4-methoxybenzyl Alcohol Research Articles

Photoelectrocatalytic selective oxidation of 4-methoxybenzyl alcohol in water by TiO2 supported on titanium anodes

The photoelectrocatalytic partial oxidation of 4-methoxybenzyl alcohol in aqueous solution irradiated by near-UV light was carried out in a three-electrode batch reactor. TiO2 films were either deposited by dip-coating of a TiO2 sol onto a Ti foil and subsequent calcination or generated on Ti plates by thermal oxidation in air at 400–700°C. The effects of the anode preparation method and bias potential values on conversion and selectivity to the corresponding aldehyde were investigated. The photoelectrocatalytic results were compared with the photocatalytic and electrocatalytic ones. The results indicated that no reaction occurred during the electrocatalytic experiments, whereas the photocatalytic reactivity was positively influenced by the application of a small bias (0.75V vs. saturated calomel electrode). By applying the previous bias the highest initial degradation rates, r0, and selectivities to 4-methoxybenzaldehyde, S, were obtained in photoelectrocatalytic experiments by using Ti plate calcined at 500°C (r0=0.1507mMh−1, S=85%) or TiO2 film prepared by dip-coating and calcined at 700°C (r0=0.1339mMh−1, S=90%). In order to investigate the influence of the substituent groups on reactivity and selectivity, photoelectrocatalytic runs with benzyl alcohol, 2-methoxybenzyl alcohol, 3-methoxybenzyl alcohol, 2,4-dimethoxybenzyl alcohol, 2,3,4-trimethoxybenzyl alcohol, 4-nitrobenzyl alcohol and 4-hydroxybenzyl alcohol have been performed. The selectivity and reactivity values increased by increasing the electron-donor properties of the substituent groups. This positive effect, however, was obtained only if the electron-donor group is in para or ortho position with respect to the alcoholic group.

Aggregation Effects in Visible‐Light Flavin Photocatalysts: Synthesis, Structure, and Catalytic Activity of 10‐Arylflavins

A series of 10-arylflavins (10-phenyl-, 10-(2',6'-dimethylphenyl)-, 10-(2',6'-diethylphenyl)-, 10-(2',6'-diisopropylphenyl)-, 10-(2'-tert-butylphenyl)-, and 10-(2',6'-dimethylphenyl)-3-methylisoalloxazine (2 a-f)) was prepared as potentially nonaggregating flavin photocatalysts. The investigation of their structures in the crystalline phase combined with (1)H-DOSY NMR spectroscopic experiments in CD(3)CN, CD(3)CN/D(2)O (1:1), and D(2)O confirm the decreased ability of 10-arylflavins 2 to form aggregates relative to tetra-O-acetyl riboflavin (1). 10-Arylflavins 2 a-d do not interact by π-π interactions, which are restricted by the 10-phenyl ring oriented perpendicularly to the isoalloxazine skeleton. On the other hand, N3-H⋅⋅⋅O hydrogen bonds were detected in their crystal structures. In the structure of 10-aryl-3-methylflavin (2 f) with a substituted N3 position, weak C-H⋅⋅⋅O bonds and weak π-π interactions were found. 10-Arylflavins 2 were tested as photoredox catalysts for the aerial oxidation of 4-methoxybenzyl alcohol to the corresponding aldehyde (model reaction), thus showing higher efficiency relative to 1. The quantum yields of 4-methoxybenzyl alcohol oxidation reactions mediated by arylflavins 2 were higher by almost one order of magnitude relative to values in the presence of 1.

