Trypsin-catalyzed direct asymmetric aldol reaction

Article abstract of DOI:10.1016/j.molcatb.2012.10.014

Unnatural catalytic activity of trypsin from porcine pancreas for direct asymmetric aldol reaction was discovered. The reactions between aromatic aldehydes and various ketones gave products in moderate yields and enantioselectivities in the presence of a

Full text of DOI:10.1016/j.molcatb.2012.10.014

Accepted Manuscript
Title: Trypsin-catalyzed direct asymmetric aldol reaction
Authors: Yan-Li Chen, Wei Li, Yan Liu, Zhi Guan, Yan-Hong
He
PII:
DOI:
Reference:
S1381-1177(12)00282-2
doi:10.1016/j.molcatb.2012.10.014
MOLCAB 2566
To appear in:
Journal of Molecular Catalysis B: Enzymatic
Received date:
Revised date:
Accepted date:
24-9-2012
20-10-2012
26-10-2012
Pleasecitethisarticleas:Y.-L. Chen, W. Li, Y. Liu, Z. Guan, Y.-H. He, Trypsin-catalyzed
direct asymmetric aldol reaction, Journal of Molecular Catalysis B: Enzymatic (2010),
doi:10.1016/j.molcatb.2012.10.014
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*Graphical Abstract (for review)
Trypsin-catalyzed direct asymmetric aldol reaction
Yan-Li Chena, Wei Lib, Yan Liub, Zhi Guana*, Yan-Hong Hea*
a School of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, China
b School of Life Science, Southwest University, Chongqing, 400715, China
Fax: +86-23-68254091; e-mail: heyh@swu.edu.cn (for Yan-Hong He); guanzhi@swu.edu.cn for (Zhi Guan)
Page 1 of 18

*Manuscript
Trypsin-catalyzed direct asymmetric aldol reaction
Yan-Li Chena, Wei Lib, Yan Liub, Zhi Guana*, Yan-Hong Hea*
a School of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, China
b School of Life Science, Southwest University, Chongqing, 400715, China
Fax: +86-23-68254091; e-mail: heyh@swu.edu.cn (for Yan-Hong He); guanzhi@swu.edu.cn for (Zhi Guan)
Abstract: Unnatural catalytic activity of trypsin from porcine pancreas for direct asymmetric
aldol reaction was discovered. The reactions between aromatic aldehydes and various ketones
gave products in moderate yields and enantioselectivities in the presence of a small amount of
water. The influences of solvent, water content, temperature, mole ratio of substrates, and enzyme
concentration were investigated. The mechanism of trypsin-catalyzed aldol reaction was discussed.
This enzymatic promiscuity widens the application of trypsin to new chemical transformations.
Keywords: enzymatic promiscuity; trypsin; aldol reaction; asymmetric synthesis; biocatalysis
1. Introduction
As one of the most powerful C-C bond forming reactions for the production of chiral
1,3-dioxygenated compounds, the asymmetric direct aldol reaction is one of the central study
issues in the field of asymmetric synthesis [1]. Since the first proline-catalyzed aldol reactions
obtained the breakthrough, there has been a blossoming of general asymmetric organocatalyzed
reactions. Though (S)-proline is readily available at low price, this merit is not a comment to all
organocatalysts, let alone ecological sustainability of organocatalysts [3,4]. On the other hand,
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enzymes as potential environmentally-friendly alternatives to chemical catalysts in organic
synthesis have attracted more and more attention recently. Since enzymes were discovered to be
active in nearly anhydrous organic media by Klibanov in the early 1980s [5], chemists and
biochemists have focused on the notion of enzymatic promiscuity, which means that one single
active site of a given enzyme can catalyze different chemical transformations of natural or
non-natural substrates [6,7]. Enzyme catalytic promiscuity largely expands the application of
biocatalysts and provides a useful approach for organic synthesis. Some elegant works on
enzymatic promiscuity have been reported [8-10], and several aldol reactions catalyzed by
promiscuous enzymes were available. Berglund and co-workers used mutant CAL-B (lipase from
Candida antarctica) to catalyze aldol additions in cyclohexane in 2003 [11]. Wang and Yu
reported the asymmetric aldol reactions catalyzed by lipase in “wet” acetone in 2008 [12], and
they also found pepsin-catalyzed aldol reactions in aqueous media in 2010 [13]. More recently,
our group found that nuclease p1 from Penicillium citrinum, alkaline protease from Bacillus
licheniformis, chymopapain from Carica papaya, acidic protease from Aspergillus usamii and
lipase from porcine pancreas could catalyze direct asymmetric aldol reactions with good to
excellent selectivities [14-18]. However, since enzymatic promiscuity is still in its exploratory
stage, no general methods are available to profile enzyme catalytic promiscuity [19]. Accordingly,
it is still a formidable and significant task to discover enzyme-catalyzed asymmetric aldol and
other reactions as many as possible. Herein, we wish to report the trypsin (from porcine pancreas)-
catalyzed asymmetric direct aldol reaction in the presence of a small amount of water.
