(+)-Ascosalitoxin and Vermelhotin, a Calmodulin Inhibitor, from an Endophytic Fungus Isolated from <i>Hintonia latiflora</i>

Abstract

Chemical investigation of the endophytic MEXU 26343, isolated from the medicinal plant <i>Hintonia latiflora</i>, yielded the known polyketide vermelhotin (<b>1</b>) and a new salicylic aldehyde derivative, namely, 9<i>S</i>,11<i>R</i>-(+)-ascosalitoxin (<b>2</b>). The structure and absolute configuration of the new compound were established through extensive NMR spectroscopy and molecular modeling calculations at the DFT B3LYP/DGDZVP level, which included the comparison between theoretical and experimental optical rotation values. In addition, chemical transformations of <b>2</b> yielded suitable derivatives for NOESY and ¹H–¹H NMR coupling constant analyses, which reinforce the stereochemical assignment. The potential affinity of <b>1</b> and <b>2</b> with (Ca²⁺)₄-<i>h</i>CaM in solution was measured using the fluorescent biosensor <i>h</i>CaM M124C-<i>mBBr</i>. The results showed that <b>1</b> bound to the protein with a dissociation constant (<i>K</i>_d) of 0.25 ± 0.04 μM, close to that of chlorpromazine (<i>K</i>_d = 0.64 ± 0.03 μM), a classical CaM inhibitor. The stoichiometry ratio of <b>1</b> to (Ca²⁺)₄-<i>h</i>CaM was 1:4, similar to other well-known CaM ligands