Exploration of Vanilla pompona from the Peruvian Amazon as a potential source of vanilla essence: Quantification of phenolics by HPLC-DAD

This study provides the first chemical investigation of wild-harvested fruits of Vanilla pompona ssp. grandiflora (Lindl.) Soto-Arenas developed in their natural habitat in the Peruvian Amazon. Flowers were hand-pollinated and the resulting fruits were analysed at different developmental stages using an HPLC-DAD method validated for the quantification of glucovanillin and seven other compounds. The method showed satisfactory linearity (r2>0.9969), precision (coefficient of variation <2%), recoveries (70–100%), limit of detection (0.008–0.212μg/ml), and limit of quantification (0.027–0.707μg/ml). The evaluation of crude and enzyme-hydrolyzed Soxhlet-extracted samples confirmed the leading role of glucosides in fruit development. LC–ESI-MS studies corroborated the identities of four glucosides and seven aglycones, among them vanillin (5.7/100g), 4-hydroxybenzyl alcohol (3.6/100g), and anisyl alcohol (7.1/100g) were found in high concentrations. The attractive flavor/aroma profile exhibited by wild V. pompona fruits supports studies focused on the development of this species as a specialty crop.

A very practical and selective method for PMB protection of alcohols

A very simple, practical and efficient one-step heterogeneous protocol for the PMB protection of alcohols using Amberlyst-15 has been developed. The stability and hazard issues regarding PMBCl and PMBBr are totally avoided by directly using anisyl alcohol for the protection. Alcohols are protected in very good yields. The selective mono-PMB protection of diols as well as di-PMB protection of diols was achieved in good yields, along with the demonstration of recyclability of the catalyst.

Aerobic Oxidation Reactions Catalyzed by Vanadium Complexes of Bis(Phenolate) Ligands

Vanadium(V) complexes of the tridentate bis(phenolate)pyridine ligand H(2)BPP (H(2)BPP = 2,6-(HOC(6)H(2)-2,4-(t)Bu(2))(2)NC(5)H(3)) and the bis(phenolate)amine ligand H(2)BPA (H(2)BPA = N,N-bis(2-hydroxy-4,5-dimethylbenzyl)propylamine) have been synthesized and characterized. The ability of the complexes to mediate the oxidative C-C bond cleavage of pinacol was tested. Reaction of the complex (BPP)V(V)(O)(O(i)Pr) (4) with pinacol afforded the monomeric vanadium(IV) product (BPP)V(IV)(O)(HO(i)Pr) (6) and acetone. Vanadium(IV) complex 6 was oxidized rapidly by air at room temperature in the presence of NEt(3), yielding the vanadium(V) cis-dioxo complex [(BPP)V(V)(O)(2)]HNEt(3). Complex (BPA)V(V)(O)(O(i)Pr) (5) reacted with pinacol at room temperature, to afford acetone and the vanadium(IV) dimer [(BPA)V(IV)(O)(HO(i)Pr)](2). Complexes 4 and 5 were evaluated as catalysts for the aerobic oxidation of 4-methoxybenzyl alcohol and arylglycerol β-aryl ether lignin model compounds. Although both 4 and 5 catalyzed the aerobic oxidation of 4-methoxybenzyl alcohol, complex 4 was found to be a more active and robust catalyst for oxidation of the lignin model compounds. The catalytic activities and selectivities of the bis(phenolate) complexes are compared to previously reported catalysts.

Triplet state of 4-methoxybenzyl alcohol chemisorbed on silica nanoparticles

The knowledge of photochemical kinetics in colloidal systems is important in understanding environmental photochemistry on dispersed solid surfaces. As model materials for the chemically sorbed organic compounds present in natural environments, modified silica nanoparticles (NPs) were obtained here by condensation of the silanol groups of fumed silica nanoparticles with 4-methoxybenzyl alcohol. These particles were characterized by different techniques. To evaluate their toxicity, the inhibition of the natural luminescence emission of the marine bacterium Vibrio fischeri in suspensions of the particles was measured. Laser flash-photolysis experiments (λ(exc) = 266 nm) performed with NP suspensions in acetonitrile-aqueous phosphate buffer mixtures showed the formation of the lowest triplet excited state of the chemisorbed organic groups (λ(max) = 390 nm). DFT calculations of the absorption spectrum of this radical support the assignment. From the calculated triplet energy, a thermodynamically favorable energy transfer from these triplet states to oxygen to yield singlet molecular oxygen is predicted. A value of 0.09 was measured for the quantum yield of singlet molecular oxygen generation by air-saturated suspensions of the nanoparticles in the mixture of solvents acetonitrile-aqueous phosphate buffer. The quantum yield of singlet molecular oxygen generation by the free 4-methoxybenzyl alcohol in the same solvent is 0.31.

