O

rectly from one atom or molecule to another without a water-dissolved intermediate. In this type of oxidation reaction, electrons often are transferred to small electron-carrying molecules, sometimes referred to as coenzymes. The most common of these electron carriers are NAD+ (nicotinamide adenine dinucleotide), which is reduced to NADH, and FAD (flavin adenine dinucleotide), which is reduced to FADH2 (Figure 2-26). The reduced forms of these coenzymes can transfer protons and electrons to other molecules, thereby reducing them.

To describe redox reactions, such as the reaction of ferrous ion (Fe2+) and oxygen (O2), it is easiest to divide them into two half-reactions:

Oxidation of Fe

2 Fe2

In this case, the reduced oxygen (O2-) readily reacts with two protons to form one water molecule (H2O). The readiness with which an atom or a molecule gains an electron is its reduction potential E. The tendency to lose electrons, the oxidation potential, has the same magnitude but opposite sign as the reduction potential for the reverse reaction.

Reduction potentials are measured in volts (V) from an arbitrary zero point set at the reduction potential of the following half-reaction under standard conditions (25 °C, 1 atm, and reactants at 1 M):

reduction

oxidation

The value of E for a molecule or an atom under standard conditions is its standard reduction potential E'0. A molecule or ion with a positive E'0 has a higher affinity for electrons than the H+ ion does under standard conditions. Conversely, a molecule or ion with a negative E'0 has a lower affinity for electrons than the H+ ion does under standard conditions. Like the values of AG0', standard reduction potentials may differ somewhat from those found under the conditions in a cell because the concentrations of reactants in a cell are not 1 M.

In a redox reaction, electrons move spontaneously toward atoms or molecules having more positive reduction potentials. In other words, a compound having a more negative reduction potential can transfer electrons to (i.e., reduce) a compound with a more positive reduction potential. In this type of reaction, the change in electric potential AE is the sum of the reduction and oxidation potentials for the two half-reactions. The AE for a redox reaction is related to the change in free energy AG by the following expression:

where n is the number of electrons transferred. Note that a redox reaction with a positive AE value will have a negative AG and thus will tend to proceed from left to right.

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