Page 236 - Advances in Textile Biotechnology

Functionalisation of wool and silk fi bres using enzymes 217

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band at 1615 cm , assigned to νC=C modes (where ν indicates stretching)
of both aromatic amino acids and conjugated dienes, increased in intensity,

confirming the formation of quinone species and their tautomers. The trend
of the Raman I 852 /I 830 intensity ratio, which increased upon oxidation, indi-
cated a shift towards a more hydrophobic environment for non-oxidised

Tyr residues, confirming that only those more exposed to the solvent and
more accessible to the enzyme were oxidised. Broadening of FTIR band at
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1650 cm (νC=O mode) confirmed the presence of Tyr oxidation products.
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Moreover, the appearance of a band at 1045 cm (νCO mode of conjugated
secondary alcohols) disclosed the formation of a certain amount of the
quinone methide tautomer.
In a second stage of the study, the efficiency of the tyrosinase-mediated

grafting of chitosan onto silk proteins was investigated (Anghileri et al.,
2007; Freddi et al., 2006; Sampaio et al., 2005). pH had a strong effect on the
yield of grafting. A pH value close to 7 (i.e. more favourable to tyrosinase

activity) allowed a higher grafting yield, confirming that the determining
step of the whole grafting reaction is the formation of the o-quinone species.
FTIR spectra of chitosan-grafted silk fibroin and sericin showed a signifi -

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cant increase in intensity of the bands near 1400 and 1000 cm , where
chitosan absorptions fall. Accordingly, the Raman spectra also showed
several bands ascribable to chitosan. Although the mechanism of the non-
enzymatic grafting step is still poorly understood, some spectroscopic data
seem to support the occurrence of a Schiff base reaction. In fact, the pres-
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ence of a shoulder at about 1646 and 1650 cm in Raman and FTIR spectra,
respectively, may be attributed to the imine group, whose νC=N mode falls
in this region. However, the Michael addition-type mechanism cannot be
completely excluded, as indicated by the trend of the νNH FTIR region,
where the grafted samples showed higher wavenumbers broadening and
the presence of a single νNH band. These spectral features are consistent
with the conversion of some primary amino groups of chitosan into second-
ary amino groups (according to the Michael addition-type pathway).
The yield of grafting was dependent on the extent of oxidation and fol-

lowed the order: sericin/fibroin aqueous solution >> hydrated gel > dried

gel >> mechanically generated fibroin powder. Grafting onto silk fi bres did
not occur, or occurred at an undetectable level. The results reported on the
silk proteins/chitosan model system demonstrate the potential of the tyros-
inase-initiated grafting for the formation of novel biopolymer conjugates
suitable for a range of applications (e.g. textile, coating, packaging, bio-

medical). Grafting chitosan onto sericin or fibroin polypeptides may com-
plement the outstanding properties of the polysaccharide (antimicrobial
activity) with those of the silk proteins (antioxidant, UV-resistant, moist-
urising properties of sericin; strength, biocompatibility, biodegradability of
fibroin), resulting in the production of valuable bio-based polymers under

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