
0.2 mM Ox-CD + 0.05 mM indole
0.2 mM Ox-CD + 0.05 mM indole
0.5 mM Ox + 0.05 mM indole
0.13
0.01
<0.01
0.27
0.01
0.08
2b
3
4
0.2 mM Ox-CD + 0.05 mM DMA
0.2 mM Ox-CD + 0.05 mM DMA
0.2 mM TD-CD + 0.05 mM Indole
0.2 mM TD-CD + 0.05 mM Indole
0.5 mM TD + 0.05 mM Indole
5b
6
7b
8
0.01
<0.01
a Irradiation with 295 nm light for the indole-containing systems and
310 nm light for the DMA-containing systems, 10 nm bandwidths. b 50%
Methanol aqueous solution.
6 with 3 and 8, respectively, in Table 2). This shows that interactions
exist between the modified b-CDs and indole or DMA. However,
a very low efficiency for the TD-CD system was observed in the
presence of DMA, similar to the TD system.
Splitting quantum yields of the dimer/oxetane unit
The splitting quantum yields of the dimer/oxetane unit in the
systems of Ox-CD/TD-CD and DMA/indole were measured,
and are listed in Table 2. Data in Table 2 result in three eductions,
as follows: (1) a strong concentration dependence (Fig. 4); (2)
more efficient splitting over the systems of free Ox/TD and
indole/DMA; and (3) a solvent-dependent splitting efficiency
(entries 1/2, 4/5 and 6/7), much more efficient in water over that in
the binary solvent of water–methanol (v/v, 50 : 50) due to a higher
binding constant for a CD complex in water. The results showed
the formation of supramolecular complexes between Ox-CD/TD-
CD and the chromophores, in which the photosensitized splittings
of the dimer/oxetane unit occur. In addition, the splitting of Ox-
CD by DMA is more efficient than the indole system. A larger
driving force for electron transfer between Ox-CD and DMA over
indole may be an important reason for it.
Chart 2
A recent ultrafast spectroscopic study14 showed the charge shift
with an quantum efficiency of 0.87, consistent with the repair
quantum yield of 0.89, implying that the efficiency of the ring
cleavage is nearly 100%. The active-site solvation was thought to
be critical to strategically slowing down the charge recombination
by dynamically tuning the redox potentials of reaction species and
stabilizing the charge-shift radical intermediates, leaving enough
time to cleave the cyclobutane ring to reach a maxium-repair
quantum yield.
Furthermore, a titration experiment was carried out and is
shown in Fig. 4. The splitting quantum efficiency did not reach
There may be a similar effect with the active-site solvation in the
complex of Ox-CD/TD-CD and indole/DMA. After the forward
electron transfer from the excited donor to the dimer/oxetane,
the solvation of formed radical ions – a TD-/Ox- unit radical
anion and a sensitizer radical cation – would occur, and more
solvent molecules move to the interspace of the radical ion pair.
This may change the redox potentials of the species, enlarge
the distance of the two charge centers and stabilize the charge-
separated radical pair. These would lead to a decrease in the driving
force of back electron transfer, i.e. the charge recombination would
be suppressed. However, the effects in a zwiterionic intermediate
may not be well achieved for a covalently-linked model, at least
the through-bond distance between a TD-/Ox- unit radical anion
and a chromophore radical cation is constant, and the through-
bond back electron transfer is the special mechanism for the
Fig. 4 Dependence of the quantum yield of splitting on concentration
of Ox-CD aqueous solution containing 0.05 mM indole (kir = 295 nm) or
DMA (kir = 310 nm).
