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

摘要

The ~(13)C NMR chemical shifts of a range of flavonoids are predicted by the Mnova NMRPredict software and related to their radical scavenging activity (RSA). ~(13)C NMR chemical shifts of G 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 ~(13)C 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, ~(13)C 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 rale, 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 ~(13)C 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 ~(13)C 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.