Physicochemical Properties of Oosporein, the Major Secreted Metabolite of the Entomopathogenic Fungus Beauveria brongniartii.

Abstract

Oosporein (1) is the major secondary metabolite excreted by Beauveria brongniartii (Sacc.) Petch (Ascomycota: Clavicipitaceae), an entomopathogenic fungus infesting the larvae of Melolontha melolontha L. (Coleoptera: Scarabaeidae; common European cockchafer). Physicochemical parameters were established to allow a better understanding of the spatiotemporal distribution of this marker substance in the environment. i) The dependence of the oosporein solubility (Cs) from proton concentration and temperature was assessed. Oosporein is a rather strong organic acid. The basal solubility of oosporein (C in aqueous citrate/HCl buffer at pH 1.23) was found to be 24.8±0.3 μM. The pKa of the first deprotonation step was determined to be 2.42±0.02. A nonlinear van't Hoff equation was established for the temperature dependency of the oosporein solubility (lnCs=a+bT−1+clnT, with a=−123.3, b=2678.9, and c=19.5). ii) A second and third deprotonation step was found in Britton–Robinson-buffered aqueous solution. The associated pKa values were determined as 6.79±0.08 and 9.19±0.03. iii) The stability of oosporein at different pH values and temperatures was addressed. Oosporein degraded quickly under moderate alkaline conditions and with increased temperatures. The half-life dropped below 20 h at pH≥8.0 and T≥43°. The analysis of the Arrhenius plot allowed to calculate the activation energies (Ea) as 102±17 (pH 6), 100±5 (pH 8), and 91±6 kJ/mol (pH 10). iiii) The octanol/water partition coefficient (PO) and its pH dependence were also determined. The distribution coefficient (DO) for oosporein at pH 1.2 was found to be 53.7±4.1 (logDO=1.73±0.03). The partition coefficient PO for undissociated oosporein was calculated as 56.6 (logPO=1.75). From the physicochemical properties, it can be concluded that oosporein can hardly be adsorbed by organisms.