Hydrolases in Polymer Chemistry: Part III: Synthesis and Limited Surface Hydrolysis of Polyesters and Other Polymers - Enzymatic Polymerisation - page 121

analysis of the release of polyacrylic acid and on microscopic inspection, bacteria
such as Micrococcus luteus were demonstrated to degrade PAN [ 5 ].
Generally, enzymatic hydrolysis of nitriles to the corresponding acids can either
proceed stepwise, which is the case for catalysis by the nitrile hydratase/amidase
enzyme system, or in one step in the case of nitrilases. Both systems have been
investigated for surface hydrolysis of PAN [ 10 ] . Complete hydrolysis with either
system was monitored by quantification of ammonia and/or polyacrylic acid formed
as a consequence of hydrolysis of nitrile groups [ 70 - 72 ] . As a result, considerable
increases in colour levels (e.g. 156% for commercial nitrilase) were found upon
dyeing [ 72 ] .
XPS analysis and FTIR were used to quantify chemical changes on the surface
of PAN materials [ 5 , 16 ]. For PAN treated with nitrilases from Arthrobacter sp.
and Agrobacterium tumefaciens , increases of the O/C ratio of 60-80%, respectively,
were measured with XPS [ 17 ]. The conversion of nitrile groups into amide groups
was demonstrated on the basis of formation of new bands at 1649 and 1529 cm − 1
in FTIR analysis [ 16 ] . Generally, as with PET and PA, lower hydrolysis rates were
measured for crystalline PAN [ 5 , 16 ]. Interestingly, depending on the enzyme sys-
tem used, either hydrolysis to the corresponding amides or further hydrolysis to
the acids can be achieved. Nitrile hydratases from Rhodococcus rhodochrous , Bre-
vibacterium imperiale and Corynebacterium nitrilophilus lead to the corresponding
amides, whereas further hydrolysis by the amidase seems to be slower [ 16 , 71 , 73 ] .
Apart from nitrile-hydrolyzing enzymes, some esterases and cutinases have been
used for surface hydrolysis of PAN [ 74 ] . These enzyme were shown to specifically
hydrolyse vinyl acetate moieties present as co-monomer in many commercial PAN
materials, with no changes in crystallinity as determined by X-ray diffraction [ 74 ] .
Summarising the recent progress in enzymatic surface hydrolysis of PA, PET
and PAN, recent detailed mechanistic studies using distinct analytical tools have
contributed considerably to our mechanistic understanding of the enzyme action
of these polymers. Investigations of structure-function relationships and enzyme
homologies yield important information as a basis for enzyme engineering and
screening for more efficient enzymes.
Acknowledgement The work was financed by the SFG, the FFG, the city of Graz and the province
of Styria within the MacroFun project and supported by the European COST868 program.
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