Development of a Synthetic Receptor for the Food Toxin Beauvericin: A Tale of Carbazole and Steroids

Abstract

The synthesis of the first synthetic receptor showing high affinity for the toxic ionophoric cyclodepsipeptide beauvericin is described. Binding results in a pronounced increase in fluorescence intensity of the receptor, while this increase is not observed for a very similar ionophore such as valinomycin. Experiments that shed light on the nature of this selectivity are discussed.

Counteracting in Vitro Toxicity of the Ionophoric Mycotoxin Beauvericin—Synthetic Receptors to the Rescue

“…Subsequent second-order nucleophilic substitution reaction with NaN 3 gave us steroid 3 in good yield. The carbazole fragment 5 was synthesized as described previously . Briefly, alkylation of 2,7-dibromocarbazole 4 was followed by a Sonogashira reaction using 2-methyl-3-butyn-2-ol.…”

“…An association constant of 1.1 × 10 5 M –1 ± 4.5 × 10 4 [ K a ± standard deviation (SD)] was obtained, based on the mean of three independent titrations. This value was determined by fitting the fluorescence titration data at 389 nm to a theoretical binding isotherm for a 1:1 binding model …”

“…We have previously shown that titration of the carbazole fragment alone, without any steroid side arms, yielded a 1000-fold lower binding affinity for BEA compared to receptor R2 (Figure S3 in SI). On the other hand, when the Boc-protected receptor was tested, no change in fluorescence intensity was observed upon BEA addition (Figure S4 in SI).…”

“…These results seem to indicate that the use of a scaffold containing the trans -decalin system is clearly advantageous compared to one containing a cis-fused ring (e.g., lithocholic acid), when higher affinities are required. Finally, none of the receptors showed appreciable interaction with the larger ionophore valinomycin, which is in line with our previous observation for receptor R1 . For further applications, the more complicated synthesis of receptors R3 and R4 is not outweighted by the small increase in affinity compared to receptor R2 .…”

“…Compound 5 (synthesized as reported previously starting from 2,7-dibromocarbazole 4 ) (24 mg, 0.066 mmol) and compound 3 (78 mg, 0.166 mmol, 2.5 equiv) were dissolved in 4 mL of DMF, followed by addition of 6 mL of t BuOH. CuSO 4 ·5H 2 O (33 mg, 0.132 mmol, 2 equiv) and Na- l -ascorbate (52 mg, 0.264 mmol, 4 equiv) were separately dissolved in 1 mL of H 2 O each and added to the reaction mixture.…”