Fig. 6.

Estimated reduction potential of enediol(ate) forms of ribulose 1,5-bisphosphate (vs. SHE). For simplicity, R and R′ represent C1 (CH₂O–PO₄²⁻) and C4–C5 groups (CHOH–CH₂O–PO₄²⁻). The thermodynamic (equilibrium) H/D isotope effect associated with the redox half reaction (^HK/^DK) is shown in blue (positions of isotopic substitution recalled below; M stand for the nonsubstituted [monoisotopic] form). Here, calculations were carried out assuming that phosphate groups were deprotonated. Note that the form deprotonated in C2 (C2–O⁻) is less oxidant than oxygen while the form protonated in C2 is more oxidizing than oxygen. Reduction potentials of couples involving O₂ as an oxidant (62) are indicated in red. Further calculations with variations in the point charge accommodating O2 are provided in SI Appendix, Fig. S3.