@article{guerra_norambuena_freer_argyropoulos_2008, title={Determination of arylglycerol-beta-aryl ether linkages in enzymatic mild acidolysis lignins (EMAL): Comparison of DFRC/P-31 NMR with thioacidolysis}, volume={71}, ISSN={["1520-6025"]}, DOI={10.1021/np800080s}, abstractNote={Enzymatic mild acidolysis lignins (EMAL) isolated from different species of softwood and Eucalyptus globulus were submitted to comparative analysis that included thioacidolysis, derivatization followed by reductive cleavage (DFRC), and DFRC followed by quantitative (31)P NMR (DFRC/(31)P NMR). While gas chromatography (GC) was used to determine the monomer yields from both thioacidolysis and DFRC, (31)P NMR studies quantified the various phenolic hydroxy groups released by DFRC. The monomer yields from thioacidolysis and DFRC were substantially different, with thioacidolysis resulting in higher yields. In contrast, an excellent agreement was obtained in the total number of beta-aryl ether structures determined by thioacidolysis and DFRC/(31)P NMR, indicating that the combination of DFRC with quantitative (31)P NMR overcomes, at least in part, the limitations presented by the DFRC method. Both thioacidolysis and DFRC/(31)P NMR were further used to better understand the lignin isolation process from wood. The results show that mild rotary ball milling minimizes, but does not prevent, the degradation of beta-O-4 structures during the early stages of wood pulverization. The extent of such degradation was found to be higher for E. globulus than for a variety of softwoods examined. Furthermore, the structures of the EMALs isolated at yields ranging from 20% to 62% were very similar, indicating structural homogeneity in the lignin biopolymer within the secondary wall.}, number={5}, journal={JOURNAL OF NATURAL PRODUCTS}, author={Guerra, Anderson and Norambuena, Marcela and Freer, Juanita and Argyropoulos, Dimitris S.}, year={2008}, month={May}, pages={836–841} }
 @article{guerra_lucia_argyropoulos_2008, title={Isolation and characterization of lignins from Eucalyptus grandis Hill ex Maiden and Eucalyptus globulus Labill. by enzymatic mild acidolysis (EMAL)}, volume={62}, ISSN={["1437-434X"]}, DOI={10.1515/HF.2008.004}, abstractNote={Abstract}, number={1}, journal={HOLZFORSCHUNG}, author={Guerra, Anderson and Lucia, Lucian A. and Argyropoulos, Dimitris S.}, year={2008}, pages={24–30} }
 @article{contreras_gaspar_guerra_lucia_argyropoulos_2008, title={Propensity of Lignin to Associate: Light Scattering Photometry Study with Native Lignins}, volume={9}, ISSN={["1526-4602"]}, DOI={10.1021/bm800673a}, abstractNote={Many studies of lignins in solution invoke association and aggregation phenomena to explain their solution behavior (e.g., reprecipitation onto pulp fibers, condensation, etc.). Following their colloidal (apparent) molecular weights in solution as a function of time allows us to explore observable dissociation phenomena. These measurements were carried out using multiple angle laser light scattering (MALLS) photometry in the static mode. The challenges and opportunities of measuring the specific refractive index increment (dn/dC) of lignin solutions and determining the kinetics of the dissociation process were thus investigated. Hardwood and softwood representative lignins were isolated, and method for their full dissolution in THF was further developed, which then lead to accurate dn/dC values being obtained as a function of time. When coupled to additional work using light scattering static measurements and Zimm plots for the same solutions, this effort offers insight into the aggregation and ensuing dissociative events that operate within the lignin macromolecules.}, number={12}, journal={BIOMACROMOLECULES}, author={Contreras, Sofia and Gaspar, Armindo R. and Guerra, Anderson and Lucia, Lucian A. and Argyropoulos, Dimitris S.}, year={2008}, month={Dec}, pages={3362–3369} }
 @article{guerra_gaspar_contreras_lucia_crestini_argyropoulos_2007, title={On the propensity of lignin to associate: A size exclusion chromatography study with lignin derivatives isolated from different plant species}, volume={68}, ISSN={["0031-9422"]}, DOI={10.1016/j.phytochem.2007.05.026}, abstractNote={Despite evidence that lignin associates under both aqueous and organic media, the magnitude and nature of the underlying driving forces are still a matter of discussion. The present paper addresses this issue by examining both solution properties and size exclusion behaviour of lignins isolated from five different species of softwoods, as well as from the angiosperms Eucalyptus globulus and wheat straw. This investigation has used the recently described protocol for isolating enzymatic mild acidolysis lignin (EMAL), which offers lignin samples highly representative of the overall lignin present in the wood cell wall. The molecular weight distributions of these EMALs were found to be dependent upon the wood species from which they were isolated and upon the incubation conditions used prior to size exclusion chromatography. While the chromatograms of EMALs isolated from softwoods displayed a bimodal behaviour, the elution profiles of EMAL from E. globulus and straw were nearly unimodal. A marked tendency to dissociate prevailed under incubation at room temperature for all examined species with the exception of the straw lignin preparation; furthermore, lignin solutions incubated at 4 degrees C showed an associative behaviour manifested by an increase in the weight and number average molecular weights for some species. The extent of such association/dissociation, as well as the time needed for the process to reach completion, was also found to depend upon the wood species, i.e. lignins from softwoods were found to associate/dissociate to a greater extent than lignins from E. globulus and straw. The origin of such effects within the lignin structure is also discussed.}, number={20}, journal={PHYTOCHEMISTRY}, author={Guerra, Anderson and Gaspar, Armindo R. and Contreras, Soffa and Lucia, Lucian A. and Crestini, Claudia and ArgyropouloS, Dimitris S.}, year={2007}, month={Oct}, pages={2570–2583} }
