W. Colin McRoberts

Methoxyl groups of plant pectin as a precursor of atmospheric methane: evidence from deuterium labelling studies

* The observation that plants produce methane (CH4) under aerobic conditions has caused considerable controversy among the scientific community and the general public. It led to much discussion and debate not only about its contribution to the global CH4 budget but also about the authenticity of the observation itself. Previous results suggested that methoxyl groups of the abundant plant structural component pectin might play a key role in the in situ formation process of CH4. Here, this effect is investigated using an isotope labelling study. * Polysaccharides, pectin and polygalacturonic acid, with varying degrees of trideuterium-labelled methyl groups in the methoxyl moieties, were investigated for CH4 formation under UV irradiation and heating. * A strong deuterium signal in the emitted CH4 was observed from these labelled polysaccharides. * Results clearly demonstrate that ester methyl groups of pectin can serve as a precursor of CH4, supporting the idea of a novel chemical route of CH4 formation in plants under oxic environmental conditions.

2‐Alkylcyclobutanones as markers for the detection of irradiated mango, papaya, Camembert cheese and salmon meat

Experimental work was carried out in order to determine the usefulness of the 2-alkylcyclobutanones as markers for irradiated Camembert cheese, salmon meat, mango and papaya. Both 2-dodecylcyclobutanone (2-DCB) and 2-tetradecylcyclobutanone (2-TCB) were readily detected in Camembert cheese even after storage for 26 days at 10 °C. A linear relationship was observed between irradiation dose (0.5–5 kGy) and the amount of cyclobutanone produced in the cheese. 2-DCB and 2-TCB were both identified in salmon meat irradiated in either the chilled (4 °C) or frozen state (−40 °C), although it was noted that less 2-DCB was measured in the frozen samples. A linear response to increasing irradiation dose was demonstrated for salmon over the experimental range of 1–10 kGy. 2-TCB was identified as the main marker for irradiated mango and could be detected in samples following storage for 14 days at 10 °C at doses as low as 0.1 kGy. As for the other products investigated, the concentration of this cyclobutanone increased linearly with increasing dose (0.1–2 kGy). With regard to papaya, 2-DCB was identified as the principal irradiation marker. However, the concentration of this cyclobutanone decreased significantly with time, so that by day 21 of storage at 10 °C it could only be detected at the 2 kGy dose level. 2-Tetradecenylcyclobutanone (2-TDCB) was also detected in irradiated mango and papaya. © 2000 Society of Chemical Industry

Fluoroacetaldehyde: a precursor of both fluoroacetateand 4-fluorothreonine in Streptomyces cattleya

Fluoroacetaldehyde is converted to fluoroacetate and 4-fluorothreonine in Streptomyces cattleya indicating that it is the biosynthetic precursor of both of these secondary metabolites.

Enzyme-catalysed synthesis and reactions of benzene oxide/oxepine derivatives of methyl benzoates

A series of twelve benzoate esters was metabolised, by species of the Phellinus genus of wood-rotting fungi, to yield the corresponding benzyl alcohol derivatives and eight salicylates. The isolation of a stable oxepine metabolite, from methyl benzoate, allied to evidence of the migration and retention of a carbomethoxy group (the NIH Shift), during enzyme-catalysed ortho-hydroxylation of alkyl benzoates to form salicylates, is consistent with a mechanism involving an initial arene epoxidation step. This mechanism was confirmed by the isolation of a remarkably stable, optically active, substituted benzene oxide metabolite of methyl 2-(trifluoromethyl)benzoate, which slowly converted into the racemic form. The arene oxide was found to undergo a cycloaddition reaction with 4-phenyl-1,2,4-triazoline-3,5-dione to yield a crystalline cycloadduct whose structure and racemic nature was established by X-ray crystallography. The metabolite was also found to undergo some novel benzene oxide reactions, including epoxidation to give an anti-diepoxide, base-catalysed hydrolysis to form a trans-dihydrodiol and acid-catalysed aromatisation to yield a salicylate derivative via the NIH Shift of a carbomethoxy group.

Isolation of a stable benzene oxide from a fungal biotransformation and evidence for an ‘NIH shift’ of the carbomethoxy group during hydroxylation of methyl benzoates

Substituted methyl benzoates are biotransformed in the fungus Phellinus ribis by enzyme-catalysed epoxidation to yield an isolable benzene oxide and transient benzene oxides, whose presence was inferred from isomerisation to the corresponding methyl salicylates with concomitant migration of the carbomethoxy group.

The influence of culture vessel head-space volatiles on somatic embryo maturation in Sitka spruce [Picea sitchensis (Bong.) Carr.]

