Hydrolases in Polymer Chemistry: Chemoenzymatic Approaches to Polymeric Materials - Enzymatic Polymerisation - page 97

positive and negative charges are effectively stabilized by Asp187 and the oxyanion
hole, respectively. Then, the proton is transferred from the His224 residue to the
Ser105 alkyl oxygen while restoring the carbonyl bond of the bound substrate. As
a result, a weakly bound enzyme-product complex is formed and the free enzyme
species is regenerated after release of the reaction product.

3.2.1 Enantioselectivity of Lipases Towards the Nucleophile

The enantioselectivity of lipases towards secondary alcohols and amines is well-
documented in the literature [ 68 - 71 ]. In general, lipases display a strong -

selectivity towards secondary alcohols. In the case of CALB, for example, an E
of 10 6 has been reported in the kinetic resolution of ( rac )-1-phenylethanol [ 72 ] .
The active site of CALB contains a small cavity called the stereospecificity pocket,
which can hold a methyl- or ethyl-sized substituent. The smaller substituent of the
secondary alcohol can be placed in this stereospecificity pocket during reaction,
while the larger substituent resides in the larger cavity. The spatial orientation of
these cavities implies that

( R )

( R ) -secondary alcohols are highly preferred; the docking

of an -secondary alcohol leads to significant steric hindrance. This behavior was

empirically recognized by Kazlauskas [ 73 ] and was later confirmed on a molecular
level by Uppenberg et al. [ 74 ]. Similarly, the

( S )

-enantiomer in primary amines next
to a chiral center is also the faster reacting enantiomer.

( R )

3.2.2 Enantioselectivity of Lipases Towards the Acyl Donor

Although many publications have covered the enantioselectivity of lipases in the
deacylation step, their enantioselectivity in the acylation step (i.e., towards the acyl
donor) has received much less attention. Generally, the selectivity of lipases towards
racemic esters or acids is low to moderate [ 75 - 77 ] . Directed evolution and site-
directed mutagenesis lead to a significant increase in the selectivity of the wild-type
enzymes [ 78 - 80 ] . However, the enantiomeric ratios attained are still well below
those typically obtained in kinetic resolutions of secondary alcohols.

3.2.3 Enantioselectivity in the Ring-Opening of Substituted Lactones

Lactones are ideal substrates for preparing polymers because they incorporate both
the acyl donor as well as the nucleophile (after ring-opening) in one molecule. Intro-
ducing a substituent on the lactone ring inevitably generates a chiral center. Since
the enantioselectivity of an enzyme can be determined both in the acylation and
in the deacylation step, the question is how the position and the size of a substituent
on the lactone ring affects the selectivity. In addition, the size of the lactone ring
may play an important role because the ring conformation of the ester bond in the
lactone ring changes from cisoid (ring size < 8) to transoid (ring size > 8) [ 81 ] .