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Correlation Between 13C NMR Chemical Shifts and Antiradical Activity of Flavonoids

The purpose of this report is to relate 13C NMR chemical shifts to antiradical activity of flavonoids. Because the substitution pattern causes the changes of the 13C NMR spectral assignments of flavonoids, the 13C NMR chemical shifts may spread some light on antiradical potency of flavonoids. The 13C NMR chemical shifts of flavonoids are predicted by the Mnova NMRPredict software1 and related to their free radical scavenging activities. For dana set of 24 flavonoids2 modest correlation (r = −0.876) exist between minimal value of 13C NMR chemical shift (13C NMRmin) and antiradical potency. Antiradical activity (%) = 506.13(±52.61) − 3.064(±0.359) 13C NMRmin N = 24, r = 0.876, rcv = 0.858, s = 18.91, scv = 20.20, F = 72.92 Radical scavenging potency of flavonoids is mostly related to the presence of OH group(s) at specific position(s) on the flavonoid core. 13C NMR chemical shift for carbon atom adjacent to the phenolic OH group, associated with radical attack, tends to decrease with increasing antiradical activity. Minimal value of 13C NMR chemical shift is related to 3’, 4’- dihydroxy moiety in the B ring or to phenolic 3-OH group in the C ring, i.e., to the preferred sites of radical attack.3 A good linear correlation also exists between 13C NMRmin and the minimal value of bond dissociation enthalpy (BDEmin) of OH groups (r = 0.822). Here we introduce the 13C NMR chemical shift of carbon atom bearing phenolic OH group as a new descriptor for antiradical activity modeling. The antiradical activity of flavonoids could be related to 13C NMRmin though modification of electron density on corresponding C atom in aromatic ring depending on electronegativity of oxygen atom in adjacent OH group. Presented analysis could help in understanding of antiradical action mechanism of flavonoids, but more detailed theoretical basis for this relationship remains to be elucidated. References 1. Mestrelab Research (www.mestrelab.com) 2. Burda, S. ; Oleszek, W. J. Agric. Food Chem. 49 (2001) 2774. 3. Pietta, P. ; G. J. Nat. Prod. 63 (2000) 1035.

flavonoids ; antiradical activity ; 13C NMR chemical shift ; molecular modeling

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