Polyoxometalate (POM) Oxidation of Milled Wood Lignin (MWL)

Abstract

The chemical structural changes of Lodgepole pine milled wood lignin (MWL) before and after polyoxometalate (POM) oxidation were investigated using gel permeation chromatography (GPC), Fourier Transform infrared spectroscopy (FTIR), and solution‐state nuclear magnetic resonance spectroscopy (NMR). ¹³C NMR spectroscopic data revealed an approximately 28% decrease in α‐OH/β‐O‐4 inter‐unit linkages after POM treatment. This was accompanied by an increase in carbonyl and phenolic hydroxyl content. These results suggest POM oxidation involved side chain (such as α‐OH/β‐O‐4) oxidation and/or degradation of some of the inter‐unit linkages. Quantitative ¹³C NMR along with GPC analysis revealed an increase in the degree of condensation of the MWL as a result of POM treatment, suggesting radical coupling as a major reaction pathway.     

Isolation and Structural Characterization of Lignin from Cotton Stalk Treated in an Ammonia Hydrothermal System

To investigate the potential for the utilization of cotton stalk, ammonia hydrothermal treatment was applied to fractionate the samples into aqueous ammonia-soluble and ammonia-insoluble portions. The ammonia-soluble portion was purified to yield lignin fractions. The lignin fractions obtained were characterized by wet chemistry (carbohydrate analysis) and spectroscopy methods (FT-IR, ¹³C and ¹H-¹³C HSQC NMR spectroscopy) as well as gel permeation chromatography (GPC). The results showed that the cotton stalk lignin fractions were almost absent of neutral sugars (0.43%–1.29%) and had relatively low average molecular weights (1255–1746 g/mol). The lignin fractions belonged to typical G-S lignin, which was composed predominately of G-type units (59%) and noticeable amounts of S-type units (40%) together with a small amount of H-type units (~1%). Furthermore, the ammonia-extractable lignin fractions were mainly composed of β-O-4′ inter-unit linkages (75.6%), and small quantities of β-β′ (12.2%), together with lower amounts of β-5′ carbon-carbon linkages (7.4%) and p-hydroxycinnamyl alcohol end groups.     

Effects of Solvation and Ionization on 13C NMR Spectra of Lignin Model Compounds,Spruce Milled Wood Lignin and Kraft Lignin

The effects of alkali and polar aprotic solvent on the aromatic carbons signals in ¹³C NMR (Carbon-13 nuclear magnetic resonance) spectra of lignin model compounds and spruce milled wood lignin (MWL) were studied. It was found that in 1 M aqueous NaOH signal shifts of C-1 and C-4 carbon atoms in the aromatic ring were the most noticeable in lignin models with free phenolic hydroxyl groups, which are ionized under the conditions. A similar effect in the spectra of the studied model compounds was observed in 0.5 M aqueous NaOD-deuterated dimethyl sulfoxide (DMSO) mixture (DMSO: water ratio 3:7 v/v). The model data help explaining changes in the ¹³C NMR spectra of MWL and lignin in situ dissolved in spruce kraft black liquor caused by ionization. In the ¹³C NMR spectra of spruce black liquor the signals of phenolic and non-phenolic lignin units are clearly separated and do not overlap with the signals of the carbon atoms of carbohydrates and other aliphatic products of wood degradation. The data obtained are useful in understanding the important role of solvation and ionization processes leading to lignin solubilization.     

An NMR Comparison of the Whole Lignin from Milled Wood,MWL, and REL Dissolved by the DMSO/NMI Procedure

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.     

Fractional Separation of Hemicelluloses and Lignin in High Yield and Purity from Mild Ball-Milled Periploca sepium

