Charles P. Gorst-Allman

Structure elucidation of the fumonisins, mycotoxins from Fusarium moniliforme

The structures of the fumonisins, a family of structurally related mycotoxins isolated from cultures of Fusarium moniliforme, were elucidated by mass spectrometry and 1H and 13C n.m.r. spectroscopy as the diester of propane-1,2,3-tricarboxylic acid and either 2-acetylamino- or 2-amino-12,16-dimethyl-3,5,10,14,15-pentahydroxyicosane as well as in each case the C-10 deoxy analogue; in all cases both the C-14 and C-15 hydroxy groups are involved in ester formation with the terminal carboxy group of propane-1,2,3-tricarboxylic acid.

Screening methods for the detection of thirteen common mycotoxins

A study of screening methods for thirteen mycotoxins showed that they can be separated as neutral and acidic metabolites. RF values were determined in several solvent systems. The reactions of the mycotoxins with well known spray reagents were investigated, and their detection limits were established. A general procedure for the extraction of mycotoxins from contaminated samples is described.

Tremorgenic mycotoxins from Penicillium crustosum. Biosynthesis of penitrem A

The biosynthesis of penitrem A has been studied with both 13C- and 2H-labelled precursors, viz. [1-13C]-, [2-13C]-, [1,2-13C2]-, and [1-13C,2-2H3]-acetate, [2-13C]-, [2,3-13C2]-, [2-2H2]- and [5-2H2]-mevalonate. The results show that penitrem A is derived from tryptophan, which contributes the indole moiety of the metabolite, geranylgeranylpyrophosphate, and two isopentenylpyrophosphate units. A 1,2-bond migration, involving the 2,3-bond of a mevalonate unit, occurs in the course of the biosynthesis and results in the observation of a one-bond (C,C) coupling between two [1-13C]acetate-derived carbon atoms and between two [2-13C]acetate-derived carbon atoms. Analysis of the one-bond (C,C)coupling constants in [2-13C]acetate-derived penitrem A showed that [1,2-13C2]acetate was formed during the fermentation. Although loss of water from an hydroxyisopropyl group to form the isopropenyl function present in penitrem A should proceed with retention of the stereochemical integrity of the two methyl groups, isomerization of the double bond causes equal distribution of 13C label between C-36 and C-38 and precludes any stereochemical deductions.

Structure elucidation and absolute configuration of phomopsin a, a hexapeptide mycotoxin produced by phomopsis leptostromiformis

Abstract Phomopsin A, C36H45CIN6O12, the main mycotoxin isolated from cultures of Phomopsis leptostromiformis and the cause of lupinosis disease, is a linear hexapeptide containing 3-hydroxy-L-isoleucine, 3,4-didehydrovaline, N-methyl-3-(3-chloro-4,5-dihydroxyphenyl)serine, E-2,3-didehydroaspartic acid, E-2,3-didehydroisoleucine, and 3,4-didehydro-L-proline. The L-configuration of the indicated amino acids was established by a comparison of the N-trifluoroacetyl n-butylester derivatives of the acid hydrolysis products of phomopsin A with samples prepared from authentic amino acids, using capillary gas chromatography on a chiral stationary phase. The E configuration of the two 2,3-didehydro amino acids is based on the products obtained by catalytic hydrogenation and sodium borohydride reduction of phomopsin A followed by acid hydrolysis (for 2,3-didehydroisoleucine) or by analysis of the coupled 13C n.m.r. spectrum of phomopsin A (for 2,3-didehydroaspartic acid). Evidence is presented which shows that the glycine formed during the acid hydrolysis of phomopsin A is derived from the 3,4-didehydrovaline moiety. The sequence of the amino acids was established by heteronuclear 13C{1H} selective population inversion (SPI) experiments and by fast atom bombardment (f.a.b.) mass spectrometry of phomopsin A and its derivatives. An X-ray crystallographic study of phomopsin A confirmed the amino acid sequence and showed that the linear hexapeptide is modified by an ether bridge in place of the 5-hydroxy group of the N-methyl-3-(3-chloro-4,5-dihydroxyphenyl)serine and the hydroxy group of the 3-hydroxyisoleucine units. In addition, the X-ray study specified the absolute configuration of phomopsin A as 22E, 25E, 3R, 4S, 7S, 10S, 11S, 19S.

