A new phenolic diglycoside produced in response to copper toxicity and a new flavan dimer from the leaves of Viburnum ichangense (Hemsl.) Rehd.

Article abstract of DOI:10.1002/hlca.201100177

A new phenolic digycoside 1 was produced as stress metabolite in the fresh leaves of Viburnum ichangense (Hemsl.) Rehd., in response to abiotic stress elicitation by CuCl₂. The stress metabolite was characterized as 1-O-[α-L-arabinofuranosyl(1→6)-β-D-glucopyranosyl]-erythro-1, 2-bis(4-hydroxy-3-methoxyphenyl)propane-1,3-diol (1). A new flavan dimer, 2,3-epoxyflavan-3′,4′,5,7-tetraol-(4→8″)-flavan-3″, 3?,4?,5?,6″-pentaol (2), and two known compounds, hovetrichoside A (3) and asperglaucide (4), were also isolated. Their structures were established by spectroscopic means. Copyright

Full text of DOI:10.1002/hlca.201100177

Helvetica Chimica Acta – Vol. 94 (2011)
1677
A New Phenolic Diglycoside Produced in Response to Copper Toxicity and a
New Flavan Dimer from the Leaves of Viburnum ichangense (Hemsl.) Rehd.
a
b
a
by Bin Wu* ), Kuiwu Wang ), and Xiaodan Wu )
a) Institute of Ocean Resources, Department of Ocean Science and Engineering, Zhejiang University,
Hangzhou 310058, P. R. China (phone/fax: þ 86-571-88208540; e-mail: wubin@zju.edu.cn)
b) College of Food and Biology Engineering, Zhejiang Gongshang University, Hangzhou 310035,
P. R. China
A new phenolic digycoside 1 was produced as stress metabolite in the fresh leaves of Viburnum
ichangense (Hemsl.) Rehd., in response to abiotic stress elicitation by CuCl . The stress metabolite was
2
characterized as 1-O-[a-l-arabinofuranosyl(1 ! 6)-b-d-glucopyranosyl]-erythro-1,2-bis(4-hydroxy-3-
methoxyphenyl)propane-1,3-diol (1). A new flavan dimer, 2,3-epoxyflavan-3’,4’,5,7-tetraol-(4 ! 8’’)-
flavan-3’’,3’’’,4’’’,5’’’,6’’-pentaol (2), and two known compounds, hovetrichoside A (3) and asperglaucide
(4), were also isolated. Their structures were established by spectroscopic means.
Introduction. – The genus Viburnum comprises more than 230 species mainly
distributed in either the temperate or subtropical zones, from South America (Peru) to
South-East Asia (Philippines, Malaysia), and the majority of them are endemic [1].
Viburnum species are commonly used in folk medicine for their diuretic, antispas-
modic, and sedative properties, mainly as uterine excitability [2][4], and the dry
powder of V. tinus leaves was reported as an effective molluscicidal agent [5]. The
genus Viburnum is known to contain iridoids, iridoid glycosides [6 – 9], flavonoids, and
biflavonoids [10], monoterpene, triterpenoids, and diterpenoids [11 – 15], and other
constituents [16 – 18]. In our previous phytochemical studies on the genus Viburnum,
phenolic glycosides, including stilbene glycoside and lignan glycosides, were isolated
from V. fordiae [19], and salicin analog, quercetin derivatives, and quinic acid
derivatives were isolated from V. dilatatum [20][21]. In Traditional Chinese Medicine,
this genus is the resource of many medicinal herbs [22]. V. ichangense (Hemsl.) Rehd.,
Chinese name ꢀYichang Jiamiꢁ, is an evergreen shrub mainly distributed in south China
[
22]. Previously, we isolated lignan, flavan, monoterpene glycoside, and quercetin
derivatives from this plant [23].
