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Lignin radicals are crucial intermediates for lignin biosynthesis in the cell wall of vascular plants. In this work they were for the first time, to our knowledge, selectively observed in wood cell walls by laser-based Kerr-gated resonance Raman spectroscopy, and the observations were supported by density functional theory prediction of their vibrational properties. For dry wood cells a lignin radical Raman band is observed at 1,570 cm(-1) irrespective of species. For wet beech cells they were generated in situ and observed at 1,606 cm(-1). DFT/B3LYP/6-31+G(d) modeling results support that in beech they are formed from syringyl (S) phenolic moieties and in spruce from guaiacyl (G) phenolic moieties. The observed lignin radical band is predicted as G is approximately 1,597 cm(-1) and S is approximately 1,599 cm(-1), respectively, and is assigned the (Wilson notation) nu(8a) phenyl ring mode. The RR band probes lignin radical properties, e.g., spin density distribution, and these respond to charge polarization or hydrogen bonding to proximate water molecules. These observations can be crucial for an understanding of the factors that control cell wall structure during biosynthesis of vascular plants and demonstrate the unique potential of RR spectroscopy of lignin radicals.

Original publication

DOI

10.1529/biophysj.105.070391

Type

Journal

Biophysical journal

Publication Date

04/2006

Volume

90

Pages

2978 - 2986

Addresses

Danish Centre for Forest, Landscape and Planning, The Royal Veterinary and Agricultural University, DK-2630 Taastrup, Denmark. sbar@kvl.dk

Keywords

Cell Wall, Fagus, Picea, Free Radicals, Water, Lignin, Laccase, Spectrum Analysis, Raman, Hydrogen Bonding