Stereoselective Total Synthesis of (−)‐Thallusin for Bioactivity Profiling

Abstract

Chemical mediators are key compounds for controlling symbiotic interactions in the environment.Here, we disclose a fully stereoselective total synthesis of the algae differentiation factor (À )-thallusin that utilizes sophisticated 6-endo-cyclization chemistry and effective late-stage sp 2 -sp 2 -couplings using non-toxic reagents. An EC 50 of 4.8 pM was determined by quantitative phenotype profiling in the green seaweed Ulva mutabilis (Chlorophyte), underscoring this potent mediator's enormous, pan-species bioactivity produced by symbiotic bacteria. SAR investigations indicate that (À )-thallusin triggers at least two different pathways in Ulva that may be separated by chemical editing of the mediator compound structure.

Thallusin Quantification in Marine Bacteria and Algae Cultures

“…to 8.22 µg L −1 (46.9 µmol L −1 , in Pseudomonas sp.) during the stationary phase by gas chromatographymass spectrometry (GC-MS) analysis [42]; these are still 10 3 -to 10 4 -times higher than the measured concentrations of thallusin and its EC 50 value (4.9 ± 0.1 pmol L −1 ) [9]. Therefore, the strong contrast between the high biological activity of thallusin and its comparatively low production yields is striking.…”

“…Thallusin induces the development of basal rhizoids and healthy cell wall formation [6,9] in U. compressa, but also possesses distinguishable context-dependent functions as known for hormones in Plantae [6,30]. For plant hormone profiling, our approach should be thus included in the workflow for the qualitative and quantitative analysis of phytohormones in marine bacteria [31,32] and algae [20,33,34].…”

“…A co-injection experiment allowed for the identification of the peak of the thallusin trimethyl ester by comparison with the synthetic standard (Figure 6). The thallusin concentration in the pooled stationary culture of a closed cultivation system (U. compressa, three months old, 17.2 g L −1 fresh weight biomass) was 0.22 ± 0.04 nmol L −1 , implying that under the selected laboratory conditions, the actual thallusin concentration in the algal culture was approximately 30-times higher than the EC90 = 7.6 pmol L −1 [9], indicating strong oversaturation.…”

“…The thallusin trimethyl ester showed increased peak sharpness, improving the detection limit and calibration capability (Figure 1B,C). The peak was identified by comparing the associated retention time and the mass spectrum with those of a synthetic reference standard of thallusin trimethyl ester (Figure 1B-E) [9]. The added internal standard became methylated as well, thus providing evidence of successful derivatization in the analytical procedure.…”

“…releases the hormone-like compound thallusin, a terpenoid hybrid metabolite, which can functionally replace the bacterium [6]. With the half-maximal effective concentration (EC 50 ) of 4.9 ± 0.1 pmol L −1 [9], thallusin is one of the most bioactive natural products which induces cell wall and rhizoid formation in U. compressa ("rhizoin") [6,9] and thallus formation in Gayralia oxyspermum ("thallusin") [10,11]. Thallusin is thus a versatile chemical regulator of plant development at low concentrations and reveals, like plant hormones [12,13], context-dependent, morphogenetic bioactivities [6,14,15] that need further investigation.…”