L-enantioselective oligomerization of alkyl D- and L-lactates (RDLa and RLLa), in
contrast to the lipase (Novozym 435)-catalyzed perfect D-enantioselective reaction of
Scheme 7. The four proteases examined preferentially gave oligo(L-lactic acid)s
(oligoLLAs; dimer ~ pentamer), with moderate to high yields. The enantioselection
was L-/D-selective (56/28 to 25/4 in conversion % ratio), showing an opposite direction
in enantioselection to that of the lipase [54].
Hydrolysis reaction of ethyl D- and L-lactates (EtLa)s catalyzed by protease were
studied; EtLLa was consumed a little faster than EtDLa. The mechanism of the
protease-catalyzed oligomerization was similar to that of lipase (as seen in Figs. 3
and 4), but in an L-selective manner; the enantioselection is governed by the
deacylation step.
The opposite enantioselection of enzymatic catalysis by protease and lipase has
been discussed in the case of PLA depolymerizing hydrolysis [58]. These two
classes of enzymes are both serine hydrolases, possessing a catalytic triad of serine,
histidine, and aspartic acid; the catalytic active site of the two classes, however,
are topological mirror images [59–61]. This difference in the catalytic sites
was considered responsible for the opposite selection, where protease was
PLLA-preferential and was PDLA-specific [53, 54, 58]. The results of
enantioselective oligomerization of alkyl lactates catalyzed by protease and lipase,
therefore, may be similarly understood. The enantioselection of Novozym 435 was
perfect, and lipases of other origin were not so strong. Proteases were less selective.
This selectivity difference is probably because in living systems the substrate of
lipase is an ester having an ester linkage like that of RLa, whereas the substrate of
protease is a protein having an amide linkage.
3.3 Lipase-Catalyzed Degradation and Polymerization
of Polyesters: New Method of Polymer Recycling
Using the characteristics of lipase catalysis, a new method of polymer chemical
recycling was proposed [8]. The polyester samples used were poly(ε-caprolactone)
(PCL), poly(12-docecanolide) (PDDL), and poly(1,4-butane adipate) (PBA). First,
lipase CA-catalyzed degradation of PCL with molecular weight 6.0 Â 10
4 at 60
C
was performed in toluene. After 24 h, PCL almost disappeared via hydrolysis to
give oligoCL with molecular weight of less than 500. A small amount of water in
the reaction mixture is probably involved in the hydrolysis. The solvent was then
removed under reduced pressure to give a waxy oligomer mixture containing lipase
CA. The mixture was then kept at 60
C for 8 h, yielding a polymer with molecular
weight 8 Â 10
3 .
The cycle of degradation–polymerization could be performed repeatedly and
controlled by the presence or absence of the solvent, using the same catalyst in one
pot. This method provided a concept for an environmentally benign process of
Green Polymer Chemistry: Recent Developments
159
contrast to the lipase (Novozym 435)-catalyzed perfect D-enantioselective reaction of
Scheme 7. The four proteases examined preferentially gave oligo(L-lactic acid)s
(oligoLLAs; dimer ~ pentamer), with moderate to high yields. The enantioselection
was L-/D-selective (56/28 to 25/4 in conversion % ratio), showing an opposite direction
in enantioselection to that of the lipase [54].
Hydrolysis reaction of ethyl D- and L-lactates (EtLa)s catalyzed by protease were
studied; EtLLa was consumed a little faster than EtDLa. The mechanism of the
protease-catalyzed oligomerization was similar to that of lipase (as seen in Figs. 3
and 4), but in an L-selective manner; the enantioselection is governed by the
deacylation step.
The opposite enantioselection of enzymatic catalysis by protease and lipase has
been discussed in the case of PLA depolymerizing hydrolysis [58]. These two
classes of enzymes are both serine hydrolases, possessing a catalytic triad of serine,
histidine, and aspartic acid; the catalytic active site of the two classes, however,
are topological mirror images [59–61]. This difference in the catalytic sites
was considered responsible for the opposite selection, where protease was
PLLA-preferential and was PDLA-specific [53, 54, 58]. The results of
enantioselective oligomerization of alkyl lactates catalyzed by protease and lipase,
therefore, may be similarly understood. The enantioselection of Novozym 435 was
perfect, and lipases of other origin were not so strong. Proteases were less selective.
This selectivity difference is probably because in living systems the substrate of
lipase is an ester having an ester linkage like that of RLa, whereas the substrate of
protease is a protein having an amide linkage.
3.3 Lipase-Catalyzed Degradation and Polymerization
of Polyesters: New Method of Polymer Recycling
Using the characteristics of lipase catalysis, a new method of polymer chemical
recycling was proposed [8]. The polyester samples used were poly(ε-caprolactone)
(PCL), poly(12-docecanolide) (PDDL), and poly(1,4-butane adipate) (PBA). First,
lipase CA-catalyzed degradation of PCL with molecular weight 6.0 Â 10
4 at 60
C
was performed in toluene. After 24 h, PCL almost disappeared via hydrolysis to
give oligoCL with molecular weight of less than 500. A small amount of water in
the reaction mixture is probably involved in the hydrolysis. The solvent was then
removed under reduced pressure to give a waxy oligomer mixture containing lipase
CA. The mixture was then kept at 60
C for 8 h, yielding a polymer with molecular
weight 8 Â 10
3 .
The cycle of degradation–polymerization could be performed repeatedly and
controlled by the presence or absence of the solvent, using the same catalyst in one
pot. This method provided a concept for an environmentally benign process of
Green Polymer Chemistry: Recent Developments
159