Electron Transfer Reactions Summary

Chemical vs Biological Oxidants

  • Chemistry oxidants (e.g., MnO<em>4−MnO<em>4^-, Cr</em>2O72−Cr</em>2O_7^{2-}) are toxic and unselective for biological systems.

Biological Oxidants

  • Coenzyme Q: Eo′=0.060extVE'_{o} = 0.060 ext{ V}
  • Flavine adenine dinucleotide (FAD): Eo′=−0.219extVE'_{o} = -0.219 ext{ V}
  • Nicotinamide adenine dinucleotide (NADH): Eo′=−0.320extVE'_{o} = -0.320 ext{ V}

Oxidation of Food

  • Food is oxidized in series, transferring electrons to Oxygen (O2) to produce water (H2O).
  • Biological oxidants must be able to be reduced and re-oxidized during electron transfer.

Transition Metal Ions

  • Cytochromes contain iron: Fe3++e−⇌Fe2+Fe^{3+} + e^{-} ⇌ Fe^{2+}.
  • Important in enzyme catalysis (e.g., copper/zinc superoxide dismutase).

Ligands

  • Definition: Molecules/ions with donor atoms (e.g., nitrogen, oxygen) that bond to transition metals.
  • Transition metals are Lewis acids, while ligands are Lewis bases.

Biological Ligands

  • In metalloproteins, donor atoms arise from amino acid side chains (e.g., glutamic acid, cysteine, histidine).
  • Ligands can form large complexes with transition metals.

Heme Ligands

  • Heme ligands have cyclic nitrogen arrangements, bonding to Fe2+/Fe3+.

Influence of Ligands on Reduction Potential

  • Ligands affect electron attraction to the metal ion, altering reduction potentials significantly for various complexes:
    • E<em>o([Fe(H</em>2O)<em>6]3+/[Fe(H</em>2O)6]2+)=0.77extVE<em>{o}([Fe(H</em>2O)<em>6]^{3+}/[Fe(H</em>2O)_6]^{2+}) = 0.77 ext{ V}
    • E<em>o([Fe(CN)</em>6]3+/[Fe(CN)6]2+)=0.28extVE<em>{o}([Fe(CN)</em>6]^{3+}/[Fe(CN)_6]^{2+}) = 0.28 ext{ V}
    • E<em>o([Fe(bpy)</em>3]3+/[Fe(bpy)3]2+)=1.18extVE<em>{o}([Fe(bpy)</em>3]^{3+}/[Fe(bpy)_3]^{2+}) = 1.18 ext{ V}

Electron Transfers in Cytochromes

  • Example reactions show electron transfer between cytochromes (e.g., Fe3+(extcyta)+Fe2+(extcytb)Fe^{3+}( ext{cyt a}) + Fe^{2+}( ext{cyt b})) with respective potentials:
    • Eo′([Fe3+(extcytb)]/[Fe2+(extcytb)])=0.050extVE_{o'}([Fe^{3+}( ext{cyt b})]/[Fe^{2+}( ext{cyt b})]) = 0.050 ext{ V}
    • Eo′([Fe3+(extcyta)]/[Fe2+(extcyta)])=0.254extVE_{o'}([Fe^{3+}( ext{cyt a})]/[Fe^{2+}( ext{cyt a})]) = 0.254 ext{ V}
    • Eo′([Fe3+(extcytc)]/[Fe2+(extcytc)])=0.290extVE_{o'}([Fe^{3+}( ext{cyt c})]/[Fe^{2+}( ext{cyt c})]) = 0.290 ext{ V}
  • Final electron transfer to produce water:
    • O<em>2+4H++4e−−>2H</em>2OO<em>2 + 4H^{+} + 4e^{-} -> 2H</em>2O