Biomacromolecules 2003, 4, 64-69
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Oxidative Quenching of Spruce Thermomechanical Pulp Fiber Autofluorescence Monitored in Real Time by Confocal Laser Scanning MicroscopysImplications for Lignin Autofluorescence Søren Barsberg*,† and Kirsten A. Nielsen‡ Plant Fibre Laboratory, The Royal Veterinary and Agricultural University, 10 Agrovej, DK-2630 Taastrup, Denmark, and Plant Biochemistry Laboratory, Department of Plant Biology, The Royal Veterinary and Agricultural University, 40 Thorvalsensvej, DK-1871 Frederiksberg C, Denmark Received July 31, 2002; Revised Manuscript Received October 15, 2002
Confocal laser scanning microscopy (CLSM) was used to monitor real-time lignin autofluorescence intensities from different cell wall compartments of spruce fibers during oxidation by laccase-2,2′-azinobis-3ethylbenzthiazoline-6-sulfonate (ABTS) treatment. CLSM data revealed an instant emission quenching from all cell wall compartments, including those not physically accessible to enzyme or ABTS, followed by an additional decrease simultaneously in all cell wall compartments over a 45 min time course. The importance of site-to-site excitation energy transfer in lignin efficient over long distance is suggested. Introduction Lignin is a main constituent of the plant cell wall. It is synthesized in vivo from three different types of phenolic monomers (coniferyl, sinapyl, and p-coumaryl alcohol).1-3 The aromatic polymer is branched and disordered, but structural constraints occur by close association with cell wall carbohydrates.4-6 Certain electronic properties of synthetic aromatic polymers are not found for corresponding monomer models and pertain solely to macromolecular systems.7 Such polymers contain, for example, polaron or bipolaron species after oxidative doping. For monomers, equivalent species are, for example, cation radicals (polarons) or dications (bipolarons), which differ by a single charge in oxidation state. In polymer systems, the presence of neighboring monomer units enables polaron and bipolaron charges to migrate (inter- or intrachain) via electron transfer from one location to another.8-10 As an equivalent option for electronically excited states, these may in such systems migrate by excitation energy transfer (EET).11-13 These phenomena do not involve physical migration of molecules but electron- and excited-state migration, respectively. Like synthetic aromatic polymers, lignin exhibits locations for electronic states present in close proximity to each other.14 In principle, this shouldsjust as for synthetic polymerss enable site-to-site transfer of electrons or excited states within cell wall lignin. The last process (EET) might play a dominant role for the luminescence of lignin because it allows an emitting or nonemitting lignin moiety to accept excitation energy from other moieties within an effective transfer distance, which is typically
