Structural characterization of p-benzosemiquinone radical in a solid state: the radical stabilization by a low-barrier hydrogen bond.

Semiquinone (p-benzosemiquinone), a transient organic radical, was detected in the solid state by EPR spectroscopy revealing four symmetrically equivalent protons. A variable-temperature X-ray diffraction analysis (293 and 90 K) and EPR data support a dynamical disorder of the proton. A low-barrier O-H...O hydrogen bond stabilizes the radical. The C-O bond length is 1.297 (4) A, corresponding to a bond order of ca 1.5. The geometry of the radical implies an electron delocalization throughout the benzenoid ring. Two polymorphs of semiquinone, monoclinic and triclinic, were observed and their structures determined. Their crystal packings were compared with those of quinhydrone polymorphs.

Evaluation of the antioxidant activity by flow injection analysis method with electrochemically generated ABTS radical cation.

A Trolox equivalent antioxidant capacity (TEAC) decolourisation assay was adapted to a flow injection analysis (FIA) system and a simple and rapid method for antioxidant activity evaluation was developed. To avoid the time consuming step of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)(ABTS) radical cation preparation by chemical oxidation of ABTS, as in the original TAEC assay, and hence, to shorten the analysis time, the ABTS radical cation was generated on-line by electrochemical oxidation of ABTS in the flow-through electrolysis cell forming a part of the FIA system. The proposed method was optimised with respect to a flow rate, injection volume and ABTS radical cation/carrier ratio. Under the optimised conditions linear calibration graphs for Trolox were obtained over the range 10-100 microM, with a limit of detection 1.6 microM. Good reproducibility (relative standard deviation 1.95%) and sample throughput (32 samples per hour) were achieved. The developed method was applied to the evaluation of the antioxidant activity of pure compounds and samples of some common beverages. In both cases a good correlation between the results obtained by the proposed method and TEAC values evaluated by the classic TAEC decolourisation assay was obtained (r(2)= 0.996 for pure compounds and r(2)= 0.957 for beverage samples).