Adducts of Arzanol with Explicit Water Molecules: An Ab Initio and DFT Study

Arzanol (C₂₂H₂₆O₇) is a naturally occurring acylphloroglucinol present in Helichrysum italicum. It is the major responsible of its medicinal properties, which include anti-oxidant properties. In the arzanol molecule, the R of the COR group characterising acylphloroglucinols is a methyl group, and the two substituents in meta to COR are an α-pyrone ring, bonded to the benzene ring through a methylene bridge, and a prenyl chain. The high number of hydrogen bond donor and acceptor sites in the molecule entails an investigation taking into account solute-solvent hydrogen bonds in an explicit manner. The current work considers adducts of arzanol with explicit water molecules for a representative selection of its conformers. Adducts with one water molecule attached in turn to each of the H-bond donors or acceptors were calculated to estimate the strength with which each site can bind a water molecule. Adducts with varying numbers of water molecules were calculated to identify preferred arrangements of the water molecules around the various sites and around the molecule as a whole. These adducts also suggest possible geometries for the first solvation layer. All the adducts were calculated at the HF/6-31G(d, p) and the DFT/B3LYP/6-31+G(d, p) levels, with fully relaxed geometry.