Partial oxidation of aromatic alcohols via TiO2 photocatalysis: the influence of substituent groups on the activity and selectivity

Aromatic alcohols with substituent groups in different positions have been partially oxidised to the corresponding aldehydes in a photocatalytic system in order to investigate the influence of the substituents on reactivity and selectivity to aldehyde. To this aim benzyl alcohol, 2-methoxybenzyl alcohol, 3-methoxybenzyl alcohol, 4-methoxybenzyl alcohol, 2,4-dimethoxybenzyl alcohol, 4-hydroxybenzyl alcohol and 4-hydroxy-3-methoxybenzyl alcohol have been photocatalytically oxidised to their corresponding aldehydes in aqueous TiO2 suspensions under near-UV irradiation. Home-made and commercial rutile TiO2 samples were used as photocatalysts. The catalysts were characterized by XRD, BET, SEM, TEM and TGA measurements. For all the used substrates the main oxidation products were the corresponding aldehydes and CO2. The aromatic alcohols showed selectivity values decreasing with the substituent position on the aromatic ring according the following order: para > ortho > meta. In the presence of two substituent groups, the overall oxidation rate increased while the selectivity decreased. The home-made catalyst generally showed selectivity higher but activity lower than those of the commercial one. The results showed that the reaction rate and selectivity were dependent not only on the catalyst properties such as crystallinity and hydrophilicity but also on the kind and position of the substituent groups of the aromatic alcohols.

Enhancing selectivity in photocatalytic formation of p-anisaldehyde in aqueous suspension under solar light irradiation via TiO2 N-doping

The photocatalytic partial oxidation of 4-methoxybenzyl alcohol to the corresponding aldehyde (p-anisaldehyde) was performed under simulated solar irradiation by using home prepared N-doped TiO2 catalysts. The photocatalysts were prepared by a sol–gel method, using TiCl4 as TiO2 precursor and NH4Cl, urea or NH4OH as N-doping sources. A commercial TiO2 (Degussa P25) was also used for comparison aims. The prepared catalysts were characterized by BET specific surface area, XRD, ESEM and UV-vis spectroscopy. The reactivity results show that (i) the doped catalysts are predominantly amorphous, and they show selectivity values far higher than those of the corresponding undoped ones and of well crystallized catalysts – even if the last ones show a higher activity – and (ii) exploitation of solar light significantly increases the reaction selectivity. In addition, different light sources were also used in order to investigate the effect of radiation wavelength ranges on the reactivity and selectivity to aldehyde.

Osmium-Catalyzed Oxidation of Primary Alcohols with Molecular Oxygen

OsHCl(CO)(PiPr3)2 (1) reacts with O2 to afford the dioxygen derivative OsHCl(CO)(η2-O2)(PiPr3)2 (2), which under atmospheric pressure of oxygen promotes the oxidation of 4-methoxybenzyl alcohol, benzyl alcohol, and 4-chlorobenzyl alcohol to the corresponding aldehydes. Under the same conditions, 1-hexanol and 1-octanol give hexyl hexanoate and octyl octanoate, respectively. On the other hand, cinnamyl alcohol undergoes disproportionation to 3-phenylpropanol and cinnamaldehyde.

