13C-Enriched Dehydrogenation Polymers Used for Studying Light-Induced Yellowing of Lignocellulosic Materials
This study deals with the use of selectively 13C-enriched dehydrogenation polymers (DHPs) as models of lignin in lignocellulosic materials for elucidating the reaction pathways involved in light-induced yellowing. Methods for the synthesis of coniferin, a precursor to lignin and DHP, selectively 13C-enriched in the aromatic ring (positions 1, 3, 4, and 5) have been developed.
The photoyellowing experiments were performed in solid state using UV/VIS-light and the chemical changes were traced using solution and solid state NMR spectroscopy. The results show that unsaturated end groups of the cinnamyl alcohol type are photodegraded under the formation of end groups of the benzaldehyde and benzoic acid types. The findings regarding cinnamaldehyde end groups were, however, not unanimous in the case of photoyellowed DHPs, possibly due to different experimental conditions. Studies of monomeric lignin model compounds of the non-phenolic cinnamyl alcohol and cinnamaldehyde types [(E)-3-(3,4-dimethoxyphenyl)-2-propen-1-ol and (E)-3-(3,4-dimethoxyphenyl)propenal] indicate that both types of end groups generate color and degradation products during irradiation. Irradiation with UV/VIS-light induces E/Z-isomerization of the starting materials, as well as photo-oxidation of the cinnamyl alcohol to the corresponding cinnamaldehyde.
No conclusive evidence of any significant β-ether cleavage was obtained from the experiments, either with CW-DHP (cell wall-dehydrogenation polymer) or with DHP. The relative amount of β-ethers was not extensively changed, although very small amounts of a suggested cleavage product (of the 1-aryl-3-hydroxy-1-propanone type) was indicated in the β- and γ-13C-enriched samples. The observed decrease in the relative amount of Cα in arylglycerol-β-aryl ether structures, indicated in the difference spectrum (13C-enriched - unenriched) corresponding to α-13C-enriched DHP (applied to filter paper, irradiated, and extracted), could be due to changes other than β-ether cleavage. A moderate formation of α-carbonyls occurred, although the exact nature of these structures is unknown, at present. However, studies of dimeric lignin model compounds of the non-phenolic β-ether type, with and without an α-carbonyl group [1-(3,4-dimethoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)-1-propanone and erythro-1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol], showed that both models form colored materials upon irradiation. More color was formed in the case of the α-carbonylic model. Apart from monomeric degradation products, dimers of the β-5 and β-1 types were formed during irradiation of the α-carbonylic β-ether model, indicative of a homolytic cleavage of the Cβ-O bond and recombination of the initially formed radicals (β-5 recombination more favored).
In all investigated lignin model compounds, photoproducts indicative of Cα-Cβ cleavage (e.g., structures of the benzaldehyde type) were formed. No conclusive evidence for the formation of quinones was obtained.
lignin model compounds
13C NMR spectroscopy