*3.5. UV-VIS Spectra of Fagopyrin Conformers*

Different conformation of the investigated fagopyrins is reflected in their electron structures. In Figure 10 are presented the HOMO and LUMO orbitals for the conformers of fagopyrin F—the most popular in the plant material. For other fagopyrins, the HOMO and LUMO orbitals are collected in Supplementary Materials. It is characteristic that for all conformers, the HOMO orbital is located mainly on the outer A, C, F, and H rings and on the oxygen atoms of the hydroxyl group. Only for conformers 34 and 36, the HOMO orbital is more concentrated on rings A and F than on C and H. The transfer of electrons to the LUMO orbital is connected with the shifting of electrons to the B and G rings, the oxygen of the carbonyl group, and outer bonds of the A, C, F, and H rings. Since the arrangement of the HOMO and LUMO orbitals is similar for all conformers, the HOMO–LUMO gap

energy is also similar. This is persistent for all the analyzed fagopyrins. In Table 3 are collected the HOMO–LUMO gap energies for all the analyzed fagopyrins.

**Figure 9.** NCI plots for (**a**) **31**, (**b**) **32**, (**c**) **33**, (**d**) **34**, (**e**) **35**, and (**f**) **36** fagopyrin F conformers. Colors used for isosurfaces are: red for repulsive, green for dispersive, and blue for attractive interactions.


**Table 3.** HOMO–LUMO gap energy for calculated fagopyrin structures.

**Table 3.** *Cont.*


**Figure 10.** HOMO and LUMO orbitals for the fagopyrin F conformers **31**, **32**, **33**, **34**, **35**, and **36**.

The UV spectra for fagopyrin F shown in Figure 11 are characterized by the presence of two intense bands. For the conformers 31, 32, and 33, the most intensive band shifts from 556 nm to 552 nm. The second intensive band is located at 457, 455, and 459 nm. For conformer 34, except for the most intensive band at 546 nm, two bands with similar intensity at 477 and 427 nm are present. The last band at 427 nm is visible in the UV-VIS spectra of 31, 32, 34, and 36 conformers; however, it is significantly lower compared to other bands. For conformers 31, 32, 33, and 34, the most intensive band is related to HOMO–LUMO transition. For conformer 35, this band is shifted to 539 nm, for conformer 36 to 503 nm, and the intensity of this band is lower than the bands at 437 and 421, respectively. For the conformers 35 and 36, transitions from lower energy orbitals are more intense than for the HOMO–LUMO transition. The electron transition participating in the bands for fagopyrin F conformers are collected in Table 4. The shape of the orbitals involved in the electron transitions in structure 32 (the lowest energy structure of fagopyrin F) is shown in Figure 12. The shape of the orbitals involved in UV-VIS transitions for fagopyrin F conformers except the presented in the text (Figure 12) is shown in Supplementary Materials.

**Figure 11.** Theoretical excitation spectra for fagopyrin F conformers. Structure **31**—red, **32**—blue, **33**—black, **34**—green, **35**—orange, and **36**—purple.

**Figure 12.** The shape of the orbitals for structure **32** (the lowest energy structure of fagopyrin F). 176—HOMO and 177—LUMO orbitals.


**Table 4.** The electron transition contribution and orbitals for fagopyrin F conformers.

A comparison of calculated fagopyrin F and experimental spectra [19] for the plant material suggests that in the plant material, many fagopyrin structures may be present. It is not clear which version of fagopyrin in the experimental spectra was registered; however, in the experimental spectra, the most intensive theoretically calculated bands are visible.
