**4. Conclusions**

Theoretical calculations can provide information on the molecular structure when the structure is unknown, which is often the case with plant material. Fagopyrin compounds may exist as conformers characterized by a different energy. The presence of the piperidine and pyrrolidine ring in fagopyrin introduces novel intramolecular interactions compared to

the double anthrone molecules. Fagopyrin A–F structures are characterized by the presence of a number of substituents and strong hydrogen bonds in the anthrone moiety. Although the OHO hydrogen bonds in the anthrone moiety are characterized as very strong, both the OHO and OHN hydrogen bonds may exist in the fagopyrin A–F structure. It is possible to break the strong OHO hydrogen bonds in the anthrone moiety in favor of interactions with the nitrogen atom in piperidine or pyrrolidine substituent. Changes in the molecular geometry are related to the changes in the orbital localization, which is reflected in the UV–VIS spectra of fagopyrin conformers.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27123689/s1. Figure S1. HOMO (left) and LUMO (right) orbitals for the 1, 2, 3, 4, 5, and 6 Fagopyrin A conformers. Figure S2. HOMO (left) and LUMO (right) orbitals for the 7, 8, 9, 10, 11, and 12 Fagopyrin B conformers. Figure S3. HOMO (left) and LUMO (right) orbitals for the 13, 14, 15, 16, and 17 Fagopyrin C conformers. Figure S4. HOMO (left) and LUMO (right) orbitals for the 19, 20, 21, 22, 23, and 24 Fagopyrin D conformers. Figure S5. HOMO (left) and LUMO (right) orbitals for the 25, 26, 27, 28, 29, and 30 Fagopyrin E conformers. Figure S6. The shape of the orbitals for structure 31 (Fagopyrin F). 176—HOMO and 177—LUMO orbitals. Figure S7. The shape of the orbitals for structure 33 (Fagopyrin F). 176—HOMO and 177—LUMO orbitals. Figure S8. The shape of the orbitals for structure 34 (Fagopyrin F). 176— HOMO and 177—LUMO orbitals. Figure S9. The shape of the orbitals for structure 35 (Fagopyrin F). 176—HOMO and 177—LUMO orbitals. Figure S10. The shape of the orbitals for structure 36 (Fagopyrin F). 176—HOMO and 177—LUMO orbitals.

**Author Contributions:** S.S., I.M. contributed to the conceptualization, methodology, and writing of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by a Ministry of Health grant number SUBK.D050. 22.003 from the IT Simple system of Wroclaw Medical University.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The Wroclaw Center for Networking and Supercomputing is acknowledged for generous allocations of computer time.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

**Sample Availability:** Samples of the compounds are not available from the authors.
