Dynamic Applications of an Underutilized Renewable Biopolymer with Simple Modifications

Lignin, an abundant renewable dendritic polymer, is the largest naturally occurring source of aromatic structures on Earth.  Expanding upon established pathways to alkylate at the phenolic position to create amphiphilic host-guest delivery systems, the projects discussed herein explore the incorporation of copper-catalyzed azide-alkyne cycloaddition into synthesis methods.  Kraft lignin was modified to create derivatives with extended aliphatic carbon chains and delocalized cationic groups known for accumulating in mitochondria, including the triphenylphosphonium cation.  These nanoparticles have been confirmed by ¹H-NMR and FTIR, with further characterization by DLS for size and organizational behavior.  While the polymers do not reliably accumulate in mitochondria in vitro, they are capable of encapsulating the lipophilic cyanine dye DiD and enhancing its optical properties.  Another set of lignin compounds was reported to demonstrate functionality as independently fluorescent nanoparticles.  The polymers utilize the alkylation at the phenolic -OH position to create an azide-modified lignin precursor, which can be introduced to terminal alkynes in presence of a copper catalyst and base to append stilbazolium chromophores via Huisgen cycloaddition.  The lignins are multifunctional, also bearing extended aliphatic tails to aid in organization into amphiphilic nanostructures.  All compounds are characterized by ¹H-NMR and FTIR.  Their size and organization was investigated through DLS.  Tethering to lignin was found to increase the quantum yield of dyes BMH+ and HNEP+.  The derivative featuring HNEP+ also demonstrates the unique solvatochromic properties of its small molecule companion, suggesting decoration of the nanoparticle surface due to accessibility of solvent.  The "L-DYEs" were investigated as imaging agents via confocal microscopy, and are all capable of internalizing into HEK-293 cells.  One derivative modified with a redox-sensitive boronic pinacol ester responds to H₂O₂ with increased fluorescence in vitro, including in live cell culture.