 @article{guerra_filpponen_lucia_argyropoulos_2006, title={Comparative evaluation of three lignin isolation protocols for various wood species}, volume={54}, ISSN={["1520-5118"]}, DOI={10.1021/jf062433c}, abstractNote={Milled wood lignin (MWL), cellulolytic enzyme lignin (CEL), and enzymatic mild acidolysis lignin (EMAL) were isolated from different wood species and characterized by various techniques. The EMAL protocol offered gravimetric lignin yields 2-5 times greater than those of the corresponding MWL and CEL. The purities of the EMALs were 3.75-10.6% higher than those of their corresponding CELs, depending upon the wood species from which they were isolated. Molecular weight analyses showed that the EMAL protocol isolates lignin fractions that are not accessed by the other procedures evaluated, while 31P NMR spectroscopy revealed that MWL is more condensed and bears more phenolic hydroxyl groups than EMAL and CEL. The yields and purities of EMAL, MWL, and CEL from hardwood were greater than those obtained for the examined softwoods. Structural details obtained by DFRC (derivatization followed by reductive cleavage)/31P NMR revealed different contents of condensed and uncondensed beta-O-aryl ether structures, dibenzodioxocins, and condensed and uncondensed phenolic hydroxyl and carboxylic acid groups within lignins isolated from different wood species.}, number={26}, journal={JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY}, author={Guerra, Anderson and Filpponen, Ilari and Lucia, Lucian A. and Argyropoulos, Dimitris S.}, year={2006}, month={Dec}, pages={9696–9705} }
 @article{filpponen_guerra_hai_lucia_argyropoulos_2006, title={Spectral monitoring of the formation and degradation of polysulfide ions in alkaline conditions}, volume={45}, ISSN={["0888-5885"]}, DOI={10.1021/ie060651p}, abstractNote={The recently developed protocol for the absolute determination of the various polysulfide species based by 1H NMR was applied to monitor the amount of such ions in alkaline media produced at different conditions. The thermal stability of each individual polysulfide species was also evaluated in temperature ranges representative of a typical alkaline pulping process. Overall, the species, S12-, S22-, S32-, S42-, S52-, and S62- were detected and quantified in all alkaline conditions evaluated. The concentration of sodium sulfide and the temperature were found to have a significant effect on the total amount of polysulfides detected. In summary, the di- and trisulfide anions were found to predominate over the other polysulfide species in all examined experimental conditions. Additionally, the experimental values of Ea for the thermal degradation of polysulfide species correlate with the observed stability of the individual species; i.e., S22- has the highest Ea, followed by S32-.}, number={22}, journal={INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH}, author={Filpponen, Ilari and Guerra, Anderson and Hai, Abdul and Lucia, n A. Lucia and Argyropoulos, Dimitris S.}, year={2006}, month={Oct}, pages={7388–7392} }
 @article{guerra_filpponen_lucia_saquing_baumberger_argyropoulos_2006, title={Toward a better understanding of the lignin isolation process from wood}, volume={54}, ISSN={["1520-5118"]}, DOI={10.1021/jf060722v}, abstractNote={The recently developed protocol for isolating enzymatic mild acidolysis lignins (EMAL) coupled with the novel combination of derivatization followed by reductive cleavage (DFRC) and quantitative (31)P NMR spectroscopy were used to better understand the lignin isolation process from wood. The EMAL protocol is shown to offer access at lignin samples that are more representative of the overall lignin present in milled wood. The combination of DFRC/(31)P NMR provided a detailed picture on the effects of the isolation conditions on the lignin structure. More specifically, we have used vibratory and ball milling as the two methods of wood pulverization and have compared their effects on the lignin structures and molecular weights. Vibratory-milling conditions cause substantial lignin depolymerization. Lignin depolymerization occurs via the cleavage of uncondensed beta-aryl ether linkages, while condensed beta-aryl ethers and dibenzodioxocins were found to be resistant to such mechanical action. Condensation and side chain oxidations were induced mechanochemically under vibratory-milling conditions as evidenced by the increased amounts of condensed phenolic hydroxyl and carboxylic acid groups. Alternatively, the mild mechanical treatment offered by ball milling was found not to affect the isolated lignin macromolecular structure. However, the overall lignin yields were found to be compromised when the mechanical action was less intense, necessitating longer milling times under ball-milling conditions. As compared to other lignin preparations isolated from the same batch of milled wood, the yield of EMAL was about four times greater than the corresponding milled wood lignin (MWL) and about two times greater as compared to cellulolytic enzyme lignin (CEL). Molecular weight distribution analyses also pointed out that the EMAL protocol allows the isolation of lignin fractions that are not accessed by any other lignin isolation procedures.}, number={16}, journal={JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY}, author={Guerra, Anderson and Filpponen, Ilari and Lucia, Lucian A. and Saquing, Carl and Baumberger, Stephanie and Argyropoulos, Dimitris S.}, year={2006}, month={Aug}, pages={5939–5947} }