Abstract

A novel three-step procedure for separation of hemicelluloses and lignin with high yield and purity was proposed in this study, where wood is mildly milled and successively extracted to produce three hemicellulosic and lignin fractions representing the total hemicelluloses and lignin in wood. The sequential treatments of the mild ball-milled Periploca sepium with 80% aqueous dioxane containing 0.05 M HCl at 85°C for 4 h, DMSO at 85°C for 4 h, and 8% NaOH at 50°C for 3 h resulted in a total release of over 85% of the original hemicelluloses and 86% of the original lignin. In particular, approximately 36% of the original hemicelluloses and 50% of the original lignin were separated during the first mild acidolytic hydrolysis process after low intensity milling. The structure of the acidic dioxane-, DMSO-, and alkali-soluble hemicellulosic and lignin fractions were elucidated using wet chemical analysis, FT-IR, and solution-state ¹H, ¹³C, and ³¹P NMR techniques. Results showed that both the mild ball milling and the mild acidolysis under the conditions given did not affect the separated lignin macromolecular structure. On the other hand, the mild acidolytic hydrolysis condition did cause substantial hemicellulosic depolymerization exception for a significant cleavage the ether linkages between lignin and hemicelluloses. The acidic dioxane-soluble lignin fraction was structurally different from the DMSO- and alkali-soluble lignin preparations and may originate mainly from the primary wall, while the alkali-soluble lignin preparation was mainly released from the secondary wall of Periploca sepium. Furthermore, it was found that the acidic dioxane-soluble hemicelluloses mainly contained more branched and less acidic arabinoxylans, and the 8% NaOH-soluble hemicellulosic fraction H₃ was both less branched and less acidic in structure, whereas the DMSO-soluble hemicelluloses were more acidic but less branched and consisted mainly of 4-O-methylglucuronoarabinoxylan.     

Sequence analysis of poly(ethylene/1,4-cyclohexanedimethylene terephthalate) copolymer using H and C NMR

The triad-level sequence analysis of poly(ethylene/1,4-cyclohexanedimethylene terephthalate) copolymer was reported in a solvent system of o-chlorophenol/deuterated chloroform mixture (50/50 v/v) at 80 °C using 600 MHz ¹H NMR. The well-resolved alcoholic CH₂ proton peak of the glycol units was observed, which made the detailed sequence analysis possible. The peaks of the cis- and trans-forms of the 1,4-cyclohexanedimethylene glycol units were split into the triad sequence in the chain and could be assigned by a comparison of the spectra with those of homopolymers and by an additional two-dimensional heteronuclear multiple bond correlation observation. The triad sequence distributions centered on 1,4-cyclohexanedimethylene glycol units were determined, which was independent of the cis- and trans-forms of the units and controlled according to Bernoullian statistics.     

Multinuclear magnetic resonance study of equilibria between aluminum(III) and sulfur-containing amino acids in aqueous solutions

Aluminum(III) is generally found in small amounts in living organisms where it interacts with several biomolecules. Low mass biomolecules can form complexes associated with absorption and distribution processes of this toxic ion. However, investigating aluminum(III) complexes in solution poses singular difficulties, because it is highly hydrolyzable. Amino acids constitute possible ligands for aluminum(III). Methionine, cysteine and homocysteine are found in healthy human body. Penicillamine is a drug used in several circumstances, such as in treating metal poisoning. These four biomolecules share similar donor atoms involved in complexation reactions: a carboxylate oxygen, an amine nitrogen and sulfur. In this study, four binary complexes formed by aluminum(III) and amino acids in solution at a metal-to-ligand ratio of 1:1 were analyzed through potentiometry and multinuclear magnetic resonance (¹³C and ²⁷Al). Potentiometry data consistently demonstrated complexion formation. Distributions of species revealed the coexistence of several species in ample pH ranges and pointed out the preferable value to perform the NMR analyses. ¹³C NMR data were used to confirm complexion formation, while ²⁷Al NMR data indicated the geometric arrangement adopted by the complexes. Structures were proposed for each complex.     

Synthesis of dimer and oligomers from (R)-methyl hydnocarpate

We report synthesis and characterization of dimer and oligomer acids from chaulmoogra oil. (R)-Methyl hydnocarpate (methyl ester of the major fatty acid component of chaulmoogra oil) was brominated to give threo-2,3-dibromocyclopentane-1-methyl undecanoate. Formation of two diastereoisomers, viz., threo-2(R),3(R)-dibromocyclopentane-1(R)-methyl undecanoate and threo-2(S),3(S)-dibromocyclopentane-(R)-1-methyl undecanoate, was observed. Dehydrobromination of bromo derivatives using alcoholic KOH gave a cyclopentadiene derivative as intermediate, which underwent Diels-Alder reaction to give dimer and oligomer fatty acids. The products were characterized by ultraviolet, direct exposure probe-mass spectroscopy, ¹H nuclear magnetic resonance (NMR), and ¹³C NMR spectroscopic techniques.     

