Studies of the chemoenzymatic modification of cellulosic pulps by the laccase-TEMPO system

The chemoenzymatic modification of cellulosic pulps by the laccase-mediator system (LMS) consisting of laccase (EC 1.10.3.2) and the 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) has been investigated. The reaction proceeds under mild aqueous conditions (sodium citrate buffer pH 6, 30 degrees C) and introduces primarily aldehyde groups into cellulose so that carboxyl groups amount to one-third to one-fifth of the carbonyl groups only. LMS treatment caused uniform oxidation of the material, also in the high-molecular weight area, which is a non-typical behavior compared to other chemical oxidations of cellulose. Treatment of the pulp only with TEMPO or only with laccase caused no changes whatsoever, whereas treatment with the TEMPO-derived oxoammonium ion (in the absence of laccase) introduced carbonyl groups into the pulp, but no carboxyls. This chemoenzymatic approach was compared to the well-known chemical approach by means of TEMPO and hypohalite. Both approaches yielded comparable distributions of functional groups at the low oxidation degrees studied, indicating a similar reaction mechanism with the TEMPO-derived oxoammonium ion being the actual oxidant. The laccase is able to generate this oxoammonium ion, which in turn oxidized the 6-hydroxymethyl group into the corresponding aldehyde.