Thermodynamic and Mechanistic Study of the Electrochemical Oxidation of Rosmarinic Acid

Abstract

Electrochemical oxidation of rosmarinic acid (RA) in aqueous solutions has been studied using cyclic voltammetry, linear sweep voltammetry and differential pulse voltammetry. RA (1) has two catechol rings, and the electrochemical study of it shows two oxidation-reduction peaks (E r E r mechanism). In the first step oxidation RA (1) converted to OX 1-1 , and in the second step of oxidation the produced OX 1-1 oxidized to OX 2 . Two possible structures were designed for the first step of oxidation and optimized at the B3LYP/6-311 G (d, p) level of theory. To confirm the results, the oxidation potential of RA (1) was compared with some catechol species at pH = 0.0. Plotted diagram of the oxidation potential versus G tot showed a good correlation between the electrochemical oxidation potential (E pA0 ) and calculated G tot of electrochemical oxidation of studied species. The voltammetric study in the basic solutions shows a dimerization reaction after electron transfer process. Analysis of the voltammograms shows a partially adsorption behavior in the electrochemical oxidation of RA (1). Also, pKa of RA (1) was determined by the use of acid-base titration and effect of deprotonation of RA (1) on voltammograms was studied.

A Comprehensive Study of the Radical Scavenging Activity of Rosmarinic Acid

“…p K a values were calculated to be 3.07 (p K a1 ) (vs experimental value of 3.16, validating the method) and 8.28 (p K a2 ). Thus, in the aqueous solution at pH = 7.4, RA exists in two states: monoanion (HA – , 88.6%) and dianion (A 2– , 11.4%).…”

“…Calculated ΔG ≠ (kcal/mol), κ, k Eck (M −1 s −1 ), and Γ (%) for the HO values were computed following the literature, 46,47 and the results are shown in Figure 3. pK a values were calculated to be 3.07 (pK a1 ) (vs experimental value of 3.16, validating the method) 15 and 8.28 (pK a2 ). Thus, in the aqueous solution at pH = 7.4, RA exists in two states: monoanion (HA − , 88.6%) and dianion (A 2− , 11.4%).…”

“…Rosmarinic acid ( RA ) is an ester of caffeic acid and 3,4-dihydroxyphenyllactic acid (Figure ) that is found in plants such as savory, lemon balm, rosemary, sage, lavender, oregano, mint, and salvia − This compound was reported to have various proven and/or assumed biological activities such as antimutagenic, antiviral, antihepatitis, anticarcinogenic, antibacterial, antiallergic, and anti-inflammatory properties. − RA could inhibit HIV-1 and blood clotting and alleviate seasonal allergic rhinoconjuctivitis. , RA is also known as a potent natural antioxidant as confirmed in many studies. − Yet, RA was also implicated in pro-oxidant activity under at least some of the studied conditions. ,,, …”

“…Not only was the antiradical activity of RA thoroughly examined experimentally, but a few studies also attempted to model the antioxidant activity of this compound. ,,, Cao et al reported the role of aromatic rings on the H-abstraction of RA in the gas phase using the B3LYP/6-31G* method, whereas Beiginejad and coauthors studied the oxidation–reduction of anion states of RA by calculations at the B3LYP/6-311 G (d,p) level of theory . Swislocka et al investigated the structures and antioxidant effects of rosmarinic acid and its alkali metal salts using experiments and theories .…”

“…8,13,15,19 Cao et al reported the role of aromatic rings on the H-abstraction of RA in the gas phase using the B3LYP/6-31G* method, 8 whereas Beiginejad and coauthors studied the oxidation− reduction of anion states of RA by calculations at the B3LYP/ 6-311 G (d,p) level of theory. 15 Swislocka et al investigated the structures and antioxidant effects of rosmarinic acid and its alkali metal salts using experiments and theories. 13 However, these studies were limited in scope, approximating activity based on thermodynamic calculations.…”