Cα Cβ Research Articles

Investigate cleavage of β-O-4 linkage in lignin model compounds by aerobic oxidation of Cα and Cγ hydroxyl groups

The selective cleavage of common linkages in lignin polymers is a promising approach to generate valuable aromatic hydrocarbons. Herein, we found that on oxidation of Cα and Cγ hydroxyl groups in β-O-4 lignin model compounds with TEMPO catalyst resulted in the formation of 1,3-dicarbonyl TEMPO adduct. These oxidized products readily underwent fragmentation at Cα–Cβ bond in the presence of a catalytic amount of acid to generate corresponding carboxylic acid and phenol monomers.

Unusual Regioselectivity in the Gold(I)‐Catalyzed [3+2] Carbocycloaddition Reaction of Vinyldiazo Compounds and N‐Allenamides

AbstractThe reaction of N‐allenamides with alkenyl diazo compounds in the presence of gold catalysts provided methylidenecyclopentene derivatives resulting from a formal intermolecular [3+2] carbocyclization, a rare process in the gold chemistry of allenes. The participation of the CαCβ bond of the allenamide represents a very unusual regioselectivity in gold‐catalyzed cycloaddition reactions of this type of allenic scaffolds. A stepwise mechanism involving initial activation of the diazo component has been proposed.magnified image

Combined gas-phase electron diffraction/mass spectrometry and DFT study of the molecular structure of zinc(II) etioporphyrin-II

The molecular structure and conformational variety of the zinc(II) etioporphyrin have been studied by the combination of gas-phase electron diffraction (GED), mass spectrometry and quantum chemical calculations. The DFT (B3LYP and PBE) calculations predict six conformers, which differ in the orientation of the ethyl groups relative to the porphyrin skeleton. The energy differences between conformers I–V do not exceed 0.3 kJ mol−1; the conformer VI has relatively high energy. Our investigation has shown that GED is not able to solve the conformational problem of the isolated zinc(II) etioporphyrin molecules. At the same time, this method allows to obtain reliable structural parameters of the macrocycle, as well as to determine the position of the ethyl groups with respect to the porphyrin core. Selected experimental parameters are as follows: rh1(Zn–N) = 2.042 (5) A, rh1(N–Cα) = 1.374 (5) A, rh1(Cα–Cβ) = 1.453 (4) A, rh1(Cα–Cm) = 1.396 (4) A, rh1(Cβ–Cβ) = 1.376(4) A, ∠ZnNCα = 126.8 (2)°, ∠CαNCα = 106.3 (3)°, ∠CαCmCα = 127.0 (4)°.

Conformational Switching in Heterochiral α,β2,3‐Hybrid Peptides in Response to Solvent Polarity

AbstractThe ability of α,β2,3‐hybrid peptides with a heterochiral backbone to exist in a helical, extended, or partially folded state depending on the solvation conditions employed was investigated. The preference was found to be directly dictated by Cα–Cβ torsions in the β‐residues.

Manganese peroxidase and biomimetic systems applied toin vitrolignin degradation inEucalyptus grandismilled wood and kraft pulps

BACKGROUND Organoperoxyl radicals generated from unsaturated fatty acids can initiate lignin degradation reactions. RESULTS Manganese peroxidase (MnP, EC 1.11.1.13) produced by the basidiomycete Ceriporiopsis subvermispora and biomimetic systems were explored to produce organoperoxyl radicals for lignin degradation of eucalypt wood and unbleached kraft pulps. Treatment of milled-wood with MnP–, Mn3+– or Fe2+–linoleic acid systems transformed the lignin structure and released water-soluble low molar mass aromatic fragments. Lignin transformations initiated by MnP were the most intense. Residual lignins from MnP- or Mn3+-treated wood exhibited diminished yields for thioacidolysis monomers and increased carboxyl content, suggesting β-O-4 and Cα–Cβ cleavage followed by side chain oxidation. MnP-based biobleaching of eucalypt kraft pulp corresponded to savings of 3.7 kg ClO2 ton−1 pulp during elemental chlorine free (ECF) bleaching to 88–90% brightness. Another interesting characteristic of the MnP-treated pulp was a slightly higher drainage before refining, suggesting that this pulp would require less energy during drying in industrial processes. CONCLUSION Lignin was degraded in vitro by the action of MnP and biomimetic systems initiating lipid peroxidation reactions. Application of the most reactive system, based on MnP, provided savings of 3.7 kg ClO2 ton−1 pulp during ECF bleaching to 88–90% ISO brightness. © 2015 Society of Chemical Industry

