Effect of conformation and configuration of arabinose residues on the circular dichroism of C-glycosylflavones

Examination of a variety of arabinose containing C-glycosylflavones has shown that the sign and intensity of the CD band at 250–275 nm (charge-transfer band) reflect not only the point of attachment of the sugar to the flavone but also depend upon the absolute and anomeric configuration, ring-size and ring-conformation in addition to the preferred rotameric conformation of the sugar about the C-aryl, C-l″ bond. A change in stereochemistry of arabinose from the α to β anomer resulted in sign inversion of the 250–275 nm CD band for 6-C-l-arabinosylflavones. Furthermore, a 6-C-arabinosylflavone containing α-l-arabinose exhibited an oppositely signed charge-transfer CD band in comparison to one which contained α-d-arabinose. 6,8-Di-C-glycosylflavones containing arabinose and glucose exhibited CD bands resulting from contributions due to both sugars, if the arabinose was not present as the β-pyranose form (¹C₄, conformation).