WANG Qinqqin et al. / Chinese Journal of Catalysis, 2012, 33: 275–280
larimeter (Rudolph Research Analytical, Inc. USA). For sur-
mond glucosidase.
face tension measurements [27], the surfactant was dissolved in
double-distilled water. Surface tension was measured at 25 °C
with a DCA315 system (Thermo-Cahn Instruments, Inc. USA).
Before the measurement, the equipment was calibrated using
double-distilled water at 25 °C.
The purity of all these alkyl glucosides was detected by
HPLC in Table
1 while propyl ꢀ-D-glucoside, butyl
ꢀ-D-glucoside, and pentyl ꢀ-D-glucoside were also character-
ized by 1H NMR as follows.
Propyl ꢀ-D-glucoside. 1H NMR (500 MHz, D2O): ꢂ
0.85–0.87 (m, 3H), 1.54–1.62 (m, 2H), 3.19–3.22 (m, 1H),
3.30–3.34 (m, 1H), 3.38–3.45 (m, 2H), 3.56–3.61 (m, 1H),
3.64–3.68 (m, 1H), 3.80–3.88 (m, 2H), 4.41 (d, J = 8.0 Hz, 1H).
Butyl ꢀ-D-glucoside. 1H NMR (500 MHz, D2O): ꢂ 0.84–0.87
(m, 3H), 1.28–1.36 (m, 2H), 1.53–1.58 (m, 2H), 3.18–3.22 (m,
1H), 3.28–3.34 (m, 1H), 3.38–3.45 (m, 2H), 3.60–3.68 (m,
2H), 3.85–3.90 (m, 2H), 4.40 (d, J = 8.0 Hz, 1H).
A plot of surface tension versus concentration was used to
determine the critical micelle concentration (cmc). The free
energy associated with the micelle formation per mole of
monomer unit (G) was evaluated according to the equation
0
[28]: ꢁGm = RTln(cmc).
2 Results and discussion
Pentyl ꢀ-D-glucoside. 1H NMR (500 MHz, D2O): ꢂ
0.83–0.85 (m, 3H), 1.27–1.29 (m, 4H), 1.57–1.58 (m, 2H),
3.19–3.22 (m, 1H), 3.31–3.35 (m, 1H), 3.38–3.45 (m, 2H),
3.60–3.68 (m, 2H), 3.85–3.89 (m, 2H), 4.40 (d, J = 8.0 Hz, 1H).
2.1 Synthesis of alkyl ꢀ-D-glucosides
Synthetic reaction profiles for the ten n-alkyl ꢀ-D-glucosides
(C1–C10) are shown in Fig. 1. Their final yields (Table 1) were
significantly affected by the alkyl chain lengths of the alcohols
that functioned as glycosidyl acceptors [29]. Longer alkyl
chains led to lower reaction rates and final yields with the
exception of the synthesis of methyl ꢀ-D-glucoside. Both the
rate and the yield for the synthesis of methyl ꢀ-D-glucoside
were lower than that of the ethyl, propyl, and butyl glucosides.
This may be due to the severe toxicity of methanol to the al-
2.2 Equilibrium between glucoside hydrolysis and reverse
hydrolysis
The yield of alkyl ꢀ-D-glucoside is controlled by the ther-
modynamic equilibrium and the biocatalyst activity [16]. To
verify the equilibrium yield of alkyl ꢀ-D-glucoside, both the
hydrolysis and reverse-hydrolysis of propyl, hexyl, and octyl
glucosides were performed (Fig. 2). On this basis, 30 mg al-
mond meal was added twice at 72 and 144 h to avoid the effect
of biocatalyst activity loss. With an increase in the alkyl chain
length the yield of n-alkyl ꢀ-D-glucosides synthesis decreased
sharply while the yield and rate of glucoside hydrolysis in-
creased greatly. The equilibrium yield did not change after the
addition of almond meal. Therefore, the final yield of alkyl
ꢀ-D-glucosides was affected not by the residual activity of the
biocatalyst but by the thermodynamic equilibrium.
70
(2)
(3)
60
50
40
30
20
10
(4)
(1)
(5)
Table 1 Thermodynamic data for the synthesis of alkyl-ꢀ-D-glucosides
Product
Yield (%)
27.6
62.5
58.7
36.1
13.7
9.17
4.57
4.05
K
1.03
ꢁGmꢂ/(kJ/mol)
–0.073
0.609
1.02
0
10
Methyl ꢀ-D-glucoside
Ethyl ꢀ-D-glucoside
Propyl ꢀ-D-glucoside
Butyl ꢀ-D-glucoside
Pentyl ꢀ-D-glucoside
Hexyl ꢀ-D-glucoside
Heptyl ꢀ-D-glucoside
Octyl ꢀ-D-glucoside
Nonyl ꢀ-D-glucoside
Decyl ꢀ-D-glucoside
(6)
(7)
0.797
0.685
0.326
0.107
0.079
0.042
0.041
0.033
0.023
8
6
4
2
0
3.01
6.01
6.82
8.51
8.58
9.20
10.1
(8)
(9)
2.92
1.93
(10)
Yield = CP/CG0, where CP is the final concentration of glucoside and CG0
is the initial concentration of glucose.
0
24
48
72
96
Time (h)
120 144 168
K is the equilibrium constants of ten n-alkyl ꢀ-D-glucosides; K =
CPCW/(CGCA), where CP is the final concentration of glucoside, CW is the
final concentration of water, CG is the final concentration of glucose, and
CA is the final concentration of n-alcohol.
Fig. 1. Progress curves for n-alkyl ꢀ-D-glucosides synthesis (C1–C10). (1)
Methyl glucoside; (2) Ethyl glucoside; (3) Propyl glucoside; (4) Butyl
glucoside; (5) Pentyl glucoside; (6) Hexyl glucoside; (7) Heptyl glucoside;
(8) Octyl glucoside; (9) Nonyl glucoside; (10) Decyl glucoside.
ꢁGmꢂ = –RTlnK, where the temperature was 50 °C.