Development of Singlet Oxygen Absorption Capacity (SOAC) Assay Method. 2. Measurements of the SOAC Values for Carotenoids and Food Extracts

Abstract

Recently a new assay method that can quantify the singlet oxygen absorption capacity (SOAC) of antioxidants was proposed. In the present work, kinetic study of the reaction of singlet oxygen ((1)O(2)) with carotenoids and vegetable extracts has been performed in ethanol/chloroform/D(2)O (50:50:1, v/v/v) solution at 35 °C. Measurements of the second-order rate constants (k(Q)(S)) and the SOAC values were performed for eight kinds of carotenoids and three kinds of vegetable extracts (red paprika, carrot, and tomato). Furthermore, measurements of the concentrations of the carotenoids included in vegetable extracts were performed, using a HPLC technique. From the results, it has been clarified that the total (1)O(2)-quenching activity (that is, the SOAC value) for vegetable extracts may be explained as the sum of the product {Σ k(Q)(Car-i)(S) [Car-i](i)} of the rate constant (k(Q)(Car-i)(S)) and the concentration ([Car (i)]) of carotenoids included in vegetable extracts.

Kinetic study of the quenching reaction of singlet oxygen by α-, β-, γ-, δ-tocotrienols, and palm oil and soybean extracts in solution

“…These carotenoids will also contribute to the quenching of 1 O 2 because carotenoids show about a hundred times larger quenching rates k Q (S) than vitamin E homologues. 27,28,30,31) On the other hand, lower concentrations of carotenoids were included in extract 2. Carotenoids were not observed in extracts 3-5, as expected from the measurement of UV-vis absorption spectrum.…”

“…Experimental errors in the rate constants (k Q AO (S) and k Q AO (t 1/2 )) and relative SOAC values were estimated to be < 10%. [29][30][31] Further, the ratios (k Q AO (S)/k Q α-Toc (S)) and (k Q Toc-3 (S)/k Q Toc (S)) are also listed in Table 3. The values of the ratios (k Q Toc-3 (S)/k Q Toc (S)) for α-, β-, γ-, and δ-Tocs and -Toc-3s are 0.931, 1.11, 0.998, and 1.04, respectively.…”

“…[26][27][28] Recently, kinetic studies of the quenching reaction of 1 O 2 with natural antioxidants (AOs) (such as carotenoids, tocopherol homologues (Tocs), catechins, and caffeic acids) were performed in ethanol/chloroform/ D 2 O (50:50:1, v/v/v) (hereafter abbreviated as "mixed solvent") and ethanol solutions at 35°C. [29][30][31] The overall rate constants, k Q (=k q + k r , physical quenching + chemical reaction), for the reaction of AOs with 1 O 2 were measured, using the competition reaction method, where endoperoxide was used as a singlet oxygen generator and 2,5-diphenyl-3,4-benzofuran (DPBF) as an UV-vis absorption probe (Scheme 1). [29][30][31] 1 O 2 þ AO À!…”

“…[29][30][31] The overall rate constants, k Q (=k q + k r , physical quenching + chemical reaction), for the reaction of AOs with 1 O 2 were measured, using the competition reaction method, where endoperoxide was used as a singlet oxygen generator and 2,5-diphenyl-3,4-benzofuran (DPBF) as an UV-vis absorption probe (Scheme 1). [29][30][31] 1 O 2 þ AO À! k Q physical quenching ðk q Þ þ chemical reaction ðk r Þ…”

“…From the results, a new assay method, which can quantify the singlet oxygen absorption capacity (SOAC) of antioxidants, including carotenoids, phenolic antioxidants, and vegetable extracts, was proposed. [29][30][31] Relative SOAC value was defined in the following way:…”