Demonstration of isospecific free radical polymerization of acrylate controlled by conformation and chirality of monomer

ABSTRACTRadical polymerization of lactic acid‐based chiral and achiral methylene dioxolanones, a model for conformationally s‐cis locked acrylate, was carried out with AIBN to demonstrate an isospecific free radical polymerization controlled by chirality and conformation of monomer. Polymerization of the dioxolanones proceeded smoothly without ring opening to give a polymer with moderate molecular weight and 100% of maximum isotacticity. ESR spectrum indicated a twisted conformation of the growing poly(methylene dioxolanone) radical in contrast to an acyclic analogous radical, suggesting a restriction of the free rotation around main chain CαCβ bond of the growing radical center. Chirality as well as the polarity and bulkiness of monomer affected the polymer tacticity, and chiral alkyl substituent would afford a high isotactic polymer, in which higher the enantiomeric excess of the monomer was, higher the isotacticity of the polymer was. While, achiral or polar substituents including dibenzyl and trichloromethyl groups would afford an atactic polymer. In addition, glass transition temperature (Tg) of the resulting polymers was significantly high, ranging from 172.2 to 229.8 °C, and even for an isotactic polymer Tg was as high as 206.8 °C. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2007–2016

To the limit of gas-phase electron diffraction: Molecular structure of magnesium octa(m-trifluoromethylphenyl)porphyrazine

The gas-phase molecular structure of the magnesium octa(m-trifluoromethylphenyl)porphyrazine (MgC72H32N8F24) has been studied by a synchronous gas-phase electron diffraction and mass spectrometric experiment at T=667(10) K in combination with DFT calculations using B3LYP hybrid method and triple-ζ valence basis sets. The molecule has an equilibrium structure of D4 symmetry. The following values of selected internuclear distances have been determined: rh1, Å: r(Mg–N)=1.979(5), r(N–C)=1.363(3), r(Nmezo–C)=1.334(4), r(Cpyr–CPh)=1.469(3), r(CPh–CF3)=1.510(5), r(C–F)=1.349(3), r(Cα–Cβ)=1.466(3), r(Cβ–Cβ)=1.380(7). A slight (less than 1 to 2 degrees) twisting deformation of the macrocycle from planarity, caused by the presence of the eight bulky PhCF3 substituents, planes of which are turned by 132.6(9) degrees relative to the adjacent pyrrole rings, has been found. The deviation of phenyl ring planes from 90 degrees orientation is caused by stabilizing donor–acceptor interactions between π-natural orbitals of pyrrole and phenyl moieties. Substitution effects and coordination bonding in magnesium porphyrazine complexes, MgPz, MgPzPh8 and MgPz(CF3Ph)8, are discussed. Sensitivity of GED data to long range interatomic distances of large molecules has been shown.

Effect of UV-light on the uniaxial tensile properties and structure of uncoated and TiO2 coated Bombyx mori silk fibers

The effect of UV-light on the uniaxial tensile properties and the structure of uncoated and TiO2 coated silk fibers in the bave form by using sol-gel method was investigated with tensile testing and FT-IR/ATR spectroscopy methods after the silk filaments were exposed to UV-light with high intensity of 760W/m2 for different times from 0.5h to 1day. It was clearly observed that TiO2 coating considerably increased the Young’s modulus of the uncoated silk single filament by around 17% before the UV-irradiation. The yield point and the post yield region disappeared on the stress-strain curves of both uncoated and TiO2 coated silk filaments after UV-irradiation time higher than 1h. Except for the Young’s modulus, most of the tensile characteristics of both uncoated and TiO2 coated silk filaments decreased remarkably with increasing UV-irradiation time, e.g., after 1h irradiation, although the Young’s modulus slightly changed and ultimate tensile strength decreased by only around 18% and 23%, for the uncoated and TiO2 coated silk filaments, respectively; breaking extension decreased dramatically by 67% and 72%, respectively, for uncoated and TiO2 coated silk filaments. Only the Young’s modulus of TiO2 coated silk filaments which can be considered as a more stable tensile characteristic became significantly higher than that of the uncoated silk filaments with increasing UV-irradiation time. After 1day irradiation, even though the uncoated silk filaments could not be tested and completely lost of their fiber properties, the TiO2 coated silk filaments showed a stress-strain curve in initial elastic region with Young’s modulus of ∼13GPa which indicates considerable protective effect of TiO2 on the silk fiber structure, especially on the β-sheet microcrystals against UV-radiation. The FT-IR/ATR spectral results showed that significant photodegradation took place in not only crystalline but also amorphous regions which were deduced from the decrease in the absorbance ratios of the bands assigned to CH3 rocking, Cα–Cβ, Cα–C stretching vibrations in β-sheet crystalline regions as well as the Amide I, II, and III bands for both crystalline and amorphous regions. Even though the ratio of crystalline to amorphous regions in uncoated silk filaments decreased significantly, the ratio in TiO2 coated silk filaments became almost constant with increasing UV-irradiation time which may indicate more stable β-sheet microcrystals against photodegradation.

