6. The water coupling/dioxygen evolving reaction promoted by photosynthetic orga

Question

6. The water coupling/dioxygen evolving reaction promoted by photosynthetic organisms (involves the
proton-coupled four electron oxidation of two water molecules along with O–O bond formation to
yield dioxygen (O2). This is the dominant global process responsible for replenishing atmospheric oxygen essential for life.
a. What would the redox potential of the photosynthetic reaction site have to be in order to drive this reaction under physiological conditions (ie pH= 7)?
b. Would you want to raise or lower the pH in order to make this reaction more favorable? Explain.
c. The catalytic site that promotes this reaction (O2 formation from H2O) consists of a cluster of four Mn ions, in a mixture of +3, +4, and possibly +5 oxidation states. Can you think of a reason why nature would devise such a complex system, when according to Table 5.2 , MnO4– would capable of promoting this reaction?

Table 5.2 Selected standard potentials at 298 K; further values are included in Resource section 2 Eo /V +2.87 +1.76 +1.51 +1.36 +1.23 +0.87 +0.77 +0.60 +0.54 . +-0.36 +0.22 Couple F(g) +2e- 2 F-(aq) Ce4+(aq) + e- Ce3+ (aq) MnO, (aq) + 8 H+(aq) + 5 e- Mn2+(aq) + 4 H20(1) Cl2(g) + 2 e- 2 Cl-(aq) 02(g) + 4 H+ (aq) + 4 e- 2 H2O(l) Fe3+(aq) + e- Fe2+(aq) [PtC14] 2-(aq) + 2 e- Pt(s) + 4 Cl-(aq) (aq) + 2 e- 3 I-1 (aq) [Fe(CN)6]3-(aq) + e- [Fe(CN)6]4-(aq) AgCl (s) + e- Ag(s) + Cl- (aq) 2H+(aq) + 2 e- H2(g Agl (s) + e- Ag(s) + I-(aq) Zn2+(aq) + 2 e- Zn(s) Al3+ (aq) + 3 e- Al(s) Ca2+(aq) + 2 e- Ca(s) Li+ (aq) + e- Li(s) 0 -0.15 -0.76 1.68 2.84 3.04

Explanation / Answer

a. Redox porential of the photosynthetic reaction site must be equal to

E = -1.23 V

b. An decrease in the pH of the reaction would favor the forward reaction.

c. Nature uses things which are benign and sustainable. MnO4- is toxic, strong and hazardous. Hence, it is not the reagent used for photosynthesis.

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