CP/MAS C NMR analysis of the structure and hydrogen bonding of melt-crystallized poly(vinyl alcohol) films

The structure and hydrogen bonding of the melt-crystallized atactic poly(vinyl alcohol) (A-PVA) films, which were carefully prepared without significant thermal degradation, have been characterized by CP/MAS ¹³C NMR spectroscopy. The ¹³C spin-lattice relaxation analysis has revealed that there exist three components with different T_1C values, the crystalline, less mobile noncrystalline and mobile noncrystalline components, in good accord with the results for different PVA samples previously reported. It should be noted that the T_1C values of the crystalline and noncrystalline components are appreciably smaller for the melt-crystallized films than those for the un-annealed and annealed samples prepared by casting from the aqueous solution. The ¹³C NMR spectra of the crystalline and noncrystalline components are separately recorded by using the difference in T_1C and their CH lines are successfully resolved into three and seven constituent lines by the least-squares curve fitting, respectively. Moreover, the statistical analysis of the integrated intensities of the constituent lines thus obtained enables to determine the probability f_a for the formation of intramolecular hydrogen bonding in the successive two OH groups along each chain and another probability f_t of the trans conformation for the crystalline and noncrystalline components. It is found that the f_a value is relatively larger for the melt-crystallized films than those for the un-annealed and annealed samples. On the basis of these results, the features of the melt-crystallization and the resulting crystalline-noncrystalline structure are discussed by particularly considering effects of intra- and inter-molecular hydrogen bonding on the crystallization.     

Change in the structure of poly(tetramethylene succinate) under tensile stress monitored with solid state C NMR

A new solid-state NMR experimental equipment was developed to monitor the structural change of polymer and protein fibers under active uniaxial deformation. The equipment was applied to monitoring change in the structure of a biodegradable aliphatic polyester fiber, poly(tetramethylene succinate) (PTMS; [-O(CH₂)₄OCO-(CH₂)₂CO-]_n), as a function of tensile stress. The crystal transition from α to β forms for PTMS was induced by stress and monitored by the change of the methylene region in the ¹³C NMR spectra. The ¹³C chemical shielding constant for the model compound of poly(tetramethylene succinate) was calculated using the GIAO-CHF (gauge invariant atomic orbitals-coupled Hartree-Fock) with ab initio 6-311G^∗∗ basis set. This calculation acceptably explains the experimental results.     

Highly isotactic poly(vinyl alcohol) derived from tert-butyl vinyl ether. Part IV. Some physical properties, structure and hydrogen bonding of highly isotactic poly(vinyl alcohol) films

Some basic physical properties, structure and hydrogen bonding have been characterized for different stereoregular PVA films including highly isotactic PVAs (HI-PVAs), which were recently succeeded in synthesis, as functions of the mm fraction by using different analytical methods. The melting temperature, degree of crystallinity, and ¹³C spin-lattice relaxation time of the crystalline component are found to have their own clear minima at the mm fraction of about 0.4-0.5. This fact suggests that structural disordering associated with the decrease in crystallinity may be most strongly induced at this mm fraction. The formation of the new crystal form of PVA has been reconfirmed for HI-PVAs with the mm fractions higher than about 0.55 by FTIR spectroscopy and the structure and hydrogen bonding have been investigated in detail by solid-state ¹³C NMR spectroscopy. It is found that all OH groups are allowed to form successive intramolecular hydrogen bonding along the respective chains in the crystalline region for HI-PVAs with the mm fractions higher than about 0.7. Since these chains should contain some amount of r units even in the crystalline region, a slightly helical structure with a considerably long period may be adopted by them as an energetically stable state. On the basis of the line shape analysis of the CP/MAS ¹³C NMR spectra of the crystalline components, structural causes of the appearance of the minima of the physical values described above are also discussed in relation to the introduction of disordered units mainly associated with hydrogen bonding to the syndiotactic or isotactic sequences forming successive intermolecular or intramolecular hydrogen bonding, respectively.     

Solid-state C NMR investigation of the structure and hydrogen bonding for stereoregular poly(vinyl alcohol) films in the hydrated state

The structure and hydrogen bonding of the hydrated stereoregular poly(vinyl alcohol) (PVA) films have been investigated by high-resolution solid-state ¹³C NMR spectroscopy. It is found by the ¹³C spin-lattice relaxation analysis that there exist three components with different T_1C values assigned to the crystalline, less mobile and mobile components for the hydrated syndiotactic PVA (S-PVA) and highly isotactic PVA (HI-PVA) films. The line shape analysis indicates that the probability of intramolecular hydrogen bonding is appreciably increased in the crystalline region for the S-PVA films by the hydration but a slightly helical structure, which is probably allowed by the formation of the successive intramolecular hydrogen bondings along the chains in the crystalline region, seems not to undergo any significant change by the hydration for HI-PVA. This fact indicates that intramolecular hydrogen bonding is more stable in the hydrated state in the crystalline region. As for the less mobile component, the line shape of the CH resonance line for the hydrated S-PVA or HI-PVA films is found to be very similar to that of the corresponding crystalline component, probably being due to the successive formation of intermolecular or intramolecular hydrogen bonding in the interfacial region, which mainly contributes to the less mobile component, for the S-PVA or HI-PVA films even in the hydrated state. The mole fractions of the mm, mr and rr sequences are also estimated for the mobile component that is produced in each stereoregular PVA sample by swelling with water and it is concluded that no prominent preferential partitioning of the mm, mr and rr sequences occurs in the crystalline and noncrystalline regions for the PVA films with different tacticities.     

