Alternatives to the 'water oxidation pathway' of biological ozone formation

Abstract

Recent studies have shown that ozone (O3) is endogenously generated in living tissues, where it makes both positive and negative physiological contributions. A pathway for the formation of both O3 and hydrogen peroxide (H2O2) was previously proposed, beginning with the antibody or amino acid-catalyzed oxidation of water by singlet oxygen ((1)O2) to form hydrogen trioxide (H2O3) as a key intermediate. A key pillar of this hypothesis is that some of the H2O2 molecules incorporate water-derived oxygen atoms. However, H2O3 decomposes extremely readily in water to form (1)O2 and water, rather than O3 and H2O2. This article highlights key literature indicating that the oxidation of organic molecules such as the amino acids methionine, tryptophan, histidine, and cysteine by (1)O2 is involved in ozone formation. Based on this, an alternative hypothesis for ozone formation is developed involving a further reaction of singlet oxygen with various oxidized organic intermediates. H2O2 having water-derived oxygen atoms is subsequently formed during ozone decomposition in water by known reactions.

Keywords: Amino acid oxidation; Baeyer-Villiger oxidation; Hydrogen peroxide; Ozone; Singlet oxygen.

Fig. 1

The water oxidation pathway involving the initial formation of H₂O₃ followed by decomposition of the latter by (i) reaction with singlet oxygen to form H₂O₂ and O₃, (ii) reaction with another H₂O₃ molecule to form 2H₂O₂ and ¹O₂ with the intermediacy of ozone, or (iii) the water catalyzed decomposition to ¹O₂ and H₂O [4,15,16]. The latter reaction is extremely facile under aqueous conditions [16]

Fig. 2

Proposed mechanism of formation of ozone through reactions of formaldehyde (1) and singlet oxygen (¹O₂)

Fig. 3

Proposed pathway of O₃ formation via the successive reactions of methionine (4) and methionine sulfoxide (7) with singlet oxygen (¹O₂). The competing conversion of 7 to sulfone 10 normally occurs to a limited extent

Fig. 4

Proposed pathways of O₃ formation via reactions of tryptophan 11 and its oxidation products with singlet oxygen (¹O₂)

Fig. 5

Proposed pathways of O₃ formation via reactions of histidine 23 and its oxidation products with singlet oxygen (¹O₂)

Fig. 6

Proposed formation of H₂O₂ containing water-derived oxygen atoms as a consequence of the decomposition of O₃