Selective photocatalytic oxidation at TiO 2/Ti anodes of 4-methoxybenzyl alcohol to the corresponding benzaldehyde in “green” conditions

The photosensitized electrochemical oxidation at a TiO 2/Ti anode is, in itself, a “green” chemical strategy as the semiconductor remains unchanged during the experiments, the metal is safe, low energies are involved and experimental conditions are mild. In this context, the oxidation of 4-methoxybenzyl alcohol to 4-methoxybenzaldehyde has been carried out on the light of further “green chemistry” requirements: water as medium, NaCl as supporting electrolyte, exclusion of the reference electrode, Ti cathode (less expensive than the traditional Pt one) and Ti mesh (to optimize the cell geometry). In this study some chemical (alcohol conversion to aldehyde and material recovery) and electrochemical (current efficiency and density) parameters have been used to select the reaction conditions that are more suitable for this purpose. As concern the cell, the best geometry was realized with an immersion lamp, coaxial with a mesh cylindrical TiO 2/Ti anode and a mesh Ti cathode.

Substrate diffusion and oxidation in GMC oxidoreductases: an experimental and computational study on fungal aryl-alcohol oxidase

AAO (aryl-alcohol oxidase) provides H₂O₂ in fungal degradation of lignin, a process of high biotechnological interest. The crystal structure of AAO does not show open access to the active site, where different aromatic alcohols are oxidized. In the present study we investigated substrate diffusion and oxidation in AAO compared with the structurally related CHO (choline oxidase). Cavity finder and ligand diffusion simulations indicate the substrate-entrance channel, requiring side-chain displacements and involving a stacking interaction with Tyr⁹². Mixed QM (quantum mechanics)/MM (molecular mechanics) studies combined with site-directed mutagenesis showed two active-site catalytic histidine residues, whose substitution strongly decreased both catalytic and transient-state reduction constants for p-anisyl alcohol in the H502A (over 1800-fold) and H546A (over 35-fold) variants. Combination of QM/MM energy profiles, protonation predictors, molecular dynamics, mutagenesis and pH profiles provide a robust answer regarding the nature of the catalytic base. The histidine residue in front of the FAD ring, AAO His⁵⁰² (and CHO His⁴⁶⁶), acts as a base. For the two substrates assayed, it was shown that proton transfer preceded hydride transfer, although both processes are highly coupled. No stable intermediate was observed in the energy profiles, in contrast with that observed for CHO. QM/MM, together with solvent KIE (kinetic isotope effect) results, suggest a non-synchronous concerted mechanism for alcohol oxidation by AAO.

Porosity driven photocatalytic activity of wormhole mesoporous TiO2-xNx in direct sunlight

Results obtained by combining four important factors simultaneously, namely, wormhole mesoporosity with low diffusion length for charge carriers, high surface area, nanoparticles with high crystallinity, and visible light absorption due to N-doping, in titania (meso-TiO2-xNx) are reported. Meso-TiO2-xNx materials have been prepared by a combustion method within 10 min and by varying urea : Ti(NO3)4 between 1 (UT1) and 10 (UT10). All of the prepared materials have been thoroughly characterised. Nanocrystalline anatase phase with high surface area (234 m2 g−1), and type-IV H3-mesoporosity is observed with UT10. Photocatalytic rhodamine-B degradation was employed to screen for the activity of the materials, and p-anisyl alcohol oxidation to p-anisaldehyde was carried out successfully in aqueous solution under direct sunlight. High photocatalytic activity of UT10 in direct sunlight, in spite of high band gap (3.24 eV), is attributed to the better utilization of holes due to the low charge diffusion barrier associated with wormhole mesoporosity along with highly crystalline, however, nanoparticulate TiO2-xNx.

