@article{holtman_chang_kadla_2007, title={An NMR comparison of the whole lignin from milled wood, MWL, and REL dissolved by the DMSO/NMI procedure}, volume={27}, ISSN={["1532-2319"]}, DOI={10.1080/02773810701700828}, abstractNote={Abstract Lignins isolated from pine milled wood, milled wood lignin (MWL), and residual enzyme lignin (REL) were compared using modified thioacidolysis, modified DFRC, gel permeation chromatography (GPC), two‐dimensional Heteronuclear Multiple Quantum Coherence (HMQC) NMR, and quantitative 13C NMR. Dissolution of the lignin for solution‐state NMR was accomplished by utilizing the recently reported DMSO/N‐methylimidazole/acetic anhydride solvent system. Contrary to previous reports, comparison of the lignin preparations by thioacidolysis indicated that REL was more structurally similar to the lignin in the milled wood and Wiley wood meal than MWL. Total monomer yields indicated that the MWL was lower in β‐aryl ether content than the other preparations, and this was verified by quantitative 13C NMR. NMR analysis indicated that the inter‐unit linkages present in all the lignin preparations are consistent with the present knowledge about lignin biosynthesis. The contribution of minor end group structures in the MWL are further decreased in the milled wood, indicating that they are preferentially isolated as low molecular weight material, possibly generated during the milling process. All other structural moieties were similar in all preparations. GPC data indicated that the milled wood and REL both contain a portion of lignin with a molecular weight of 55,000 g/mol. Data indicate that the inefficiency of the DFRC method may be related to molecular mobility or accessibility in higher molecular weight portions of the lignin polymer.}, number={3-4}, journal={JOURNAL OF WOOD CHEMISTRY AND TECHNOLOGY}, author={Holtman, Kevin M. and Chang, Hou-Min and Kadla, John F.}, year={2007}, pages={179–200} }
 @article{holtman_chang_jameel_kadla_2006, title={Quantitative C-13 NMR characterization of milled wood lignins isolated by different milling techniques}, volume={26}, ISSN={["1532-2319"]}, DOI={10.1080/02773810600582152}, abstractNote={Abstract Milled wood lignins (MWL) prepared from finely milled wood flour produced by different milling techniques were compared by quantitative 13C NMR. Wood meal produced in a Wiley mill was milled for either six weeks in a porcelain rotary mill with porcelain balls, or by two variations of our standard technique. Specifically the Wiley wood meal was milled for one week in the rotary mill followed by 48 h of vibratory ball‐milling with steel balls either in toluene or under a N2 atmosphere. Results showed that the vibratory‐milled samples were similar in structure with the exception that the preparation milled under N2 had higher aliphatic and phenolic hydroxyl contents. The rotary‐milled sample on the other hand had a much lower β‐O‐4′ and hydroxyl content along with a higher degree of condensation and oxidized side chain structures.}, number={1}, journal={JOURNAL OF WOOD CHEMISTRY AND TECHNOLOGY}, author={Holtman, KM and Chang, HM and Jameel, H and Kadla, JF}, year={2006}, pages={21–34} }
 @article{holtman_chang_kadla_2004, title={Solution-state nuclear magnetic resonance study of the similarities between milled wood lignin and cellulolytic enzyme lignin}, volume={52}, ISSN={["1520-5118"]}, DOI={10.1021/jf035084k}, abstractNote={The structures of milled wood lignin (MWL) and cellulolytic enzyme lignin (CEL) have been analyzed using traditional chemical methods and solution-state NMR techniques. Comparisons of the results obtained reveal that subtle differences exist between the two lignin preparations. Thioacidolysis produced higher monomer yields from CEL than MWL, suggesting MWL has a more condensed structure. Quantitative (13)C NMR determined the degree of condensation in MWL to be 0.43 unit per aromatic moiety as compared to 0.36 in CEL. The MWL also contained a lower amount of beta-O-4' substructures per aromatic ring than CEL, 0.41 versus 0.47, respectively. Carbohydrate analysis revealed that the MWL may contain a higher proportion of middle lamella material as compared to the CEL. Because the middle lamella is considered to have a more condensed lignin structure, on the basis of the bulk polymerization theory, these results could explain the differences in beta-O-4' and degree of condensation.}, number={4}, journal={JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY}, author={Holtman, KM and Chang, HM and Kadla, JF}, year={2004}, month={Feb}, pages={720–726} }
 @article{holtman_chang_jameel_kadla_2003, title={Elucidation of lignin structure through degradative methods: Comparison of modified DFRC and thioacidolysis}, volume={51}, ISSN={["0021-8561"]}, DOI={10.1021/jf0340411}, abstractNote={Milled wood and milled wood lignin (MWL) samples were subjected to DFRC and thioacidolysis. Despite the fact that both methods selectively cleave aryl ether bonds, substantial differences in results were obtained. Lignin thioacidolysis gave total molar yields of degradation monomer products in the range of 3.5-7 mol % higher than DFRC. GPC analysis showed that the thioacidolysis-treated lignin was degraded to a lower average molecular weight than that treated by DFRC. Contrary to results reported for lignin model compounds, these results indicate that the DFRC method does not completely or efficiently degrade the lignin polymer. In fact, the DFRC-degraded lignin retained much of the characteristics of the original MWL. Elemental analysis revealed the presence of bromine in the DFRC-treated lignin, and two-dimensional (1)H-(13)C HMQC NMR spectroscopy showed the presence of beta-O-4 linkages in the DFRC-treated lignin. No beta-O-4 interunit linkages were detected in the thioacidolysis-treated lignin. These results are consistent with the lower monomer yields and the higher average molecular weight of the DFRC-treated lignin and indicate inefficiency in the chemistry of the method, probably due to steric constraints of the polymeric nature of lignin.}, number={12}, journal={JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY}, author={Holtman, KM and Chang, HM and Jameel, H and Kadla, JF}, year={2003}, month={Jun}, pages={3535–3540} }
 @article{ikeda_holtman_kadla_chang_jameel_2002, title={Studies on the effect of ball milling on lignin structure using a modified DFRC method}, volume={50}, ISSN={["0021-8561"]}, DOI={10.1021/jf010870f}, abstractNote={The structures of milled wood lignin (MWL), cellulolytic enzyme lignin (CEL), and residual lignin (REL) from a loblolly pine were analyzed using a modified derivatization followed by reductive cleavage (DFRC) method developed to allow the quantitative determination of three different structural monomeric products originating in lignin: phenolic beta-O-4, alpha-O-4, and etherified beta-O-4 structures. Results show that MWL and CEL are structurally identical, with an increased phenolic beta-O-4 content compared to that of the original Wiley milled wood. These results indicate that the portion of lignin linked to carbohydrates and that not linked to carbohydrates are structurally the same. Modified DFRC analysis of the effect of ball milling on the structure of lignin in wood, MWL, CEL, and REL indicate that vibratory ball milling does not change the lignin structure provided certain precautions are taken. Specifically, dry vibratory ball milling under a nitrogen atmosphere causes substantial structural changes including condensation, whereas vibratory ball milling in toluene had little effect on the lignin structure. This indicates that the structural differences observed in MWL and CEL arise because of the extraction procedure, which preferentially extracts phenolic lignin structures. MWL and CEL are representative of the total lignin in wood; however, due primarily to the solvent extraction process, higher phenolic hydroxyl contents are observed. Nitrobenzene oxidation showed structural results similar to those from the modified DFRC method.}, number={1}, journal={JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY}, author={Ikeda, T and Holtman, K and Kadla, JF and Chang, HM and Jameel, H}, year={2002}, month={Jan}, pages={129–135} }