Copper is an essential trace element for all higher plants, and it has several roles in
metabolic processes in plants [24]. Copper concentrations in healthy plant tissues range
ꢀ1
from 20 – 30 mg g dry weight. However, excess concentrations are assumed to
generate oxidative stress due to an increase in the levels of reactive oxygen species
(
ROS) within subcellular compartments [25]. To find stress metabolites from V.
ichangense, a stress application on the leaves of this plant was carried out. In the present
study, application of the abiotic stress agent CuCl to the leaves of V. ichangense
2
resulted in the emergence of one additional spot in the TLC of the extract of the treated
ꢂ 2011 Verlag Helvetica Chimica Acta AG, Zꢃrich

1
678
Helvetica Chimica Acta – Vol. 94 (2011)
plants in comparison with that of the corresponding control extract. The new phenolic
diglycoside, ichangoside (1), produced in response to abiotic stress treatment, a new
flavan dimer, ichangol (2), and two known compounds, hovetrichoside A (3) [26] and
asperglaucide (4) [27], were separated by preparative TLC, and purified by Sephadex
LH-20 column chromatography. Their structures and relative configurations were
established by spectroscopic means.
Results and Discussion. – The AcOEt- and BuOH-soluble fractions of the MeOH
extract of the leaves of copper stressed V. ichangense were subjected to repeated
chromatography to afford new stress metabolite 1. Compounds occurring in both
treated and untreated leaves were also isolated, and identified as a new flavan dimer 2
and two known compounds, hovetrichoside A (3) [26] and asperglaucide (4) [27] by
analysis of their NMR and MS data, and by comparison of their spectroscopic data with
literature values.
Compound 1 was isolated as a yellow gum. The HR-FT-ICR-MS exhibited a
ꢀ
pseudo-molecular-ion peak at m/z 613.2141 ([M ꢀ H] ; calc. 613.2138), providing the
ꢀ
1
molecular formula, C H O . The IR absorption maxima at 3415 and 1614 cm
28
38 15
suggested the presence of a OH group and an aromatic ring. The phenolic nature of the
compound was also indicated by its characteristic color reactions (FeCl : purple;
3
1
phosphomolybdic acid reagent: deep blue). The H-NMR spectrum (Table 1) of 1
exhibited two sets of ABX-type signals (d(H) 6.83 (d, J ¼ 2.1), 6.63 (overlap), 6.55 (d,
J ¼ 8.2), and d(H) 6.74 (d, J ¼ 2.1), 6.63 (overlap), 6.47 (dd, J ¼ 8.2, 2.1)) and two
MeO signals (d(H) 3.63 (s) and 3.66 (s)), revealing the presence of two 1,3,4-
trisubsituted phenyl groups [28][29]. In the aliphatic region, ABMX-type signals (d(H)

Helvetica Chimica Acta – Vol. 94 (2011)
1679
Table 1. NMR Data for Compound 1 (in (D )DMSO; d in ppm, J in Hz)
6
a
b
d(H)c)
1
1
HMBCd)
Position
d(C) ) )
H, H-COSY
1
2
3
77.9 (d)
54.4 (d)
63.2 (t)
5.13 (d, J ¼ 5.5)
2
1, 3
2
2, 3, 2’, 6’
1, 3, 1’, 1’’
1, 2, 1’’
2.88 (ddd, J ¼ 10.5, 8.0, 5.5)
3.78 – 3.81 (m), 3.35 – 3.39 (m)
1
2
3
4
5
6
1
2
3
4
5
6
3
3
’
’
’
’
’
’
’’
’’
’’
’’
’’
’’
131.9 (s)
112.1 (d)
147.6 (s)
145.6 (s)
115.0 (d)
120.1 (d)
131.1 (s)
114.2 (d)
147.3 (s)
145.0 (s)
115.0 (d)
122.5 (d)
55.8 (q)
55.9 (q)
6.83 (d, J ¼ 2.1)
1, 3’, 4’
6.55 (d, J ¼ 8.2)
6’
5’
1’, 3’
1, 4’
6.63 (overlap)
6.74 (d, J ¼ 2.1)
6.63 (overlap)
6.47 (dd, J ¼ 8.2, 2.1)
3.63 (s)
6’’
5’’
1’’, 3’’
2, 2’’, 4’’
’-MeO
’’-MeO
3.66 (s)
Glucose
1
2
3
4
5
6
101.4 (d)
73.7 (d)
77.4 (d)
70.9 (d)
75.6 (d)
67.7 (t)
4.80 (d, J ¼ 7.5)
3.02 – 3.07 (m)
3.10 – 3.15 (m)
3.00 – 3.06 (m)
3.20 – 3.25 (m)
Glu-2
1, Glu-3
Glu-5
Glu-1, Glu-5
Glu-2, Glu-6
Glu-3
Glu-3, Glu-1
Glu-2, Glu-4
Glu-3, Glu-5
Glu-4, Glu-6
Glu-5
3.78 – 3.82 (m), 3.37 – 3.40 (m)
Glu-4, Ara-1
Arabinose
1
2
3
4
5
109.0 (d)
81.3 (d)
76.1 (d)
83.0 (d)
60.6 (t)
4.74 (s)
3.80 – 3.84 (m)
3.82 – 3.87 (m)
3.97 – 4.02 (m)
Ara-2
Glu-6
Ara-4
Ara-1, Ara-5
Ara-2
Ara-3
Ara-1, Ara-3
Ara-2, Ara-4
Ara-3, Ara-5
Ara-4
3.58 – 3.62 (m), 3.48 – 3.53 (m)
a) Recorded at 125 MHz. ) Multiplicities inferred from DEPTand HMQC experiments. ) Recorded at
