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Oxidation Reduction reactions

B R. A R. A ox. +. B ox. +. Transfer of electrons. +. Fe 2+. +. Cu 2+. Cu +. Fe 3+. Transfer of H + + e -. AH 2. A. +. B. +. BH 2. Transfer of hydride ions :H -. Addition of O 2 RCH3 + 1/2O 2 RCH 2 OH. AH 2. +. NAD. A. +. NADH + H +.

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Oxidation Reduction reactions

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  1. BR AR A ox + Box + • Transfer of electrons + Fe2+ + Cu2+ Cu+ Fe3+ • Transfer of H+ + e- AH2 A + B + BH2 • Transfer of hydride ions :H- • Addition of O2 • RCH3 + 1/2O2 RCH2OH AH2 + NAD A + NADH + H+ Oxidation Reduction reactions (see Horton: p319-323) Dehydrogenases

  2. Redox potential Eo’ the standard redox potential at pH 7 and 1 M concentrations • measure of the affinity for electrons • units of volts • the greater the affinity, the larger the • value of Eo’

  3. Eo’ -0.12V A + 2H+ + 2 e- B + 2H+ + 2 e- AH2 BH2 +0.10V AH2 A + B + BH2 Half cells or redox couples DEo’ = Eo’ acceptor – Eo’donar = + 0.1 -(-0.12) = + 0.22V

  4. V Salt bridge KCl . 1 MX + 1MX- 1M H2 + 1 atm H+ Agar bridge Eo of the H half cell is defined as 0 For biochemical reactions Eo’= -0.42V

  5. V V . . +0.771v Fe3+ H2 2H+ Fe2+ -0.197v H2 CH2OH 2H+ CH2O

  6. BR AR Aox + Box + DGo’ = -nFDEo’ For a reaction involving the transfer of 2 electrons: DGo’ = -nFDEo’ = (-2 x 96.5)DEo’ = - 193DEo’ n = number of electrons transferred F – Faraday’s constant = 96.485 kJ/Vmol DEo’ = difference in redox potential of e- accepterand e- donar = Eo’acceptor - Eo’ donar

  7. CH3CHO + NADH + H+ CH3CH2OH + NAD 2 half cells: CH3CHO + 2H+ + 2 e- Eo’ -0.197V +0.320 CH3CH2OH NAD + 2H+ + 2 e- NADH + H+ Example of calculation of Eo’ DEo’ = Eo’ acceptor - Eo’donor = 0.320 –(-0.197) = 0.320 + 0.197 Positive DEo’ = spontaneous rx = +0.517 DGo’ = -nFD Eo’ = -2 x 96.5 x 0.517 = -99.8kJ/mol Negative DGo’ = spontaneous rx

  8. NADH + H+ NAD + 2H+ + 2e- Eo’ -0.32V +0.82V 1/2 O2 + 2H+ + 2e- H2O Oxidation of NADH by oxygen NADH + H+ + O2 NAD + H2O DEo’ = Eo’ acceptor - Eo’donor = +0.82 - (-0.32) = +1.14V DGo’ = -nFD Eo’ = -2 x 96.5 x 1.14 = -220kJ/mol

  9. RT RT nF F Nernst equation (see page 333 Horton) • for determination of actual DEo’ DE = DEo’ - ln Q At 25oc (298oK) for 1 e transfer = 0.026 and equation becomes 0.026 DE = DEo’ - ln Q n

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