Theoretical calculation of electronic circular dichroism of a hexahydroxydiphenoyl-containing flavanone glycoside.

Time-dependent density functional theory (TDDFT) was employed for theoretical calculation of electronic circular dichroism (ECD) of a hexahydroxydiphenoyl (HHDP)-containing flavanone glycoside, mattucinol-7-O-[4'',6''-O-(aS)-hexahydroxydiphenoyl]-beta-d-glucopyranoside (2). It identified the roles of the (2S)-flavanone and (aS)-HHDP moieties in generating the ECD spectrum of 2 and provided theoretical evidence for the empirical ECD rules applicable to monomeric flavanones and HHDP-containing compounds. The experimentally observed high-amplitude positive Cotton effect (CE) around 240 nm in 2 is derived from the (aS)-HHDP chromophore, while the low-amplitude negative CE in the 260-300 nm region is contributed by both the (aS)-HHDP and (2S)-flavanone moieties. The "linker" glucosyl moiety has little effect on the overall ECD. It appears that the respective chromophores in 2 contribute additively to the overall ECD, and the empirical rules are applicable for configurational assignment. However, if an (aR)-HHDP chromophore is present, as shown in mattucinol-7-O-[4'',6''-O-(aR)-hexahydroxydiphenoyl]-beta-d-glucopyranoside (3), the dominant role of the (aR)-HHDP and interaction between the (aR)-HHDP and (2S)-flavanone moieties in its overall ECD may be confusing when applying the empirical rules to experimental ECD interpretation. Thus, theoretical calculation of the ECD that quantifies the contributions and interactions of different chromophores is essential for the assignment of the absolute configuration of such molecules.