ChemComm
Communication
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from 15.6 to 5.5 U mg only after five cycles though the enantio-
selectivity remained unchanged (99%) during the process. Enzyme
leaching and enzyme denaturation are generally regarded as the two
main reasons accounting for the decreased catalytic efficiency of
immobilized enzyme in reusability assay. In each cycle, the PCL
content in the water phase of the broken emulsion after centrifuga-
tion was detected and the results suggested that enzyme leaching can
be effectively inhibited owing to the silylation treatment as discussed
above (Fig. S9, ESI†). Therefore, in our case enzyme denaturation is
probably the principal factor resulting in the low reusability. The
homogenizing process performed at 8000 rpm for 1 min to prepare a
Pickering emulsion could cause notable PCL inactivation considering
that most enzyme proteins including PCL are shear sensitive. More-
Fig. 2 (A) Photographs of (left) 200 mg of PCL–FDU12–HMDS introduced into a
biphasic system consisting of water (1.5 mL) and hexane (1.5 mL), and (right) over, the slightly smaller pore entrance of FDU-12 in comparison with
further homogenized using an Ultra Turrax homogenizer at 8000 rpm for 1 min. the molecular dimension of PCL might make the PCL proteins
(
B) and (C) Optical micrographs of the Pickering emulsion stabilized by PCL–FDU12–
undergo some conformational changes during translocation into
the FDU-12 interior. The resultant non-native protein conformation
could also lead to the tertiary structure destabilization of PCL.
In conclusion, through a facile silylation procedure using HMDS
as the silylation agent, the encapsulation of PCL into the nanocages
of FDU-12 and the amphiphilic surface modification of FDU-12 can
HMDS particles. The inset shows a photograph of the broken Pickering emulsion
after centrifugation. (D) Specific activities of free PCL in a regular biphasic water–
hexane system and PCL–FDU12–HMDS in a Pickering emulsion system in kinetic
resolution of (R,S)-1-phenylethanol with vinyl acetate as the acyl donor.
10
that in our research the water-in-oil (w/o) Pickering emulsion be accomplished at the same time. This protocol is beneficial for
was created. the construction of the Pickering emulsion stabilized by modified
To study the utilization of the Pickering emulsion in biphasic bio- FDU-12 particles hosting high-performance PCL in the nanocages
catalysis, the PCL-catalyzed kinetic resolution of (R,S)-1-phenylethanol with invisible protein leaching. Compared to the biphasic system,
was again chosen as the model reaction. Following the afore- the Pickering emulsion fabricated in our research showed much
mentioned approach, a Pickering emulsion system is emulsified enhanced catalytic performance probably profiting from the substan-
wherein PCL–FDU12–HMDS particles adsorbed at the w/o interface tially increased area of the water–oil interface, and allowed simple
with phosphate buffer solution as the water phase and hexane product separation as well as recycling of the enzyme-loaded stabi-
containing substrates as the oil phase. The PCL proteins encapsu- lizer. The versatility of such an emulsion system and its utilization in
lated in the nanocages of the stabilizer were responsible for trans- more typical biocatalytic reactions suffering the immiscibility between
forming the substrates to the reaction products. The reaction took substrates can be envisioned.
place for 10 min and then was stopped by centrifugation to cause
This work was supported by the National Natural Science
macroscopic phase separation. Three phases could be observed in Foundation of China (21232008, 21273226).
the broken emulsion, the top phase was hexane comprising the
products and unreacted substrates, the middle phase was buffer Notes and references
solution and the bottom phase was the PCL–FDU12–HMDS solid
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À1
1
.1 U mg (Fig. 1D). This result indicates that the Pickering
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1
activity of PCL encapsulated in the nanocages of FDU-12 declined
This journal is c The Royal Society of Chemistry 2013
Chem. Commun.