Reduction titration curve computed

Shown in Fig. 11.12 is the computed reduction titration curve of a model groundwater, assuming the titrant is dissolved organic carbon (DOC). Note that the pH remains in a narrow range between 6.5 and 7.5 as the water drops in pE and Eh. 

Redox potentials have been determined for each of the steps of reduction of the trypsin-solubilized reductase (403) step 1, one electron consumed, Eo = —109 mV step 2, two electrons consumed. Eg = —276 mV and step 3, one electron consumed. Eg = —371 mV at pH 7.0, 26°. As expected, the redox potential of step 3 is more negative than the potential of the NADPH-NADP+ couple and was determined from the dithio-nite titration. The overall potentiometric—spectrophotometric titration curves could be very closely fitted with a computer-generated curve based on the assumptions of four one-electron reduction steps and octinction coefficients of 4.9 and 4.5 mM cm for the semiquinones, FliH and rijH the Eg values assumed for steps 2 and 3 were —270 and —290 mV. The precise fit was very sensitive to all of the assumptions (40 ) ... 

Fig. 6. (A) Plot of amplitude of light-induced pheophytin-reduction signal vs. ambient potential of the medium. (B) Effed of ambient redox potential on the extent of light-induced PS-II reaction-center triplet signal in pea chloroplast particles. (C) Plot of the extent of the light-induced triplet EPR signal in (B) vs. redox potential. Open and closed circles are for reductive and oxidative titrations, respedively. The solid curve is a computer fit of the Nernst equation with n=1 and E , was estimated to be -604 mV. Figure source (A) Klimov. Allakhverdiev. Demeter and Krasnovsky (1979) Photoreduction ofpheophytin in photosystem 2 ofchloroplasts with respect to the redox potential of the medium. DokI Akad NaukSSSR 249 229 (B and C) Rutherford, Mullet and Crofts (1981) Measurement of the midpoint potential of the pheophytin acceptor of photosystem II. FEBS Lett 123 236,237...

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