Nature of serum ascorbate radical and its quantitative estimation.

A doublet signal was observed in human serum by the ESR technique at room temperature. This radical had a g value of 2.0054 and a hyperfine splitting constant of 1.84 gauss and was assigned to ascorbate radical. The ascorbate radical in serum was very stable. The intensity of ESR signal showed no differences between serum and plasma of the same individual. Photosensitivity of the ascorbate radical in serum and sodium ascorbate solution was examined and enhancement of ESR signal by irradiation was observed, although the responses in serum and in ascorbate solution were considerably different. The intensity of ESR signal was proportional to the concentration of ascorbate solution. The ESR intensity of ascorbate radical can be used as a reliable method for quantitative estimation of ascorbate radical.