An investigation of water splitting in the Kok scheme of photosynthetic oxygen evolution

The involvement of OH bond breaking in the 4 dark reactions of the Kok scheme of photosynthetic oxygen evolution was investigated using Chlorella and spinach chloroplasts. When the photosynthetic material was suspended in a 2H2O based medium, the reaction rates in all 4 cases were only slightly reduced as compared to the rates observed in an H2O based medium. This was evidence that these rate processes were probably not limited by the breaking of an OH bond. Observations were also made on the yields of O2 from dark adapted Chlorella subjected to a sequence of brief saturating light flashes. The oscillating flash yield sequence observed with algai suspended in 2H2O showed greater damping of the oscillations than when the algae were suspended in H2O. A computer fit of the Kok model to these results revealed a slightly higher proportion of misses, alpha (i.e. absorbed quanta that do not drive photochemistry) in the 2H2O case.