5
b
c
d
00 MHz. ) H-Atom showing long-range correlation with indicated C-atoms.
5
.13 (d, J ¼ 5.5), 2.88 (ddd, J ¼ 10.5, 8.0, 5.5), 3.78 – 3.81 (m), and 3.35 – 3.39 (m)) were
13
observed. The C-NMR spectrum displayed eleven signals for a b-glucopyranose
moiety and a terminal arabinose moiety, together with the twelve signals corresponding
to two aromatic rings, and three aliphatic C-atom signals, including those of one CHꢀO
(
d(C) 77.9), one CH ꢀO (d(C) 63.2), and one benzylic CH group (d(C) 54.4).
2
Comparing the NMR data of 1 with those of hovetrichoside A (3) isolated from the
same plant, the 1,2-bis(4-hydroxy-3-methoxyphenyl)propane-l,3-diol moiety was
determined as the aglycone of 1. The carbohydrate moieties were identified by
comparison with authentic glucose and arabinose on TLC after acid hydrolysis, and d-
13
glucose and l-arabinose were confirmed by GC analysis. The C-NMR data of the
arabinose unit were in good agreement with those reported for a-l-arabinofuranoside
[
30]. The HMBC cross-peaks from the CHꢀO H-atom signals at d(H) 5.13 (d, J ¼ 5.5,

1
680
Helvetica Chimica Acta – Vol. 94 (2011)
HꢀC(1)) to the C-atom signal at d(C) 101.4 (d, C(Glu-1)), and the anomeric H-atom
signal of the glucopyranose moiety at d(H) 4.80 (d, J ¼ 7.5, HꢀC(Glu-1)) to the C-atom
signal at d(C) 77.9 (d, C(1)) indicated that the glucosyloxy unit was at benzylic C(1).
The remaining terminal arabinose residue was at C(6) of glucose, based on the
observation of long-range correlation from the anomeric H-atom signal of the
arabinose moiety at d(H) 4.74 (s, HꢀC(Ara-1)) to the downfield-shifted glucosyl C-
atom signal at d(C) 67.7 (t, C(Glu-6)). The configurations of glycosidic linkages in 1
were determined as b for glucose and a for arabinose units on the basis of the J values
(
d(H) 4.80 (d, J ¼ 7.5, HꢀC(Glu-1)) and 4.74 (s, HꢀC(Ara-1))) of the two anomeric H-
atoms. Comparison of the coupling constant J(1,2) ¼ 5.5 with published data of known
,2-bis(4-hydroxy-3-methoxyphenyl)propane-1,3-diol isomers provided evidence for
the determination of the erythro-configuration of the substituents at C(1) and C(2)
26][31]. Thus, compound 1 was elucidated as 1-O-[a-l-arabinofuranosyl(1 ! 6)-b-d-
glucopyranosyl]-erythro-1,2-bis(4-hydroxy-3-methoxyphenyl)propane-1,3-diol, and
1
[
was named ichangoside. Ichangoside (1) was considered as a stress metabolite in
response to copper toxicity, and the production of this new phenolic diglycoside was
assumed to be an antioxidant activity-promoting transformation, which was hypothe-
sized to play a role in the antioxidative defense system of the plant.
