Reactive Absorption of Ozone by Aqueous Biomolecule Solutions: Implications for the Role of Sulfhydryl Compounds as Targets for Ozone

The rates of reactive absorption of ozone by various biomolecule solutions were measured. At pH 7, the ability of various biomolecules to reactively absorb ozone was in the following sequence: thiosulfate > ascorbate > cysteine ≍ methionine > glutathione. The rates of reactive absorption under a variety of conditions were then analyzed using a mathematical model in order to estimate the reaction rate constants for the various ozone-biomolecule reactions. Compared to the ozone-methionine rate constant, the relative rate constants for thiosulfate, ascorbate, cysteine, and glutathione reactions were 18 ± 2, 12 ± 1, 1.1 ± 0.1, and 0.62 ± 0.03, respectively. Using an ozone-methionine reaction rate constant of 4 × 106 M−1 s−1, the rate constants for thiosulfate, ascorbate, cysteine, and glutathione were 7.2 × 107, 4.8 × 107, 4.4 × 106, and 2.5 × 106 M−1 s−1, respectively. Competitive studies using tryptophan as a standard ozone target were consistent with these rate constants. Compared to tryptophan, the relative ozone reaction rate constants for methionine, cysteine, and glutathione were 0.77 ± 0.08, 0.88 ± 0.19, and 0.42 ± 0.01, respectively. These relative rate constants refer to ozone consumption rather than biomolecule consumption, so they may be compared with the relative rate constants obtained from reactive absorption. In addition, various inconsistencies in the literature regarding the rates of the ozone-cysteine and the ozone-glutathione reactions were reviewed.