Thielavins: tuned biosynthesis and LR-HSQMBC for structure elucidation

Abstract

A series of thielavins I, V, and Q (1-3) and the previously undescribed thielavin Z₈ (4) were isolated from cultures of a fungal Shiraia-like sp. (strain MSX60519) that were grown under a suite of media and light conditions, with enhanced biosynthesis noted using rice as a substrate with 12:12 h light:dark cycles. Conversely, oatmeal medium and continuous white light-emitting diode light exposure negatively affected the production of these compounds, at least by strain MSX60519. The structure of 4 was determined using NMR spectroscopic data and mass fragmentation patterns. Of note, the utility of LR-HSQMBC and NOESY NMR experiments in the structural elucidation of these hydrogen-deficient natural products was demonstrated. Compounds 1-4 exhibited cytotoxic activity at the micromolar level against human breast, ovarian, and melanoma cancer cell lines.

Fig. 1.

Structures of compounds 1-4.

Fig. 2.

Panels A-D show the relative percentages of 1-4, respectively, across cultures grown on rice, Cheerios, and oatmeal media under 12:12 h light:dark cycles, continuous LED, or in darkness. The relative percentages were measured by LC-HRMS in three biological replicates and multiplied by the extract weight, and then normalized according to the extract with highest abundance. * Indicates significantly higher productions of 1-4 by cultures grown on rice under 12:12 h light:dark cycles as compared to other growth conditions (p < 0.05). # Indicates statistically no difference in thielavin production as compared to culture grown rice under 12:12 h light:dark cycles. Data are presented as mean ± SD.

Fig. 3.

Key LR-HSQMBC correlations of compound 1.

Fig. 4.

Key HMBC correlations of compound 4.

Fig. 5.

Intra- and inter-ring NOESY correlations for 3 and 4.