A new atisane diterpene: ent-16α-hydroxyatis-13-en-3-one from Androstachys johnsonii prain

The title diterpene, a rare example of the naturally occurring ent-atis-13-ene type, has been isolated as the 2-benzylidene derivative from the heartwood hexane extract of Androstachys johnsonii Prain. Its structure was determined by n.m.r. and X-ray crystallography. It is suggested that the title compound is biogenetically produced from an ent-12-oxybeyer-15-ene precursor. An interesting reaction reported here was found to convert a vicinal diol into a monoketone using the diethyl azodicarboxylate–triphenylphosphine–benzoic acid system.

Carbon-13 nuclear magnetic resonance assignments of some fungal C20 anthraquinones; their biosynthesis in relation to that of aflatoxin B1

The 13C n.m.r. spectra of four related secondary fungal metabolites, averufin, averufanin, averantin, and 1′-methoxyaverantin, have been assigned. The distributions of 13C label and of the intact acetate units have been determined from the 13C n.m.r. spectra of averufin derived from single and doubly labelled [13C]acetate, These data enabled a prediction of the mode of folding of the single chain acetate–polymalonate-derived polyketide precursor of these C20 anthraquinones. A modified pathway which involves an epoxide intermediate for the conversion of averufin into the versicolorins, and, therefore, into the aflatoxins is proposed.

Structure elucidation of a novel trichothecene glycoside using 1H and 13C nuclear magnetic resonance spectroscopy

The structure elucidation of a novel trichothecene glycoside, 15-acetoxy-3α-hydroxy-4β-(α-D-glucopyranosyloxy)-12,13-epoxytrichothec-9-ene, is based on a detailed study of its one- and two-dimensional 1H and 13C n.m.r. spectra, and chemical reactions. The metabolite displays a reduced toxicity when compared with other trichothecene metabolites in several biological systems.

Oxidation of the bisdihydrofuran moieties of aflatoxin B1 and sterigmatocystin; conformation of tetrahydrofurobenzofurans

Attempts to prepare and isolate the products of epoxidation of the vinyl ether moieties of the two metabolites aflatoxin B1(1) and sterigmatocystin (2) gave only trans-glycol derivatives. The rates of consumption of peroxy-acid in the presence of compounds (1) and (2) were measured. Aflatoxin B1 dichloride and dibromide were prepared and their susceptibilities to nucleophilic attack were examined. The conformations of various di-substituted tetrahydrofurobenzofuran derivatives obtained were studied by 1H n.m.r. spectroscopy.

Structure elucidation of phomopsin A, a novel cyclic hexapeptide mycotoxin produced by Phomopsis leptostromiformis

Phomopsin A, the main mycotoxin isolated from cultures of Phomopsis leptostromiformis and the cause of lupinosis disease in animals grazing infected lupins, is a cyclic hexapeptide containing 3-hydroxyisoleucine, 3,4-didehydrovaline, N-methyl-3-(3-chloro-4,5-dihydroxyphenyl)-3-hydroxyalanine, E-2,3-didehydroaspartic acid, E-2,3-didehydroisoleucine, and 3,4-didehydroproline; its 13C n.m.r. spectrum was completely assigned and the amino-acid sequence established unambiguously by extensive heteronuclear 13C-{1H} selective population inversion n.m.r. experiments.

Biosynthesis of PR toxin by Penicillium roqueforti

The biosynthesis of PR toxin, a metabolite produced by Penicillium roqueforti, has been studied utilizing both 14C- and 13C-labelled precursors. The results indicate that PR toxin is formed via an isoprene biosynthetic pathway.