Excited-state deactivation mechanisms of protonated and neutral phenylalanine: a theoretical study

Minimum energy paths (MEPs) of protonated phenylalanine (PheH+) at the electronic ground and S1 (1ππ*) excited states along the Cα–Cβ bond stretching coordinate, following proton transfer to the aromatic chromophore.

Conformational preferences of β-sheet structures in cyclopropane-containing γ-peptides

Oligo-γ-peptides based on 2-(aminomethyl)cyclopropanecarboxylic acid (γAmc3) with a cyclopropane constraint on the Cα–Cβ bond preferentially formed parallel β-sheets rather than antiparallel β-sheets due to the stronger N–H⋯O H-bonds in the parallel conformation.

A computational study on thermal decomposition mechanism of β-1 linkage lignin dimer

In order to gain a better understanding of the detailed mechanism of lignin pyrolysis and the formation mechanism of its main products, the thermal decomposition processes of a β-1 linkage lignin dimer model compound 1 (1,2-p-hydroxyphenyl-1,3-propanediol) were theoretically investigated by employing density functional theory methods at B3LYP/6-31G(d, p) level. Three possible pyrolytic pathways: the homolytic cleavage of Cα–Cβ bond, the homolytic cleavage of Cβ–Cγ bond and the concerted reactions, were proposed. The activation energies of each reaction step in the pyrolysis processes were calculated and the temperature effect on the pyrolysis processes was analyzed. The calculation results indicate that the homolytic cleavage reaction of Cα–Cβ bond and concerted reaction pathway (3) could be the major reaction channels, and the homolytic cleavage reaction of Cβ–Cγ bond and concerted reaction pathway (1) could be the competitive reaction channels in the pyrolysis processes of β-1 linkage lignin dimer. The concerted reactions would dominate over the free-radical homolytic reactions at lower temperatures, while at high temperatures the free-radical reaction would dominate over the concerted reactions in the β-1 linkage lignin dimer pyrolysis processes.

Hydroxyalkoxy radicals: importance of intramolecular hydrogen bonding on chain branching reactions in the combustion and atmospheric decomposition of hydrocarbons.

During both the atmospheric oxidation and combustion of volatile organic compounds, sequential addition of oxygen can lead to compounds that contain multiple hydrogen-bonding sites. The presence of two or more of these sites on a hydrocarbon introduces the possibility of intramolecular H-bonding, which can have a stabilizing effect on the reactants, products, and transition states of subsequent reactions. The present work compares the absolute energies of two sets of conformations, those that contain intramolecular H-bonds and those that lack intramolecular H-bonds, for each reactant, product, and transition state species in the 1,2 through 1,7 H-migrations and Cα-Cβ, Cα-H, and Cα-OH-bond scission reactions in the n-hydroxyeth-1-oxy through n-hydroxyhex-1-oxy radicals, for n ranging from 1 to 6. The difference in energy between the two conformations represents the balance between the stabilizing effects of H-bonds and the steric cost of bringing the two H-bonding sites together. The effect of intramolecular H-bonding and the OH group is assessed by comparing the net intramolecular H-bond stabilization energies, the reaction enthalpies, and barrier heights of the n-hydroxyalkoxy radical reactions with the corresponding alkoxy radicals values. The results suggest that there is a complex dependence on the location of the two H-bonding groups, the location of the abstraction or bond scission, and the shape of the transition state that dictates the extent to which intramolecular H-bonding effects the relative importance of H-migration and bond scission reactions for each n-hydroxyalkoxy radical. These findings have important implications for future studies on hydrocarbons with multiple H-bonding sites.

