BIOSYNTHESIS OF THE BORON-CONTAINING MACRODIOLIDE ANTIBIOTICS APLASMOMYCIN AND BOROMYCIN

TOM SHIEH-SHUNG CHEN, Purdue University

Abstract

The biosynthesis of the boron-containing macrodiolide antibiotics aplasmomycin and boromycin, isolated from Streptomyces griseus SS20 and Streptomyces spec. MA4423, respectively, has been studied using a combination of radiotracer and stable isotope techniques. A complete assignment of all the carbon signals in the spectrum of aplasmomycin was made on the basis of chemical shift theory, multiplicity analysis, single frequency proton decoupling experiments, comparison with several derivatives and model compounds and the analysis of one bond carbon-carbon coupling constants obtained from a sample of the antibiotic biosynthesized from {1,2-('13)C(,2)}acetate. The ('13)C NMR assignment of desvalino-boromycin was based on chemical shift theory, multiplicity analysis, and comparison with aplasmomycin and model compounds. The ten methyl carbons in boromycin were assigned by comparison with desvalino-boromycin. These assignments were then used to analyze the spectra of the compounds obtained from feeding experiments with ('13)C-labeled precursors for determination of the label distribution. ('13)C NMR analysis of aplasmomycin samples biosynthesized from {1-('13)C}-, {2-('13)C}-, and {1,2-('13)C(,2)}acetate and from {('13)CH(,3)}methionine showed that each half of the molecule is made up from 7 intact acetate units providing carbons 1-14 and 3 methyl groups from methionine, which are located in positions 18, 19 and 20. The most specific precursor of the 3-carbon starter unit, C-15, C-16 and C-17, so far found is glycerol. The boron was inserted back into deboroaplasmomycin by treatment with boric acid at pH6 and pH8. The results favor the possibility that the boron is introduced into the carbon skeleton after the macrocyclic ring is formed. In the work on boromycin, ('13)C NMR analysis of the sample derived from L-{('13)CH(,3)}methionine revealed that 6 methyl groups corresponding to carbons 18, 18', 19, 19', 20 and 20' are methionine derived. ('13)C NMR analysis of the desvalinoboromycin sample derived from {2-('13)C}malonate showed enrichment in 14 carbon atoms corresponding to carbons 2, 2', 4, 4', 6, 6', 8, 8', 10, 10', 12, 12', 14 and 14'. The results so far are indicative of a similar mode of formation as for aplasmomycin. Preliminary result indicates that D-valine is the more immediate precursor of D-valine moiety. The interpretation of the 360 MHz ('1)H NMR spectra of aplasmomycin and deboroaplasmomycin was made to extract most of the parameters revealing their conformations in CDCl(,3) solution. It was found that the conformation of aplasmomycin in CDCl(,3) is identical to that for the solid state and removal of boron atom from aplasmomycin has led to slight conformational change of the molecule in CDCl(,3).

Degree

Ph.D.

Subject Area

Biochemistry

COinS