Biosynthesis of elsinochromes C and D. Pattern of acetate incorporation determined by 13C and 2H nmr

Elsinochromes C and D have been isotopically labelled by supplementing cultures of the producing fungus with sodium [1- 13 C]-, [2- 13 C]-, [1,2- 13 C]-, or [2- 13 C, 2- 2 H 3 ]acetate and the distribution of isotope has been determined by 13 C and 2 H nuclear magnetic resonance spectroscopy. The pattern of 13 C labelling is consistent with assembly of the elsinochrome carbon skeleton from two heptaketide chains with loss of the terminal carboxyl groups, and dimerization to generate the 1,2-dihydrobenzo (ghi)-perylene ring system. Elsinochrome D was converted to the more soluble triacetate to examine the fate of 2 H presented to the culture as [2- 13 C, 2- 2 H 3 ]acetate. Deuterium labelling was restricted to the C-14 and C-16 methyl groups, a result consistent with these belonging to the "starter" C 2 units of polyketide intermediates. Carbon-13 enrichment data for elsinochromes labelled from [1,2- 13 C]acetate indicated that the polyketide chains were formed from a highly enriched precursor pool with subsequent dilution accounting for a much lower average incorporation. Presence of spin–spin coupling between linked carbons derived from separate polyketide precursors implies that dilution occurs after the dimerization step. From the combined evidence a plausible pathway for elsinochrome biosynthesis is deduced.