Inhibition of the Hill Reaction by Tris and Restoration by Electron Donation to Photosystem II.

Experiments in which chloroplasts were washed with tris and tricine buffers at different pH's indicated that the non-protonated (uncharged) form of tris was inhibitory to the Hill reaction while the protonated form of tris and the zwitterionic forms of tricine were non-inhibitory. Buffers analogous to tris and tricine gave similar results. Photoreduction of NADP could be restored to the inhibited chloroplasts by adding the reduced forms of p-hydroquinone, p-aminophenol, p-phenylenediamine, benzidine, semicarbazide, and dihydroxydiphenyl, all of which donated electrons to photosystem II. Photoreduction of ferricyanide was shown with those donor systems (benzidine and semicarbazide) which did not react chemically with ferricyanide. Photophosphorylation was also restored with all of the electron donors except semicarbazide.