thank you but the problem is a bit the choice of dissolution media. so far, I only seem to find that isoquercetin is soluble in alkaline solutions, but without solubility data...
“Selected Physical and Chemical Properties of Commercial Hypericum perforatum Extracts Relevant for Formulated Product Quality and Performance,” S. Kopelman et al, The AAPS Journal, 3 (4) article 26 (2001). Isoquercetin was one of the components of Hypericin perforatum extract assayed in this paper.