Surface Modification of Cellulose Acetate with Cutinase and ...

Subchapter 2.5
The acetic acid release was not directly proportional to all the tested enzyme
concentrations as it would be expected (Tipton, 2002; Lee and Fan, 1982). The
observed upward curvature could be caused by several factors like inadequate
sensitivity of the method used to quantify the acetic acid and/or, most probably, by a
very slow enzyme reaction. The higher level of released acetic acid from the less
substituted cellulose acetate was according to the irreversible relation between the
degree of substitution and the degree of bio-deacetylation (Samios et al., 1997; Altaner
et al., 2001, 2003b; Moriyoshi et al., 1999, 2002). Steric hindrance and crystallinity are
considered important factors in the adsorption and mostly in the effectiveness of the
adsorbed enzyme to promote the hydrolysis (Lee and Fan, 1982). These factors should
favour CDA over CTA. At the maximum concentration used, the enzyme activity was
0.17 nkat and 0.12 nkat (nmol/s of acetic acid) while only 0.54% and 0.36% of the
acetyl groups were released from CDA and CTA, respectively. These values were
obtained considering a DS 2.4 for CDA and a minimum DS 2.7 for CTA commercial
fibres (Steinmann, 1998; Zugenmaier, 2004). A very low yield in deacetylation is not
uncommon for highly substituted cellulose acetates treated with cell-free enzymes (Puls
et al., 2004). By comparison, in view of the fact that at least one of the cellulose acetate
used has higher DS, cutinase showed potential as cellulose acetate esterase. Altaner et al
(2001) reported that acetylesterases from 13 different commercial origins could
significantly use cellulose acetates with DS ≤1.4 as substrates. Only one enzyme from
Humicola insolens was able to release a small amount (10%) of acetyl groups from a
cellulose acetate DS 1.8, after 220 hours. Another enzyme purified from a commercial
preparation, derived from Aspergillus niger, was able to hydrolyse 5% of the existing
acetyl groups on a cellulose acetate DS 1.8 after 140 hours (Altaner et al., 2003b).
Considering the values found in the literature, the percentage of acetic acid released
obtained with cutinase was not insignificant having in consideration that the final
purpose of this modification is not biodegradation of the substrate, but the modification
of the fibre surface. The amount of cutinase was a limiting factor, therefore amounts of
enzyme used in subsequent treatments were ≤50 U mL -1 .
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