NMR study of the gamma radiolysis of poly(dimethyl siloxane) under vacuum at 303 K

The structural changes which occur on the γ-radiolysis of poly(dimethyl siloxane) (PDMS) under vacuum at 303 K have been investigated using ²⁹Si and ¹³C NMR. New structural units consistent with main chain scission and crosslinking through both H-linking and Y-linking reactions have been identified. The results obtained at various absorbed doses have been used to calculate the G-values for scission and crosslinking. G-values for scission of G(S)=1.3±0.2, for H-linking of G(D^CH₂-R)=0.34±0.02 and for Y-linking of G(Y)=1.70±0.09 were obtained for radiolysis under vacuum at 303 K. Thus crosslinking predominates over scission for radiolysis of PDMS under these conditions, and, by contrast with previous studies, Y-links have been shown to be the predominant form of crosslinks.     

Quantitative high-resolution13C and1H nuclear magnetic resonance of ω3 fatty acids from white muscle of atlantic salmon (Salmo salar)

High-resolution¹³C nuclear magnetic resonance (NMR) spectra have been obtained and used to define the ω3 (n-3) fatty acid distribution in lipid extract and white muscle from Atlantic salmon (Salmo salar). The¹³C spectrum of lipid extracted from muscle gives quantitative information about the individual n-3 fatty acids, 18:2n-6, 20:1/22:1 and groups of fatty acids. The quantitative data compare favorably with those obtained by gas-liquid chromatography. The¹H NMR spectrum of the lipid extract gives information about the amount of 22:6n-3 and the total content of n-3 fatty acids. The¹³C NMR technique also revealed the positional distribution (1,3- and 2-acyl) of the important 20:5n-3 and 22:6n-3 acids in the triacylglycerol molecules. In the quantitative¹³C NMR spectrum of white muscle, the methyl region of the acyl chains of triacylglycerols gave rise to sufficiently resolved signals to permit estimation of the total concentration of lipids and the n-3 fatty acid content. The NMR data are in good agreement with corresponding data obtained by traditional methods.     

Network structures and dynamics of dry and swollen poly(acrylate)s. Characterization of high- and low-frequency motions as revealed by suppressed or recovered intensities (SRI) analysis of C NMR

We recorded temperature-dependent high-resolution ¹³C NMR spectra of dry and swollen poly(acrylate)s [poly(2-methoxyethyl acrylate) (PMEA), poly(2-hydroxyethyl methacrylate) (PHEMA), and poly(tetrahydrofurfuryl acrylate) (PTHFA)] by dipolar decoupled-magic angle spinning (DD-MAS) and cross-polarization-magic angle spinning (CP-MAS) methods, to gain insight into their network structures and dynamics. Suppressed or recovered intensities (SRI) analysis of ¹³C CP-MAS and DD-MAS NMR was successfully utilized, to reveal portions of dry and swollen polymers which undergo fast and slow motions with fluctuation frequencies in the order of 10⁸ Hz and 10⁴-10⁵ Hz, respectively. Fast isotropic motions with frequency higher than 10⁸ Hz at ambient temperature were located to the portions in which ¹³C CP-MAS NMR signals of swollen PMEA were selectively suppressed. In contrast, low-frequency motion was identified to the portions in which ¹³C DD-MAS (and CP-MAS) signals are most suppressed at the characteristic suppression temperature(s) T_s. Network of PMEA gels (containing 7 wt% of water) turns out to be formed by partial association of backbones only, as manifested from their T_s gradient at lowered temperature, whereas networks of PHEMA (containing 40 wt% of water) and PTHFA (9 wt% of water) gels are tightly formed through mutual inter-chain associations of both backbones and side-chains, as viewed from the raised T_s values for both near at ambient temperature. It is also interesting to note that flexibility of gel network (PMEA > PTHFA > PHEMA) characterized by the suppression temperature T_s (PMEA < PTHFA < PHEMA) is well related with a characteristic parameter for biocompatibility such as the production of TAT (thrombin-antithrombin III complex) as a marker of activation of the coagulation system.     