Compound 2 was obtained as white powder. The HR-FT-ICR-MS exhibited a
ꢀ
molecular-ion peak at m/z 575.1198 ([M ꢀ H] ; calc. 575.1195), corresponding to the
molecular formula C H O . The UV spectrum showed typical benzenoid absorptions
30
24 12
at 210, 250, and 281 nm. Analysis of the IR spectrum of 2 indicated the presence of a
ꢀ
1
ꢀ1
OH group (3459 cm ) and an aromatic ring (1617 cm ), suggesting the presence of
phenol moieties, which was further confirmed by the characteristic color reactions. The
13
C-NMR spectrum (Table 1) displayed 30 signals, among which six were aliphatic C-
atom signals, indicating a dimeric flavan, as confirmed further by the molecular formula
C H O . The 2,3-dioxygenated 4-substituted flavan unit was deduced from analysis of
3
0
24 12
13
resonances in the C-NMR spectrum (Table 2) at d(C) 100.2 (s), 67.7 (d), and 29.1 (d)
1
1
attributable to C(2), C(3), and C(4), respectively, and confirmed by the H, H-COSY
correlation from HꢀC(4) to HꢀC(3), and the HMBC correlations from HꢀC(3) and
HꢀC(4) to the dioxygenated C(2). Further analysis of the NMR data, aided by the
molecular formula, evidenced the presence of an epoxide ring at C(2) and C(3). The C-
atom signals at d(C) 84.3 (d), 68.0 (d) and 28.8 (t) were assigned to C(2’’), C(3’’), and
1
1
C(4’’), respectively, of another flavan unit, confirmed by the H, H-COSY correlation
of HꢀC(2’’)/HꢀC(3’’)/CH (4’’) and HMBC correlations HꢀC(3’’)/C(2’’) and CH (4’’)/
2
2
C(2’’). The C-atom resonance of C(4) was attributed to a methine in the DEPT
spectrum of 2, indicating that C(4) in ring B was one of the connecting positions of the
two flavans. Another linkage position in the flavan unit was deduced by analysis of the
HMBC correlations. The HMBC correlations from HꢀC(4) at the connecting position
to the oxygenated C(9’’) and quaternary C(8’’) indicated that the flavans units were
linked at C(4) (ring B) and C(8’’) (ring A’). The assignment of proposed C(8’’) linkage
13
position was supported from analysis of the C-NMR spectrum, where the C(8’’)
resonance was observed at d(C) 107.0 (s) [32]. Besides the dimeric flavan skeleton with
an epoxide ring at C(2) and C(3), and one OH group at C(3’’), there were eight
phenolic OH groups in the structure of 2. The assignment of a phenolic OH group was
determined mainly from the HMBC spectrum of 2. The signal of HꢀC(4) showed a

Helvetica Chimica Acta – Vol. 94 (2011)
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cross-peak with the C-atom resonance of the oxygenated C(5), indicating a OH group
at C(5). The signals of HꢀC(6) and HꢀC(8), meta to each other, displayed correlations
with C(10), indicating that C(7) was oxygenated. The relatively upfield signals of C(6)
(d(C) 96.5) and C(8) (d(C) 98.1) supported this inference [33]. The HMBC
correlations from the double doublet resonance of HꢀC(6’) and doublet resonance of
HꢀC(2’) to the dioxygenated C-atom signal of C(2) indicated that ring C was 1,3,4-
trisubstituted, and two phenolic OH groups were located at C(3’) and C(4’). The OH
group in ring A’ of another flavan unit was inferred to be located at C(6’’) from the
HMBC correlations from HꢀC(5’’) and HꢀC(7’’), which were meta to each other, to
oxygenated C(9’’). The remaining three OH groups were assigned to C(3’’’), C(4’’’),
and C(5’’’) from the observation of HMBC correlation of HꢀC(2’’) to C(2’’’ and 6’’’).
