ASSOCIATIVE AND COLLOIDAL BEHAVIOR OF LIGNIN AND IMPLICATIONS FOR ITS BIODEGRADATION IN-VITRO

Lignin is a relatively complex aromatic biopolymer that occurs in woody plant cell walls. This biomaterial can be degraded by extracellular enzymes from white rot fungi, a process that has attracted much attention. Although the enzymes that catalyze such a biodegradation process have been identified and isolated, the rate of lignin biodegradation in vitro has been found to be quite low. The view presented in this paper is that this may be due to the way that the substrate, lignin, is presented to the enzyme. In fact, little is known about the substrate in its natural physical state or about the modifications that it undergoes during the solubilization process. In this paper a solvent-extracted lignin from spruce has been characterized in terms of its molecular weight and size. Evidence is presented that shows that this lignin sample forms aggregates in a dioxane-water (5:1) solvent system, having a hydrodynamic radius of 60 nm. Each aggregate is formed with an average of 256 molecules and has a cross-sectional area of 190 nm2, as determined using a Langmuir-Blodgett trough. Furthermore, in water solutions the pH was found to influence the molecular weight of lignin, as seen by high-pressure liquid chromatography and scanning electron microscopy. As the pH was lowered from 12 to 2, lignin aggregates with a discrete pattern of molecular weight increments. These results are interpreted in terms of electrostatic and van der Waals forces interacting between polar and apolar groups of the lignin molecules. These findings stress the importance of controlling the conditions under which lignin is presented to the ligninolytic enzymes during in vitro degradation experiments.