Copper Catalysts for Selective CC Bond Cleavage of β‐O‐4 Lignin Model Compounds

AbstractThe reactivity of homogeneous copper catalysts towards the selective CC bond cleavage of both phenolic and non‐phenolic arylglycerol β‐aryl ether lignin model compounds has been explored. Several copper precursors, nitrogen ligands, and solvents were evaluated in order to optimize the catalyst system. Using the optimized catalyst system, copper(I) trifluoromethanesulfonate [Cu(OTf)]/L/TEMPO (L=2,6‐lutidine, TEMPO=2,2,6,6‐tetramethyl‐piperidin‐1‐yl‐oxyl), aerobic oxidation of the non‐phenolic β‐O‐4 lignin model compound proceeded with good selectivity for CαCβ bond cleavage, affording 3,5‐dimethoxybenzaldehyde as the major product. Aerobic oxidation of the corresponding phenolic β‐O‐4 lignin model proceeded with different selectivity, affording 2,6‐dimethoxybenzoquinone and α,β‐unsaturated aldehyde products resulting from cleavage of the CαCaryl bond. At low catalyst concentrations, however, a change in selectivity was observed as oxidation of the benzylic secondary alcohol predominated with both substrates.magnified image

Crystal Structure of Tryptophan Lyase (NosL): Evidence for Radical Formation at the Amino Group of Tryptophan

AbstractStreptomyces actuosus tryptophan lyase (NosL) is a radical SAM enzyme which catalyzes the synthesis of 3‐methyl‐2‐indolic acid, a precursor in the synthesis of the promising antibiotic nosiheptide. The reaction involves cleavage of the tryptophan CαCβ bond and recombination of the amino‐acid‐derived ‐COOH fragment at the indole ring. Reported herein is the 1.8 Å resolution crystal structure of NosL complexed with its substrate. Unexpectedly, only one of the tryptophan amino hydrogen atoms is optimally placed for H abstraction by the SAM‐derived 5′‐deoxyadenosyl radical. This orientation, in turn, rules out the previously proposed delocalized indole radical as the species which undergoes CαCβ bond cleavage. Instead, stereochemical considerations indicate that the reactive intermediate is a .NH tryptophanyl radical. A structure‐based amino acid sequence comparison of NosL with the tyrosine lyases ThiH and HydG strongly suggests that an equivalent .NH radical operates in the latter enzymes.

Crystal structure of tryptophan lyase (NosL): evidence for radical formation at the amino group of tryptophan.

Streptomyces actuosus tryptophan lyase (NosL) is a radical SAM enzyme which catalyzes the synthesis of 3-methyl-2-indolic acid, a precursor in the synthesis of the promising antibiotic nosiheptide. The reaction involves cleavage of the tryptophan Cα-Cβ bond and recombination of the amino-acid-derived -COOH fragment at the indole ring. Reported herein is the 1.8 Å resolution crystal structure of NosL complexed with its substrate. Unexpectedly, only one of the tryptophan amino hydrogen atoms is optimally placed for H abstraction by the SAM-derived 5'-deoxyadenosyl radical. This orientation, in turn, rules out the previously proposed delocalized indole radical as the species which undergoes Cα-Cβ bond cleavage. Instead, stereochemical considerations indicate that the reactive intermediate is a (·)NH tryptophanyl radical. A structure-based amino acid sequence comparison of NosL with the tyrosine lyases ThiH and HydG strongly suggests that an equivalent (·)NH radical operates in the latter enzymes.

An unusual conformation of gabapentin (Gpn) in Pyr-Gpn-NH-NH-Pyr stabilized by weak interactions.

The crystal structure of N-[(1-{2-oxo-2-[2-(pyrazin-2-ylcarbonyl)hydrazin-1-yl]ethyl}cyclohexyl)methyl]pyrazine-2-carboxamide monohydrate (Pyr-Gpn-NN-NH-Pyr·H2O), C19H23N7O3·H2O, reveals an unusual trans-gauche (tg(-)) conformation for the gabapentin (Gpn) residue around the C(γ)-C(β) (θ1) and C(β)-C(α) (θ2) bonds. The molecular conformation is stabilized by intramolecular N-H...N hydrogen bonds and weak C-H...O interactions. The packing of the molecules in the crystal lattice shows a network of strong N-H...O and O-H...O hydrogen bonds together with weak C-H...O and π-π inteactions.

