Antioxidant Properties of Dihydroxyfumaric Acid and Its Dimethyl Ether: a Comparative DFT Study of Their Reactions with the Stable Radical DPPH*

Abstract

Abstract. The preferred mechanism of the reaction of dihydroxyfumaric acid and its dimethyl ether with the free stable radical 1,1-diphenyl-2-picrylhydrazyl (DPPH*) was revealed by means of Density Functional Theory (DFT) calculations. The proposed mechanism has an ionic character and includes the formation of charge-transfer complexes as the main stage. It is also shown that the lower antioxidant activity of dimethyl ether of dihydroxyfumaric acid is caused by both its lower acidity (as compared with its precursor acid) and formation of more stable intermediates during its reaction with DPPH*. Our results allow one to rationalize the available experimental data.Keywords: antioxidant activity, DPPH*, dihydroxyfumaric acid, dimethyl ether of dihydroxyfumaric acid, DFT calculations. IntroductionMany organic compounds with enol or phenol groups are effective antioxidants which transform various free radicals into their diamagnetic derivatives [1]. Usually their relative antioxidant activity is evaluated by the reaction rate with the free stable radical -DPPH* (1,1-diphenyl-2-picrylhydrazyl) in the methanol-water medium. It was shown [2] that DPPH* reacts with compounds containing active hydrogen atoms (phenols, mineral and organic acids, etc.) forming 1,1-diphenyl-2-picrylhydrazine. According to [2], DPPH* solutions immediately change their color by adding of diluted aqueous mineral acids therein, in methanol or ethanol (whose acidity is lower) they demonstrate change of the color within several days, while solutions of DPPH* in liquid media without active hydrogen atoms (for instance, in benzene, chloroform, acetone, etc.) remain unchanged within a period of 50 days. Thus, the higher acidity of liquid media leads to the faster decomposition of DPPH*.In its turn, the rate of the reaction of DPPH* with some phenols depends on the stability of the free radicals formed from the above phenol diamagnetic molecules when their hydrogen atoms migrate to DPPH* ones. So, for example, isoeugenol reacts very quickly (immediately) with DPPH*, while the reaction between DPPH* and eugenol (whose double bond is not conjugated with its benzene cycle) requires several hours (see [1]). It has been also shown [1] that the compounds containing enol groups (ascorbic acid and isoascorbic one) possess the maximum antioxidant activity, i. e. they are the best scavengers of the free radical DPPH*.The other effective antioxidant compounds containing in their structures enol groups are the dihydroxyfumaric acid -DHFA and its derivatives (for instance, its dialkylethers). In the work [3] it was shown by means of the method of stopped-fl ow that in the methanol-water medium the reaction between DHFA and DPPH* includes two stages. During the fi rst stage which requires only 7-10 s at T = 293.15K one half of DPPH* reacts with DHFA. The second reaction step is slower and requires around 1-2 hours [3]. Basing on these facts, some theoretical suppositions were done [3] in order to describe the reaction mechanism between DPPH* and DHFA. So, it was a...

Quantum-chemical study of the electronic structure - antioxidant action relationship of some chemical compounds

“…Anterior, [19], o situație similară a fost găsită și pentru reacția acidului dihidroxifumaric cu radicalul stabil DPPH • . Rezultatele acestei lucrări sunt în acord cu datele lucrărilor teoretice anterioare dedicate studiului activității antioxidante a tiaflavanelor pe baza reacției lor cu cation-radicalul ABTS •+ [20].…”

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