1
13
Thus, the structure of 2 was established as shown. The complete H- and C-NMR
spectroscopic assignments are compiled in Table 2. The relative configuration of 2 was
Table 2. NMR Data for Compound 2 (in (D )DMSO; d in ppm, J in Hz)
6
a
b
d(H)c)
1
1
HMBCd)
Position
d(C) ) )
H, H-COSY
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
2
3
4
100.2 (s)
67.7 (d)
29.1 (d)
154.1 (s)
96.5 (d)
158.0 (s)
98.1 (d)
156.6 (s)
104.0 (s)
132.2 (s)
115.6 (d)
146.2 (s)
145.5 (s)
120.6 (d)
119.8 (d)
84.3 (d)
68.0 (d)
28.8 (t)
4.08 (d, J ¼ 3.2)
4.24 (d, J ¼ 3.2)
4
3
2, 4, 10, 1’, 8’’
2, 3, 5, 9, 10, 7’’, 8’’, 9’’
6.08 (d, J ¼ 2.5)
5.97 (d, J ¼ 2.5)
5, 7, 8, 10
6, 7, 9, 10
1
’
’
’
’
’
’
’’
’’
’’
7.14 (d, J ¼ 2.5)
2, 1’, 3’, 4’, 6’
6.80 (overlap)
6’
5’
3’’
2’’, 4’’
3’’
1’ , 3’, 4’, 6’
2, 1’, 2’, 4’, 5’
3’’, 4’’, 9’’, 1’’’, 2’’’, 6’’’
2’’, 4’’, 10’’, 1’’’
2’’, 3’’, 5’’, 9’’, 10’’
7.03 (dd, J ¼ 8.0, 2.5)
4.75 (d, J ¼ 9.0)
4.13 – 4.17 (m)
2.94 (dd, J ¼ 16.2, 5.2, H
a
),
2
.59 (dd, J ¼ 16.2, 8.5, H )
b
5
6
7
8
9
’’
’’
’’
’’
’’
96.5 (d)
156.0 (s)
115.7 (s)
107.0 (s)
152.0 (s)
103.0 (s)
130.5 (s)
116.3 (s)
146.6 (s)
151.3 (s)
6.09 (d, J ¼ 2.5)
4’’, 6’’, 7’’, 9’’, 10’’
5’’, 6’’, 8’’, 9’’
6.91 (d, J ¼ 2.5)
1
0’’
1
2
3
4
’’’
’’’ (6’’’)
’’’ (5’’’)
’’’
6.80 (overlap)
2’’, 1’’’, 3’’’ (5’’’), 4’’’
a) Recorded at 125 MHz. ) Multiplicities inferred from DEPTand HMQC experiments. ) Recorded at
5
b
c
d
00 MHz. ) H-Atom showing long-range correlation with indicated C-atoms.

1
682
Helvetica Chimica Acta – Vol. 94 (2011)
determined by analysis of H-atom coupling patterns. The coupling constant observed
for HꢀC(2’’) is in close agreement with those reported in the literature [34][35] for the
2,3-trans-configuration. The epoxide ring and the flavan unit at C(4) were established
to be on the same face as depicted, which was deduced from coupling constant observed
for HꢀC(3) (d(H) 4.08 (d, J ¼ 3.2)) and HꢀC(4) (d(H) 4.24 (d, J ¼ 3.2)) [36]. From the
above information, compound 2 was deduced to be a new flavan dimer, 2,3-
epoxyflavan-3’,4’,5,7-tetraol-(4 ! 8’’)-flavan-3’’,3’’’,4’’’,5’’’,6’’-pentaol, and was assigned
the trivial name ichangol. Ichangol was also been detected in the roots and leaves of
other species of the same genus, V. chinshanense and V. dilatatum.
Experimental Part
General: Merck precoated plates (silica gel 60 F254) of 0.25-mm thickness. Prep. HPLC: Agilent 1100
with a Shim-pack PREP-ODS (250 ꢁ 20 mm) column. Column chromatography (CC): Sephadex LH-20
(
(
Amersham). M.p.: Reichert apparatus; uncorrected. UV Spectra: Jasco UV-2200 spectrophotometer; l
log e) in nm. Optical rotations: Perkin-Elmer-341 polarimeter. IR Spectra: Nicolet AVTAR-360 FT-IR
ꢀ1
1
13
spectrometer; ˜n in cm . H- (500 MHz) and C-NMR (125 MHz) spectra: Bruker Avance DMX 500
NMR spectrometer, with TMS as internal standard, at 258, d in ppm, J in Hz. HR-FT-ICR-MS: Bruker
plus
Apex III spectrometer; ESI-MS: Bruker Esquire-3000
spectrometer; m/z.
