Development of Free Radical Products during the Greening Reaction of Caffeic Acid Esters (or Chlorogenic Acid) and a Primary Amino Compound
ESR spectra were measured directly on a marked greening reaction mixture of Et-caffeate and a primary amino compound in alkali solution under aeration. A clear hyperfine structure was commonly detected early in the greening reaction with different amino compounds. Its hyperfine spectrum split into seven peaks was analyzed and found to be due to the oxidized free radical product of the Et-caffeate using an authentic sample system. Another type of hyperfine ESR spectrum was observed later in the reaction, and was altered with different amino compounds. The hyperfine structure for n-butylamine split into 12 lines. The latter type of free radical products were assumed to be a semiquinone type radical compounds of the trihydroxy benzacridine derivative, which was identified as the principal structure of the green and yellow pigments formed by this greening reaction system. A formation mechanism of the green pigment and related products involving these free radical products is proposed.
Antioxidant Activity of Caffeic Acid and Dihydrocaffeic Acid in Lard and Human Low-Density Lipoprotein†
Caffeic acid (CA) has been implied as an important source of natural antioxidants in various agricultural products. We compared the antioxidative activity of CA and dihydrocaffeic acid (HCA) in lard and the low-density lipoprotein (LDL) system to know the role of the 2,3-double bond on the appearance of its antioxidative property. HCA was more effective than CA in enhancing oxidative stability of lard at 60 °C. Inhibition by HCA of copper ion-induced oxidation of human LDL was less effective than that by CA, whereas there was no significant difference between the two compounds in the capacity of 1,1-diphenyl-2-picrylhydrazyl radical scavenging and the activity of lipid peroxyl radical scavenging in solution. It can be concluded that the 2,3-double bond in the structure of CA affects the efficiency of antioxidative activity depending on the environment where the oxidation happens, although it is rarely responsible for its inherent activity. Keywords: Antioxidants; caffeic acid; dihydrocaffeic acid; free ra...
ESR Spectrum in the Course of Autoxidation of Chlorogenic Acid
ESR spectra were estimated by the static or flow method in the course of the alkaline autoxidation of chlorogenic acid which is one of the substrates in enzymatic browning of apple.When water, methyl alcohol or N, N-dimethylformamide was used as solvent in the autoxidation of chlorogenic acid, no signal was observed by the static method, but in case of alkaline ethanolic solution, ESR spectrum composed of 14-16 signals was observed and also during the course of the oxidation, the number of signals did not change but only the intensity decreased with time.On the other hand, when above mentioned solvent had been employed, 8 signals were observed by the flow method. The ESR spectrum consisting of 8 signals has the coupling constants 5.0 gauss, 2.4 gauss and 1.2 gauss, which were presumed to be attributed mainly to an interaction of an unpaired electron with three unequivalent protons at position 5, 6 and 3, respectively. Thus, it has been assumed to be the hyperfine structure of o-semiquinone type anion radical of chlorogenic acxd.