Study On The Antioxidant Capacities Of Several Plant Phenolic Compounds By Potentiometric And Spectrophotometric Titrations

Plant phenolic compounds are the secondary metabolites of plants and have important research value.In this thesis,we briefly review the research progress of plant phenolic compounds,antioxidants and the methods of evaluating antioxidant capacity.Potentiometric and spectrophotometric titration methods for evaluating the antioxidant capacity were studied on the basis of the redox reactions of several phenolic compounds with some free radicals.The main contents are as follows:1.The antioxidant capacity（AOC）of chicoric acid（Ch A,the main antioxidant component of Echinacea）or an ethanol/water-extract of Echinacea flowers was determined by potentiometric and UV-Vis absorption spectrophotometric titrations with ABTS·⁺radical cations as the oxidizing probe.The potentiometric and spectrophotometric titration results agreed well with each other.The trolox-equivalent antioxidant capacity（TEAC）of Ch A was found to be 5.00±0.07（potentiometry）and 4.81±0.06（spectrophotometry）at p H 7.4,and the TEAC value has been found and proven to be actually equal to the ratio of the stoichiometric ratio of the ABTS·⁺–Ch A redox reaction to that of the ABTS·⁺–trolox redox reaction.The TEAC of the ethanol/water-extract of Echinacea flowers,expressed in m M（trolox）per gram per liter（Echinacea extract）,was found to be 0.241±0.006 mmol g^-1（potentiometry）and 0.240±0.007 mmol g^-1（spectrophotometry）at p H7.4.The stoichiometric ratio of the ABTS·⁺–Ch A redox reaction varied from 10.8 to 3.2,depending on the solution p H and the final ABTS·⁺–Ch A concentration ratio.However,the stoichiometric ratio of the ABTS·⁺–trolox redox reaction remained ca.2.0 at various solution-p H values and final ABTS·⁺–trolox concentration ratios.The unusual stoichiometric ratio of the ABTS·⁺–Ch A redox reaction is examined by potentiometric/spectrophotometric titrations and cyclic voltammetry,clearly revealing the new mechanism of a rapid redox process followed by a slow redox process for the ABTS·⁺-Ch A redox system at p H 7.4 and9.0 when the ABTS·⁺–Ch A molar concentration ratio is greater than 4.2.Taking DPPH·radical as the oxidizing probe,the AOC values of several phenolic compounds and ascorbic acid were determined by potentiometric and spectrophotometric titrations in ethanol-containing Tris-HCl aqueous buffer.The potentiometric and spectrophotometric titration results agreed well with each other.The trolox-equivalent antioxidant capacity（TEAC）values of ascorbic acid,chlorogenic acid,gallic acid and quercetin were found to be 1.04,1.94,2.76 and 3.35（potentiometry）,as well as 1.01,1.92,2.72 and 3.46（spectrophotometry）,respectively,in 0.2 M p H 7.4 Tris-HCl buffer containing 50%（v/v）anhydrous ethanol solution.The two methods gave comparable limits of detection for DPPH·radical.The two methods were successfully applied to determine the total AOC values of the real sample Chinese herb extracts.