Correlation between ¹³C NMR chemical shifts and antiradical activity of flavonoids

Abstract

The ¹³C NMR chemical shifts of a range of flavonoids are predicted by the Mnova NMRPredict software and related to their radical scavenging activity (RSA). ¹³C NMR chemical shifts of C atoms bearing phenolic OH groups associated with radical attack tend to decrease with increasing antiradical activity. For a data set of 27 flavonoids, fair correlation (r = −0.881) was found between the antiradical activity and minimal value of the ¹³C NMR chemical shift (NMR _min), and it was similar to the correlation (r = −0.850) obtained with the minimal O–H bond dissociation enthalpy (BDE _min) calculated by the PM7 method. For a particular flavonoid molecule, ¹³C NMR chemical shifts of C atoms bearing phenolic OH groups correlate nicely with the corresponding O–H BDEs (e.g., for robinetin r = 0.953). For the complete data set, there is a similar correlation between NMR _min and BDE _min values (r = 0.944). As a rule, NMR _min is related to nuclei bearing a 3′,4′-dihydroxy moiety in the B ring or 3-OH phenolic group in the C ring, i.e., to the preferred sites of radical attack. Thus, the ¹³C NMR chemical shifts of C atoms bearing phenolic OH groups are in accordance with the O–H BDEs, i.e., describe the H atom donor ability of phenolic OH groups. The statistical significance of the relationship between the minimal ¹³C NMR chemical shift and RSA was verified by comparison with correlations between RSA and each of 1,140 Dragon molecular descriptors, where the highest correlation coefficient of 0.812 was obtained.

Graphical abstract