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covalently-linked models. Therefore, it is further demonstrated
that CPD photolyase binding the substrate and the active-
site solvation in photoreactivation are important reasons why
CPD photolyases can repair the substrates with high quantum
efficiencies.
was recorded by a Shimadzu UV-2401PC spectrometer. The rates
of the dimer/oxetane unit split were measured by the monitoring
the increase in absorbance at 270 nm due to the regeneration
of the 5,6-double bond of the thymines and benzophenone. The
intensity of the light beam (I0 einsteins/min) was measured by
ferrioxalate actinometry.13 Thus, the rate of photons absorbed
was obtained from the absorbance (A) at 295 or 310 nm in terms
of Beer’s law, Ia = I0(1 − 10− A). The observed quantum yields
of dimer/oxetane splitting of TD-CD/Ox-CD were calculated
according to U = (rate of dimer/oxetane split)/(rate of photon
absorbed). In order to avoid competition of absorbing between the
model compound and the photosplitting products, the splitting
reaction was controlled within 10% yield in the measurements.
Despite the forward electron transfer (charge separation) in
the tryptophan-containing model systems with similar quantum
efficiencies (ca. 0.8)3c,4 to CPD photolyase,2c the splitting quantum
yields of the dimer or the oxetane are very low, ca. 0.1 for dimer–
tryptophan models3c and ca. 0.3 for oxetane–tryptophan models4
due to unproductive back electron transfer (charge recombina-
tion). In the case of flavin-containing systems, the deactivation of
the excited flavin by non-radiative processes, which compete with
forward electron transfer, is another important reason causing
low splitting quantum yields.4 Therefore, inefficient suppression
of non-radiative processes and/or charge recombination may be
responsible for the low repair efficiency of (6–4) photolyase. The
mechanistic detail warrants co-crystal structures of photolyase–
DNA complexes and ultrafast spectroscopic studies on (6–4)
photolyase.
Mono-6-deoxy-6-amino-b-cyclodextrin. 1H NMR (300 MHz,
D2O, [D6]acetone): d = 3.46–3.69 (m, 14H), 3.87–4.02 (m, 28H),
5.09 (d, 7H). 13C NMR (75 MHz, D2O, [D6]acetone): d = 61.47,
73.05, 73.33, 74.52, 82.42, 82.59, 84.34, 103.08, 103.28.
6-Deoxy-6-cis-syn thymine dimer-b-cyclodextrin (TD-CD).
A
dimethylformide (DMF, 20 mL) solution of the cis-syn thymine
dimer diacid (220 mg, 0.55 mmol) and 6-amino-b-cyclodextrin
(0.57 g, 0.5 mmol) was stirred at 0–5 ◦C for 30 min. Dicyclohexyl-
carbodiimide (DCC) (160 mg, 0.75 mmol) and hydroxybenzotria-
zole (HOBt) (110 mg, 0.75 mmol) were added to the above mixture,
Experimental
General
◦
and the solution was stirred at 0–5 C for 2 h and then at room
b-Cyclodextrin was recrystallized three times and dried in vacuo at
100 ◦C for 12 h before use. Indole was purified by recrystallization
from EtOH–water (1 : 10) before use. N,N-Dimethylaniline
(DMA) was purified by distillation in vacuo. Deionized water was
used throughout the experiments. The samples are fully aqueous
temperature for 2 d. After the insoluble material was removed
by filtration, the filtrate was poured into acetone (200 mL) to
precipitate the product. After being washed with acetone twice, the
crude product was absorbed on a column of macroporous resin
and eluted with an aqueous solution of ethanol (0 to 20%, v/v).
The eluted solution was concentrated to obtain the desired product
solutions unless otherwise indicated. H and 13C NMR spectra
1
were recorded on a Bruker AV (300 MHz for 1H, 75 MHz for 13C)
spectrometer. The chemical shifts were referenced to acetone (d
2.05, 29.8) in [D6]acetone and DMSO (d 2.50, 39.5) in [D6]DMSO
as a white powder (140 mg, 19%). Mp > 250 ◦C; mmax(KBr)/cm−1
=
3419 s, 1702 s, 1646 s, 1155 m, 1081 m, 1034 s; 1H NMR (300 MHz,
D2O, [D6]acetone): d = 1.30 (s, 6 H, CH3), 2.47 (m, 4H, CH2), 3.02
(m, 2H, CH2), 3.11–3.79 (m, majority, proton of CD unit + CH2),
4.02 (s, 2 H, CH), 4.90 (broad s, 7 H, H-1); 13C NMR (75 MHz,
D2O, [D6]acetone): d = 17.05 (2C, CH3), 33.00, 39.58, 39.73, 42.96,
47.34 (C), 47.42 (C), 59.20 (CH), 59.31 (CH), 59.64, 59.87, 69.68,
71.36, 71.48, 71.61, 72.32, 72.63, 80.46, 80.67, 82.39, 82.76, 101.41,
152.35, 152.39, 172.08, 172.73, 172.83; TOFMS (MALDI) calc.