The maturation of somatic embryos of Sitka spruce [Picea sitchensis (Bong.) Carr.] was found to be highly dependent on the method used to seal plastic Petri dishes. Large numbers of well-formed mature embryos developed if dishes were sealed with PVC cling-film (CF) whilst sealing with Parafilm M (PF) greatly reduced the numbers of embryos forming. Inclusion of potassium permanganate oxidation traps, normally used to deplete the atmospheric ethylene, greatly stimulated somatic embryo maturation under PF sealing. Similarly, traps of adsorption agents (Tenax, activated charcoal or soft white paraffin), capable of removing volatiles from the culture vessel head-space, stimulated somatic embryo maturation under PF sealing although to a lesser extent than the oxidation traps. Incorporation of silver nitrate or 2-chloroethylphosphonic acid (ethephon) in the culture medium indicated that ethylene was not the agent supressing somatic embryo maturation under PF sealing.

Absolute configuration assignment and enantiopurity determination of chiral alkaloids and coumarins derived from O- and C-prenyl epoxidesElectronic supplementary information (ESI) available: crystal data for 17. See http://www.rsc.org/suppdata/cc/b2/b208978e/

A combination method of ozonolysis and chiral stationary phase (CSP)-GC-MS analysis has been developed to determine the enantiopurity values and absolute configurations of a range of alkaloid and coumarin hemiterpenoids derived from C- and O-prenyl epoxides.

Detection of 2-substituted cyclobutanones as irradiation products of lipid-containing foods: synthesis and applications of cis- and trans-2-(tetradec-5′-enyl)cyclobutanones and 11-(2′-oxocyclobutyl)undecanoic acid

cis-(3cis) and trans-2-(tetradec-5′-enyl)cyclobutanone (3trans) have been chemically synthesised and used in the unambiguous identification of the cis isomer 3cis in irradiated meat (example chicken) and fruit (example papaya). 11-(2′-Oxocyclobutyl)undecanoic acid 5 has been chemically synthesised, conjugated to bovine thyroglobulin and used to generate polyclonal antibodies in rabbits, which have been used in the development of an enzyme-linked immunosorbent assay for the detection of 2-substituted cyclobutanones in irradiated chicken meat.

Inhibition of somatic embryo maturation in Sitka spruce [Picea sitchensis (Bong.) Carr.] by butylated hydroxytoluene, a volatile antioxidant released by parafilm

Head-space volatiles above embryogenicPicea sitchensis (Bong.) Carr. (Sitka spruce) tissues cultured in glass Petri dishes sealed with Parafilm M or cling-film, were captured on Tenax adsorption traps and analysed by gas chromatography / mass spectrometry. Each sealing system released a single major compound into the head-space; butylated hydroxytoluene from Parafilm M and 2-ethyl-1-hexanol from cling-film. After two weeks sealed under Parafilm M butylated hydroxytoluene accumulated to 1.1 μg g−1 FW in tissues and subsequent somatic embryo maturation was prevented. When butylated hydroxytoluene was supplied via the head-space (100 μg/250 ml flask) 0.5 μg g−1 FW accumulated in tissues after two weeks and no somatic embryo maturation occurred. Potentially phytotoxic metabolites of butylated hydroxytoluene included a substituted stilbenequinone, butylated hydroxytoluene quinone methide and butylated hydroxytoluene dimer.

Seasonal changes in chlorine and methoxyl content of leaves of deciduous trees and their impact on release of chloromethane and methanol at elevated temperatures

Environmental context Chloromethane is the most abundant naturally produced chlorine-containing organic compound, responsible for ~16% of chlorine-catalysed stratospheric ozone destruction. A significant source of this gas is emission from biomass by reaction between chloride ion and methoxyl groups of the biopolymers pectin and lignin. The seasonal changes in the chlorine and methoxyl pools observed in leaves of several deciduous tree species have implications for understanding chlorine volatilisation during biomass burning and estimation of the global chloromethane budget. Abstract Atmospheric chloromethane (CH3Cl) plays a role in the destruction of stratospheric ozone. Previous studies suggest an important source of this gas is emission from leaves and leaf litter at ambient and elevated (150–350°C) temperatures. In this study, the total chlorine and OCH3 content of leaves of the deciduous temperate tree species ash, beech, Norway maple and oak were measured throughout the 2004 and 2005 growing seasons. The total chlorine content increased with leaf age. The overall seasonal accumulation varied between five- and twenty-one fold, dependent on both year and species. Throughout the 2004 growing season, the OCH3 pool and the release of CH3Cl and methanol (CH3OH) from leaves of ash and Norway maple were monitored on heating to 350°C. The amounts of CH3Cl released increased linearly as leaf chlorine accumulated whereas emissions of CH3OH did not substantially change. Conversion of chlorine to CH3Cl was lower in the spring than during the summer and autumnal senescence period, ranging from 22 to 58%. No correlation was found between leaf OCH3 content and either CH3Cl or CH3OH release. The percentage conversion of OCH3 to the summed concentrations of CH3OH and CH3Cl ranged from 41 to 66%. The plant components pectin and lignin were identified as two major sources of the CH3 group in CH3Cl and CH3OH and emissions ceased when the OCH3 pool contributing the methyl moiety was exhausted (>350°C). These findings have implications for estimation of CH3Cl release during biomass burning and for our understanding of chlorine volatilisation during energy production from biomass.