Mechanisms of wood degradation by brown-rot fungi: chelator-mediated cellulose degradation and binding of iron by cellulose

Iron, hydrogen peroxide, biochelators and oxalate are believed to play impo rtant roles in cellulose degradation by brown-rot fungi. The effect of thes e compounds in an 'enhanced' Fenton system on alpha -cellulose degradation was investigated specifically in regard to molecular weight distribution an d cellulose-iron affinity. This study shows that the degradative ability of an ultrafiltered low molecular weight preparation of chelating compounds i solated from the brown-rot fungus Gloeophyllum trabeum (termed 'Gt chelator ') increased with increasing Gt chelator concentration when the FeIII to Gt chelator ratio was greater than about 30:1. When this ratio was less than 30.1, increasing Gt chelator concentration did not accelerate cellulose deg radation. In excess hydrogen peroxide, cellulose degradation increased and then decreased with increasing iron concentration when FeIII was present in excess of the Gt chelator. The critical ratio of FeIII to Gt chelator vari ed depending on the concentration of hydrogen peroxide in the system. Incre asing iron concentration above a critical iron:chelator ratio inhibited cel lulose degradation. The optimum pH for cellulose degradation mediated by Gt chelator was around 4.0. A comparison of the effects of 2.3-DHBA (a chelat or that reduces iron similarly to Gt chelator) and ct chelator with respect to cellulose degradation demonstrated the same pattern of cellulose degrad ation. Cellulose-iron affinity studies were conducted at three pH levels (3 .6, 3.8, 4.1). and the binding constants for cellulose-FeIII, cellulose-FeI I and cellulose FeIII in the presence of Gt chelator were calculated. The b inding constants for cellulose-FeIII at ail three pH levels were much highe r than those for cellulose-FeII, and the binding constants for cellulose Fe III in the presence of Gt chelator were very close to those for cellulose-F ell. This is probably the result of FeIII reduction to Fell by Gt chelator and suggests that chelators from the fungus may be able to sequester iron f rom cellulose and reduce it in near proximity to the cellulose and thereby better promote depolymerization. The free radical generating system describ ed has potential for use in a variety of industrial processing and pollutio n control applications. (C) 2001 Elsevier Science B.V. All rights reserved.