for [M + Na]+ C58H89N5O41: 1535.3, found 1534.5.
1
for H and 13C NMR, respectively. Mass spectra were measured
on a Bruker BIFLEXTM III mass spectrometer. Elemental analysis
was performed at the Analytic Center of University of Science and
Technology of China. FTIR spectra were recorded on a BRUKER
VECTOR22 infrared spectrometer. UV-Vis spectra were measured
on a Shimadzu UV-2401PC spectrometer. Fluorescence emission
spectra were measured on a Shimadzu RF-5301PC fluorescence
spectrometer.
6-Deoxy-6-oxetane-b-cyclodextrin (Ox-CD). Using the oxe-
tane acid instead of the cis-syn thymine dimer diacid (183 mg,
0.5 mmol), the same procedure was performed, and the desired
product Ox-CD was obtained as a white powder (115 mg, 16%).
Rf = 0.18 (EtOAc–MeOH–AcOH 3 : 4 : 1); mp > 250 ◦C;
Measurement of quantum yield of splitting
The photosensitized splitting of the dimer/oxetane unit of TD-
CD/Ox-CD by DMA or indole was first performed through
1
determining the H NMR spectra of solutions irradiated with
1
a 300 W high pressure Hg lamp (k > 290 nm) for 20 min. The
solutions were prepared through dissolving TD-CD/Ox-CD and
DMA/indole in D2O in a Pyrex NMR tube, irradiating under
ultrasonic waves for 15 min, then allowing to stand overnight.
The sample solutions (3 mL) of indole and DMA added to TD-
CD or Ox-CD aqueous solutions were placed in quartz cuvettes
(10 × 10 mm) with a Teflon stopper, and after standing for sev-
eral hours at room temperature after ultrasonification for 15 min
were then irradiated with 295 nm (for indole) or 310 nm (for DMA)
light from a Shimadzu RF-5301PC spectrofluorophotomer. After
certain time intervals, the absorbance of the irradiated solutions
mmax(KBr)/cm−1 = 3424 s, 1705 s, 1155 m, 1081 m, 1032 s; H
NMR (300 MHz, [D6]DMSO): d = 1.58 (s, 3 H, CH3), 3.22–3.76
(m, majority, proton of CD unit), 4.45 (m, 2 H), 4.84 (broad s,
7H, H-1), 5.73 (m, proton of CD unit), 7.27–7.44 (m, 10 H,
Ar–H), 8.06 (s, 1 H, CONH), 10.39 (s, 1 H, NH); 13C NMR
(75 MHz, [D6]DMSO): d = 23.28 (CH3), 47.68 (CH2), 59.82,
59.93, 60.01, 65.11 (CN), 69.54, 72.08, 72.38, 72.99, 76.12 (CCH3),
81.33, 81.54, 81.74, 83.37, 90.83 (OCC), 101.97, 102.07, 102.36,
124.89 (2C), 124.97, 125.51, 127.77, 128.25, 128.60 (2C), 139.61,
144.60, 151.26, 167.70, 169.86; Anal. calc. for C62H87N3O38·6H2O
(1590.46): C, 46.82; H, 6.27; N, 2.64. Found: C, 47.07; H, 6.41;
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Acknowledgements
The authors are grateful for the financial support by the National
Natural Science Foundation of China (Grant No. 30470444,
20332020).
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