Antioxidant activity of various lignins and lignin-related phenylpropanoid units with high and low molecular weight

The antioxidant activities (AoAs) of 50 different technical lignins have been determined. The lignins of various botanical origins (annual plants, coniferous trees, and deciduous trees) were isolated and fractionated by different techniques (delignification by alkali, kraft process, fast pyrolysis, and hydrolysis). The structure and the functionality of lignins were characterized by functional group analyses (phenolic OH, carboxyl, and methoxyl groups), analytical pyrolysis pyrolysis/gas chromatography/mass spectrometry/flame ionization detector (Py-GC/MS/FID), electron paramagnetic resonance, size exclusion chromatography, and titrimetric methods, and the AoAs were evaluated as the capacity to scavenge the DPPH· and ABTS·+ free radicals. The relationship between the lignin structure and the AoA was characterized by pair correlation, partial correlation, and multivariate regression analyses, including correlated components regression. The results were compared with those of lignin model compounds and low molecular weight phenylpropanoids. It has been shown that molecular weight does not influence essentially the AoA of lignins. There is a relationship between the activities of low and high molecular weight polyphenols; their mechanisms of action are also similar. The structure-related AoA of lignins has been quantified for the first time.