Exergonic reactions defined

CL is the emission of light by molecules that are excited by participation in a highly exergonic reaction, usually an oxidation. It can also be defined as the fluorescence of the electronically excited product of a chemical reaction however, unlike fluorescence, no external light source is used. 

Since we have defined active transport as a mechanism requiring an expenditure of energy, it is obvious that it must be coupled to an exergonic reaction in order to comply with the second law of thermodynamics. What is the nature of the free energy source utilized in the active transport ... 

Consider a reduction reaction like the ORR, where electrons are transferred from the snrface to the reactant (this corresponds to a + sign in Equation (11.8) and a - sign in Equation (11.9)). The minimum of the (7. s for the elementary steps making up the reaction defines the potential where all steps are exergonic, and this potential is termed the limiting potential for the reaction... 

Likewise, for an oxidation reaction like the OER, the maximum value of for which aU reaction steps are exergonic will define the limiting potential for the full reaction ... 

There is often a confusion between enthalpy, A,//, and Gibbs energy, A G defined in Equation (1). Negative values of A G correspond to exergonic reactions that proceed spontaneously in the direct sense, and conversely, positive values of A G correspond to endergonic reaction that will not proceed unless they are... 

The next point takes the standard-state idea and makes it more suitable for biological processes by defining a new AG°, called AG°. This new standard state is one with a pH of 7. This is the standard state used most of the time for biochemical reactions and is the one we will use. Not only does it make a big difference in reactions in which is consumed or produced, it also requir es us to be aware of the form in which various species exist at a pH of 7. A reaction that is endergonic at [H" ] = 1 M can easily become exergonic at [H" ] = 10 M (pH = 7) and vice versa. 

A consequence of Eq. (18) is that the maximum electron transfer rate occurs when AG° = —X. A plot of ln(ket) versus AG° is shown in Fig. 4. Electron transfer reactions with AG° more positive than —X define the normal region, where the rate of electron transfer increases with increasing exergonicity, whereas free energies negative relative to —X define the inverted region in which the rate decreases as AG° becomes more negative. For most ECL reactions, the... 

Since the key to understanding AG diagrams is the realization that they are relevant only to the standard state, we can ask what will happen if we change our definition of the standard state. This is very seldom done, but it can be quite instructive to do so. During this exercise, it is important to state right up front that any mathematical analysis will not affect the intrinsic stabilities of H, G, and H G. Yet, as you will see, it does effect what we define as an exergonic or endergonic reaction. 

Notice that the terms exergonic and endergonic refer to whether the reaction has a negative AG° or a positive AG°. Do not confuse these terms with exothermic and endothermic, which we will define later in this section.)... 

The reaction energy A is defined here as the difference in electronic energy between the initial (donor) and final (acceptor) states in their ground vibrational states, and is positive when the energy of the donor is higher than that of the acceptor, i.e., A > 0 for exergonic processes. It is more common to write the classical ET rate constant in terms of free-energies... 

---