2. Experimental section
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2.1 Materials
The trypsin from porcine pancreas was purchased from Sigma-Aldrich, Shanghai, China
(catalogue number: 93615, ~1500 U/mg). 1 U corresponds to the amount of enzyme which
increases the absorbance at 253 nm by 0.001 per minute at pH 7.6 and 25°C
(N-benzoyl-L-arginine ethyl ester, Fluka No. 12880, as substrate). Unless otherwise noted, all
reagents were purchased from commercial suppliers and used without further purification.
2.2 Analytical methods
Reactions were monitored by thin-layer chromatography (TLC) with Haiyang GF254 silica
gel plates (Qingdao Haiyang chemical industry Co Ltd, Qingdao, China) using UV light and
vanillic aldehyde as visualizing agents. Flash column chromatography was performed using
100-200 mesh silica gel at increased pressure. 1H NMR and 13C NMR spectra were recorded on
Bruker-AM 300 (300 MHz) (Bruker BioSpin AG Ltd., Beijing, China). Chemical shifts were
reported in ppm from TMS with the solvent resonance as the internal standard. Data were reported
as follows: chemical shifts (δ) in ppm, coupling constants (J) in Hz, and solvent (CDCl3). The
following abbreviations were used to indicate the multiplicity: s, singlet; d, doublet; m, multiplet;
br, broad signal. The enantiomeric excess (ee) of aldol products was determined by chiral HPLC
analysis performed using Chiralcel OD-H, Chiralpak AD-H and AS-H columns (Daicel Chiral
Technologies CO., LTD., Shanghai, China). Relative and absolute configurations of the products
1
were determined by comparison with the known H NMR, 13C NMR and chiral HPLC analysis.
All the aldol reaction products are known compounds (for details about references, please see the
Supporting Information).
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2.3 General procedure for the trypsin-catalyzed aldol reactions:
A round-bottom flask was charged with trypsin (100 mg), aldehyde (0.5 mmol), and ketone
(5 mmol), to which deionized water (0.15 mL) was introduced. The resultant mixture was stirred
at 30 °C for the specified reaction time and monitored by TLC. The reaction was terminated by
filtering the enzyme (with 40 mm buchner funnel and qualitative filter paper), and ethyl acetate
(20 mL) was employed to wash the filter paper and the residue to assure that the products were
dissolved in the filtrate. The solvents were then removed under reduced pressure. The residue was
purified by silica gel flash column chromatography with petroleum ether/ethyl acetate as eluent to
afford the product.
3. Results and Discussion
Initially, the aldol reaction of 4-nitrobenzaldehyde and cyclohexanone was chosen as the
model reaction. Since medium is one of the most important factors influencing the enzymatic
reactions, in particular, on enantioselectivity and regioselectivity of enzymes [20], we explored the
effects of various solvents on the trypsin-catalyzed model aldol reaction. As shown in Table 1, the
reaction medium played an important role in this enzymatic reaction. The best enantioselectivity
of 76 % ee was obtained with a low yield of 15 % in CH2Cl2 (Table 1, entry 1). The best yield of
51 % was received in DMF with 50 % ee (Table 1, entry 13). The reaction gave the product in a
yield of 41 % with 61 % ee under organic solvent-free conditions in the presence of a small
amount of water (0.10 mL) (Table 1, entry 3). However, when 1.0 mL of water was used as a
solvent, only 15 % yield was obtained with 61 % ee (Table 1, entry 4). Thus, considering both
enantioselectivity and yield of the reaction, organic solvent-free conditions (with a small amount
4
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of water (0.10 mL)) was selected for the further study.
