Singlet oxygen quenching and the redox properties of hydroxycinnamic acids

The singlet oxygen quenching rate constants (k(q)) for a range of hydroxyci nnamic acids in acetonitrile and D2O solutions were measured using time res olved near infrared phosphorescence in order to establish their antioxidant activity. The magnitude of k(q) observed depends on both the nature of the substituent groups and solvent polarity. The variations in k(q) depend on the energy of the hydroxycinnamic acid/molecular oxygen charge transfer sta tes, (O-2(delta-)...HCA(delta+)). In D2O the values of k(q) range from 4 x 10(7) M-1 s(-1) to 4 x 10(6) M-1 s(-1) for caffeic acid and o-coumaric acid respectively. In acetonitrile, the charge transfer energy levels are raise d and this is reflected in lower singlet oxygen quenching rate constants wi th a k(q) value of 5 x 10(6) M-1 s(-1) for caffeic acid. The phenoxyl radic al spectra derived from the hydroxycinnamic acids were determined using pul se radiolysis of aqueous solutions and the reduction potentials were found to range from 534 to 596 mV. A linear correlation is observed between reduc tion potential, and hence free energy for electron transfer, and log k(q). These correlations suggest a charge transfer mechanism for the quenching of singlet oxygen by the hydroxycinnamic acids. (C) 1999 Elsevier Science Inc .The singlet oxygen quenching rate constants (k(q)) for 3 range of hydroxyc innamic acids in acetonitrile and D2O solutions were measured using time re solved near infrared phosphorescence in order to establish their antioxidan t activity. The magnitude of k(q) observed depends on both the nature of th e substituent groups and solvent polarity. The variations in k(q) depend on the energy of the hydroxycinnamic acid/molecular oxygen charge transfer st ates, (O-2(delta-)...HCA(delta+)). In D2O the values of k(q) range from 4 x 10(7) M-1 s(-1) to 4 x 10(6) M-1 s(-1) for caffeic acid and o-coumaric aci d respectively. In acetonitrile, the charge transfer energy levels are rais ed and this is reflected in lower singlet oxygen quenching rate constants w ith a k(q) value of 5 x 10(6) M-1 s(-1) for caffeic acid. The phenoxyl radi cal spectra derived from the hydroxycinnamic acids were determined using pu lse radiolysis of aqueous solutions and the reduction potentials were found to range from 534 to 596 mV. A linear correlation is observed between redu ction potential, and hence free energy for electron transfer, and log k(q). These correlations suggest a charge transfer mechanism for the quenching o f singlet oxygen by the hydroxycinnamic acids. (C) 1999 Elsevier Science In c.