Reactions of reducing and oxidizing radicals with caffeic acid: a pulse radiolysis and theoretical study

Molecular calculations coupled with pulse radiolysis studies are performed to understand the reactions of radicals with caffeic acid. From molecular calculation, we find that e(aq)(-) and OH tend to form adducts with caffeic acid, while N-3(.) tends to abstract H from I-hydroxyl group in benzene ring, generating a semi-quinoid radical. Based on comparison of the heat of formation, the most favorable radical attack sites and the most stable radical structures are predicted. The calculation results suggest that the stability of the electron adducts < semi-quinoid radicals < OH adducts of caffeic ions, in good agreement with their experimental second-order decay rate constants (2k = (1.1+/-0.2) x 10(9) (6.0 +/- 0.4) x 10(7) and (2.0 +/- 0.2) x 10(7) M-1 s(-1), respectively), dietermined by pulse radiolysis. Molecular calculations seem to be a powerful tool to predict the stability and structures of transient radicals. (C) 2001 Elsevier Science Ltd. All rights reserved.