Determination of soluble lignin and proteins in the presence of each other

Several methods for the differential determination of lignin and of proteins solubilized together in the same solution are compared. It is shown that lignin is best determined by spectrophotometry at 280 nm, the absorptivity of the polymer being an order of magnitude greater than those of proteins. In contrast, a turbidimetric method applied to acid-precipitated lignin is strongly influenced by both the nature and the amounts of the proteins present in the mixture. Ninhydrin reagents permits the determination of proteins without interference from soluble lignin. Phenol and Coomassie Blue reagents react strongly with lignin, thus masking their interaction with proteins. The soluble lignin preparations used were solubilized from stake lignin in buffers adjusted to different pH values. They were analysed by exclusion chromatography and the results suggest that the lignin fragments obtained at higher pH are larger than those solubilized at lower pH. Turbidimetry showed that these larger soluble fragments form larger aggregates after acid precipitation.