To verify the specific catalytic effect of trypsin on the model aldol reaction, some control
experiments were performed (Table 1, entries 15-21). In the absence of enzyme, the reaction only
gave a trace amount of aldol product (Table 1, entry 15). When non-enzyme protein bovine serum
albumin was used as a catalyst instead of trypsin, the aldol product was obtained in a yield of
68 % with 0 % ee (Table 1, entry 16), indicating that a non-enzyme protein also could catalyze
aldol reaction, but it did not have any enantioselectivity for the aldol product. Moreover,
urea-denatured trypsin was used to catalyse the model reaction, which only gave the product in
15 % yield with 0 % ee (Table 1, entry 17). To exclude the effect of extra urea on the reaction, the
urea-pretreated trypsin was dialysed against water to remove the extra urea, and used to catalyze
the reaction, which gave product in 10% yield with 24% ee (Table 1, entry 18). Meanwhile, urea
alone was used to catalyze the reaction; it only produced the product in 4% yield, proving that
urea nearly did not catalyse this transformation (Table 1, entry 19). These results demonstrated
that the urea-denatured trypsin lost its most activity and selectivity towards aldol reaction. Thus, it
can be inferred that the tertiary structure of trypsin is necessary for this reaction. Furthermore,
when the reaction was performed with the trypsin inhibited by PMSF (phenylmethanesulfonyl
fluoride, an irreversible serine protease inhibitor), only a trace amount of product was detected
(Table 1, entry 20), which demonstrated that the reaction took place at the active site of trypsin.
Since the trypsin we used is a commercial enzyme preparation containing impurities, to
further rule out the possibility of the catalysis of some impure proteins or other impurities, we
5
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purified the enzyme (for the details of enzyme purification, please see the Supporting Information).
From SDS-PAGE of the enzyme preparation (Figure 1), it can be seen that trypsin appeared as a
clear band, but some protein impurities were also observed. After purification, only one band was
obtained, which had the same molecular mass as trypsin (Figure 1). The protein solution was
desalted and then lyophilized to give purified trypsin as white solid. The purified trypsin was used
to catalyze the model aldol reaction under organic solvent-free conditions in the presence of a
small amount of water (0.10 mL). The experiment with only 35 mg of purified trypsin gave aldol
adduct in 41 % yield with 50 % ee (Table 1, entry 21), which was almost 3 times more active than
commercial enzyme (as described above, 100 mg of commercial enzyme preparation of trypsin
gave the product in a yield of 41 % with 61% ee (Table 1, entry 3)). The experiment with purified
trypsin clearly confirmed that trypsin indeed has the promiscuity for the catalysis of asymmetric
direct aldol reaction. Therefore, we continued using the commercially available enzyme
preparation of trypsin in the following investigation.
Table 1 Solvent screening for the trypsin-catalyzed aldol reaction and control experiments a
Entry
Solvent
CH2Cl2
Yield (%) b
dr (anti:syn) c
67:33
ee % (anti) c
1
2
3
4
5
6
7
8
15
14
41
15
13
26
16
40
76
69
61
61
59
57
51
55
i-PrOH
organic solvent-free d
70:30
64:36
H2O
71:29
MeCN
68:32
cyclohexane
CHCl3
70:30
57:43
EtOH
72:28
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9
dioxane
THF
14
25
56:44
64:36
57:43
65:35
66:34
62:38
--
54
53
51
51
50
40
--
10
11
12
13
14
15
16
17
n-hexane
14
MTBE
18
DMF
51
DMSO
40
organic solvent-free (no enzyme) e
organic solvent-free (bovine serum albumin) f
organic solvent-free (urea-denatured trypsin) g
organic solvent-free (urea-denatured trypsin
with extra urea removed) h
organic solvent-free (urea) i
organic solvent-free (PMSF-inhibited trypsin) j
organic solvent-free (purified trypsin) k
trace
68
53:47
60:40
0
15
0
18
10
4
51:49
44:56
--
24
0
19
20
21
trace
41
--
66:34
50
a Reaction conditions (except the reactions under organic solvent-free conditions): 4-nitrobenzaldehyde (0.5 mmol),
cyclohexanone (2.5 mmol), trypsin (100 mg), deionized water (0.10 mL) and solvent (0.90 mL) at 25 ˚C for 143 h.
