~1H and ~(13)C NMR spectroscopy of both fumonisin B_3 and B_4, as well as high-performance liquid chromatography (HPLC) analysis of samples of fumonisin B_3 used as standards, showed in each case the presence of two stereoisomers, which could not be separated by preparative chromatography. The 2,3-anti relative configuration for the two minor stereoisomers of fumonisin B_3 and B_4 was deduced from the NMR data, and their 2S,3R absolute configurations were established by application of Mosher's method using the fumonisin B_3 sample. Samples of fumonisin B_3 and B_4 can contain between 10 and 40% of fumonisin B compounds of the 3-epi series. The 3-epi-FB_3, determined by HPLC with fluorescence detection of the omicron-phthaldialdehyde derivative and confirmed by liquid chromatography-tandem mass spectrometry, was found to occur naturally in a range of maize samples at levels much lower than FB_3 (<20%). The identification of members of the 3-epi-fumonisin B series provides insight into the order and selectivity of steps in fumonisin biosynthesis.
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