Correlation between 13C NMR chemical shifts and antiradical activity of flavonoids
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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.
KeywordsFlavonoids Antiradical activity 13C NMR chemical shift MNova QSAR BDE
This work was supported by grants 098-1770495-2919, 098-0982929-2917, 098-0982464-2511 and 079-0000000-3211 awarded by the Ministry of Science, Education and Sports of the Republic of Croatia. The authors thank Mestrelab Research S.L. for the free demonstration version of the MestReNova LITE program. We also gratefully acknowledge the anonymous reviewers for their constructive comments.
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