This review will focus on recent progress regarding the mechanisms of light-induced discoloration of mechanical and chemimechanical pulps and on the proposed preventive treatments. It is evident that the mechanisms behind photoyellowing of lignin-rich pulps are complex and that several types of reaction pathways may coexist. Photoyellowing proceeds via one initial fast phase and a slower following phase. The fast phase has been ascribed to oxidation of free phenolic groups and/or hydroquinones and catechols to photoproducts of mainly quinonoid character. A multitude of reactions involving several lignin subunits are possible. Important intermediates are phenoxyl radicals, and to some extent ketyl radicals. The importance of the phenacyl aryl ether pathway might be more important than previously thought, even though the original content of such groups is low in lignin. Even though many preventive methods against photoyellowing have been suggested, no cost-efficient treatment is available to hinder photoreversion of lignincontaining paper permanently. Suggested methods for stabilization include chemical modification (etherification and esterification), coating the paper product, addition of radical scavengers, excited state quenchers, or ultraviolet absorbing compounds.

In the structural analysis of lignins by 13 C-NMR, signal overlap limits definitive assignment and accurate intensity measurement. Selective labeling by 13 C-enrichment of a specific carbon in lignin enhances its signal intensity in the spectrum. Further enhancement of the specifically labeled carbons can be realized via difference spectra created from the enriched and unenriched samples. Difference 2D 13 C-1 H correlation (HSQC) NMR spectra, derived from the spectra of specifically 13 C-enriched lignin model polymers (so-called dehydrogenation polymers) and their unenriched counterparts, take advantage of the enhanced dispersion afforded by both 13 C and 1 H chemical shifts, diminishing the difficulties arising from the signal-overlap problem and aiding in definitive signal assignments. In this research, protolignin in xylem cell walls was specifically 13 C-enriched at all of the individual phenylpropanoid side-chain carbons by feeding 13 Cenriched coniferins to growing stems of Ginkgo biloba. The whole xylem fractions containing 13 C-enriched and unenriched protolignins were dissolved in a mixture of N-methylimidazole and DMSO, and then acetylated. Solution state 2D-NMR (HSQC) spectra of the acetylated whole cell wall were acquired. Difference spectra between the walls containing 13 C-enriched and unenriched lignins afforded simplified 2D spectra in which well-separated signals were assigned exclusively to the specifically enriched carbons. This novel NMR technique provides a useful tool for elucidation of entire protolignin in the cell wall of ginkgo xylem.

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The conformations of a variety of crystalline lignin models representing different diastereomeric forms of structural elements in lignin of the arylglycerol b-guaiacyl ether type have been studied using X-ray crystallography. Based on X-ray crystallographic data, sequences of units attached to each other by b-guaiacyl ether linkages were constructed. The appearance of the resulting oligomers shows that stereoisomerism can be expected to influence the shape of the lignin molecules to a great extent. The constructed oligomers provide an idea about the contribution to the complexity of lignins by stereochemistry alone. It is proposed that structural differences, together with conformational variations, are the main reasons for the broadness of the signals in NMR spectra of lignins.

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