b Yield of the isolated product after silica gel chromatography.
c Determined by chiral HPLC analysis using a Chiralpak AD-H column [21].
d
Reaction conditions: 4-nitrobenzaldehyde (0.5 mmol), cyclohexanone (5 mmol), deionized water (0.10 mL) and
trypsin (100 mg) at 25 ˚C for 143 h.
e Reaction conditions: 4-nitrobenzaldehyde (0.5 mmol), cyclohexanone (5 mmol), deionized water (0.10 mL) at 25
˚C for 143 h.
f Bovine serum albumin (100 mg) was used instead of trypsin under organic solvent-free conditions.
g
The trypsin was pre-treated with urea (100 mg) in water (3 ml) at 100 ˚C for 24 h, and the water was removed
under reduced pressure before use.
h
The trypsin was pre-treated with urea (100 mg) in water (3 ml) at 100 ˚C for 24 h; extra urea was removed by
dialysis against water, and then water was removed under reduced pressure before use.
i Urea (100 mg) was used instead of trypsin under organic solvent-free conditions.
j The trypsin was pre-treated with PMSF (100 mg) in anhydrous MeCN (3 ml) at 25 ˚C for 24 h, and MeCN was
7
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removed under reduced pressure before use.
k
Reaction conditions: purified trypsin (35 mg), 4-nitrobenzaldehyde (0.5 mmol), cyclohexanone (5 mmol),
deionized water (0.10 mL) at 25 ˚C for 144 h.
Enzymes require a specific amount of water bound to them to maintain activities, and water
content affects both the stereoselectivity and activity of enzymes [22,23]. Therefore, to confirm the
optimal water content in the reaction system, the trypsin-catalyzed model aldol reaction was
carried out with different amounts of water under organic solvent-free conditions. As shown in
Table 2, in the absence of water, the reaction gave the product in a relatively low yield of 40%
with 54% ee and 64:36 dr (Table 2, entry 1). When the ratios of enzyme to water were 1:1.5 and
1:2.0 (enzyme/water, w/w), the reaction afforded the product in the best yield of 51 % with
slightly better selectivity (Table 2, entries 4 and 5). Further increase of water caused a decrease of
yield. Thus, we chose the ratio of enzyme to water 1:1.5 as the optimal condition for the following
studies.
Table 2 Effect of water contents on the trypsin-catalyzed aldol reaction a
Entry
enzyme : water (w/w)
Yield (%) b
dr (anti:syn) c
64:36
ee % (anti) c
1
2
3
4
5
6
7
1:0
40
43
46
51
51
50
46
54
57
59
58
56
58
58
1:0.5
1:1.0
1:1.5
1:2.0
1:2.5
1:3.0
67:33
71:29
73:27
73:27
74:26
74:26
a Reaction conditions: 4-nitrobenzaldehyde (0.5 mmol), cyclohexanone (5 mmol), trypsin (100 mg) and deionized
water (0-300 mg, water/enzyme (w/w) = 0-3.0) at 25 ˚C for 144 h.
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b Yield of the isolated product after silica gel chromatography.
c Determined by chiral HPLC analysis using a Chiralpak AD-H column [21].
Temperature has effects on enzymatic reaction rates, and the selectivity and stability of
enzymes, so the influence of temperature on the trypsin-catalyzed model aldol reaction was
investigated (Table 3). The increase of temperature from 10 °C to 40 °C led to a rise in the yield
(from 18 % to 69 %) but a decrease in the diastereoselectivity (from 89:11 to 59:41) and
enantioselectivity (from 84% ee to 42% ee) (Table 3, entries 1-6). As a compromise between yield
and selectivity, we chose 30 °C for further studies.
