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To investigate how solubility and steric issues affect the laccase-catalysedoxidationof phenols, a series of oligomeric polyphenol compounds, having increasing size and decreasing solubility in water, was incubated with laccase. The extent of substrate conversion, and the nature of the products formed in buffered aqueous solutions, were com-pared to those obtained in the presence of an organic cosolvent, and also in the presence of twomediating species, i.e. N-hydroxyphthalimide (HPI) and 2,2,6,6-tetramethyl-piperidin-1-yloxy (TEMPO). . | Eur. J. Biochem. 269 5330-5335 2002 FEBS 2002 doi 10.1046 j.1432-1033.2002.03256.x Oxidation of phenols by laccase and laccase-mediator systems Solubility and steric issues Francesca d Acunzo Carlo Galli and Bernardo Masci Dipartimento di Chimica and Centro CNR Meccanismi di Reazione Universita La Sapienza 00185 Roma Italy To investigate how solubility and steric issues affect the laccase-catalysed oxidation of phenols a series of oligomeric polyphenol compounds having increasing size and decreasing solubility in water was incubated with laccase. The extent of substrate conversion and the nature of the products formed in buffered aqueous solutions were compared to those obtained in the presence of an organic cosolvent and also in the presence of two mediating species i.e. N-hydroxyphthalimide HPI and 2 2 6 6-tetramethyl-piperidin-1-yloxy TEMPO . This approach showed not only an obvious role of solubility but also a significant role of the dimension of the substrate upon the enzymatic reactivity. In fact reactivity decreases as substrate size increases even when solubility is enhanced by a cosolvent. This effect may be ascribed to limited accessibility of encumbered substrates to the enzyme active site and can be compensated through the use of the appropriate mediator. While TEMPO was highly efficient at enhancing the reactivity of large less soluble substrates HPI proved less effective. In addition whereas the laccase HPI system afforded the same products as laccase alone the use of TEMPO provided a different product with high specificity. These results offer the first evidence of the role of oxidation shuttles that the mediators of laccase may have but also suggest two promising routes towards an environmentally friendly process for kraft pulp bleaching a the identification of mediators which once oxidized by laccase are able to target strategic functional groups present in lignin and b the introduction of those strategic functional groups in an appropriate .
