Oxo-Hydrazyl as a Substitute for the Stable Free Radicals Employed in Measuring Total Antioxidant Activity

Abstract

Numerous standardized methods for evaluating antioxidant capacity are available, some of the most used methods in this regard employ stable free radicals, like 2,2-diphenyl-1-picrilhydrazyl free radical (DPPH·) or 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid cation radical (ABTS·⁺). However, some challenges can arise in taking correct and reproducible measurements due to the well-known unspecific reactivity of these free radicals. In pursuit of improving and expanding such methods, in this work is proposed a highly intense colored zwitterionic derivative of the DPPH· free radical, as a replacement for DPPH· and ABTS·⁺ derivatives. A discussion and comparison of the recognized methods are presented, demonstrating the very good potential of this non-radical compound.

Keywords: ABTS; DPPH; TAC; antioxidant; betaine; radical; zwitterionic.

Figure 1

Chemical structure of DPPH·, ABTS·⁺, and oxo-DPPH.

Figure 2

Reversible redox processes of DPPH· and oxo-DPPH.

Figure 3

Typical UV–Vis spectra of a solution of (a) ABTS·⁺ (in water), (b) DPPH· (in methanol), and (c) oxo-DPPH (in methanol).

Figure 3

Typical UV–Vis spectra of a solution of (a) ABTS·⁺ (in water), (b) DPPH· (in methanol), and (c) oxo-DPPH (in methanol).

Figure 4

Percentage (%) of decomposition of an aqueous solution for ABTS·⁺ free radical in time (at different wavelengths).

Figure 5

(a) Percentage (%) of decomposition of a methanolic solution of DPPH· (blue line) and oxo-DPPH (orange line); (b) percentage (%) of decomposition of a DCM solution of DPPH· (blue line) and oxo-DPPH (orange line).

Figure 6

Spectrophotometric titration (y-axis Abs as a.u.) of a solution of DPPH· (blue line) and oxo-DPPH (orange line) with ascorbic acid (as equivalent mol).

Figure 7

Possible mechanism of action of oxo-DPPH as a non-radical compound used in TAC measurements.