Local Structure and Dynamics of Serine in the Heterogeneous Structure of the Crystalline Domain of Bombyx mori Silk Fibroin in Silk II Form Studied by 2D 13C–13C Homonuclear Correlation NMR and Relaxation Time Observation

The crystalline fraction (Cp fraction) of silk fibroin in silk II form from the silkworm Bombyx mori is the classic example of antiparallel β-sheet and consists mainly of Ala, Ser, and Gly. In the 13C CP/MAS NMR spectrum, the Ala Cβ, Ser Cα, and Ser Cβ peaks are asymmetric, showing the heterogeneous nature of the structure, which is shown to consist of two different packing geometries, denoted domains A and B. In this work, these peaks were resolved and assigned using 2D 13C–13C homonuclear correlation NMR spectra collected with different dipolar- assisted rotational resonance (DARR) mixing times. The local structure and dynamics of serine in the Cp fraction are discussed in detail. Model peptides of the Cp fraction, [3-13C]Ser-(AGSGAG)5 with different [3-13C]Ser labeling positions, were prepared to clarify that the individual peaks of the Ser Cβ carbons come from different domains, not from different positions within a chain. The dynamics of these individual peaks were studied by measuring 13C T1 values, which provide information on serine side chain dynamics and thus on the formation of intermolecular hydrogen bonding through the Ser OH group. Moreover, cross peaks between domains A and B were observed in the Ala Cβ DARR spectrum with a long mixing time of 400 ms, indicating a close contact between the two geometries. We conclude that the crystalline region is heterogeneous, comprising two closely associated packing geometries that form separate but small domains.

Highly Enantioselective Bioinspired Epoxidation of Electron-Deficient Olefins with H2O2on Aminopyridine Mn Catalysts

The asymmetric epoxidation of various electron-deficient olefins with H2O2 in the presence of a novel family of chiral bioinspired bipyrrolidine-derived aminopyridine manganese(II) complexes [LMII(OTf)2] is reported. High enantioselectivities (up to 99% ee) and epoxide selectivities (up to 100%), unprecedented for catalysts of this type, have been achieved; the catalysts perform up to 8500 catalytic turnovers. The presence of electron donors in the catalyst structure substantially enhances the enantioselectivity. Isotopic (18O) labeling studies provide evidence of the formation of the oxomanganese(V) active species. Hammett analysis suggests that the enantioselective epoxidation is rate-limited by the transfer of an electron to the MnVO intermediate, to form a short-lived acyclic (carbocationic) intermediate. In effect, the epoxide stereoconfiguration may be affected by the competition between the rotation around the Cα–Cβ single bond and the epoxide ring collapse.

A Metal‐Free, Carbon‐Based Catalytic System for the Oxidation of Lignin Model Compounds and Lignin

AbstractNitrogen‐containing graphene material (LCN) has been identified as an effective catalyst for the oxidation of β‐O‐4 and α‐O‐4 types of lignin model compounds in the presence of tert‐butyl hydroperoxide, to provide aromatic aldehydes, acids and other organic chemicals in high yield. The transformations of five lignin model compounds over LCN were investigated systematically. Instrumentation analysis, kinetic study and radical trapping experiments highlight the mechanistic features of the reaction, including: 1) the reaction pathway starts by benzylic CH or COH bond activation, followed by CαCβ or CαO bond cleavage, and finally further oxidation of intermediate aromatics; and 2) the reaction follows a free‐radical mechanism with all the key steps involving radical species. In addition, the LCN proved to be a highly stable catalyst; no significant activity decrease was observed for four consecutive runs, and X‐ray photoelectron spectroscopy analysis indicates negligible decrease in the content of the active nitrogen species in the catalyst. Notably, this new catalytic system can be extended to the oxidative depolymerisation of real lignin, to produce a significant portion of liquefied, low‐molecular‐mass products.

A Mass Spectrometric Approach for Probing the Stability of Bioorganic Radicals

Glycyl radicals are important bioorganic radical species involved in enzymatic catalysis. Herein, we demonstrate that the stability of glycyl-type radicals (X-(.) CH-Y) can be tuned on a molecular level by varying the X and Y substituents and experimentally probed by mass spectrometry. This approach is based on the gas-phase dissociation of cysteine sulfinyl radical (X-Cys SO .-Y) ions through homolysis of a Cα Cβ bond. This fragmentation produces a glycyl-type radical upon losing CH2 SO, and the degree of this loss is closely tied to the stability of the as-formed radical. Theoretical calculations indicate that the energy of the Cα Cβ bond homolysis is predominantly affected by the stability of the glycyl radical product through the captodative effect, rather than that of the parent sulfinyl radical. This finding suggests a novel experimental method to probe the stability of bioorganic radicals, which can potentially broaden our understanding of these important reactive intermediates.