Chemical Composition and Antibacterial Activities of Illicium verum Against Antibiotic-Resistant Pathogens

In recent years, human pathogenic microorganisms have developed multiple drug resistance and caused serious nosocomial infections. In this study, we identified four new antimicrobial compounds from the Chinese herbal medicine Illicium verum and assessed their antibacterial efficacies. The supercritical CO₂ and ethanol extracts of Illicium verum showed substantial antibacterial activity against 67 clinical drug-resistant isolates, including 27 Acinetobacter baumannii, 20 Pseudomonas aeruginosa, and 20 methicillin-resistant Staphylococcus aureus. The diethyl ether (EE) fraction obtained from partition extraction and supercritical CO₂ extracts revealed an antibacterial activity with a minimum inhibitory concentration value of 0.15-0.70 mg/mL and 0.11 mg/mL, respectively. The EE fraction of I. verum showed synergetic effects with some commercial antibiotics. The antimicrobial mechanism was investigated with killing curves and scanning electron microscopy observation. The chemical components of the extracts were analyzed by spectrophotometry; (E)-anethole, anisyl acetone, anisyl alcohol, and anisyl aldehyde exhibited antibacterial activity against different clinical isolates. These extracts from I. verum can be further developed into antibiotic medicines due to their proven antibacterial activity.

SYNTHESIS AND ANTIPLASMODIAL ACTIVITY TESTING OF (1)-N-(4-METHOXYBENZYL)-1,10-PHENANTHROLINIUM BROMIDE

Synthesis of (1)-N-(4-methoxybenzyl)-1,10-phenanthroline bromide from 1,10-phenanthroline monohydrate and 4-methoxybenzaldehyde as starting material and evaluation of its antiplasmodial activities have been carried out. The 4-methoxybenzyl alcohol was prepared from 4-methoxy-benzaldehyde using sodium borohydride (NaBH4) reagent and ethanol absolute solution. The mixture was refluxed for 3 h. To yield colorless dilution compound with 90.41 % in efficiency. Furthermore, bromination of 4-methoxybenzyl alcohol with phosphorus bromide (PBr3) was conducted by refluxing for 3 h. The product of this reaction was yellow liquid of 4-methoxybenzyl bromide, 79.03% yield and 95.34 % purity. The final step of reaction was benzylation of 1,10-phenanthroline monohydrate with 4-methoxybenzyl bromide reagent. It was conducted by refluxing in aceton for 8 h at 55 oC. The yield of the reaction was (1)-N-(4-methoxybenzyl)-1,10-phenanthroline bromide (77.63%). It is pink solid form, and its melting point is 192-193 oC. Identification of the product was carried out by means of GC-MS, IR and 1H-NMR spectrometers. The in vitro antiplasmodial activity on chloroquine-resistant Plasmodium falciparum FCR-3 strain and chloroquine sensitive P. falciparum D10 strain for (1)-N-(4-methoxybenzyl)-1,10-phenanthroline bromide were determined by microscopic method. The result showed that after 72 h incubation, it has IC50 0.93±0.02 µM and 1.21±0.09 µM, respectively. Keywords: 1,10-phenanthroline, (1)-N-(4-methoxybenzyl)-1,10-phenanthroline bromide, 4 methoxybenzaldehyde, antiplasmodial activities

GREEN CHEMISTRY APPLICATION FOR THE SYNTHESIS OF (1)-N-4’-METHOXYBENZYL-1,10-PHENANTHROLINIUM BROMIDE

A simple, energy-efficient, and relatively quick synthetic procedure for the synthesis of (1)-N-4'-methoxybenzyl-1,10-phenanthrolinium bromide, based on green chemistry principles has been carried out. The synthesis was started by solvent-free reduction of p-anisaldehyde with NaBH4 to give 4-methoxybenzyl alcohol in 98% yield to be followed by solvent-free treatment of the resulted alcohol with PBr3 to yield 4-methoxybenzyl bromide (86%). Furthermore, the obtained bromide was reacted with 1,10-phenanthroline in acetone at reflux for 12 h to give the phenanthrolinium salt target in 68% yield. Keywords: green chemistry, p-anisaldehyde, (1)-N-4'-methoxybenzyl-1,10-phenanthrolinium bromide

Determination of suspected allergens in cosmetic products by headspace-programmed temperature vaporization–fast gas chromatography–quadrupole mass spectrometry