Plant Material and Stress Applications. Viburnum ichangense (Hemsl.) Rehd. were collected in
Linan County, Zhejiang Province, P. R. China, in June, 2010, and identified by Prof. Changxi Zhang
(
Jinhua Medical College, Jinhua, P. R. China.). Avoucher specimen (Vs121) has been deposited with the
Jinhua Medical College, Jinhua, P. R. China. The stress applications were carried out on ten plants
according to our previous method [37][38]. All the plants were separated into control (5 plants) and
stressed groups (5 plants). To elicit the stress, plants were sprayed with 2% aq. soln. of CuCl . After 48 h,
2
leaves of the control and sprayed plants were collected and dried at 608, and finely powdered in an
electronic blender and kept in separate containers for extraction.
Extraction and Isolation. The dried, powdered CuCl -treated leaves (1.1 kg) of V. ichangense and
2
untreated leaves (1.3 kg) were extracted at r.t. with MeOH (3 ꢁ 5 l), resp. The extracts were evaporated
in vacuo to afford a gummy residue (114 g) for treated and a gummy residue (122 g) for the
corresponding control. The residues were partitioned in H O (500 ml), and extracted with AcOEt (4 ꢁ
2
5
00 ml) and BuOH (4 ꢁ 500 ml), successively. The AcOEt and BuOH extracts of treated and the
corresponding control were subjected to TLC on aluminium sheets pre-coated with silica gel 60 F254
Merck). The spots were applied in as equal amounts as possible. The plates were developed in the
(
following developing solvent systems: benzene/acetone 6 :1, benzene/AcOEt 5 :1, petroleum ether/
AcOEt 5 :1 for the AcOEt extract; CHCl /MeOH 3 :1, CH Cl /MeOH 4 :1, and benzene/CHCl /MeOH
3
2
2
3
1
:3 :1 for the BuOH extract. After development, the plates were examined under UV light (250 nm) to
locate any additional spots of the different extracts of the treated leaves in comparison with that of the
corresponding control extracts. The spots on the plates were also visualized by spraying with an EtOH/
H SO soln. Several prep.-TLC plates were prepared, and the compounds were separated by prep. TLC
2
4
in different solvent systems. The crude compounds were applied to a Sephadex LH-20 column (1 ꢁ 80 cm,
3
4
8 g; Amersham) and eluted with MeOH to yield pure compounds 1 (2.6 mg), 2 (3.1 mg), 3 (2.4 mg), and
(2.5 mg). The extract of untreated leaves were separated by the same method to afford 2 (1.7 mg), 3
(
3.1 mg), and 4 (2.7 mg).
Ichangoside (¼ 3-Hydroxy-1,2-bis(4-hydroxy-3-methoxyphenyl)propyl 6-O-a-l-Arabinofuranosyl-b-
2
0
d-glucopyranoside; 1). Yellow gum. UV (MeOH): 210 (4.43), 260 (3.87), 280 (4.12). [a] ¼ ꢀ15 (c ¼
D
1
13
0
(
.15, MeOH). IR (KBr): 3415, 1614, 1460, 1455, 1230. H- and C-NMR: see Table 1. ESI-MS: 613
ꢀ
ꢀ
ꢀ
37 15
[M ꢀ H] ). HR-FT-ICR-MS: 613.2141 ([M ꢀ H] , C H O ; calc. 613.2138).
28
Ichangol (¼ rel-5-[(2R,3S)-8-[(1aR,7S,7aS)-1a-(3,4-dihydroxyphenyl)-1a,7a-dihydro-4,6-dihy-
droxy-7H-oxireno[b][1]benzopyran-7-yl]-3,4-dihydro-3,6-dihydroxy-2H-1-benzopyran-2-yl]-1,2,3-ben-
2
0
zenetriol; 2). White powder. M.p. 231 – 2338. UV (MeOH): 210 (4.10), 250 (4.02), 281 (4.18). [a] ¼ ꢀ36
D

Helvetica Chimica Acta – Vol. 94 (2011)
1683
c ¼ 0.23, MeOH). IR (KBr): 3459, 2922, 1617, 1499, 1302, 1144, 1055. H- and 13C-NMR: see Table 2.
1
(
ꢀ
ꢀ
ꢀ
ESI-MS: 575 ([M ꢀ H] ). HR-FT-ICR-MS: 575.1198 ([M ꢀ H] , C H O12 ; calc. 575.1195).
30
23
This work was supported by the NSFC (No. 30801430) and the Science Foundations of Zhejiang
Province (No. Y2110527 and Y4090121).
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Received March 18, 2011