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Biological Redox Reactions
reduction Potentials and half reactions
10
Biochemistry
03/03/2011

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Term
Redox Reaction
Definition
  • loss of electron of one species (oxidized) and the gain of electrons of another (reduced)
  • responsible indirectly for all work done by living organisims
  • electrons move from various metabolic intermediated to specialized electron carriers who in turn donate them to acceptors with a higher affinity
  • this releases energy which can be used for work
Term
The flow of Electrons can do Biological Work
Definition
  • consider a circuit with two chemical species with different electron affinities
  • because of this affinity difference, electrons flow spontaneously through the circuit driven by a force proportional to the difference in electron affinity called the emf
  • glucose is the electron source in a biological circuit and as it is oxidized it releases its electrons to carriers with higher affinity and to O2
Term

Oxidation Reduction reactions can be described as half reactions

 

Definition
  • redox reactions occur together but can be separated as 1/2 reactions:

Fe(2+) + Cu (2+) <--> Fe(3+) + Cu(+)

 

Fe(2+) <-->Fe(3+) + e(-)

Cu(2+) + e(-) <-->Cu(+)

  • electron donating agent is called the reductant, other is oxidant
  • fe(2+) and 3+ are conjugate redox pair
Term
Biological Oxidations Often Involve Dehydrogenation
Definition
  • Oxidation states of carbons based on the affinities of H<C<S<N<O
  • in bio systems, the loss of electons, oxidation, is coincident with loss of hydrogen (b/c carbon can only get electrons from hydrogen)
  • oxidation is therfore synonymous with dehydrogenation
Term
Oxidation states of Carbon
Definition
figure 13-22
Term
Electrons are transferred from One molecule to Another in one of Four Wayzzzz
Definition
  1. directly as electrons. in

    Fe(2+) + Cu (2+) <--> Fe(3+) + Cu(+) the Fe pair donates electrons to the Cu pair

  2. as hydrogen atoms. hydrogen atom  is a proton and H(+) and one electron
  3. as a hydride ion (:H-) which has 2 electrons
  4. through direct combination with oxygen. Oxygen is covalently inducted into the product and is considered the electron acceptor

reducing equivalent is pair a of electrons as a unit

Term
Reduction Potentials Measure Affinity for Electrons
Definition
  • When two conjugate redox pairs are in solution together, electron xfer may proceed spontaneously
  • tendency for such a reaction to happen depends on the relative affinity of the electron acceptor of each redox pair
  • the standard reduction potential, E^o, a measure (volts) can be determined experimentally for each half reaction
  • H(+) + e(-) --> 1/2H2 is standard of reference and is assigned 0.0v
  • when this hydrogen electrode is connected through a circuit to another half cell with another redox pair in it, electrons flow through the exernal circuit from the 1/2 cell with lower to higher  E^o.
  • a 1/2 cell that takes electrons from hydrogen cell is positive and one that loses electrons is negative
  • that with the larger E will be reduced
  • theses are at standared concentrations at std conditions
Term
Nernst equation
Definition
  • tells us that reduction potential is not affected by chemical species but by relative donor/acceptor concentrations
  • nernst equation relates standared reduction potential with actual reduction potential at any redox pair concentrations:
  • E=E^o +(RT/nFaraday)ln[electron acceptor]/[electron donor]
  • at 298K this is now
  • E=E^o + (0.026V/n)ln[electron acceptor]/[electron donor]
Term
Standard Reduction Potentials Can Be used To calculate Free Energy Change
Definition
  • energy made avialable by this electron flow is proportional to delta E:
  • delta G=-n*faraday*delta E or standard delta G and Standard delta E
  • n is number of electrons transferred in the reaction
  • can calculate the actual free energy change for any redox reaction from the E values in a reduction table
Term
Definition