Endergonic reactions defined

The Hammond and Cordes-Thornton coefficients can be utilised to characterise transition states in terms of structure reactivity surfaces as follows. The Hammond coefficient is defined so that it is positive when an increase in energy of the products (relative to reactants) accompanies an increase in the first derivative. This describes the generalisation (Hammond effect) that endergonic reactions tend to have product-like transition structures. A positive Hammond coefficient is expected for a fundamental process where structural change affects the energy of one end of a reaction coordinate, as a consequence of the maximum (negative curvature) at the saddle-point in the direction along the reaction coordinate. 

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. 

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. 

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.)... 

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