Biosynthetic pathway for halogenated methoxybenzaldehydes in the white rot fungus Bjerkandera adusta

Production of an Anise- and Woodruff-like Aroma by Monokaryotic Strains of Pleurotus sapidus Grown on Citrus Side Streams

The production of natural flavors by means of microorganisms is of great interest for the food and flavor industry, and by-products of the agro-industry are particularly suitable as substrates. In the present study, Citrus side streams were fermented using monokaryotic strains of the fungus Pleurotus sapidus. Some of the cultures exhibited a pleasant smell, reminiscent of woodruff and anise, as well as herbaceous notes. To evaluate the composition of the overall aroma, liquid/liquid extracts of submerged cultures of a selected monokaryon were prepared, and the volatiles were isolated via solvent-assisted flavor evaporation. Aroma extract dilution analyses revealed p-anisaldehyde (sweetish, anisic- and woodruff-like) with a flavor dilution factor of 2¹⁸ as a character impact compound. The coconut-like, herbaceous, and sweetish smelling acyloin identified as (2S)-hydroxy-1-(4-methoxyphenyl)-1-propanone also contributed to the overall aroma and was described as an aroma-active substance with an odor threshold in air of 0.2 ng L⁻¹ to 2.4 ng L⁻¹ for the first time. Supplementation of the culture medium with isotopically substituted l-tyrosine elucidated this phenolic amino acid as precursor of p-anisaldehyde as well as of (2S)-hydroxy-1-(4-methoxyphenyl)-1-propanone. Chiral analysis via HPLC revealed an enantiomeric excess of 97% for the isolated product produced by P. sapidus.

Fungal benzene carbaldehydes: occurrence, structural diversity, activities and biosynthesis

Covering: up to April 2020Fungal benzene carbaldehydes with salicylaldehydes as predominant representatives carry usually hydroxyl groups, prenyl moieties and alkyl side chains. They are found in both basidiomycetes and ascomycetes as key intermediates or end products of various biosynthetic pathways and exhibit diverse biological and pharmacological activities. The skeletons of the benzene carbaldehydes are usually derived from polyketide pathways catalysed by iterative fungal polyketide synthases. The aldehyde groups are formed by direct PKS releasing, reduction of benzoic acids or oxidation of benzyl alcohols.

Key Odorants from the Fermentation Broth of the Edible Mushroom Ischnoderma resinosum

Eighteen odorants were identified by applying aroma extract dilution analysis in the liquid fermentation broth of the edible mushroom Ischnoderma resinosum (P. Karst). Eight compounds with flavor dilution factors ≥16 were quantitated in a 16-day sample using stable isotope dilution assays. Odor activity values (OAV) revealed anise-smelling 4-methoxybenzaldehyde (OAV; 1639), vanilla-smelling 3,4-dimethoxybenzaldehyde (OAV; 51), and cherry-smelling benzaldehyde (OAV; 14) as key contributors to the pleasant "candy-like" odor of the fermentation broth. Odor simulation experiments revealed a mixture of five compounds in their natural concentrations mimicked the odor of a 16-day-old fermentation broth. A 30-day time course study was conducted to monitor the production of three odorants in the fungal fermentation broth, where it was revealed that both 3,4-dimethoxybenzaldehyde (10.7 ± 1.0 mg/kg) and benzaldehyde (4.5 ± 0.1 mg/kg) peaked on day 16, whereas 4-methoxybenzaldehyde peaked on day 24 (104.9 ± 4.9 mg/kg).

Stereoselective biosynthesis of chloroarylpropane diols by the basidiomyceteBjerkandera adusta: exploring the roles of amino acids, pyruvate, glycerol and phenyl acetyl carbinol

Bjerkandera adusta produces many chlorometabolites including chlorinated anisyl metabolites (CAMs) and 1-arylpropane-1,2-diols (1, 2, 3, 4) as idiophasic metabolic products of L-phenylalanine. These diols are stereoselectively biosynthesized from a C7-unit (benzylic, from L-phenylalanine) and a C2-unit, of unknown origin, as predominantly erythro (1R,2S) enantiomers. Of the labeled amino acids tested as possible C2-units, at the 4-10 mM level, none were found to efficiently label the 2,3-propane carbons of the diols. However, glycine (2-13C), L-serine (2,3,3-d3) and L-methionine (methyl-d3) entered the biomethylation pathway. Neither pyruvate (2,3-13C2), acetate (1,2-13C2), acetaldehyde (d4) nor ethanol (ethyl-d5) labeled the 2,3-propane carbons of the diols at the 4-10 mM level. Pyruvate (2,3-13C2) and L-serine (2,3,3-d3) (which also entered the biomethylation pathway) did, however, effectively label the 2,3-propane carbons of the alpha-ketols and diols at the 40 mM level as evidenced by mass spectrometry. Glycerol (1,1,2,3,3-d5) also appeared to label one of the 2,3-propane carbons (ca. 5% as 2H2 in the C3 side chain) as suggested by mass spectrometric data and also entered the biomethylation pathway, likely via amino acid synthesis. Glycerol (through pyruvate), therefore, likely supplies C2 and C3 of the propane side chain with arylpropane diol biosynthesis. Incubation of B. adusta with synthetic [2-2H1, 2-18O]-glycerol showed that neither 2H nor 18O were incorporated in the alpha-ketols or diols. The oxygen atom on the C2 of the ketols/diols, therefore, does not appear to come from the oxygen atom on the C2 of glycerol. Glycerol, however, can readily form L-serine (which can then form pyruvate via PLP/serine dehydratase and involve transamination washing out the 18O label and providing the oxygen from water), and can then go on to label the C2-unit. Labeled alpha-ketol, phenyl acetyl carbinol (5) (PAC; ring-d(5), 2,3-13C2 propane) cultured with B. adusta leads to stereospecific reduction to the (1R,2S)-diol (6) (ring-d5 and 2,3-13C2); in all other metabolites produced, the 2,3-13C2) label is washed out. Incubation of the fungus with 4-fluorobenzaldehyde (13) produces a pooling of predominantly erythro (1R,2S) 1-(4'-fluorophenyl)-1,2-propane diol (18 as diacetate) (through the corresponding alpha-ketols 16, 17). Blocking the para-position with fluorine thus appears to prevent ring oxygenation and also chlorination, forcing the conclusion that para-ring oxygenation precedes meta-chlorination.

