Density functional theory-based first principles calculations of rhododendrol-quinone reactions: Preference to thiol binding over cyclization

Using density functional theory-based first principles calculations, we investigated the changes in the energetics and electronic structures of rhododendrol (RD)-quinone for the initial step of two important reactions, viz., cyclization and thiol binding, to give significant insights into the mechanism of the cause of cytotoxic effects. We found that RD-quinone in the electroneutral structure cannot undergo cyclization, indicating a slow cyclization of RD-quinone at neutral pH. Furthermore, using methane thiolate ion as a model thiol, we found that the oxidized form of the cyclized RD-quinone, namely RD-cyclic quinone, exhibited a reduced binding energy for thiols. However, this reduction of binding energy is clearly smaller than the case of dopaquinone, which is a molecule originally involved in the melanin synthesis. This study clearly shows that RD-quinone has a preference toward thiol bindings than cyclization compared to the case of dopaquinone. Considering that thiol bindings have been reported to induce cytotoxic effects in various ways, the preference toward thiol bindings is an important chemical property for the cytotoxicity caused by RD.