Table 3 Effect of temperature on the trypsin-catalyzed aldol reaction a
Entry
Temp. (˚C)
Time (h)
168
Yield (%) b
dr (anti:syn) c
89:11
ee % (anti) c
1
2
3
4
5
6
10
20
25
30
35
40
18
41
49
60
63
69
84
62
60
59
54
42
144
78:22
144
74:26
144
73:27
144
67:33
144
59:41
a
Reaction conditions: 4-nitrobenzaldehyde (0.5 mmol), cyclohexanone (5 mmol), trypsin (100 mg), and deionized
water (0.15 mL) at temperature (10-40 ˚C)
b Yield of the isolated product after silica gel chromatography.
c Determined by chiral HPLC analysis using a Chiralpak AD-H column [21].
Molar ratio of substrates and enzyme loading are also important factors in enzymatic reactions,
and their influence on the trypsin-catalyzed model aldol reaction was investigated next. When the
molar ratio of 4-nitrobenzaldehyde to cyclohexanone was 1:10, the best yield of 46 % was obtained.
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Higher ratio led to a slight decrease of the yield. Molar ratio of substrates nearly had no effect on
the selectivity of the reaction. Thus, a molar ratio of 1:10 (4-nitrobenzaldehyde to cyclohexanone)
was selected as the optimal condition. The influence of catalyst loading on the aldol reaction
between 4-nitrobenzaldehyde (0.5 mmol) and cyclohexanone (5 mmol) showed that an increase of
trypsin loading (from 25 mg to 150 mg) led to a rise in the yield (from 23% to 55%) with a slight
increase of selectivity (from 68:32 to 73:27 dr, and from 50% to 60% ee). When the dosage
surpassed 100 mg, no increase of the ee and dr values was observed. So we chose enzyme loading
of 100 mg as the optimal condition. Finally, the time course of the trypsin-catalyzed model aldol
reaction was also investigated. The yield increased as the reaction time prolonged and reached the
highest (59%) at 144 h, while the ee value reached the peak (61% ee) at 120 h and then decreased
(for details of molar ratio, enzyme loading and time course, please see the Supporting Information).
To investigate the generality and scope of this biocatalytic promiscuity, several other
substrates were tested to expand upon trypsin-catalyzed asymmetric direct aldol reaction (Table 4,
entries 1-12). Generally, trypsin showed the ability to tolerate five-, six- cyclic ketones and
acetone as aldol donors. The effect of substituents on benzaldehyde had a great impact on both the
yield and selectivity of the trypsin-catalyzed aldol reactions. For instance, reactions of the
aromatic aldehydes with electron-withdrawing substituents provided products in higher yields
(Table 4, entries 1-9) than those with electron-donating substituents (Table 4, entry 10).
4-Nitrobenzaldehyde (Table 4, entry 1) gave a better yield, and better dr and ee values than
4-halogen aromatic aldhydes (Table 4, entries 4-6) when reacting with cyclohexanone. Moreover,
the reaction of 4-(trifluoromethyl)benzaldehyde with cyclohexanone gave the best ee value of
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65 % with a low yield of 34 % (Table 4, entry 7). However, the best diastereoselectivity of 89:11
was achieved by using the most sterically hindered substrate 2,6-dichlorobenzaldehyde (Table 4,
entry 9). Besides, when reacting with 4-nitrobenzaldehyde, the moderate yield and ee value were
received by using cyclopentanone (Table 4, entry 11), but only low yield and poor ee value were
gained by using acetone as an aldol donor (Table 4, entry 12). Surprisingly, when reacting with
cyclohexanone, 4-tolualdehyde gave diastereoselectivity for syn-isomer (Table 4, entry 10).
Otherwise, trypsin displayed different degrees of diastereoselectivities for anti-isomers. Most of
the reactions we investigated showed enantioselectivities for anti-isomers, but low or no
enantioselectivities for syn-isomers, potentially indicating that the catalytic site of trypsin had a
specific selectivity for the aldol reaction.
Table 4 Substrate scope of the trypsin-catalyzed direct asymmetric aldol reactions a
Entry
1
R1 R2
R3
Prod.
