Hydrolases in Polymer Chemistry: Part III: Synthesis and Limited Surface Hydrolysis of Polyesters and Other Polymers - Enzymatic Polymerisation - page 119

Fig. 2 Electrostatic surface rendering of Aspergillus oryzae ( left )and Fusarium solani ( right )
cutinases. The solid density illustrates the groove on the surface proximal to the active site [ 53 ]

activity. Unlike cutinases, lipases are “interfacially activated” in the presence of a
water-lipid interface [ 61 - 63 ] . Consequently PET hydrolase activity was enhanced
in the presence of detergents [ 2 , 64 ].
Several studies have reported on hydrophilicity increases of PET after limited
enzyme hydrolysis [ 14 , 46 , 47 , 54 , 65 ] . For instance, hydrolysis of PET fabrics
with cutinases or lipases resulted in a wetting time of around 100 s compared to
45 min for the untreated material [ 1 ] and in considerable decreases of the WCA
[ 14 ]. Derivatisation with BrNP [ 14 ] or sulphobenzoic acid anhydride [ 11 ] indicated
a concomitant increase in the number of carboxyl and hydroxyl groups, respec-
tively. XPS analysis yielded the same message, based on broader carbon peaks
after enzyme treatment of PET [ 2 , 13 ]. Likewise, enzyme hydrolysis of PET was
followed by using dye-binding assays, with basic dyes resulting in colour shade in-
creases of up to 200% (according to the Kubelka Monk theory) [ 1 , 54 , 60 ] . Limited
surface hydrolysis did not reveal morphological changes according to SEM inspec-
tions, except for prolonged incubation of low crystallinity PET (Fig. 3 ) . In contrast
to the considerable weight losses ( 6% for 1 M NaOH) and crater-like structures

(Fig. 3 ) reported after alkaline hydrolysis, most authors reported only insignificant
weight losses (

>

1%) for enzyme hydrolysis yet obtained similar hydrophilicity in-
creases [ 1 , 14 ]. Obviously, this is due to the fact that (successful) enzymes hydrolyse
PET endo -wise. Only recently was this demonstrated for PET ( M W

=

3500) by us-
ing MALDI-TOF MS, whereas a different behaviour for cutinases and lipases was
seen [ 2 ]. Nevertheless, in several studies the release of mono- and oligomeric reac-
tion products from PET hydrolysis was demonstrated [ 13 , 45 - 47 , 49 , 54 , 55 , 58 ] .
For example, a lipase from T. lanuginosus released higher amounts of mono(2-
hydroxyethyl) terephthalate (MHET) than of terephthalic acid (TA), whereas the
amounts of TA and MHET were similar in the case of a cutinase from T. fusca .
Small amounts of bis(2-hydroxyethyl) terephthalate (BHET) were detected for both
enzymes [ 1 ].