In the present work, a strategy for the qualitative and quantitative analysis of 24 volatile compounds listed as suspected allergens in cosmetics by the European Union is reported. The list includes benzyl alcohol, limonene, linalool, methyl 2-octynoate, beta-citronellol, geraniol, citral (two isomers), 7-hydroxycitronellal, anisyl alcohol, cinnamal, cinnamyl alcohol, eugenol, isoeugenol (two isomers), coumarin, alpha-isomethyl ionone, lilial, alpha-amylcinnamal, lyral, alpha-amylcinnamyl alcohol, farnesol (three isomers), alpha-hexyl cinnamal, benzyl cinnamate, benzyl benzoate, and benzyl salicylate. The applicability of a headspace (HS) autosampler in combination with a gas chromatograph (GC) equipped with a programmable temperature vaporizer (PTV) and a quadrupole mass spectrometry (qMS) detector is explored. By using a headspace sampler, sample preparation is reduced to introducing the sample into the vial. This reduces the analysis time and the experimental errors associated with this step of the analytical process. Two different injection techniques were used: solvent-vent injection and hot-split injection. The first offers a way to improve sensitivity at the same time maintaining the simple headspace instrumentation and it is recommended for compounds at trace levels. The use of a liner packed with Tenax-TA allowed the compounds of interest to be retained during the venting process. The signals obtained when hot-split injection was used allowed quantification of all the compounds according to the thresholds of the European Cosmetics Directive. Monodimensional gas chromatography coupled to a conventional quadrupole mass spectrometry detector was used and the 24 analytes were separated appropriately along a run time of about 12 min. Use of the standard addition procedure as a quantification technique overcame the matrix effect. It should be emphasized that the method showed good precision and accuracy. Furthermore, it is rapid, simple, and--in view of the results--highly suitable for the determination of suspected allergens in different cosmetic products.

Simple cyclodextrin aldehydes as excellent artificial oxidases

Cyclodextrin based oxidases, with a ketone as functional group are well known as good artificial enzyme mimics (Fenger et al. Org Biomol Chem 7:933–943; Marinescu and Bols Angew Chem Int Ed 45:4590–4593; Bjerre et al. Eur J Org Chem 704–710; Marinescu et al. J Am Chem Soc 127:17578–17579). We here report a series of modified cyclodextrins, having aldehydes as functional groups. The aldehyde based artificial enzymes have, in most cases, better catalysis than the ketones, because of their powerful covalent binding of hydrogen peroxide. Among the modified cyclodextrins studied are mono and di aldehydes on the 6 positions, with or without methylated hydroxyl groups. The aldehyde functionality was also introduced close to the secondary side, by attaching ethoxy-2-al or propoxy-3-al to the 2 position. The modified cyclodextrins showed excellent enzymatic activity towards oxidation of different aminophenols, and 4-methoxy benzyl alcohol with hydrogen peroxide as a stoichiometric oxidant. Rate enhancements up to 4,600 were achieved for oxidation of 4-methoxy benzyl alcohol, where as oxidation of amines gave rate enhancements up to 3,400. The artificial oxidases catalyses oxidations under enzymatic conditions (water, pH 7, 25 °C), following Michaelis–Menten kinetics. To confirm the enzyme activity, inhibition studies with sodium naphthalene-2-sulfonate were carried out. These studies showed competitive inhibition of the enzymes, verifying the cyclodextrins enzyme like character.

Quantitative determination of some volatile suspected allergens in cosmetic creams spread on skin by direct contact sorptive tape extraction–gas chromatography–mass spectrometry