Stereoselective biosynthesis of chloroarylpropane diols by the basidiomycete Bjerkandera adusta

Previously we have shown that 1-arylpropane-1,2-diols are catabolic products of L-phenylalanine during idiophasic metabolism of B. adusta that are stereoselectively biosynthesized from a C(7)-unit (ring+benzylic carbon) and a C(2)-unit as predominantly erythro 1R, 2S enantiomers.In order to probe the mechanism of 1-arylpropane-1,2-diol formation, the products of the incubation of isotopically labelled aromatic aldehydes as substrates with Bjerkandera adusta (DAOM 215869) have been characterized. The aromatic aldehydes were benzaldehyde (ring D(5)) and 4-methoxy- and 4-hydroxybenzaldehydes (ring 13C(6)). These aldehydes were all stereoselectively incorporated into the corresponding 1-arylpropane-1,2-diols, including the chloro analogues, as well as into the corresponding alpha-ketols (phenyl acetyl carbinols (PAC's) and 2-hydroxy propiophenones (2-HPP's)) the presumed precursors of the diols. Benzoic acid (ring D(5)) was likewise incorporated into the diols, chlorodiols and alpha-ketols. These results lead us to conclude that the aromatic aldehydes benzaldehyde, 4-hydroxybenzaldehyde and 4-methoxybenzaldehyde are likely C(7)-unit precursors in the carboligation reaction(s) that leads to 1-arylpropane-1,2-diol biosynthesis. The metabolic role of the diols remains to be elucidated but they may be important intermediates in CAM (chlorinated anisyl metabolite) aldehyde-alcohol cycling and also act as substrates for the chlorination/hydroxylation enzymes yet to be identified in white rot fungi.

Novel pyrazine metabolites found in polymyxin biosynthesis by Paenibacillus polymyxa

A complex mixture of methyl-branched alkyl-substituted pyrazines was found in the growth medium of the polymyxin-producing bacterium Paenibacillus polymyxa, and of these, seven are new natural compounds. A total of 19 pyrazine metabolites were identified. The dominant metabolite was 2,5-diisopropylpyrazine as identified using a combination of high-resolution mass spectrometry, (1)H- and (13)C-nuclear magnetic resonance, gas chromatography-mass spectrometry as well as co-elution with an authentic standard. Its biosynthesis was correlated with growth and production was strongly stimulated by valine supplementation. The other pyrazine metabolites, all related pyrazines with either one, two or three alkyl substituents, were identified by means of their mass spectral data and/or co-elution with authentic standards.

Membrane introduction mass spectrometry: trends and applications

Recent advances in membrane introduction mass spectrometry (MIMS) are reviewed. On-line monitoring is treated by focusing on critical variables, including the nature and dimensions of the membrane, and the analyte vapor pressure, diffusivity, and solubility in the membrane barrier. Sample introduction by MIMS is applied in (i) on-line monitoring of chemical and biological reactors, (ii) analysis of volatile organic compounds in environmental matrices, including air, water and soil, and (iii) in more fundamental studies, such as measurements of thermochemical properties, reaction mechanisms, and kinetics. New semipermeable membranes are discussed, including those consisting of thin polymers, low vapor pressure liquids, and zeolites. These membranes have been used to monitor polar compounds, selectively differentiate compounds through affinity-binding, and provide isomer differentiation based on molecular size. Measurements at high spatial resolution, for example, using silicone-capped hypodermic needle inlets, are also covered, as is electrically driven sampling through microporous membranes. Other variations on the basic MIMS experiment include analyte preconcentration through cryotrapping (CT-MIMS) or trapping in the membrane (trap-and-release), as well as differential thermal release methods and reverse phase (i.e., organic solvent) MIMS. Method limitations center on semivolatile compounds and complex mixture analysis, and novel solutions are discussed. Semivolatile compounds have been monitored with thermally assisted desorption, ultrathin membranes and derivatization techniques. Taking advantage of the differences in time of membrane permeation, mixtures of structurally similar compounds have been differentiated by using sample modulation techniques and by temperature-programmed desorption from a membrane interface. Selective ionization techniques that increase instrument sensitivity towards polar compounds are also described, and comparisons are made with other direct sampling (nonchromatographic) methods that are useful in mixture analysis.

Basidiomycetes as new biotechnological tools to generate natural aromatic flavours for the food industry

Consumer preference for natural food additives has led to an increasing demand for natural aromatic compounds. An alternative production process to plant and chemical sources is the use of biotechnological methods involving microorganisms, which ensure a stable supply, quality and price. Among filamentous fungi, white-rot basidiomycetes represent an important group that generate a wide range of flavouring compounds, particularly aromatic molecules. Their biotechnological potential to produce natural aromatic flavours de novo or by biotransformation thus presents a very interesting challenge.