Elongation-induced phase separation of poly(vinyl alcohol)/poly(acrylic-acid) blends as studied by C CP/MAS NMR and wide-angle X-ray diffraction

The membrane samples of poly(vinyl alcohol)/poly(acrylic-acid) (PVA/PAA) blend with different draw ratios were studied by both ¹³C CP/MAS NMR and wide-angle X-ray diffraction (WAXD) measurements. Phase separation induced by elongation of the sample was observed and the change of the phase structure with draw ratio was found to be dependent on the composition of the blend samples.     

NMR study of temperature-induced phase separation and polymer-solvent interactions in poly(vinyl methyl ether)/D2O/ethanol solutions

The changes in the dynamic structure during temperature-induced phase transition in D₂O/ethanol solutions of poly(vinyl methyl ether) (PVME) were studied using NMR methods. The effect of polymer concentration and ethanol (EtOH) content in D₂O/EtOH mixtures on the appearance and extent of the phase separation was determined. Measurements of ¹H and ¹³C spin-spin and spin-lattice relaxations showed the presence of two kinds of EtOH molecules: besides the free EtOH expelled from the PVME mesoglobules there are also EtOH molecules bound in PVME mesoglobules. The existence of two different types of EtOH molecules at temperatures above the phase transition was in solutions with polymer concentration 20 wt% manifested by two well-resolved NMR signals (corresponding to free and bound EtOH) in ¹³C and ¹H NMR spectra. With time the originally bound EtOH is slowly released from globular-like structures. From the point of view of polymer-solvent interactions in the phase-separated PVME solutions both EtOH and water (HDO) molecules show a similar behaviour so indicating that the decisive factor in this behaviour is a polar character of these molecules and hydrogen bonding.     

Synthesis and spectroscopic studies of new adducts of organotin(IV) chlorides with a polydentate N,S ligand

The reaction of a polydentate N,S ligand, 1′-amino-1′-cyclohexyl 2-cyclohexylideneamino-1-cyclohexene-1-dithiocarboxylate (ACCCD), with Ph₂SnCl_2, Ph₃SnCl and Me₂SnCl₂ was investigated. The new adducts have been characterized by UV–Vis, ¹H NMR, IR, mass spectroscopy and elemental analysis. These data are consistent with 1:1 adducts. The ligand behaves as a bidentate and shows both nitrogen and sulfur coordination. There is also evidence that the ligand ACCCD exhibits some interest in forming possible inter-/intra-molecular hydrogen bonds through ClH₂N interactions in Me₂SnCl₂ and Ph₂SnCl₂ adducts. The UV–Vis spectra indicate that the compounds partially dissociate in chloroform.     

The Palladium(II) Complex of Aβ4−16 as Suitable Model for Structural Studies of Biorelevant Copper(II) Complexes of N-Truncated Beta-Amyloids

The Aβ4−42 peptide is a major beta-amyloid species in the human brain, forming toxic aggregates related to Alzheimer’s Disease. It also strongly chelates Cu(II) at the N-terminal Phe-Arg-His ATCUN motif, as demonstrated in Aβ4−16 and Aβ4−9 model peptides. The resulting complex resists ROS generation and exchange processes and may help protect synapses from copper-related oxidative damage. Structural characterization of Cu(II)Aβ4−x complexes by NMR would help elucidate their biological function, but is precluded by Cu(II) paramagneticism. Instead we used an isostructural diamagnetic Pd(II)-Aβ4−16 complex as a model. To avoid a kinetic trapping of Pd(II) in an inappropriate transient structure, we designed an appropriate pH-dependent synthetic procedure for ATCUN Pd(II)Aβ4−16, controlled by CD, fluorescence and ESI-MS. Its assignments and structure at pH 6.5 were obtained by TOCSY, NOESY, ROESY, 1H-13C HSQC and 1H-15N HSQC NMR experiments, for natural abundance 13C and 15N isotopes, aided by corresponding experiments for Pd(II)-Phe-Arg-His. The square-planar Pd(II)-ATCUN coordination was confirmed, with the rest of the peptide mostly unstructured. The diffusion rates of Aβ4−16, Pd(II)-Aβ4−16 and their mixture determined using PGSE-NMR experiment suggested that the Pd(II) complex forms a supramolecular assembly with the apopeptide. These results confirm that Pd(II) substitution enables NMR studies of structural aspects of Cu(II)-Aβ complexes.