NATꢂꢀAL PꢀODꢂCT ꢀꢁSꢁAꢀCH
5
(
(
1H, d, J = 17.0 Hz, H-5′a), 2.44 (1H, d, J = 17.0 Hz, H-5′b), 3.64 (1H, t, J = 9.0 Hz, H-3″), 3.44
1H, m, H-5″), 3.36 (1H, t, J = 9.0 Hz, H-2″), 3.77 (1H, dd, J = 2.0, 12.0 Hz, H-6″a), 3.67 (1H, m,
H-6″b), 4.09 (1H, t, J = 9.0 Hz, H-4″), 5.43 (1H, d, J = 8.5 Hz, H-1″), 5.73 (1H, s, H-2), 5.84 (1H, s,
1
3
H-3′), 6.23 (1H, d, J = 16.0 Hz, H-5), 7.70 (1H, d, J = 16.0 Hz, H-4). C NMꢀ (125 MHz, CD OD):
3
δ 166.3 (C-1), 118.0 (C-2), 153.7 (C-3), 129.3 (C-4), 139.2 (C-5), 21.3 (C-6), 80.6 (C-1′), 165.7
(
(
[
C-2′), 127.6 (C-3′), 200.9 (C-4′), 50.6 (C-5′), 42.8 (C-6′), 19.5 (C-7′), 23.5 (C-8′), 24.6 (C-9′), 95.0
C-1″), 73.9 (C-2″), 76.7 (C-3″), 77.0 (C-4″), 77.1 (C-5″), 62.1 (C-6″). Hꢀ-ꢁSI-MS m/z: 507.1526
+
M + H] (Calcd 507.1536 for C H O S).
2
1
31 12
3
.3.2. Acid hydrolysis of 1
Compound 1 (2 mg) was heated in 1 N HCl (1 mL) at 80 °C for 2 h, then the solution was cooled
and extracted with ethyl acetate (1 mL × 3). The organic and aqueous layers were concen-
trated to 1 mL and their optical rotations were checked. (S)-abscisic acid was obtained from
2
5
the organic layer,[ꢀ] = +107.6 (c 0.03, MeOH). The sugar product in the aqueous layer was
D
identified as d-glucose by silica gel TLC (ꢀf 0.55 developed with acetone–methanol–water
1
00:10:1 (v/v) and was sprayed with a 10% sulphuric acid solution containing 2% vanillin),
25
and by the positive value of optical rotation ([ꢀ] = +3.2 (c 0.03, H O) in comparison with
D
2
d-glucose standard. The addition of BaCl to the aqueous solution produced a white precip-
2
2
−
itate indicating the presence of SO4 anions.
3
.4. Assay for α-glucosidase inhibition
to a previously described method (Luyen et al. 2013). The sample solution (2 μL dissolved
in DMSO) and 0.5 ꢂ/mL α-glucosidase (40 μL) were mixed in 120 μL of 0.1 M phosphate
buffer (pH 7.0). After 5 min pre-incubation, 5 mM p-nitrophenyl-α-d-glucopyranoside solu-
tion (40 μL) was added and the solution was incubated at 37 °C for 30 min. The absorbance
of released 4-nitrophenol was measured at 405 nm using a microplate reader (Molecular
Devices, CA). The IC value was graphically measured using a plot of the per cent inhibition
5
0
vs. log of the concentration of the test compound.
3
.5. DPPH radical scavenging activity
The antioxidant activity of the isolated compounds was evaluated by its scavenging capacity
of the DPPH radical. Briefly, the tested samples (10 μL) at various concentrations were mixed
with 150 μM DPPH solution (190 μL) in 96 well plates. The plate was incubated in the dark at
room temperature for 30 min. Then the absorbance of the reaction mixture was measured
at 520 nm on a microplate reader. The IC value is defined as the concentration of a sample
5
0
required to scavenge 50% of the DPPH radical.
Acknowledgement
We thank the Institute of Chemistry, Vietnam Academy of Science and Technology for the NMꢀ and
MS measurements. We would like to thank Prof Jason Weibel (Shenandoah ꢂniversity, Winchester, VA,
ꢂSA) for the ꢁnglish correction of the final manuscript.