Elucidating the effect of aggregation of lignin particles on aqueous characteristics of lignin

Limited knowledge of aqueous properties of lignin is a significant bottleneck to understanding the reactivity of this complex macromolecule. Lignin is an amphipathic complex and heterogeneous polyphenolic components of plant cell wall that mainly consists of p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) sub-units linked by C-C and C-O bonds in an irregular fashion. Most analytical methods for lignin characterization are usually carried out in polar solvents which are not ideal for the aqueous conditions employed during biological deconstruction of lignin. To improve analysis of lignin samples from biological treatments, a simple aqueous assay was developed based on the reactivity of lignin functional groups. Lignin suspensions from Eucalyptus, Switchgrass and alkali lignin were evaluated and it was established that in most aqueous lignin suspensions, the insoluble lignin particles sequester soluble lignin moieties that are often released abiotically during incubation with biological agents such as microbes or enzymes. The aboitic release of these sequestered soluble lignin moieties are often erroneously attributed to the action of the biological agent. We have demonstrated the utility of this assay to monitor consumption of a monolignol (syringic acid) by Sphingomonas sp SYK-6.