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Ubiquinone-10 from the photochemical reaction center

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Ubiquinone-10 from the photochemical reaction center of Rhodobacter sphaeroides

• QH <==> Q^- + H⁺ ----- pK₁
• QH₂ <==> QH^- + H⁺ ----- pK₂
• QH^- <==> Q^2- + H⁺ ----- pK₃

Half-cell reactions:

• quinone/hydroquinone
  Midpoint redox potential: E_{m, pH 7} = 90 mV
  QH₂ ==> Q + 2H⁺ + 2e^- (below pK₂) (z=2, n=2)
  QH^- ==> Q + H⁺ + 2e^- (above pK₂) (z=2, n=1)
  Q^2- ==> Q + 2e^- (above pK₃) (z=2, n=0)
  
• quinone/semiquinone
  Potentials depend on stability of semiquinone
  QH ==> Q + H⁺ + e^- (below pK₁) (z=1, n=1)
  Q^- ==> Q + e^- (above pK₁) (z=1, n=0)
  
• semiquinone/hydroquinone
  Potentials depend on stability of semiquinone
  QH₂ ==> QH + H⁺ + e^- (below pK₁) (z=1, n=1)
  QH₂ ==> Q^- + 2H⁺ + e^- (below pK₂, but above pK₁) (z=1, n=2)
  QH^- ==> Q^- + H⁺ + e^- (below pK₃, but above pK₂) (z=1, n=1)
  Q^2- ==> Q^- + e^- (above pK₃) (z=1, n=0)