*Article* **Theoretical Studies on the Structure and Intramolecular Interactions of Fagopyrins—Natural Photosensitizers of** *Fagopyrum*

**Sebastian Szyma ´nski \* and Irena Majerz**

Faculty of Pharmacy, Wroclaw Medical University, Borowska 211a, 50-556 Wroclaw, Poland; irena.majerz@umw.edu.pl

**\*** Correspondence: sebastian.szymanski@umw.edu.pl; Tel.: +48-71-784-0305; Fax: +48-71-784-0307

**Simple Summary:** The study determines the spatial structure and intramolecular interactions of fagopyrins—natural photosensitizers of *Fagopyrum* species. In silico calculations show many fagopyrin conformers characterized by the formation of strong intramolecular interactions.

**Abstract:** Compounds characterized by a double-anthrone moiety are found in many plant species. One of them are fagopyrins—naturally occurring photosensitizers of *Fagopyrum*. The photosensitizing properties of fagopyrins are related to the selective absorption of light, which is a direct result of their spatial and electronic structure and many intramolecular interactions. The nature of the interactions varies in different parts of the molecule. The aim of this study is to determine the structure and intramolecular interactions of fagopyrin molecules. For this purpose, in silico calculations were used to perform geometry optimization in the gas phase. QTAIM and NCI analysis suggest the formation of the possible conformers in the fagopyrin molecules. The presence of a strong OHO hydrogen bond was shown in the anthrone moiety of fagopyrin. The minimum energy difference for selected conformers of fagopyrins was 1.1 kcal·mol−1, which suggested that the fagopyrin structure may exist in a different conformation in plant material. Similar interactions were observed in previously studied structures of hypericin and sennidin; however, only fagopyrin showed the possibility of brake the strong OHO hydrogen bond in favor of forming a new OHN hydrogen bond.

**Keywords:** fagopyrins; conformation; hydrogen bond; QTAIM; NCI