This study describes a method based on direct contact sorptive tape extraction followed by on-line thermal desorption gas chromatography–mass spectrometry (DC-STE–GC–MS) to detect and quantify a group of suspected volatile allergens on the European Union (E.U.) list and a related compound on the skin (the stratum corneum) of volunteers treated with a cream of known composition fortified with the reference allergens. The following compounds were tested: citronellol, Z-citral (neral), geraniol, cinnamaldehyde, anisyl alcohol, cinnamyl alcohol, eugenol, methyleugenol, coumarin, isoeugenol, α-isomethylionone, 2-(4-tert-butylbenzyl)propionaldehyde (lilial), α-amylcinnamaldehyde, α-hexylcinnamaldehyde. Sorptive tape extraction (STE) is a sorption-based sampling technique in which a flexible polydimethylsiloxane (PDMS) tape is used to recover analytes by direct contact with the surface of a solid matrix or from the headspace in equilibrium with it. The reliability of the method was confirmed by: (i) allergen recoveries varying from 52.3% for lilial to 95.7% for neral, (ii) linearity in the range 10–150 ppm, with regression coefficient R 2 always above 0.97, (iii) repeatability of each analyte, RSD% never exceeding 10%, (iv) intermediate precision, always below 15%, and (v) LOD and LOQ in the ppb range, therefore fully compatible with E.U. prescriptions (ppm). Other parameters such as substantivity analyte, approximate permeation through skin and influence of different nature of stratum corneum on recovery were also investigated. The method was also successfully applied to five commercially available creams declared to contain some of the allergens in question spread on the skin of the same volunteers.

Kinetics of 4-Methoxybenzyl Alcohol Oxidation in Aqueous Solution in a Fixed Bed Photocatalytic Reactor

The photocatalytic oxidation of 4-methoxybenzyl alcohol in water was performed in a fixed bed continuous annular reactor by using a home-prepared TiO2 catalyst supported on Pyrex glass beads. The investigation was aimed to modeling the complex kinetics of the photoprocess which occurs through two parallel pathways: (i) partial oxidation to the corresponding aldehyde and (ii) total oxidation to CO2 and H2O. On these grounds, the influence of liquid flow rate, inlet concentrations of alcohol and oxygen, catalyst amount, and irradiation power on the photoreactivity were studied. A kinetic model like that of Langmuir−Hinshelwood satisfactorily fitted the experimental results and allowed determination of the values of model parameters. It was found that all these parameters are positively affected by an increase of radiant energy absorbed by the catalyst. All the reactivity results indicate that the partial oxidation pathway is favored by the low flux of absorbed photons and by low oxygen coverage on the TiO2 ...

Aryl-alcohol Oxidase Involved in Lignin Degradation: A MECHANISTIC STUDY BASED ON STEADY AND PRE-STEADY STATE KINETICS AND PRIMARY AND SOLVENT ISOTOPE EFFECTS WITH TWO ALCOHOL SUBSTRATES

Aryl-alcohol oxidase (AAO) is a FAD-containing enzyme in the GMC (glucose-methanol-choline oxidase) family of oxidoreductases. AAO participates in fungal degradation of lignin, a process of high ecological and biotechnological relevance, by providing the hydrogen peroxide required by ligninolytic peroxidases. In the Pleurotus species, this peroxide is generated in the redox cycling of p-anisaldehyde, an extracellular fungal metabolite. In addition to p-anisyl alcohol, the enzyme also oxidizes other polyunsaturated primary alcohols. Its reaction mechanism was investigated here using p-anisyl alcohol and 2,4-hexadien-1-ol as two AAO model substrates. Steady state kinetic parameters and enzyme-monitored turnover were consistent with a sequential mechanism in which O(2) reacts with reduced AAO before release of the aldehyde product. Pre-steady state analysis revealed that the AAO reductive half-reaction is essentially irreversible and rate limiting during catalysis. Substrate and solvent kinetic isotope effects under steady and pre-steady state conditions (the latter showing approximately 9-fold slower enzyme reduction when alpha-bideuterated substrates were used, and approximately 13-fold slower reduction when both substrate and solvent effects were simultaneously evaluated) revealed a synchronous mechanism in which hydride transfer from substrate alpha-carbon to FAD and proton abstraction from hydroxyl occur simultaneously. This significantly differs from the general mechanism proposed for other members of the GMC oxidoreductase family that implies hydride transfer from a previously stabilized substrate alkoxide.