3a
3b
3c
3d
3e
3f
Time (h)
144
192
162
192
192
192
144
192
192
187
71
Yield (%) b
dr (anti:syn) c
73:27
70:30
55:45
52:48
50:50
58:42
59:41
60:40
89:11
11:89
62:38
---
ee % (anti) c
-(CH2)4- 4-NO2C6H4
-(CH2)4- 3-NO2C6H4
60
58
47
14
19
19
34
37
58
7
60
51
2
3
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
4-CNC6H4
4-FC6H4
54
4
33
5
4-ClC6H4
4-BrC6H4
4-CF3C6H4
48
6
55
7
65
3g
3h
3i
8
-(CH2)4- 2,4-Cl2C6H3
-(CH2)4- 2,6-Cl2C6H3
40
9
33
10
11
12
-(CH2)4-
-(CH2)3- 4-NO2C6H4
Me, H 4-NO2C6H4
4-MeC6H4
45 (7) d
3j
47
28
42
3k
3l
46
16
11
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a Reaction conditions: aldehyde (0.5 mmol), ketone (5 mmol), trypsin (100 mg) and deionized water (0.15 mL) at 30
˚C.
b Yield of the isolated product after silica gel chromatography.
c Determined by chiral HPLC analysis (for details, please see the Supporting Information).
d anti (45% ee), syn (7% ee)
Finally, we attempted to propose the mechanism of the trypsin-catalyzed aldol reaction. As
one of the important serine proteases, the active center of trypsin is the catalytic triad composed of
histidine-57, aspartate-102, and serine-195 [24]. In addition, trypsin contains an "oxyanion hole"
formed by the backbone amide hydrogen atoms of Gly-193 and Ser-195, in the native activity of
trypsin, which serves to stabilize the developing negative charge on the carbonyl oxygen atom of
the cleaved amides [25,26]. Based on the mechanism proposed by Berglund [11], we hypothesized
a tentative mechanism of trypsin-catalyzed aldol reaction of aldehyde with cyclohexanone shown
in Scheme 1. Firstly, the Asp-His dyad and the oxyanion hole in the active site stabilizes the
substrate cyclohexanone. Secondly, a proton is transferred from the cyclohexanone to the His
residue and enolate ion is formed. Thirdly, another substrate aldehyde accepts the proton from
imidazolium cation and combines the cyclohexanone forming a new carbon-carbon bond.
Eventually, the product is released from the oxyanion hole.
12
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Scheme 1 Proposed mechanism for the trypsin-catalyzed aldol reaction of aldehyde with cyclohexanone
4. Conclusion
In summary, we reported the trypsin-catalyzed asymmetric direct aldol reactions under
organic solvent-free conditions in the presence of a small amount of water. This was a safe,
economical, and environmentally benign method to synthesize aldol products with a wide range of
substrates. It is interesting that trypsin from porcine pancreas possesses the function of aldolase
under the present conditions. This methodology also expanded the application of trypsin in the
asymmetric reaction.
Acknowledgements
Financial support from 2011 Select Project in Scientific and Technological Activities for Returned
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Scholars of Chongqing Personnel Bureau, and the Doctoral Foundation of Southwest University
(SWU112019) is gratefully acknowledged.
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Fig. 1 SDS-PAGE analysis of trypsin
A: Premixed Protein Marker (Low)
B,C: Purified protein (23.8 kDa)
D: Trypsin preparation
16
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*Research Highlights
Trypsin-catalyzed direct asymmetric aldol reaction
Yan-Li Chena, Wei Lib, Yan Liub, Zhi Guana*, Yan-Hong Hea*
a School of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, China
b School of Life Science, Southwest University, Chongqing, 400715, China
Fax: +86-23-68254091; e-mail: heyh@swu.edu.cn (for Yan-Hong He); guanzhi@swu.edu.cn for (Zhi Guan)
Highlights
► Unnatural catalytic activity of trypsin from porcine pancreas for direct asymmetric aldol
reaction was discovered. ► Mechanism of trypsin-catalyzed aldol reaction was hypothesized.►
This enzymatic promiscuity widens the application of trypsin to new chemical transformations.
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